moulds design

Awareness on Plastics &Moulds

H & GT - GTCI - Bangalore - July 2005 2


Contents

> Introduction

> Materials

> Moulding defects and remedies

> Design Considerations

> Design Guidelines

> Mould Manufacturing

> Advanced Moulding Technology

> Injection Moulding Machine

> Design for Manufacturing and Assembly Guidelines


Introduction to MouldsWhat is mould?

Moulding is a manufacturing technique for making parts from plastic material.

•Molten plastic is injected at high pressure into a mold, which is the inverse ofthe desired shape.

•The mould is made by a mold maker from metal, usually either steel oraluminum, and precision-machined to form the features of the desired part.

•Injection moulding is very widely used for manufacturing a variety of parts,from the smallest component to entire body panels of cars.

Why is injection moulding awareness needed for Product Designing ?

Considerable thought should be put into the design of moulded parts and theirmoulds, to ensure that the parts will not be trapped in the mould, that the mouldscan be completely filled before the molten resin solidifies, and to minimizeimperfections in the parts, which can occur due to peculiarities of the DesignProcess.

ManMaterialManufacturingMachine

4 M’s to beconsidered


> General Definition of MaterialsMaterials

Materials for moulding:

1. Natural Rubber.

2. Synthetic Rubber

3. Thermoset Plastic.

4. Thermo Plastic.

Material is the substance or matter from which something is or can be made, oralso items needed for doing or creating something.


> Criteria for selecting MaterialsMaterials

Physical & Mechanical Considerations

•What are the overall part dimensions (diameter, length, width,thickness)?

•What load will the part have to carry?

•Will the design carry high loads?

•What will the highest load be?

•What is the maximum stress on the part?

•What kind of stress is it (tensile, flexural, etc.)?

•How long will the load be applied?

•Will the load be continuous or intermittent?

•Does the part have to retain its dimensional shape?

•What is the projected life of the part or design



Thermal Considerations•What temperatures will the part see and for how long?

•What is the maximum temperature the material must sustain?

•What is the minimum temperature the material will sustain?

•How long will the material be at these temperatures?

•Will the material have to withstand impact at the low temperature?

•What kind of dimensional stability is required (is thermal expansion andcontraction an issue)?

Bearing and Wear Considerations

•Will the material be used as a bearing? Will it need to resist wear?

•Will the material be expected to perform as a bearing? If so, what will the load,shaft diameter, shaft material, shaft finish, and rpm be?

•What wear or abrasion condition will the material see?

Note:

Materials filled withfriction reducers(such as PTFE,molybdenumdisulfide, orgraphite) generallyexhibit less



Chemical Considerations•Will the material be exposed to chemicals or moisture?

•Will the material be exposed to normal relative humidity?

•Will the material be submerged in water? If so, at what temperature?

•Will the material be exposed to steam?

•Will the material be painted? If so, what kind of paint?

•Will the material be glued? If so, what kind of adhesive will be used?

•Will the material be submerged or wiped with solvents or other chemicals? If so,which ones?

•Will the material be exposed to chemical or solvent vapors? If so, which If so,which ones?

•Will the material be exposed to other materials that can outgas or leachdetrimental materials, such as plasticizers or petroleum-based chemicals?



Other Miscellaneous ConsiderationsWill the part have to meet any regulatory requirements?

Is UL94 Flame retardant rating required? What level?

Should the material have a special color and/or appearance?

Natural | White | Black | Other Colors

Color match to another part or material?

Window-Clear | Transparent | Translucent | Opaque

Smooth | Polished | Textured | One-Side or Both

Will the part be used outdoors?

Is UV Resistance needed?

Is static dissipation or conductivity important?

Insulator | Static Dissipative | Conductive


> RubberMaterials

1. A naturally gifted plastic.

2. Has many applications in industrial and consumer goods.

3. Only group of materials able to provide elastic properties across a wide rangeof temperatures.

The rubber family includes a diverse range of materials - as varied as"metals" or "plastics".


> Introduction to Rubber ManufacturingMaterials

Rubber The two types of rubber in common use today are Natural and Synthetic.

Natural rubber comes from the rubber tree (Hevea brasiliensis).

When a tree matures at the age of six or seven years, the latex is collected froma diagonal incision in the tree trunk. The tapping process does not affect thehealth of the tree and the tree wound later heals itself.

Synthetic rubber is made by man from petrochemical feedstock. Crude oil isthe principal raw material.


> Uses of RubberMaterials

Rubber

It can be used over a temperaturerange from -80°C to +300°C

Designers choose rubber because ofits wide range of properties

It can be electrically insulated,conductive or anti-static



RubberIt is available in a wide range ofcolors and textures

It can withstand extremes ofweather and outdoor environment



Rubber It can withstand exposure to fuels,oils and chemicals while retaining itsproperties

It can be made flame retardant andself extinguishing, with halogen freeand smoke suppressant typesavailable

It can absorb vibration and noise and actas an insulator



Rubber It can maintain tension and compression forces indefinitely - for example inseals. It can be gas tight and used as a fluid seal or separator

Gaskets andOil Sealsused in Engine



RubberIt has low thermal conductivity andcan be used to reduce heat transfer

It has friction properties similar to humanskin and is comfortable to grip



RubberIt can have a clean, smoothsurface which is non-stick andsuitable for hygienic applications

It is compatible with otherengineering materials (e.g. metals,plastics and ceramics) and can becombined with them in manydifferent ways, including bonding.


> PlasticsMaterials

The term plastics covers a range of synthetic or semi-synthetic organiccondensation or polymerization products that can be molded or extruded intoobjects or films or fibers.

•Their name is derived from the fact that in their semi-liquid state they aremalleable, or have the property of plasticity.

•Plastics vary immensely in heat tolerance, hardness, and resiliency.

•Combined with adaptability, the general uniformity of composition and lightnessof plastics ensures their use in almost all industrial applications today.

•Plastic may also refer to any material characterized by deformation or failureunder shear stress.

•Plastics offer extraordinary advantages in product manufacturing. Because theyare easily softened or melted, they can be molded into almost any shape.

•Plastics have replaced traditional materials like metals and wood in countlessapplications because of their cost effectiveness and property attributes.


Plastics can be divided into two processing groups

Thermoplastics and Thermosets

Materials

Plastics

Thermoplastic1.It is heated and pressed into a mould.

2.No chemical reaction of any kind takes place.

3.Once the plastic has cooled and hardened in this shape, it could be reheatedand remoulded without any perceptible change in its properties.

Granules Products


Materials

Plastics

Thermosets1. Undergo chemical change while they are being formed.

2. They react by polycondensation and cross-link to form a three-dimensional lattice.

3. Once a thermoset has achieved its final shape, it cannot be reformed.

4. Examples of thermosets are phenolic resins, melamines and urea resins

Granules Products


Materials

Plastics

Advantages of PlasticsPlastics can provide the following advantages for product designers andmanufacturers:

•Design Flexibility

•High Strength and Toughness

•Corrosion Resistance

•Reduced Manufacturing Costs

•Almost Any Color or Surface Texture

•Waterproof

•Stiffness or Ductility

•Low Weight

•High Manufacturing Throughput

•High Reproducibility of Parts

•Electrical Insulation

•Thermal Insulation


Materials

Plastics

Different types of Plastics•Polyethylene (PE)•Polyurethane (PU)•Polypropylene (PP)•Polyethylene Terephthalate (PETE)•Polyamide (PA) or Nylon•Polyester•Polyvinyl Chloride (PVC)•Polycarbonate (PC)•Acrylonitrile Butadiene Styrene (ABS)•Acetal


Materials

Plastics

Polyethylene (PE)Polyethylene or polyethene is one of the simplest and most inexpensive polymers.It is a waxy, chemically inert plastic.

Properties:

•Thermoplastic.

•Toughness

•Ease of processing

•Chemical resistance

•Abrasion resistance

•Electrical properties

•Impact resistance

•Low coefficient of friction

•Near-zero moisture absorption

•Translucent.

UsesFilm, bags, pipe and tubing,insulating sleeves, bottle stoppers,lids, plastic wrap, toys.


