thiet ke khuon mau

8/7/2019 Thiet Ke Khuon Mau

1/43

1

Mold.ppt

CCOREORETTECHECHSSYSTEMYSTEM

Mold Designold DesignFundamenta lsundamenta ls


2/43

2


Mold.ppt

Basic Tasks of a MoldBasic Tasks of a Mold

q Accomodation and Distribution of the Meltq Shaping of the Molded Part

qCooling/Heating and Solidification of the Melt

q Ejection (Demolding) of the Molding

qMechanical Functions

Accomodation of forces

Transmission of motion

Guidance of the mold components

The mold is probably the most important element of a molding machine. It is a

arrangement, in one assembly, of one (or a number of) hollow cavity spaces built

to the shape of the desired product, with the purpose of producing large numbersof plastic parts. Thus the primary purpose of the injection mold is to determine

the final shape of the molded part (shaping function).

In addition to give the final shape of the molding, the mold performs several

other tasks. It conducts the hot melt from the heating cylinder in the injection

molding machine and distributes the melt to the cavity (or cavities), vents the

entrapped air or gas, cools the part until it is ejectable, and ejects the part without

leaving marks or causing damage.

The secondary tasks of a mold derived from these primary tasks include several

mechanical functions such as accommodation of forces, transmission of motion,

guidance and alignment of the mold components.

The mold design, construction, the craftsmanship largely determine the quality

of the part and it manufacturing cost.


3/43

3


Mold.ppt

Functional Systems of the InjectionFunctional Systems of the Injection

MoldsMolds

qMelt Delivery System: Sprue/Runner/GateqCavity (with Venting)

q Tempering/Heat Exchange System

q Ejection System

qGuiding and Locating System

qMachine Platen Mounts

q

Force SupplierqMotion Transmission System

An injection mold is composed of several functional units. Each unit performs

one or several task of the mold.

The melt delivery system or runner system performs the task of receiving anddistribution of the melt. The runner system is in fact a set of flow channels that

lead the melt into the cavities.

Forming/shaping the molten material into the final shape of the part is the job of

the cavity. During the filling and packing/holding stages, melt is forced by

injection/holding pressure to completely fill the cavity (or cavities).

Mold tempering or heat exchange system is used to control the mold

temperature, cool down the molten melt (or,if thermosets or elastomer are used,

heat the melt and cross-link the material) uniformly, solidify the molding to an

ejectable state. Mold tempering system design has direct impact to the productioncycle time and the quality of the molded part.

Ejector system is utilized to open the mold and remove the molded part from the

cavity. Mold mounting, alignment, and guiding are accomplished by the

guidance/ locating system and machine platen mounts. Other auxiliary units such

as force supplier and movement transmission unit are essential to accomplish the

functions of an injection mold.


4/43

4


Mold.ppt

Structure of A Mold UnitStructure of A Mold Unit

SprueSprue

Primary Runner

Secondary Runner/Sub-runner

Gate

Part

Cold-Slug Well

Cold-Slug Well

Sprue Ejector Pin Sprue Bushing

Above figure shows the layout af a typical simple injection mold, which has

four identical cavities. Melt from the nozzle enters the mold via the spure, which

has a divergent taper to facilitate removal when demolding.Opposite the sprue is a cold slug well, which serves both to accept the first

relatively cold portion of the injected material, and to allow a re-entrant shape on

the end of an ejector pin to grip the sprue when the mold opens.

The melt flows along a system of runners leading to the mold cavities. In

general, for a single cavity mold, only the sprue orprimary runnerappears in the

mold; whereas for a multicavity mold, secondary runners or subrunners are

needed to distribute the melt into each cavity.

The gates at the entries to the cavities are very narrow passages in at least one

directions, so that the molded part can be readily detachable from the runnersafter removal from the mold.

Sometimes additional cold slug wells are added in the end of primary runners to

trap the cold slug during the filling stage.

The mold is aligned with the nozzle on the injection cylinder by means of the

locating ring surrounds the sprue bushing.


5/43

5


Mold.ppt

Mold Design IssuesMold Design Issues

mold base

cooling channel/lines

runner (mainfold) system

gate

cavity

q Mold Design No.Cavity

Cavity Layout

Runner System Design

Gating Scheme

No.Gate

Gating Location

Mechanical/MechanismConsideration

q Cooling System Design

Cooling Channel Layout

Special Design

The primary tasks of an injection mold include the accomodation and

distribution of the melt, the shaping and cooling/heating of the molding,

solidification of the melt, as well as ejection of the molded part. Besides, a moldhas to provide mechaincal functions such as accomodation of forces,

transmission of motion, and guidance of mold components.

Hence the primary functional systems of a injection mold include the melt

delivery system ( sprue/runner/gate ), cavity (single-cavity or multicavity),

ejection system, guiding and locating system, as well as mold temperature

control unit (cooling system).

From the view point of mold design, we have to evaluate the suitable size and

layout of runner system and cavity, number of cavity, cooling system, etc.

We will propose a few examples to illustrate how these design parametersinfluence the productivity and quality of the moldings.


6/43

6


Mold.ppt

Determine Number of CavitiesDetermine Number of Cavities

q Single Cavity vs. Multicavity Mold Productivity and complecity consideration

q Determination of Number of Mold Cavities

Number of moldings required and period of delivery

Quality control requirements (dimensional tolerance,etc.)

Cost of the moldings

Shape, dimensions, and complexity of the molding (position ofparting line and mold release)

Size and type of the injection molding machine machine (shot

capacity, plasticizing capacity, mold release..) Plastics used (gating scheme and gate location)

Cycle time (increase in recovery time of plasticating unit,injection time, pressure drop, and mold opening time)

The multiple mold cavities can produce several article at the same time and

hence has a higher output speeds and improved productivity. However, the

greater complexity of the mold also increases significantly the manufacturingcost. The problems arising from a multicavity mold includes cavity layout, flow

balance, balanced cooling channels layout, etc.

Theoretically, for the same product, cycle time do not increase prorate with the

number of cavities because th cooling time does not change. However, one often

find that cycle time will increase as the number of cavities increases, for the

following reasons:

-Increase in recovery time of plasticating unit for the next shot and injection

time because the total shot volume is increased. These increases in time are

significant for large shots.

-Increase in pressure drop becaused of the increased flow length from sprue,

through runner system, to each cavity. The pressure drop can be a determining

factor in the evaluation of numbers of cavity.

-Increase in mold opening time because of the increased complexity.

Both the technical and economic criteria have to be considered in determining

the number of mold cavity, such as the numbers of moldings required, the cost

and time of mold construction, the complexity of the molding, cycle time, quality

requirements and the plasticating capacity of the available machine equipment,

etc.


7/43

7


Mold.ppt

Cavity LayoutCavity Layout

Layout in SeriesCircular Layout

X-style layout

H-style bridge(branching) layout

When the number of parts produced in each cycle exceeds one, a multicavity

mold have to be used. Many cavity layouts can be adopted in the production.

For example, layout in series has the advantage that there is no space restrictionfor each cavity; however, the unequal flow lengths to individual cavities may

lead to unbalanced flow and differential part weights in each cavity.

Circular layout has the advantage of equal flow length and uniform part

quality; however, only limited number of cavities can be accomodated by this

layout.

H-style layoutand X-style layoutbelongs to the so-called symmetrical layout.

They are good in flow balance. Their disadvantage is that more larger runner

volume and much scrap will be generated. Hot runner system can be adopted to

conquer this drawback.Layout of cavities not only influence the filling pattern and extent of pressure

packing, but also determines the equilibrium of injection force and clamp force

during the molding cycle.


8/43

8


Mold.ppt

Design of Runner SystemDesign of Runner System

Piston orScrew

Screw Chamber(Reservoir)

Heating ElementNozzle

Runner

Gate

Sprue

Cavity

Mold Unit

q Runner System Sprue

Runner

(Primary/Secondary)

Gate

q Goal: Accommodates the molten plastics material coming from the screw

chamber andguides/distributesit into the mold cavity

Raises the melt temperature to the proper processing range byviscous(frictional) heatingwhile the melt is flowing through the runner

q Design Consideration Quality (filling pattern...) & Economics (cycle time...)

A runner system is composed of the sprue, the runner(s), and the gate(s) that

connecting the runner with the cavity.

The primary task of a runner is the delivery and distribution of melt from thescrew chamber into the mold cavity. The runner system must be designed in such

a way that the melt fills all cavities simultaneously and uniformly under uniform

pressure and temperature. This design criterion is referred to as the flow balance

of the runner system.

Melt temperature may be significantly increased as it passes througn the narrow

runner passage or gate due to friction effect. This viscous heating is important in

raising the melt temperature and reducing the flow resistance because of the

shear-thinning character of plastic material.

The runner system has significant impact on the part quality and the economicsof manufacture. Problems such as weld lines, pressure drop, material waste,

removability of moldings, etc.,are related to the design of runner system.


9/43

9


Mold.ppt

Common Runner Cross SectionsCommon Runner Cross Sections

qCircular Runner Full Round Runner

q Parabolic Runner U-Type or Modified

Trapesoidal Runner

q Trapezoidal Runner

q

Half Round Runner

qRectangular Runner

There are several types of cross section can be adopted for a runner. The

selection of the runner cross section depends on its efficiency and ease or

difficulty of tooling.Circular or full roundcross section provides a maximum volume-to-surface

ratio and hence offers the least resistance to flow and least heat loss from the

runner. However, it requires a duplicate machining operation in the mold, since

two semi-circular sections have to be cut for both mold halves and aligned as the

mold is closed.

Parabolic or U-type runnerrepresents a best approximation of circular runner,

although more heat losses and scrab produced (mass is 35% greater), it needs

simpler machining in one (movable) mold half only.

Trepezoidal runner is an alternative modification of circular runner, itsperformance is similar to that of the parabolic runner. Trapezoidal runner is

often used in three-plate molds since sliding movements are required across the

parting-line runner face.

Half roundand rectangularcross section may lead to larger flow resistance and

are unfavorable in the runner cross section.

Normally, full round or trapesoidal runners are adopted in most practical cases.


10/43

10


Mold.ppt

Considerations in Runner DesignConsiderations in Runner Design

q Part Consideration Geometry, Volume, Wall Thickness

Quality (Dimensional,Optical, Mechanical...)

q Material Consideration

Viscosity, Composition, Fillers,Softening Range, SofteningTemperature,Thermal Sensivity, Shrinkage, Freezing Time...

q Machine Consideration

Type of Clamping, Injection Pressure, Injection Rate...

q

Mold Consideration Way of Demolding, Temperature Control...

Key factors affecting the design of a runner are summarized here.

In the aspect of part consideration, the geometric dimensions of the runner

should be such that flow restriction is at a minimum, that is, the runner shouldconvey melt rapidly and unrestricitly into the cavity in the shortest way and with

a minimum heat and pressure losses. The runner system should allow cavity

filling with a minimum numbers of weld line so that the mechanical and surface

properties of moldings can be improved. The runner should permit the

transmission of holding pressure during the packing/holding stage so that the

dimensional accuracy can be ensured.