Materials

Plastics

Polyurethane (PU)Polyurethane is a unique material that offers the elasticity of rubber combined withthe toughness and durability of metal

Properties:•Abrasion resistant•Oil and solvent resistant•Load bearing capacity•Tear resistant•Weather resistant•High flex-life•Electrical insulating properties•Heat and cold resistant

UsesBelts, Metal forming pads, Wearstrips, Bumpers, Gears, Bellows,Machinery mounts, Cutting Surfaces,Sound-dampening pads, Chute andhopper liners, Prototype machinedparts, Foam, Gaskets, Seals,Rollers, Roller covers, Sandblastcurtains, Diaphragms


Materials

Plastics

Polypropylene (PP)Polypropylene is an economical material that offers a combination of outstandingphysical, chemical, mechanical, thermal and electrical properties not found in anyother thermoplastic.Properties:•Thermoplastic• Lightweight•High tensile strength•Impact resistant•High compressive strength•Excellent dielectric properties•Resists most alkalis and acids•Resists stress cracking•Retains stiffness and flex•Low moisture absorption•Non-toxic•Non-staining•Easily fabricated•High heat resistance

UsesHousehold items, plastic wrap,automobile parts, batteries,bumpers, garden furniture, syringes,bottles, appliances


Materials

Plastics

Polystyrene (PS)Polystyrene is a polymer made from styrene, a liquid that is commerciallymanufactured from petroleum, although it is also found in plants.

Properties:

•Thermoplastic

•Transparent

•Nontoxic

•Optical and electrical properties

•Easy to color

•Resistant to X rays, oils, and grease

Uses

Plastic wrap, kitchen utensils,furniture covers, thermal insulation,toys, office supplies, disposablerazors


Materials

Plastics

Polyethylene Terephthalete (PETE)Polyethylene terephthalate (aka PET, PETE, PETP) is a plastic resin of thepolyester family, used to make some thermoforming applications. It is also one ofthe most important raw material for man-made fibers.

Its main virtue is that it is fully recyclable as you can recover its polymer chains,unlike most other plastics

Properties:

•Thermoplastic

•Extremely hard

•Wear-resistant

•Dimensionally stable

•Resistant to chemicals

•Good dielectric properties.

UsesSoft drink bottles, peanut butter jars,salad dressing bottles, nonbreakable bottles


Materials

Plastics

Polyamide (PA) or NylonNylon is a condensation polymer made of repeating units with amide linkagesbetween them: hence it is frequently referred to as a polyamide. It was the firstsynthetic fibre to be made entirely from inorganic ingredients: coal, water and air.

Properties:

• Very good physical properties

• Moisture has significant effect onproperties

• Very good heat resistance

• Excellent chemical resistance

• Excellent wear resistance

• Moderate to high price

• Fair to easy processing

UsesElectrical connectors, gear, slide, cams andbearings, cable ties and film packaging, fluidreservoirs, fishing line, brush bristles,automotive oil pans, fabric, carpeting,sportswear, sports & recreational equipment


Materials

Plastics

PolyesterPolyester is a category of polymers which contain the ester functional group intheir main chain.

Properties:

• Strong

• Resistant to stretching andshrinking

• Resistant to most chemicals

• Quick drying

• Crisp and resilient when wet or dry

• Wrinkle resistant

• Abrasion resistant

• Easily washed

UsesFilters, conveyor belts, sleeping baginsulation, coat insulation, tire cords.


Materials

Plastics

Polyvinyl Chloride (PVC)Polyvinyl chloride is produced from its monomer, vinyl chloride. PVC is a hardplastic that is made softer and more flexible by the addition of phthalates. Polyvinylchloride (PVC) is a flexible or rigid material that is chemically non reactive..

Properties:• High strength

• Economical

• Dimensional stability

• Good weather resistance

• High impact strength

• Clarity

• Colorability

• Flexible or rigid

• Chemically inert

• Ease of fabrication

• Tasteless, odorless, non-toxic

• Good electrical properties

UsesNuts, filters, signs, tanks, pipes, bolts, valves,bushings, tank and pool liners, laboratory equipmentducts, sprinkler systems, photo mounting, wallcoverings, pump parts, fittings


Materials

Plastics

Polycarbonate (PC)Polycarbonates are a particular group of polymers that are moldable under heat;as such, these plastic are very widely used in modern manufacturing. Thisversatile thermoplastic maintains its properties over a wide range of temperatures,from -40"F to 280"F.

Properties:

• Thermoplastic

• High dielectric strength

• Unbreakable

• Machinability

• High impact strength

• Dimensional stability

• Thermal stability

• Stain resistant

• Non-toxic

• Low water absorption

UsesLenses, high temperature and pressurewindows,face shields industrial equipment andhousing components, medical equipmentcomponents, instrument components, electricalinsulators and connectors


Materials

Plastics

Acrylonitrile Butadiene Styrene (ABS)Acrylonitrile butadiene styrene, or ABS is a common thermoplastic used to makelight, rigid, moulded products.

Properties:

• Good chemical resistance

• Stress cracking resistance toinorganic salt solutions, alkalis,acids, and some oils

• Excellent abrasion resistance

• Electrical properties

• Moisture

• Creep resistance .

Uses

Machine parts, prototypes, tote bins andtrays, automotive parts, business machinehousing and parts, aircraft interior trim,industrial enclosures


Materials

Plastics

AcetalAcetal is a crystalline thermoplastic polymer with a high melting point. It is suitablefor mechanical parts or electrical insulators that require structural strength atabove normal temperatures.

Properties:

• High modules of elasticity.

• High strength and stiffness.

• Low coefficient of friction.

• Good abrasion and impactresistance.

• Low moisture absorption.

• Excellent machinability.

• Natural lubricity.

• Resistant to gasoline, solvents,and other neutral chemicals.

Uses

Pump and valve components,gears,bearings,bushings, rollers, fittings,electrical insulatorparts


Materials

Sheet MouldingCompound

Special Purpose MaterialsSMC or Sheet Moulding Compound

Definition:

•A fiber glass reinforced thermosetting compound in sheet form, usually rolled intocoils interleaved with plastic film to prevent auto adhesion.

•Made by dispensing mixed resin, fillers, maturation agent, catalyst and moldrelease agent onto two moving sheets of polyethylene film.

•The lower one also contains chopped glass roving or glass mat. SMC can bemolded into complex shapes with little scrap.

Advantages

•Processing of SMC by compression or injection moulding enables the productionof bodywork or structural automotive components, and electrical or electronicmachine housings in large industrial volumes.

•The process also penetrates sectors such as sanitary ware (baths) and urbanfurniture (stadium and cinema seating) etc.


Materials


Composition of SMCSMC or Sheet Moulding Compound


Materials


SMC Manufacturing Process


Materials


Advantages of SMCPart Consolidation:A well designed composite part can easily eliminate the assembly of many metalparts by allowing you to mold them as one complete piece. In addition, insertscan be molded into the SMC material to aid in the assembly process.

Design Flexibility:Parts molded in polyester or vinylester composite materials can reproducealmost any shape desired.

Dimensional Stability:Products made from composite materials offer a greater degree of dimensionalstability when compared to thermoplastics, wood, and some metals.

Light Weight:Composite parts offer more strength per unit of weight than any un-reinforcedplastic and most metals.

High Strength:Composite parts can be designed to provide a wide range of impact, tensile, andflexural strength properties, depending on the specific requirements of theapplication.


Materials


Advantages of SMCCorrosion Resistance:Composites do not rust or corrode, and offer various levels of chemical andenvironmental resistance.

Low Electrical and Thermal Conductivity:Composites can offer a wide range of insulating properties to meet specificrequirements for electrical and thermal resistance.


Materials


SMC Components


Materials

Bulk MouldingCompound

Special Purpose MaterialsDMC or Dough Moulding Compound

Also called as BMC or Bulk Moulding Compound

Definition:

Molding compound consisting of thermosetting plastic resins mixed with strandedreinforcement, fillers, and other additives. This viscous compound can be used forcompression or injection molding.

DMC is a combination of chopped glass strands with resin in the form of a bulkprepreg.