In the aspect of material consideration, the flow character and the thermal

properties of material are related to the sizing of runner diameter and the runner

length. Long or small runner should be avoided for material with short flow

length (high viscosity). Runner should be properly sized to minimize material

waste while not cause significant pressure loss.

In the aspect of machine consideration, we should note the allowable injection

pressure, injection rate, type of clamping, etc.

The runner should be design so that demolding and removal from the molded is

easy. Location and number of runner ejectors should be considered in the mold

design phase.


11/43

11


Mold.ppt

Flow Balance in the Runner DesignFlow Balance in the Runner Design

q Flow Balance in Multi-Cavity Molds: Increase in recovery time of plasticating unit, injection time,

pressure drop, and mold opening time

PLAY412

Consider the runner system design in the multicavity mold case.

In a symmetric, naturally balanced cavity layout, all flow lengths from the

sprue to each cavity are of the same length. In this ideal case the plastic melt willfill all cavities simultaneously under the same pressure and temperature

conditions. The molded part in each cavity has the same weight and final

properties.

Unfortunately not all runners can be naturally balanced, especially for large

parts where multiple gating may be needed to produce a proper part. Moreover,

the natural flow balance is difficult for molds with a large number of cavities

and is even impossible for the so-called family mold (combination mold) where

each of the cavities is of different size and forms one component part of the

assembled finished product.In these cases we have to balance the flow artifically. Balancing ensures

virtually equal flow of plastic through each gate of a multicavity mold, and/or

through each gate (if there is more than one) into each cavity. The melt should

arrive at all gates/cavities at the same time and with the same properties so that

all molded parts have uniform characteristics. This type of runner system is

called the artifically balancedrunner systems.

On the other hand, even though the cavity layout is virtually balanced, the

desired balanced flow may not be achieved since the flow depends on the plastic

material used, the process condition setting, the accuracy of machining and the

finish inside the channel, temperature difference due to unbalanced

cooling/heating, , uneven venting, mold surface quality, etc.


12/43

12


Mold.ppt

Runner Design and Part ShrinkageRunner Design and Part Shrinkage

Runner cross-sectional Area

Part Shrinkage

Runner Length

Part Shrinkage

The runner system design has a significant impact on the quality of moldings.

For example, the part shrinkage increases as the runner length is increased since

more pressure drop in the runner system and the melt is less packed within themold. In general, the runner length should be as short as possible in order to

reduce the pressure drop and amount of scrap. However, the runners must be of

adequate length to satisfy the other conditions such as flow balance

consideration, accommodation of cooling lines and ejector pins, etc.

The part shrinkage reduces as the runner cross section is increased since the

filling process is promoted and the effective holding pressure is higher. However,

increase the runner size also produces more scrap and material waste.

The size of the runner depends on the size of the part and its wall thickness, the

design of the mold and the type of plastic being processed. Plastics with lowviscosity (high melt flow index or long flow length) permit a longer or thinner

runner.

The runner cross section should be as small as possible but still compatible with

the melt flow requirement such as pressure drop consideration.


13/43

13


Mold.ppt

Design of RunnerDesign of Runner

P las tic M a te ria ls R ec o m m en d e d R u n n e r D i a m e te r s

A B S , S A N 0 .187 -0 .375 (4 .7 -9 .5m m )

A ceta l 0 .125 -0 .375 (3 .1 -9 .5m m )

Acryl ic 0 .312 -0 .375 (7 .5 -9 .5m m )

B utyra te 0 .187 -0 .375 (4 .7 -9 .5m m )

Cellu losics 0 .187 -0 .375 (4 .7 -9 .5m m )

F luorocarbon 0 .187 -0 .375 (4 .7 -9 .5m m )

I o n o m e r 0 .093 -0 .375 (2 .3 -9 .5m m )

N ylon 0 .062 -0 .375 (1 .5 -9 .5m m )

P o l y a m i d e 0 .187 -0 .375 (4 .7 -9 .5m m )

P C 0 . 1 8 7 -0 . 3 7 5 ( 4 .7 - 9 .5 m m )

P o lyes te r 0 .187 -0 .375 (4 .7 -9 .5m m )

P E 0 .062 -0 .375 (1 .5 -9 .5m m )

P P 0 .187 -0 .375 (4 .7 -9 .5m m )

P P O 0 .250 -0 .375 (6 .3 -9 .5m m )

P o lysu l fone 0 .250 -0 .375 (6 .3 -9 .5m m )

P S 0 .125 -0 .375 (3 .1 -9 .5m m )

P U 0 .250 -0 .313 (6 .4 -8 .0m m )

P V C 0 .125 -0 .375 (3 .1 -9 .5m m )

For most thermoplastics, minimum recommended runner size=1.5mm (0.06)

This table lists the recommended runner diameters for different thermo-plastics

in injection molding industry. For most thermoplastics, the minimum

recommended dimension of runner is 1.5mm (0.06), too small the dimensionmay lead to excessive presure drop and filling difficulty.

The recommended runner size also reveals the flow ability (processability) of

the plastic material. Plastics with low viscosity (high melt flow index or long

flow length) such as polyethylene (PE) permit a smaller runner. Larger runner

should be adopted for plastics that have shorter flow lengths (higher viscosity

values), such as polycarbonate (PC).

This table serves as an initial guess for runner sizing.


14/43

14


Mold.ppt

Design of RunnerDesign of Runner

q Location and Number of Runner Ejectors

Stiffer Plastics

Ejector Pin

Softer/Flexible/StickyPlastics

Both the number and location of ejectors depend on the plastic being processed.

The stiffer the plastic is (at the moment of ejection), the fewer ejectors are

needed; also, the designer has higher degree of freedom to determine the ejectorlocations. For example, the ejectors can be placed under the connecting runners

(bridge runners) .

For soft, flexible, or sticky plastics, more ejectors have to be adopted. Care must

be taken in the ejector location so that the part can be ejected without leaving

marks or causing damage. In general, more ejectors lead to an increase in the

comlexicity of mold and the cost of the hardware and of machining.

In the design phase of the runner system, one should consider the ease of

demolding and removal from the molded part. The runner system should provide

sufficient spacing for cavity in order to accommodate cooling lines and ejector

pins and leave adequate cross section to withstand the injection pressure force.


15/43

15


Mold.ppt

Runnerless Molding TechnologyRunnerless Molding Technology

Moldings

Runner System:Scrap and material wastePressure drop

q Runnerless Molding Technology:

runners and sprues are kept a molten state during the processing runner systems are never actually ejected with the molded parts.

q Types of Runnerless Molding Technology:

Insulated Runner System

Heated/Hot Runner System

The conventional runner systemare referred to as cold runner systems since the

runners solidifies during the cooling phase of the injection molding cycle and is

ejected with the part. During the molding cycle the pressure drop increas as therunner is cooled down gradually. Degating is required during mold opening (for

three-plate molds) or separately afterwards (for two-plate molds) and the runner

system is regarded as scrap. The runner material may be reground and recycled

again, but it may have some physical properties degraded from the original,

virgin material. For small products the mass of cold runners may be as much as

80% of the mass of the total shot.

On the other hand, the so-called runnerless molding technology has been

developed to circumvent the drawbacks encountered in the cold runner systems.

In these special mold designs the runners and sprues are kept a molten state

during the processing and are never actually ejected with the molded part. Thereare no runners to be reground and recycled, thus, savings in material, labor,

and/or overhead are realized.

Typical examples of runnerless molding methods include insulated runners,

heated/hot runner systems.


16/43

16


Mold.ppt

Insulated Runner SystemInsulated Runner System

Molten state melt

Solidified resin shellCooling Lines

Emergency

parting line

Parting line

q Oversized the runner diameter (15~30mm)

q Insulation effect of frozen skin shell

q Works for most olefinic resins(PE,PP...) and PS

In the insulated runner system, the runner diameter is oversized (say, 15~30mm)

in order to maintain the molten state of the material. The large diameter runner

allows an inner molten melt to pass through during the molding cycle because ofthe insulation effect of frozen skin shell surrounding the melt core.

The insulation runner system has the advantage of extremely simple

construction, low cost tooling, and high efficiency, provided the system can be

left running undisturbed for long periods. This design is suitable for most olefinic

plastics (such as polyethylene (PE), polypropylene (PP)... ) and polystryene (PS).

The disadvantages of the insulated runner system includes:

- it requires fast cycle to maintain molten state within runner (at least 5

shots/min).

- it requires long start-up periods (15-25min) to stabilize the runner temperature(up to 150 oC)

- it needs a long color change time

- it needs very accurate gate temperature control in order to have a satisfactory

production rate.

- Additional emergency parting line is required to facilitate the removal of the

frozen runner in the case of prolonged delay in the cycle time.


17/43

17


Mold.ppt

Internally Heated Hot RunnerInternally Heated Hot Runner

SystemSystem

q Material is heated by the heating element in the center of the runner

q Annular gap for melt flow

Heater Cartridge

Heated Probe

(Torpedoe)

Part

Melt

Tempertature Profile

Vlocity Profile

In the internally heated hot runner system, the material is heated and kept at a

molten state by the heated probe (torpedoe) in the center of the runner. The melt

is allowed to flow in the cross section of the annular gap of the runner.The advantages of the internally heated hot runner systems include:

-Less heat loss and lower heating power required since the thermal insulation of

polymer melt

-Less mold components mis-matching problem arising from thermal expansion

-Inexpensive (as compared with the external heated runner system)

-Little space required.

The disadvantages of this design include:

-Higher shear rate and pressure drop since the restricted flow area

-Sophicated heat control required (temperature profile exists in the cross

section of the annular gap of the runner).


18/43

18


Mold.ppt

Externally Heated Hot RunnerExternally Heated Hot Runner

SystemSystem

q Material is heated by the cartridge-heating manifold inthe housing of the runner

q Circular cross section for melt flow

Cooling Lines

Heater Cartridge

Heated Manifold

Part

Air gap insulation

Insulation Blocks

Hot Runner

Vlocity Profile:plug-like flow

Temperature Profile:constant temperature profile

In the externally heated hot runner system the material is heated by the

cartridge-heating manifold in the housing of the runner. Thus a plug-like flow

profile and an approximately constant temperature profile across over the circularflow area is developed. Thus the flow resistance is smaller than that of the

internally heated system.

The advantages of this design are:

-More uniform temperature distribution.

-Better temperature control

-Lower melt stresses and pressure drop

-Color/material changes easily

The disadvantages of the externally heated hot runner system include:-More complicated design

-More Expensive

-Significant thermal-expansion-induced mis-match problems for various mold

components.


19/43

19


Mold.ppt

Design of GateDesign of Gate

Generalities

ease of demolding

ease of degating

weld lines

distortion

molding defects

cost

Part Design

geometry

wall thickness

direction of mechanicalloading

quality demands

(dimensions,cosmetics,

mechanics...)