BMC is suitable for either compression or injection moulding.

Injection moulding of BMC is used to produce complex components such as

1. Electrical equipment

2. Car components (headlamps are an important application for BMC)

3. Housings for electrical appliances

4. Tools, in large industrial volumes.


Composition of BMC

BMC or Dough Moulding Compound

Materials


BMC Manufacturing Machine


Materials


Advantages of BMCTechnical Advantages:

•Very rigid and stiffer than thermoplastics

•Ability to integrate all the housing functions

•No mechanical rework needed

•No paint: BMC can be mass coloured

•Reduced cycle time compared to aluminium

•Longer life of BMC tools

•Dimensional stability and higher precision compared to aluminium

•Noise reduction : BMC has a dampening effect on vibrations

Economic Advantages:

•50 % total cost savings in comparison to cast aluminium


Materials


BMC vs. Thermoplastics•Dimensional accuracy•Dimensional stability (creep resistance) over a broad range of temperatures•Good property retention over long term / high temperature conditions (ageing)•Low thermal linear expansion (same as steel)•High mechanical properties (strength, stiffness and impact)•Excellent electrical properties•Design flexibility, thin to thick variable component sections•Inserts can be integrated in the moulding process•Direct screw assembly without previous threading•Corrosion resistance in aggressive environments including solvents•Non-melting, flame retardant , low smoke density, low toxicity and no halogens•Fire resistant UL94 V-0; glow wire 9600

•Customizing to meet specific needs, good speed to market•BMC can be mass coloured to match customer specifications•Lower mould cavity pressure•Faster cycle time (at medium to high wall thickness)•Machine workability•Low cost per litre


BMC ComponentsMaterials



Materials

Mould(Manufacturing)

Types of Moulds

1. Compression Mould

2. Transfer Mould

3. Extrusion Mould

4. Injection Mould

5. Hot Runner Mould.

6. Blow Mould


Types of Moulds Compression MouldsCompression Moulding is one of three processes used to mold parts.Compression molding is the oldest and simplest way to make products. Insome specific applications, compression molding is still the best way.

To put it simply, compression molding involves squishing a chunk of uncuredmaterial into a pocket in the mould. After time, heat and pressure the materialcures in the shape of the pocket. The mold can then be opened and the partremovedStep #1 - A piece of uncured material is placed in the mold.Step #2 & 3 - The mold is closed up and held under hydraulic pressure while the

material cures.Step #4 - When the mold opens the part can be removed. The excess material,called flash, needs to be trimmed off the part.


Types of MouldsCompression Moulds

Parts of Compression Moulds

Compression Mould - Closed


Types of MouldsCompression Moulds

Advantages & DisadvantagesAdvantages of Compression Molding

Lowest cost molds

Little "throw away" material provides advantage on expensive compounds

Often better for large parts

Disadvantages of Compression Molding

Offers least product consistency

Difficult to control flash

Not suited for some types of parts


Types of Moulds Transfer MouldsTransfer Molding involves having a "piston and cylinder"-like device built into themold so that the rubber may be squirted into the cavity through small holes.

Pot Transfer Molding

Step #1 - A piece of uncured material is placed into a portion of the mold calledthe "pot." The plunger (on the top-most part of the mold) fits snugly into the "pot."Step #2 - The mold is closed up and under hydraulic pressure the material isforced through the small hole (the ”sprue") into the cavity. The mold is held closedwhile the material cures.

Step #3 - The plunger is raised up and the "transfer pad" material may beremoved and thrown away. Mold is opened and the part can be removed. Theflash and the gate may need to be trimmed.


Types of MouldsTransfer Moulds

Plunger Transfer Molding

Step #1 - A piece of uncured material is placed into a portion of the moldcalled the "pot." The plunger (on the top-most part of the mold) fits snugly intothe "pot."

Step #2 - The mold is closed up and under hydraulic pressure the material isforced through the “cull” and to the gate into the cavity. The mold is heldclosed while the material cures.

Step #3 - The plunger is raised up, Mold is opened and the part can beremoved. The flash and the Cull may need to be trimmed.



Movie - Transfer Mould Operation



Advantages & Disadvantages

Advantages of Transfer Molding

•Provides more product consistency than compression molding

•Cycle times are shorter than compression molding

•Better than compression molding for rubber-to-metal bonding

Disadvantages of Transfer Molding

•The transfer pad is scrap

•Cycle time is longer than injection molding

•Product consistency is poorer than injection molding


Types of Moulds Extrusion MouldsExtrusion moulding is a method used to form thermoplastic materials intocontinuous sheet film, tubes, rods, and other shapes, and to coat wiring and cable.

The process produces continuous two dimensional shapes like sheet, pipe, film,tubing, gasketing, etc.

The material is fed into the extruder where it is melted and pumped out of theextrusion die.


Types of MouldsExtrusion Moulds

Movie - Extrusion Mould Operation


Types of Moulds Blow MouldsBlowing molding is the primary method to form hollow plastic objects such assoda bottles.

Blow molding is another common type of plastic molding. In this process a plastictubular form, produced by extrusion or injection molding, is used to form the part.This form, called a parson, is softened inside a mold and then injected with air orother compressed gas. This expands the parson against the sides of the moldcavity, forming a hollow object the size and shape of the mold.

Step #1 - The Parision is Extruded from injection Unit

Step #2 - The mold is closed onto Parision and then the Parision is inflatedagainst the walls of the Mould by using a Air.

Step #3 - The Mould is opened and the Part is collected


Types of MouldsBlow Moulds

2 Movies - Blow Mould Operation

Blow Mould


Types of Moulds Injection MouldsInjection Molding is the most advanced typical method of molding plasticproducts. Injection molding produces the most consistent results by automating allaspects of how the material gets into the mold. In injection molding, the material isworked and warmed and then squirted into the mold at controlled speeds,pressures and temperatures.

Step #1 - Mold is closed and clamped.

Step #2 - A shot of melt is injected under high pressure into the mold cavity.

Step #3 - The screw is rotated and retracted and the polymer in the mold hascompletely solidified.

Step #4 - The mold is opened, and the part is ejected and removed.


4 major steps in Injection MouldingTypes of MouldsInjection Moulds


Types of MouldsInjection Moulds

Advantages & Disadvantages

Advantages of Injection Molding

•Provides the maximum product consistency

•Allows the most control of flash

•Because the material is warmed before going into the mold, fastest cycle times.

Disadvantages of Injection Molding

•Not suited for all compounds

•Most expensive molds

•Typically has some runners or other "throw away" portion in each shot



Parts of a typical mould base



Top Half


This part is fitted on to the front face of the mold to serve thepurpose of locating the mould in the correct position in alignmentto the machine nozzle and directly align the sprue bush hole.

Locating Ring:

Sprue Bush:It is the connecting member between nozzle and the runnersystem.The plasticized material is transferred to theimpression through a passage termed as a “Sprue”. TheSprue Bush radius is always more than the nozzle radius toavoid leakage of plastic material.

Top Clamp Plate:It is the top most part of the mould assembly and is usedfor clamping top assembly on the machine platen.



Top Half


This plate incorporates the cavity inserts and also help toincorporate cooling media into the mold

Cavity Plate ( A Plate)

Guide PillarsThe accurate mould assembly need the perfect alignmentbetween top half and bottom half at any point of time in amould cycle.The needed services is provided by guide pinswhich guided into the guide bush in other half.

CavityThe space inside a mold into which material is injected. Thematerial injected will take the form of the cavity profile.



Bottom Half


This plate incorporates the Core inserts and also help toincorporate cooling media into the mold

Core Plate ( B Plate)

Guide BushThe accurate mould assembly need the perfect alignmentbetween top half and bottom half at any point of time in amould cycle.The needed services is provided by guide bushwhich guides the guide pillar in other half.

Support PlateThis plate will give extra support for core plate. Whichreduces the deflection caused by injection force.



Bottom Half


Pins that are pushed into a mold cavity from the rear as the moldopens to force the finished part out of the mold.

Ejector Pins

Ejector Return PinPins that push the ejector assembly back as the mold closes.Also called surface pins or return pins

Sprue Puller PinThis member pulls the Sprue from the Sprue Bush. Thiscan also be used as a cold slug well by reducing the heightfrom parting line.



Bottom Half


This plate will retain the ejector pins in its position as the ejectorassembly is pushed back.

Ejector Retainer Plate

Ejector PlateThe Ejector Plate is clamped to Ejector Retainer Plate. As themold opens the ejector rod pushes the ejector plate (assembly),which results in ejection of component

Ejector HousingThis is the bottom most part of the mould assembly and isused for housing the ejector assembly and also forclamping to the moving platen of the moulding machine




Plastic Part

Locating Ring Sprue Bush

A-Plate

Cavity

Core

B-Plate

Ejector Pin

Ejector Housing

Guided Ejection Support Pillar

Ejector Plate

EjectorRetainer Plate

Retainer Pin

Guide Bush

Guide Pillar

3D View of theMould Parts



Types of Injection Moulds



Two Plate MouldA two plate mould is the simplest type of mould. It is called a two plate mouldbecause there is one parting plane, and the mould splits into two halves. Therunner system must be located on this parting plane; thus the part can only begated on its perimeter.



Two Plate Mould

Functioningof two platemould



Three Plate MouldA three plate mould differs from a two plate in that it has two parting planes, andthe mould splits into three sections every time the part is ejected. Since themould has two parting planes, the runner system can be located on one, andthe part on the other. Three plate moulds are used because of their flexibility ingating location. A part can be gated virtually anywhere along its surface.



Three Plate Mould

Functioningof three platemould



Three Plate Mould




Different Types of Mould - Movies

Two Plate mould(Pin ejection)




Two Plate mould(Stripper ejection)




Two Plate mould(Core Side Stripperejection)




Three Plate mould(Pin ejection)




Stack mould(Stripper ejection)




Shuttle mould(Pin ejection)




Unit mould(Pin ejection)



Hot Runner MouldsHot runner molds are two plate molds with a heated runner system inside onehalf of the mold. A hot runner system is divided into two parts: the manifold andthe drops. The manifold has channels that convey the plastic on a single plane,parallel to the parting line, to a point above the cavity. The drops, situatedperpendicular to the manifold, convey the plastic from the manifold to the part.



Hot Runner Mould

Hot Runner Moulds



Hot Runner Mould

Advantages & DisadvantagesWhile Hot Runner Molds are typically more expensive than "Cold Runner"molds, the cost of the mold can be offset in other ways.

Thermoplastic Hot Runner Molds can reduce costs due to :•No scraping of the the runner.•Reducing the cycle time.•Injection time is reduced due to the shot size being reduced by the eliminationof the runner.•Improves both part and mold design with flexibility of gating locations, whichprovides options for cavity orientation.•Pressure drops are greatly reduced due to the balanced melt flow as thetemperature is consistent from the machine nozzle to the gate.•Precise material temperature control is critical to successful Hot Runnerprocessing.


Over MouldingOvermoulding is an injection molding process using two separate moulds ofwhich you mould one material over another to create or touch appeal such as ahandle or knob.


Advanced MouldingTechnologies

Over molding


•Two color Over Molding

Types of Over Moulding

•Two Material Over Molding

•Insert Over Molding




Double Injection MoldingThis process produces a soft feel to the product as wellas improved impact properties and an increased value.

This process has been used for keycaps and buttons ontelephones and other products for many years. The molded-in graphics are embedded in the part and will not wear offwith use. Clear sections can be utilized with back lighting forreadability in dim or dark applications.

Two Shot Injection Molding

In Mold Decoration

This application is relatively new to the plastics industry.Using IMD, multiple colors and graphics can be added ina single operation.

Over Moulding TechnologiesTypes of MouldsInjection Moulds



Over MouldsTypes of MouldsInjection Moulds



• Grabs consumer attention - a proven concept to build market share• Improved feel and appearance• Provides a soft grip or feel• Improved ergonomics• Provides a safe, tactile grip in wet environments• Eliminates / reduces assembly labor• Eliminates need for mechanical fasteners and adhesives• Ease of color match with no secondary painting required• Improved impact resistance• Sealing in fluid environment• Provides high friction surface• Vibration damping• Sound absorption

Advantages & DisadvantagesTypes of MouldsInjection Moulds





Gas Injection MouldingThe gas injection technique (GIT) is a special injection molding method. Afterthe actual injection molding operation, a permanent cavity is created in themolding as a second step by means of an inert compressed gas (nitrogen).The plastic is pressed against the mold wall by maintaining the gas pressureduring the solidification process, thus defining the external contour of thecomponent.

Gas InjectionTechnology

Gas Injection Method Blow Out Method




Advantages & Disadvantages - Gas Injection Moulding

Advantages•Greater design freedom (thick-walled, rod-shaped parts possible)

•High degree of rigidity due to larger closed cross-section profiles

•Reduction of sink marks

•Uniform shrinkage and thus less distortion

•Shorter cycle times as compared to thick-walled compact parts

•With rod-shaped parts weight savings of up to approx. 50%

Disadvantages•Additional costs for gas, gas pressure system and injection device

•Higher expenditure for quality assurance may be necessary

•Risk of surface faults (e.g. switchover markings)

•Possibly greater startup losses with more complex moldings

•Restrictions in the selection of material and with subsequent material changeovers

•Composite molds more difficult than in the case of conventional injection molding




Rotational or Roto MouldingRotational or rotomoulding is an extremely popular and well-used process forproducing items that are usually hollow.

•Used for very large articles which are usually made in small quantities.•Items such as children's toys, garden furniture are manufactured by rotationalmoulding.

Roto moulding uses PVC in paste ( plastisol ) form which is introduced into themould along with any additives such as pigments or finishers. The mould isclosed and then spun both vertically and horizontally and moved into an oven.

As the paste starts to melt and the mould continues rotating, it's flung to thewalls of the mould by centrifugal force where it forms a skin. After a fixed period,the mould is removed from the oven and allowed to cool carefully to avoid theproduct shrinking or warping.

Movie on Roto Moulding


Injection MouldingMachine

Injection Moulding MachineThe injection molding machine converts granular or pelleted raw plastic into finalmolded parts via a melt, inject, pack, and cool cycle.

Zones in Injection Moulding Machine



Moulding Cycle

Plasticizing the Resin• The cycle begins with the extruder plasticizing the resin and accumulating it inthe forward section of the barrel.

• The heater bands maintain the melt's temperature as the shot it built up.

• The mold is closed.

• The cycle is typically timed so that there is minimal time between the closingof the mold and the next shot



Moulding Cycle

Injecting the Resin• Once the shot is ready, a valve is opened at the nozzle and the melt is quicklyinjected into the mold.

• This part of the process only takes a few seconds.

• As the melt enters the cavity, the displaced air is vented out through the holesfor the ejection pins and along the parting line.

• Proper filling of the cavity is dependant on part design as well as good gatelocation and design and proper venting.



Moulding Cycle

Cooling the PartThis is the longest portion of the molding cycle.

Once the cavity is filled, the part is allowed to cool.

If an accumulator is not used, the extruder continues to push material into the moldand maintain the proper amount of pressure until the material cools (or "freezes").



Moulding Cycle

Ejecting the Part•Once the part has cooled enough (so that it will hold its shape out of the mold,and the ejection pins won't deform the part), the mold is opened.

•The moving platen has moves backwards and the ejector pins strike the rearplate (or "ejector plate"), ejecting the part.



Moulding Cycle

Movie of Moulding Cycle



Components of Injection Moulding MachineA typical injection molding machine consists of the following major components,Injection systemHydraulic systemMold systemClamping systemCooling SystemControl system



Injection Moulding Machine



Injection System

Injection SystemThe injection system consists of a hopper, a reciprocating screw and barrelassembly, and an injection nozzle, as shown in figure. This system confines andtransports the plastic as it progresses through the feeding, compressing,degassing, melting, injection, and packing stages.