Flow length

PlasticMaterial

viscosity (MFI)

processing temperatureflow characteristic

fillers

shrinkage behavior

Then gate provides the connection between the runner and the mold cavity. It

must permit enough material to flow into the mold to fill out the cavity, raises

melt temperature by viscous (frictional) heating, and freezes-off when theholding stage is over. It should be smaller in the cross section so that it can be

easily separated from the molded part (degated).

The type of the gate and its size and location in the mold strongly affect the

molding property and the quality of the molded part. The factors which

determine the gate design is summarized here briefly.

General speaking, the gate should be small, simple to demold and easily

separated from the part. The gate should be connected to the molding in such a

manner that the latter is not distorted (the molding tends to deform concave to the

feed ) and does not exhibit blemishes. Cost of tooling is also a consideration

factor. The location of the gate must be such that weld lines are avoided or

shifted to a less critical position. Molding defects such as jetting, burning,

thermal degradation, short shot, etc. should be avoided in the production.

Gating scheme and location of gates are crucial to the quality of the molding.

Filling pattern and cavity pressure profile are closely related to the final

properties of molded parts, such an mechanical properties, cosmetics (surface

properties), dimensional accuracy. A gate should provide appropriate filling

pattern and viscous heating effect, permit effective packing and holding of the

material within the mold. These criteria depend on both part design as well as

physical properties of the plastic material.


20/43

20


Mold.ppt

Gating SchemeGating Scheme

Direct/Sprue Gate Side/Edge Gate

Pin Gate

There are several gate type can be adopted in the mold design, and each has its

own advantage for application.

The direct gate or sprue gate feeds material directly into the cavity. It is usedfor temperature-sensitive or high viscosity materials, and is suitable for

producing part with heavy sections. The direct gate can be applied in high quality

part because it allows effective holding (minimum pressure loss) and exact

dimensions can be obtained. However, it is suitable only for single-cavity molds.

Visible gate mark and the high stress concentration around the gate area are the

disadvantges.

The side gate or edge gate is the standard gate for injection molding. It is used

wherever the product can be or must be gated from the parting line and where

self-degating is not required or practical. It is carried out at the side of the partand is easy to construct and degate.

The pin gate or pinpoint gate is a kind of restricted gates that are usually

circular in cross section and for most thermoplastics do not exceed 1.5mm (0.06

in.) in diameter. It is generally used in three-plate molds (with automatic gate

removal) and hot runner construction. It provides rapid freeze-off and easy

degating of the runner from the gate. Flexibility in gate location is another

advantage of the pin gate. It can easily provide multiple gating to a cavity for

thin-walled parts. Viscous heating as the melt passing through the restricted

pinpoint gate raises melt temperature and improves the filling process since the

melt viscosity is lowered. Higher pressure drop is a drawback.


21/43

21


Mold.ppt


Fan Gate Film Gate

Tab Gate Disc Gate

Thefan or fin gate is a fanned out variation of the edge gate. It is used for large

flat parts (say,over 8cm x 8cm or 3 in x 3 in) or when there is a special reason

such as elimination of weld lines. when the danger of part warpage anddimensional change exists, the fan gate is often adopted.

Thefilm gate orflash gate involves extending the fan gate over the full length

of the part but keeping it very thin. It is used for flat molded part in the situation

that the orientation of flow pattern in one direction is required, this is important

in the applications of optical parts. It has the advantages that there is no weld

line, reduced warpage and improved part dimensional stability. However,

postoperation for gate removal is required for this type of gate.

The tab gate is used in cases where it is desirable to transfer the stress generated

in the gate to an auxiliary tab, which is removed in a postmolding operation. The

tab gate is capable of preventing the jetting problem during the filling stage. Flat

and thin parts require this type of gate.

The disc gate or its variation, the diaphragm gate, has a conical manifold. It is

used for rotationally symmetrical parts (hollow tubes) with core mounted at just

one half of the mold. The advantage of using this gate system is that there are no

weld lines, and concentricity of the molded part is ensured. This is a important

dimensional requirement for pipe fittings. The cone or diagram region eliminates

stress concentration around the gate since the whole area is removed, but the

postoperation is necessary and more difficulty.


22/43

22


Mold.ppt


Ring Gate Submarine/Tunnel Gate

The ring gate accomplishes the same purpose as gating internally in a hollow

tube, but from the outside. In the ring gate the melt reaches an annular channel

manifold next to the sprue. The gate has a small cross section and acts as athrottle. Therefore the annular channel fills before melt begins to fill the cavity. It

is adopted in the case that the core cannot be mounted on just one side of the

mold such as in the case of disc gating. The ring gate is used to produce sleeve-

like parts with core mounted at both sides of the mold.The advantages of this

gating scheme include: uniform wall thickness around circumference can be

obtained, applicable for long cylindrical part, as well as easy production.

However, final finishing of molded part is necessary and sometimes slight weld

line may appear.

The submarine or tunnel gate is used mainly for small parts in multicavity mold

where it is possible to locate the gate laterally. This gate is automatically degatedas soon as the mold opens, this is the primary advantage of this gate system.

However, it is used for simple part only because of high pressure loss as the melt

passing through the small gate cross section and the runner length. The tunnel

gate can be used only for tough, elastic materials, since the material in the tunnel

has to withstand deformation during mold opening; the tunnel could break and

plug the runner system if brittle materials are used.