Injection SystemInjection MouldingMachine

Injection System



Moulding Cycle

Movie of Injection System


Components of Injection SystemInjection MouldingMachine

Injection SystemThe Hopper:Thermoplastic material is supplied to molders in the form of small pellets. Thehopper on the injection molding machine holds these pellets. The pellets aregravity-fed from the hopper through the hopper throat into the barrel and screwassembly.

Granules in Hopper:



Injection System The Barrel: The barrel of the injection moldingmachine supports the reciprocatingplasticizing screw. It is heated by theelectric heater bands.



Injection SystemReciprocating Screw

The reciprocating screw is used to compress, melt, and convey the material. Thereciprocating screw consists of three zones:•the feeding zone•the compressing (or transition) zone•the metering zone•the injection zone



Injection SystemReciprocating Screw

The injection zone



Injection SystemThe NozzleThe nozzle connects the barrel to the spruebushing of the mold and forms a seal betweenthe barrel and the mold.


Hydraulic SystemInjection MouldingMachine

Hydraulic SystemThe hydraulic system on the injection moldingmachine provides the power to open and closethe mold, build and hold the clamping tonnage,turn the reciprocating screw, drive thereciprocating screw, and energize ejector pinsand moving mold cores. A number of hydrauliccomponents are required to provide this power,which include pumps, valves, hydraulic motors,hydraulic fittings, hydraulic tubing, and hydraulicreservoirs.


Mould SystemInjection MouldingMachine

Mould SystemThe mold system consists of tie bars, stationary and moving platens, as well asmolding plates (bases) that house the cavity, sprue and runner systems, ejectorpins, and cooling channels, as shown in figure. The mold is essentially a heatexchanger in which the molten thermoplastic solidifies to the desired shape anddimensional details defined by the cavity.



Mould System


The delivery systemInjection MouldingMachine

Delivery SystemThe delivery system, which provides passage for the molten plastic from themachine nozzle to the part cavity, generally includes:•a sprue•cold slug wells•a main runner•branch runners•gates

The deliverysystem design hasa great influence onthe filling patternand thus the qualityof the molded part.


The Delivery SystemInjection MouldingMachine

Delivery System


SprueInjection MouldingMachine

Delivery SystemA sprue is a channel through which to transfer molten plastics injected from theinjector nozzle into the mold. It is a part of sprue bush, which is a separate partfrom the mold

Sprue BushSprue Gate


RunnerInjection MouldingMachine

Delivery SystemA runner is a channel that guides molten plastics into the cavity of a mold.

Runner System


GateInjection MouldingMachine

Delivery SystemA gate is an entrance through which molten plastics enters the cavity.


Functions of GateInjection MouldingMachine

Delivery System•Restricts the flow and the direction of molten plastics.

•Simplifies cutting of a runner and moldings to simplify finishing of parts.

•Quickly cools and solidifies to avoid backflow after molten plastics has filledup in the cavity.

•Generates shear heat by going through the narrow gate, raising thetemperature of molten plastics and improving the filling in the cavity.

•Reduces residual stress, and thus reduces part defect such as warpage.

•As the cooling solidification time is shortened, molding cycle is alsoshortened.

•As the gate trace is less, it is possible to complete finishing process in ashort time.


Sprue GateInjection MouldingMachine

Delivery SystemTypes of Gate

•The Sprue gate is mainly used for cylindrical parts

•The Parts are balanced and concentric

•Have very good weld-line strength


Edge GateInjection MouldingMachine


•The most common gate.

•Put to the side of parts.

•The gate trace will be left.

•Often used for the structure with more than two cavities.


Fan GateInjection MouldingMachine


•Suitable for large and flat plate parts.

•Finishing is difficult and cost is high due to the wide gate.



Point GateInjection MouldingMachine


•Suitable for molding multiple parts.

•The position is relatively flexible.

•The structure is complicated due to Three Plate method of die


Ring GateInjection MouldingMachine


•Suitable for Round Hollow parts.

•The part consistency is more..

•The Material loss is more.


Submarine GateInjection MouldingMachine


•The gate will be automatically cut off during mold opening.

•The position is flexible (front, side, or back of parts).

•The gate needs to be thought about not to be left inside the cavity.


Film GateInjection MouldingMachine


•Suitable for thin plate parts.

•Finishing is difficult and cost is high due to the wide gate.



Cold Slug WellInjection MouldingMachine


Cold Slug Wells are are highly desirable in an Injection Mold. The Cold Slug Wellprovides a small reservoir (well) to trap air, and impurities before they enter theRunner, Gate and Cavity.

A Cold Slug Well is located above the Sprue Puller Pin. Typically, as therunner changes from a primary to secondary, and, secondary to tertiary there isalso a cold slug well at each intersection.


Clamping SystemInjection MouldingMachine

Clamping SystemThe clamping system opens and closes the mold, supports and carries theconstituent parts of the mold, and generates sufficient force to prevent the moldfrom opening. Clamping force can be generated by a mechanical (toggle) lock,hydraulic lock, or a combination of the two basic types


Cooling SystemInjection MouldingMachine

Cooling SystemCooling time is by far the most dominate time consumer in the injection moldingcycle. A long cycle time means that the molder must charge more for the samepart. During Product Design we should use thermal analysis capabilities to specifythe cooling design for the tools. The result will be a low cost tool that will run fastand keep your cost per part as low as possible.


Types of Cooling SystemInjection MouldingMachine

Cooling SystemTypes of CoolingSystem

Most commonly used cooling design.Cooling channels should beplaced close to the mold cavity surface with equal center distancesin between.

Cooling channels or lines




Baffle coolingA baffle is actually a cooling channel drilled perpendicular to a maincooling line, with a blade that separates one cooling passage intotwo semi-circular channels.

This design is usedfor core insert




Bubbler coolingA bubbler is similar to a baffle except that the blade is replacedwith a small tube.

This design is usedfor slender core insert




This design is usedfor long large core

(above 40mm)

Spiral Cooling

A spiral is similar to a bubbler except that the coolant flow in aspiral slots.




Thermal pins

This design is usedfor long slender core

A thermal pin is an alternative to baffles and bubblers. It is made ofcopper or sealed cylinder filled with a fluid.




Air Cooling

Air is blown at the cores from the outside during opening or flowsthrough a central hole from inside.

This design is usedfor very slender core

(less than 5mm)


Ejection SystemInjection MouldingMachine

Ejection SystemAfter the molding solidified andcooled down, it has to be removedfrom the mold, however, byundercuts, adhesion and internalstress the molding does not fall dueto gravity. Therefore it has to beseparated and removed from themold by special means. Ejectionequipment is usually actuatedmechanically by the opening strokeof the molding machine

The Ejector rod in turnhits the Ejector plate inthe mold base and it willmove towards theparting line and to ejectthe molded part.


Methods of EjectionInjection MouldingMachine

Ejection SystemThe method of ejection has to be adapted to the shape of the molding to preventdamage. In general, mould release is hindered by shrinkage of the part on themould cores. Large ejection areas uniformly distributed over the molding areadvised to avoid deformations.

Ejector system is normally in movable mold half, assuming the molding isconnected to the movable side of the mold in initial ejection.

Several ejector systems can be used:Pin EjectionSleeve EjectionBlades EjectionAir valve EjectionStripper plate EjectionThreads Ejection


Ejector PinsInjection MouldingMachine

Ejection SystemMethods of Ejection

•Straight, cylindrical pins are most common.

•Used where little force is needed.

Ejector Pins Movie of Pin Ejection


Blade Ejector PinsInjection MouldingMachine


•Used to eject narrow, slender, intricate shaped parts

•Avoids deep impression in the molded part during ejection in pin ejector

Blade Ejector Pins Movie of Blade Ejection


Sleeve EjectionInjection MouldingMachine


Ejector Sleeve is basically an ejector pin with a hole through the center.The holeis used for a core pin to form a portion of the desired part. The core pin touchesthe part, the other end of the core pin runs through the ejector housing andterminate near the bottom of the mold base.

Sleeve Ejector Pins Movie of Sleeve Ejection


Stripper EjectionInjection MouldingMachine


Stripper Plates are used to strip the part off the core steel. The stripper plate isactuated via stripper bolts from the A side of the mold, or by the ejector mechanismsin a variety of ways.

Stripper Plate Movie of Stripper Ejection


Air Valve EjectionInjection MouldingMachine


Air Poppets are standard components that aid the ejection of a part by usingcompressed air within the mold. The timing of the actuation of the air is controlled bythe controller of the moulding machine.

Air poppets Movie of Air Valve Ejection


Threads EjectionInjection MouldingMachine


•Used to eject threaded components.

•Too costly method but it is the only method

Movie of Collapsible Core Ejection Movie of Rotary Core Ejection


Control SystemInjection MouldingMachine

Control SystemThe control system provides consistency and repeatability in machine operation.It monitors and controls the processing parameters, including the temperature,pressure, injection speed, screw speed and position, and hydraulic position. Theprocess control has a direct impact on the final part quality and the economics ofthe process.

Moulding Machine Control System


Control SystemInjection MouldingMachine

Control System

Onboard External Control System

Internal Control System


Moulding defects and remediesMoulding defectsand remedies

•Problems can occur at all stages in the injection moulding process.The origins ofthese problems are often difficult to identify thanks to the complex interrelationshipbetween the moulded part and the mould.•Successful troubleshooting should begin at the design stage not on the shop floorso that mistakes can be identified and remedied before they become critical.As apart designer it is a very good Idea to be aware of your options in tooling and toconsider those while designing your part.•For example, have potential gate locations in mind. Try to guess where knit lineswill occur and how different gate locations will affect them.

Areas which areconcentrated during

Product Design to geta defect freecomponent


BlushMoulding defectsand remedies

Dull discolored or whitish area on the surface ofthe part, usually at the gate.

May also occur where there is a sudden changein part thickness.

BurnDiscoloration usually black, brown or darkyellow/brown depending upon severity. Feelsrough and crunchy.

Most often seen in deep, blind ribs where a lot ofair can be forced into a small space.

Cold FlowWavy or streaked appearance on part surface.Looks like a fingerprint or small waves like youwould see on the surface of water.

Low melt temperature, low injection speed orlow injection pressure.


Cold SlugMoulding defectsand remedies

Cold piece of plastic that has been forced intothe part along with the melt.

Add a cold slug well at each intersection in therunner

ContaminationForeign particles embedded in the part

DelaminationSeparation of plastic surface layer giving aflaking or onion skin effect.

Due to contaminated resin.


DiscolourationMoulding defectsand remedies

Deviation from the original intended color of thematerial as compared to the manufacturers colorchip.

Contaminated resin / Overheated resin /Incorrect regrind ratio / Incorrect color mixing orblending.

GlossSmooth shiny areas on the part surface.

Hard to fill areas.

JettingSquiggly line in part pointing to gate. Looks like aworm in the part.Incorrect gate placement orsize.The gate is positioned in such a manner as toaim the plastic straight into an open area.Theplastic launches out into the open like a piece of"silly string" and then stacks up in squiggles.


KnitlineMoulding defectsand remedies

A line where the molten polymer flow fronts meet inthe mold. Incomplete adhesion occurs along the knitline and causes a weak point in the plastic part.

Mould is not preheated to moulding temperature.

PinpushCircular or semicircular white stress rings on the side ofthe part opposite an ejector pin. May even be raisedcircular bumps.

Unpolished core or less draft on core side of component.

Inadequate ejector pins for ejection

DragFine, straight lines scraped in the line of draw.

Cavity Side happens usually from insufficient draftfor the texture.

Core side drag happens usually from inadequatedraft, rough core, or overpacking.


Sink MarksMoulding defectsand remedies

Depressions or dimples in the part that are usually adjacent to thick areas. In clearparts, bubbles can be seen in thick areas.

As the plastic cools it shrinks. If there is an area that is proportionally thicker thanthe rest of the part, then the plastic will shrink more in the thick spot causing it tocollapse inward.

Wall perpendicular to ribs or bosses that don't conform to the 66% rule.

Inconsistent wall thickness. i.e. thick areas adjacent to thin areas.


WarpageMoulding defectsand remedies

The failure to maintain flatness of a plastic part that was intended to be flat.Distortion from the intended shape of the plastic part.

The underlying cause of most part warpage is the shape of the part itself. Thepattern, shape, and thickness of ribs on the part as they undergo shrinkage havethe greatest effect upon warpage.

Present to some degree in most Injection molded parts but most easily detectedon large flat parts.

Differential mold cooling can get you parts that are flatter. Your best bet is to followthe 66% rule and minimize rib height.

Flat parts are more susceptible to warpage than curved parts.


Parting SurfaceDesign Consideration

Parting Surface is a line at which the two halves of mould meet and form a seal toprevent the escape of material. The shape of the component, method of ejection,etc. largely influence the selection of parting surface.


Avoid round edges along the Parting LineDesign Consideration

Parting Surface


Always look for simple tooling solutionsDesign Consideration

Parting Surface

Design to avoidside cam moulding

Movie of Side Cam Moulding



Parting Surface

Design to avoid side cam moulding



Parting Surface



Parting Surface

Extending vent slots over the corner edgeeliminates the need for a side action in the mold


ShrinkageDesign Consideration

In the production of plastic components melt is injected into the mold cavity. Aftercompletion of the injection phase and the hold period the molding is cooled downto the temperature for removal from the mold during the cooling period. Due tophysical factors, the plastic component undergoes a dimensional change duringthe cooling process that is specific to the material used. This dimensional changeis called shrinkage.


Effects of Shrinkage in designDesign Consideration

Shrinkage Wall thickness differences may lead to varying shrinkage behavior that is weakeror stronger depending on the plastic used. In the case of semi-crystallinematerials, a large wall thickness results in slower cooling, which then leads togreater shrinkage. The resulting shrinkage differences in the molding lead tointernal stresses in the molding, which are either absorbed through the inherentrigidity of the structure or reduced through special processing conditions.

Distortions due to differences in basicrib wall thickness Distortion in non-reinforced components


Effects of Shrinkage in design

Incorrect

Design Consideration

Shrinkage

Correct


DraftDesign Consideration

The purpose of draft is to first provide release from the cavity side of the moldupon tool opening. Then upon ejection, draft allows instant release of the plasticpart without dragging. If plastic parts have completely vertical walls, drag markswill occur on the plastic as it scrapes along the metal tool face. If money is noobject vertical faces may be obtained however with the use of slides and lifters

Without Draft

With Draft

stuck

free


Where should you give Draft ?Design Consideration

Draft The answer is it should start from parting surface.Draft must fall away from the parting lines on ALL vertical faces in the plastic part.The widest point on the part is thus the parting line. The location of parting line andsubsequent application of draft to the plastic part are design decisions that affectboth the aesthetics and functionality of the part. These decisions should be madebefore the part is sent to the mold maker if time to market is critical.


What is the correct draft angle?Design Consideration

Draft The answer is it should start from parting surface.Draft must fall away from the parting lines on ALL vertical faces in the plastic part.The widest point on the part is thus the parting line. The location of parting line andsubsequent application of draft to the plastic part are design decisions that affectboth the aesthetics and functionality of the part. These decisions should be madebefore the part is sent to the mold maker if time to market is critical.

Industrialdesigners want

0.0 degrees,mold designers

want 45.0degrees…. Use

the adjacentwrite up as a

guide.

0.0 degree:Very small details under 0.040in tall that will get polished. The act of polishing willapply some draft. Faces to be 100% relieved with side actions.1/4 degree:Emergency use only. Deep ribs, one internal side of a box where the other sideshave good draft, bosses ejected by sleeves.1/2 degree:Use sparingly and for good reason. Ribs, one internal side of a box, snaps, hooks,etc..1.0 degree:Standard draft, all features.2.0 degree:Standard draft, very light texture, cavity side to ensure good release.3.0 degree:Textured faces, faces that are in common with a shutoff.


Draft to Assembly PartsDesign Consideration

Draft Point to remember

Part mismatch due toimproper draft


Importance of draft to the CAD modelDesign Consideration

Draft

Designers whocan't draft yourparts you arecosting yourcompany timeand money andshould get a jobdoing somethingthat you CAN do.