23/43

23


Mold.ppt

Effect of Gating SchemeEffect of Gating Scheme

Side gate: possibility of jetting

Tab gate: uniform filling, no jetting

The filling pattern of melt flow is largely governed by the location and size of

the gate(s). For example, jetting of the plastic into the mold cavity may occur if a

fairly large cavity id filled through a narrow gate (such as a side gate) is used,especially in the case of low-viscosity plastic melt.

Jetting gives rise a random filling pattern: the melt no longer fills the mold by

an advancing front way but snakes it away into the cavity without wetting the

walls near the gate. Surface defects, flow lines, variations in structure, and air

entrapment are related to the jetting phenomena.

Jetting can be prevented by enlarging the gate or locating the gate in such a way

that the flow is directed against a cavity wall. For example, tab gates (or fan

gates) can minimize the potential of jetting by reducing the inertia of the inlet

melt flow.


24/43

24


Mold.ppt

Effect of Gating SchemeEffect of Gating Scheme

Time

C

avityPressure

Sprue Gate

Pinpoint Gate

Film Gate

Different influence on holding stage and effective holding time

The gating scheme has a significant influence on the holding pressure profile

during the cooling stage.

For exmple, the size of a sprue gate is large so that the holding pressure can betransmitted without difficulty. The gate freezing-off time is longer due to the

larger gate size, leads to a slower droping in the cavity pressure and a longer

effective holding time. Hence in general a sprue gate is used for part that the

dimensional accuracy is important.

On the other hand, the pinpoint gate freezes early and leads to a shorter

effective holding time. This may cause sink marks and voids in the final part.

The cavity pressure curve of part with film gate is located between that of sprue

gate and pinpoint gate.

In the mold design phase, one have to consider if the gate can provide suitablefilling pattern, viscous heating, as well as its influence on effective holding time.


25/43

25


Mold.ppt

Weld line and Gate LocationWeld line and Gate Location

q Hot Weld (Streaming Weld, Meldline) Weld lines arising from obstructions (core,insert,pin...) in the flow

the melt is split by theobsraction into two fronts

the two streams arebrought back together

the temperature at the weld linedoes not differ much

Weld lines or knit lines are formed during the mold filling process where two

melt fronts meet each other. Microscopically, in the weld lines (or weld planes)

the two fronts are made of molecules that are aligned with the front shape andwill meet tangentially. The incomplete molecular entanglement and diffusion,

unfavorable frozen-in molecular (or fiber) orientation, as well as the crack-like

V-notches at the weld surface lead to structural weaknesses in the weld line area.

The presence of weld lines causes reduced mechanical strength for structural

applications and surface visual imperfections in the part. The allowable working

stress would be reduced by at least 15% in the weld line area.

In general, the colder the merging flows of melt, the more these weld lines

become visible and the poor is their strength.

Hot weld lines (or streaming weld line, meldline) is formed in the molds with

obstructions such as core, insert, or pin. In this case the melt front is separated by

cores or obstructions and recombines at some downstream location.

Experimental results indicate that the strength of the weld would decrease as the

distance between the obstruction and the gate increases, since the average flow

front temperature has been reduced.


26/43

26


Mold.ppt

Example of Hot Weld linesExample of Hot Weld lines

Melt FrontAverage Temperature

a.weld @ 188oC

c.weld @ 184oC

b.weld @ 185oC

Consider a part has one rectangular and two circular inserts obstructing the flow

with the rest of the cavity at an uniform thickness. From the CAE analysis we can

predict the location of weld lines behind each insert.They are hot weld lines sincethey are formed due to the exist of flow obstructions and the welding temperature

is high.

The welding temperature at position a,b,and c is 188, 185, and 184 oC,

respectively. The melt front splits and recombines around each insert. Weld

strengths tend to decrease as the number of flow stream divisions and

recombinations increase. They also decay with the distance from the gating

position because the melt is cooled along the flow path. We can anticipate that

the local strength in each welding position:

Thin sections are particular prone to weak welds because of rapid melt

solidification and less chance for chain diffusion.

1 2 3> >


27/43

27


Mold.ppt

Weld line and Gate LocationWeld line and Gate Location

q Cold Weld (Butt Weld) arise from the impingment of advancing fronts from different

gates in multi-gating molds. Worst welding manner.

Melt fronts traveling in opposite

directions meet, and are almost

immediately stoped after meeting.

the temperature of the fronts has dropped

somewhat at the welding zone

On the other hand, the so-called cold weld lines or butt weld lines present in

multiple gating molds where the impingment of advancing fronts from different

gates may occur.Cold weld lines are generally considered to be the worst welding manner

because they are formed from melt fronts traveling in opposite directions, the

fronts meet and are almost immediately stoped after meeting. The temperature of

the meeting fronts has dropped somewhat at the welding zone, this leads to a

weak welding condition since the molecular diffusion and entanglement is rather

poor in the low temperature area.

For unreinforced plastics, the tensile strength in the cold weld region can be

reduced to 80%; for fiber-reinforced plastics, this value is reduced to 30% to

40%.

The melt temperature is the most significant process variable in the welding

phenomena. Hotter melt tends to improve the weld strength due to the increased

molecular chain mobility and their coupling. Increase the mold temperature is

another strategy to improve the welding strength. Besides, welding strength can

be improved by good molding venting (avoid air entrapment), high injection

speed (decrease the temperature drop).

Gate design play an important role in the removal or elimination of weld lines.


28/43

28


Mold.ppt

Weld line and Gate DesignWeld line and Gate Design

Edge gatingwill lead toa weld line opposite the gateWeld strength is weak whendiameter ( &flow length ) is increased.