-The failure to apply draft to a CAD file before sending it to the mold maker forcesthe mold designer to guess about what the part designer intended. Frequently themold designer does not even know what your part is.

-Time is consumed by someone unfamiliar with your parts applying draft arbitrarilywithout knowledge of your mating parts, sheet metal or components.

-Frequently your mold designer is using a different CAD package and if he sendsback a drafted model, it may be difficult for you to do a good interference analysis.

-Many toolers will simply slap some draft on and if the parts don't fit, its yourproblem.

-When the mold is finished and your parts don't fit, time is lost reworking the tool.

Once you understand the basics of draft and shutoffs it is very simple to applythese to your model and save time to market. If you still have questions, getthe Mold Designer to review your parts before tooling release.


Design GuidelinesDesign Guidelines

Much is writtenregarding designguidelines inlatter slides.Yet, the designguidelines canbe summed up injust a few designrules.

1. Use uniform wall thickness throughout the part.

This will minimize sinking, warping, residual stresses, and improve mold fill andcycle times.

2.Use generous radius at all corners.

The inside corner radius should be a minimum of one material thickness.

3.Use the least thickness compliant with the process, material, or productdesign requirements.

Using the least wall thickness for the process ensures rapid cooling, short cycletimes, and minimum shot weight. All these result in the least possible part cost.

4.Use ribs or gussets to improve part stiffness in bending.

This avoids the use of thick section to achieve the same, thereby saving on partweight, material costs, and cycle time costs.

5.Design parts to facilitate easy withdrawal from the mold by providingdraft (taper) in the direction of mold opening or closing.


Wall thicknessDesign Guidelines

From a coststandpoint, thethinnest wallutilizes the leastmaterial andresults in thefastest moldingcycles

The typical plastic part may be considered to have a shell type configuration witha basic surface and features which are attached to it to meet functionalrequirements.

The actual determination of the wall thickness isbased on a number of considerations.

1.Application Requirements

Structural requirements includingstrength, impact, fatigue or deflection.

2. Moldability

The size of the part and the ability of thematerial to fill the furthest point candetermine the minimum wall


Influence of wall thicknessDesign Guidelines

Wall thickness •Part characteristics•Mechanical performance •Cosmetic appearance•Mouldability•Economy

Giving wall thickness should be carefully considered in the design stage to avoidexpansive mold modifications and molding problems in productions

The optimum thickness is often a balance between opposingtendencies, like:

Strength Vs Weight

Durability Vs Cost


Avoid designs with thin areassurrounded by thick perimetersections as they are prone togas entrapment problems

Design Guidelines

Wall thickness

Maintain uniform nominal wallthickness through out the part


Avoid sudden wall thickness variation that result in filling from thin tothick sections:

Design Guidelines

Wall thickness


Core or redesign thick areas to create a more uniform wall thicknessDesign Guidelines

Wall thickness


Wall Thickness Design for Stiffness:Design Guidelines

Wall thickness Corrugation

Corrugations can add stiffness to

non cosmetic parts

Curved Side Walls

Adding curvature to the sidewalls

enhances stiffness and appearance


Wall Thickness Design for Stiffness:Design Guidelines

Wall thickness Flexible

Stiffer


RadiusDesign Guidelines

RadiusSharp corners greatly increase the stress concentration. This high amount ofstress concentration can often lead to failure of plastic parts.

High molded in stressesPoor flow characteristicsReduced mechanical propertiesIncreased tool wearSurface appearance problems, (especially with blends).

Crack due to Stress

No Crack due to Radius

Uniform coolingLess warpageLess flow resistanceEasier fillingLower stress concentrationLess notch sensitivity.


What radius should you give?Design Guidelines

RadiusStandard tables for stress concentration factors are available and should beconsulted for critical applications.

As can be seen from the above chart, the stress concentration factor is quitehigh for R/T values less than 0.5. For values of R/T over 0.5 the stressconcentration factor gets lower.


Radius should beDesign Guidelines

Radius•Radius should be between 50% of the nominal wall thickness.•If the part has a load bearing function then the upper end is recommended.•A minimum radius of 0.5mm is suggested and all sharp corners should be brokenwith at least a 0.125 mm radius.


Design Guidelines

Radius

Internal and external radii should originate from the same point


Design Guidelines

RibsRibs increase the bending stiffness of a part. Without ribs, the thickness has to beincreased to increase the bending stiffness. Adding ribs increases the moment ofinertia, which increases the bending stiffness.

Ribs

1. The rib gives stiffness and strength in molded partwithout increasing overall wall thickness.

2. Locating and captivating components of an assembly.

3. Providing alignment in matting parts.

4. Acting as stops or guide for mechanisms

Functions of Ribs


Design Guidelines

Ribs •Thickness•Height•Location•Quantity•Moldability

Rib design issues

Proper RibDesign reducesthe defects

Consider these issues carefully when designing ribs


Design Guidelines

Ribs Many factors go into determining the appropriate rib thickness. Because thickribs often cause sink and cosmetic problems on the opposite surface of the wall.

Rib thickness

If rib thickness is a constraintbut not the cosmetic

Offset Rib to reduce Sink


Design Guidelines

Ribs 1. The rib thickness should be less than the wall thickness

2. The thickness ranges from 40 to 60 % of the material thickness as per 66% rule

3. The rib should be attached to the base with .125 X thickness radius at thecorners and .5 degree draft should be given for ejection,

If rib thickness is a not a constraint but cosmetic


Design Guidelines

Ribs The divine 66% rule for ribs

The thickness of ribs should never exceed 66% of the nominal wall thickness.

What is the 66% rule ?

If your ribs never exceed 50-66% of nominal wall thickness you willnever have a problem with sink.

Sometimes you can get away with 66% to 75%of nominal wall, but it is risky. Don't do it unlessyou absolutely have to. If you do, be certainthat the area gets better than average plasticflow.


Design Guidelines

Ribs

What happens if you ignore the 66% rule ?


Design Guidelines

Ribs

Rib heightMaximum rib height should not exceed 3 times the nominal wall thickness asdeep ribs become difficult to fill and may stick in the mold during ejection.


Design Guidelines

Ribs

Effect of 1/8 rib on 1/4 thickness part


Design Guidelines

Ribs

Rib locationThe rib location is based on providing maximum bending stiffness. Dependingon orientation of the bending load, with respect to the part geometry, ribsoriented one way increase stiffness. If oriented the wrong way there is noincrease in stiffness.


Design Guidelines

Ribs

Rib locationAnother issue to be consideredIf air is trapped, it will compress and create a burn mark on the rib, which probablywon’t fill anyway.The best solution is to try to locate your ribs into the side walls or other features tohelp convey the plastic and air through the part.Another solution is to transition to a projection from the base wall with a gusset orramped rib.

Rib A will trap air in the top corner

Rib B has a better transition to the

base wall.

Rib C is the best since it's tied into

the side wall.


Design Guidelines

Ribs

Rib quantity

Multiple Ribs are better than one thick rib or one tall rib


Design Guidelines

Ribs

Rib mouldabilityRibs are preferably designed parallel to the melt flow as flow across ribs canresult in a branched flow leading to trapped gas or hesitation. Hesitation canincrease internal stresses and short shots.

Mold FlowPosition ribs inthe line of flow toimprove fillingand prevent airentrapment.


Design Guidelines

Ribs

Rib design for stiffness


Now you know the standards of rib designing. Implement it withthe help of these steps for better results.

Design Guidelines

Ribs


Design Guidelines

Holes

HolesHoles also are a major design element, The location of any holes maysignificantly affect the part's overall strength. Trying to create a hole in the sideof a part is especially challenging, and the need for side holes should beminimized in the initial design.

1. The holes are given in molded parts to accommodate rivets or pins.

2. Locating and captivating components of an assembly.

3. Providing alignment in matting parts.

The functions of the holes


Design Guidelines

Holes

Hole spacingThe minimum spacing between twoholes or between a hole and side wallshould be one diameter.

Hole locationHoles should be located threediameter or more from the edge of thepart to avoid excessive stress.