Spoke gatingwill producefour weld lines withstronger weld strength due

to shorter flow lengths

Sprue gatingat the cupbottom will eliminateweld line,gate mark problem

Consider the gate design in an injection-molded cup. This part can be produced

using a single edge gate in a two-plate mold. This gating scheme would result in

a cold weld line opposite the gate. As the diameter of the cup is increased, theweld line becomes more visible and the welding strength is decreased since the

flow length prior to welding is longer and the welding temperature is lower.

When an internal spoke gating scheme is adopted, although four weld lines will

be formed, however, each weld line is likely to be stronger (compared to the part

with a single edge gate) due to the reduction in melt flow length in the cavity.

Hence the weld line produced by the spoke gate is less visible and the welding is

stronger.

If a sprue gate at the cup bottom is used in this case. No weld line would be

produced in the final part. However, the significant gate mark is a problem andan postoperation is require to finish the product.


29/43

29


Mold.ppt

Location of Weld LineLocation of Weld Line

Possible weld line location

Possible weld line location

PLAY447

Top Cover of Scanner

In general, weld lines would be visually unacceptable, or, since they act as

stress concentrator, may be structurally unacceptable, depend on the product

specification and quality requirement.Computer analysis is capable of predicting the possible location of weld line.

According to the analysis result we can modify the gate design, part design

(modify the part thickness), or process condition, to relocate the weld lines to

visually or structurally less sensitive areas.

Consider a scanner cover that is produced by three submarine gates as an

example. In multi-gated parts the weld lines are almost unfavorable. From the

CAE analysis result we can predict the possible weld line locations and check if

they occur in critical regions. This precautions from CAE analysis in the design

phase will minimize the risk of part failure.

We can modify the design conditions to see if the weld lines can be relocated to

noncritical regions. When they are unavoidable, venting plays an important role

in improving the weld strength. That is, it is essential that air at the weld should

escape before the melt streams meet. Other techniques to improve weld strength

are to :

- Increase melt temperature (that is, chain mobility and coupling)

- Increase mold temperature (that is, chain mobility and coupling)

- Increase injection pressure (that is, lower the temperature difference)

- Avoid use of external release mold lubricant (avoid the presence of foreignsubstances at the weld interface)


30/43

30


Mold.ppt

Weld Line and Gate DesignWeld Line and Gate Design

Allow cavity filling with a minimum no. weld lines

more significantweld line

less significant weld line

more significantweld line

As a rule, if a single gate can fill the cavity without excessive injection

pressure, use it. Multiple gating always produce extra weld lines in the product.

However, two or more gates per cavity are sometimes required for very largeproducts (such as automobile products, bottle crates, etc.) where the flow lengths

from a single gate would be too long and/or too high the injection pressure is

required to fill the cavity. In some cases a multiple gating scheme is required to

avoid short-shot (incomplete filling) problems.

Consider the injection-molding of the motorcycle side cover by ABS. If two

gates per cavity is adopted, one weld line is produced in each cavity. However,

the injection pressure required is high and short-shot problem will present in the

end of filling; If triple gating scheme is employed, the cavity can be complete

filled without difficulty, except that there is an additional (less significant) weld

line in the final product.

It is important that the melt arrives at the welds (junction points) hot enough to

form an acceptable welding. Venting problem should not be overlooked in

improving the weld strength.


31/43

31


Mold.ppt

AirAir--Trap and Gate LocationTrap and Gate Location

Air Bag Housingthinner section(0.5-0.8mm)

thicker section(>10mm)

Racetrack Effect

Air-trap here

PLAY

When the plastic melt fills the mold, it displaces the air. The displaced air must

be removed quickly, or it may cause burn spot (due to the fast compression of

trapped air pocket by the low-thermal-conductivity polymer melt), or it mayrestrict the flow of the melt into the mold cavity, resulting in incomplete filling

(short-shot problem).

Consider the injection-molding of a air bag housing. Notice that the part

consists of a thin central region and a thick rim around it. A single gate is

adopted in the original design. Most of the melt flow along the part side since the

section is thicker and the flow resistance is lower than that in the central thinner

region. That is, the melt races away along the thick rim while the central region is

filling at a slow rate. The filling along the rim is dominant and finally the melt

backfills the central region and cause an entrapment of air there. In this case an

air-trap problem is caused by the racetrack effect of melt flow.

To avoid the buring or incomplete filling associated with the entrapment of air,

proper venting is required. Venting is provided by the clearance between

knockout/vent pins and their holes, parting lines, as well as additional venting

slots (in general, 0.01 to 0.02mm deep and 3mm to 6mm wide).

Gate location is directly related to the consideration of venting location. In

general, the vent is located opposite the gate, area near the end of filling, or in the

air-trap position.


32/43

32


Mold.ppt

Viscous Heating and Gate SizeViscous Heating and Gate Size

pinpoint gate (dia.=2mm)

Temperature(oC)

Gapwise Scale

inlet melt

temperature

temperature peak caused by

viscous heating effect

Melt viscosity is reduced and flowability is improved by raising themelt temperature via viscous heating effectTemperature raised


33/43

33


Mold.ppt

Gate Design vs. Part ShrinkageGate Design vs. Part Shrinkage

Gate Size

Part Shrinkage

Higher Packing Lower Packing

Demolding

Less Shrinkage Larger Shrinkage

Differential Shrinkage

Back

Gate design is important not only in controlling the filling pattern of the mold

cavity, but also in the dimensional quality of molded part.

Smaller gates freeze off sooner. Once the gates frozen, there is no melt addedduring the holding pressure stage, and the molded part will therefore shrink

more.