Design Guidelines

Holes

Blind holesThe depth of a blind hole should not exceed 3 times the diameter. Fordiameters less than 5 mm this ratio should be reduced to 2.

Core pins supported by just one side of the mold tool create blind holes. Thelength of the pins, and therefore the depth of the holes, are limited by the ability ofthe core pin to withstand any deflection imposed on it by the melt during theinjection phase.


Design Guidelines

Holes

Blind holesFor blind holes the thickness of the bottom should be greater than 20% of thehole diameter in order to eliminate surface defects on the opposite surface. Abetter design is to ensure the wall thickness remains uniform and there are nosharp corners where stress concentrations can occur.


Design Guidelines

Holes

Through holesWith through holes the cores can be longer as the opposite side of the moldcavity can support them.For through holes the length of a given size core canbe twice that of a blind hole.


Design Guidelines

Holes

Through holesAn alternative is to use a split core fixed in both halves of the mold that have agap equal to wall thickness.


Design Guidelines

Holes

Through holesAnother alternative is to use a split core fixed in both halves of the mold thatinterlock when the mold is closed.


Design Guidelines

Holes

Through holesIn cases where even longer cores are required, careful tool design is necessaryto ensure balanced pressure distribution on the core during filling to limitdeflection. Doing this the core mismatch can be reduced

Incorrect Correct


Design Guidelines

Boss

BossesBosses are used for the purpose of registration of mating parts or for attachingfasteners such as screws or accepting threaded inserts (molded-in, press-fitted,ultrasonically or thermally inserted).


Design Guidelines

Boss

The divine 66% rule for bossesThe thickness of boss wall should never exceed 66% of the nominal wall thickness

Ideal Design

Bad Design


Boss design standardsNominal boss wall thickness less than 66% nominal wall thickness,

Design Guidelines

Boss


Boss design standardsThe boss height should be 3 times of the wall thickness.

Design Guidelines

Boss


Boss design standardsThe outer diameter should be within 2 to 2.4 times of internal diameter

Design Guidelines

Boss


Boss design standardsA minimum radius of 25% the nominal wall thickness or 0.4 mm at the base ofthe boss is recommended to reduce stresses.

Design Guidelines

Boss


Boss design standardsThe core pin should be given a radius (min 0.25 mm) to reduce materialturbulence during filling and to help keep stresses to a minimum.

Design Guidelines

Boss


Boss design standardsA minimum draft of 0.5 degrees is required on the outside dimension and insidealso (if required) of the boss to ensure release from the mold on ejection.

Design Guidelines

Boss


Boss design standardsGreater wall sections for increased strength will increase molded-in stressesand result in sink marks. So use this method.

A recess around the base of a thick boss reduces sink.

Design Guidelines

Boss


Boss design standardsBosses located at corners can result in very thick walls causing sinks. Bossescan be isolated using the techniques illustrated.

Design Guidelines

Boss


Boss design standardsAlternative boss design can be used for bosses near a standing wall.

Design Guidelines

Boss


Strengthening a BossThe boss can be strengthened by gussets at the base

Design Guidelines

Boss


Strengthening a BossThe boss can be strengthened by attaching it to nearby walls with connecting ribs.

Design Guidelines

Boss


Rib designAvoid bosses that merge into side walls by connecting ribs for support.

Design Guidelines

Boss


Rib designA minimum distance of twice the nominal wall thickness should be used fordetermining the spacing between bosses. If placed too close together thin areasthat are hard to cool will be created. These will in turn affect quality andproductivity..

Design Guidelines

Boss


Failure of bossFailures of a boss are usually attributable to:1. Knit lines -these are cold lines of flow meeting at the boss from opposite sides,causing weak bonds. These can split easily when stress is applied.

Design Guidelines

Boss

Knit lines should be relocated away from the boss, if possible. If not possible,then a supporting gusset should be added near the knit line.


Failure of bossFailures of a boss are usually attributable to:2. High hoop stresses caused because of too much interference of the internaldiameter with the insert (or screw).

Design Guidelines

Boss

Hoop stresses are imposed on the boss walls by press fitting or otherwise inserting inserts.


Mating of bossesExcessively long bosses can often be replaced by two shorter bosses

Design Guidelines

Boss


GussetsGussets can be considered as a subset of ribs and the guidelines that apply toribs are also valid for gussets. This type of support is used to reinforce corners,side walls, and bosses.

Design Guidelines

Gussets

The height of the gusset can be up to 95% of the height of the boss or rib it isattached to.Depending on the height of the rib being supported gussets may be more than 4times the nominal wall thickness.Gusset base length is typically twice the nominal wall thickness.These values optimize the effectiveness of the gusset and the ease of molding andejecting the part.


Gusset designAvoid sharp corners in your gusset design.

Design Guidelines

Gussets


Design for Manufacturability and Assembly (DFM/A)Design for Manufacturability and Assembly (DFM/A) is a very broad topic coveringmany areas. Regardless, it can best be defined as any tool or process that helps adesigner or engineer think about, and therefore avoid, manufacturing andassembly problems down the road.The design of plastic parts is good discipline for the application of DFM/Aprinciples because designers must be in tune with all the factors that can cause aflawed design

Design Guidelines

DFM/A


Design for ease of assemblySimplify design and assembly so that the assembly process is unambiguous.

Design Guidelines

DFM/A


Assembly alignmentSimplify design and assembly that can be easily aligned.

Design Guidelines

DFM/A


Poka Yoke (Fail Proof design)Components should be designed so that they can only be assembled in one way;they cannot be reversed. Roll pins, dowel pins or offset mounting holes can beemployed.

Design Guidelines

DFM/A


Design for orientationDesign for components orientation and handling to minimize non-value-addedmanual effort, ambiguity or difficulty in orienting and merging parts. Basic principlesto facilitate parts handling and orienting are:•Parts must be designed to consistently orient themselves. Examples are dowel pins.•Product design must avoid parts that can become tangled, wedged or disoriented.•With hidden features that require a particular orientation, provide an externalfeature, guide surface or design alignment fixturing or tooling to correctly orient thepart.•Design in fasteners large enough that are easy to handle and install

Design Guidelines

DFM/A


Design Lettering for assemblyBase assembly component should have some sort of visual indicatives likelettering or embossing to show where other parts is to be assembled.

Design Guidelines

DFM/A


Design for efficient joining and fasteningThreaded fasteners (screws, bolts, nuts and washers) can be time-consuming toassemble. Consider design alternatives that will reduce fastener count.Snap fits are very useful because they eliminate screws, clips, adhesives, orother joining methods. The snaps are molded into the product, so additionalparts are not needed to join them together.

Design Guidelines

DFM/A


Snap fit assemblyUse of snap-fit assemblies can deliver many benefits:

•An integral element of the plastic part – no other components•Can replace screws, nuts, and washers•Easy automation can reduce assembly costs•No other fasteners, adhesives, solvents, welding, or special equipment•Design can minimize risk of improper assembly•Can be designed to engage and disengage

Things To Be Aware of When Using Snap-Fits:

•Some designs may require more complex or expensive tooling•Snap-fits that are assembled under stress will creep•It is difficult to design snap-fits with hermetic seals. If the beam or ledge relaxes,it could decrease the effectiveness of the seal.•Can be damaged by mishandling and abuse prior to assembly

Design Guidelines

DFM/ASnap design


Types of Snap fitsAnnular Snap FitAn annular snap is defined as involving a locator pair or the entire mating part-basepart system they will constrain in more then one degree of motion.

Design Guidelines

DFM/ASnap design


Types of Snap fitsCantilever Snap FitThe Cantilever Snap design is the most popular when assembling two plastic parts.A cantilever beam snap-fit assembly consists of a cantilever beam with an overhangat the end of the beam.

Design Guidelines

DFM/ASnap design


Types of Snap fitsTorsional Snap FitA Torsional snap fit involves primarily torsional deflection for assembly althoughthere is often some bending in the system as well. The torsional member is notnecessarily round; they can be round or flat. A torsional snap fit is relativelyuncommon but are useful as an alternative to the cantilever, the most used snap-fit,when clearances or access make the hook location for assembly difficult.

Design Guidelines

DFM/ASnap design


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