On the contrary, larger gates remain open longer. They freeze slowly and melt

continues to feed under holding pressure through the open gate, adding more

plastic as the melt shrinks in the cavity. Longer effective holding time and higher

holding pressure level of larger gates lead to smaller part shrinkage values.

In the mold cavity, the areas closer to the gating position are better packed than

the more remote areas, which may already have cooled down enough to prevent

additional melt to make up for volume contraction through shrinkage. The resultis that the areas near the gate shrink less than the areas farther away.

Besides, during the mold filling stage the polymer molecules undergo a

stretching that results in molecular orientation and anisotropic shrinkage

behavior: plastic materials tend to shrink more along the direction of flow ( in-

flow shrinkage) compared to the direction perpendicular to flow (cross-flow

shrinkage), while the shrinkage behavior of reinforced material is restricted

along the fiber-orientation direction.

This differential shrinkage is the primary cause of part warpage.


34/43

34


Mold.ppt

Cooling System DesignCooling System Design

molding

coolingchannels

LayoutSizeDistance to MoldingCoolant Flow RateCoolant TemperatureType of CoolantMold Material

The mold of thermoplastics receives the hot, molten plastic in its cavity and

cools it to solidify to the point of ejection. The mold is equiped with cooling

channels or cooling lines that remove heat released from the part via flowingcoolant. The mold temperature is controlled by regulating the temperature of

coolant and its flow rate through the cooling channels. Productivity (cycle time)

and quality (dimensional accuracy) of molded part depend heavily on the design

and efficiency of the cooling system.

High efficiency cooling system may cool down the part uniformly and quickly,

hence the cycle time can be shortened, this leads to an improvement of the

molding productivity.

The cooling channels should be spaced evenly to prevent uneven temper-ature

on the mold surface, they should be as close to the part surface as possible,taking into account the strength of the mold material. The cooling channels are

connected to permit a uniform flow of the coolant, and they are thermostatically

controlled to maintain a given coolant temperature.

Even mold temperature distribution is important to ensure the dimensional

accuracy of molded part. Uneven mold temperature leads to unbalanced cooling

of part surface. The thermal stresses associated with the temperature profile

across the part thickness result in part warpage or distortion.

Design parameters involved in cooling system involves the type of coolant and

mold material, coolant flow rate, coolant temperature, distance and size of

cooling channels, and their layout.


35/43

35


Mold.ppt

Cooling Channel Layout vs.Cooling Channel Layout vs.

Part WarpagePart Warpage

Lower Cooling Rate

Higher Cooling Rate

Unbalanced Cooling

Demolding

Colder surfaceSmaller Shrinkage

Hoter surfaceLarger Shrinkage

Warpage of Injection-Molded Part

Uniform cooling throughout the part is critical to the dimensional accuracy of

molded part.

Consider the cooling of an injection-molded plate part by a poor-designedcooling system. The top face of the part is cooled by three cooling channels, the

part surface temperature in higher due to the insufficient cooling; on the other

hand, the bottom face of the part is cooler since it is cooled by four cooling

channels (assume that all cooling channel has the same cooling efficiency).

The hotter top surface of the part will continue to shrink more than than the

cooler bottom surface after the gate frozen off and part ejection. This differential

shrinkage through the part thickness is caused by the differential cooling (

difference in the cooling rate between the cavity and the core) and would cause

the part to warp due to the unbalanced internal thermal stresses and theirassociated bending moments as the part is ejected from the mold.

Non-uniform cooling plays a key role in the warpage behavior of molded part,

especially in the cases of flat moldings, such as disks (records, trays, etc). The

differential cooling problem can be minimized with proper mold cooling system

design.


36/43

36


Mold.ppt

Wall Thickness and Part DesignWall Thickness and Part Design

q Flow Length/Wall Thickness Ratio (L/t Ratio) a measure of moldability of the part

( )L / t Ratio

Maximum Flow Length

From Gate to Rim

Average Wall Thickness

L

t

L/t Ratio

0 100 200 300

Heavy-walled parts

easy to mold

Most parts

relatively easy to mold

Thin-walled part

Difficult to mold,

needs special considerations

Very-difficult-to-

mold part

needs special

equipment

An important measure of the moldability of a part design is its flow length/wall

thickness ratio (L/t ratio). The L/t ratio of a part is defined as its maximum flow

length from gate (the pressure source) to the farthest point (end point of filling),to its average wall thickness.

A smaller values of the L/t ratio indicate a shorter flow length or thicker part

section, represent a smaller flow resistance and pressure loss, hence the parts are

easy to mold. On the other hand, thin-walled parts or parts with longer flow

length have larger L/t ratios and the molding is more difficult to carry out.

The L/t ratio of a given part can assist the part designer in determining the gate

locations, especially for parts of constant wall thickness. Its rather difficult to

evaluate this value for a complicated part with variable wall thicknes, this

situation is further complicated by the fact that runner systems can consume a

significant portion of the molds pressure drop.

Many factors influence the L/t ratio of a given design, such as plastic materials

processed, melt temperature, mold temperature, maximum injection pressure, and

injection velocity, etc.


37/43

37


Mold.ppt

Maximum Flow Length ofMaximum Flow Length of

Plastic MaterialsPlastic Materials

PC,PVC

Acetal

Nylon

Acrylic

ABS

PS

HDPE

PP

LDPE

25

36

38

33-38

45

51-63

57-63

63-70

70-76

0 10 20 30 40 50 60 70 80

PC,PVC

Acetal

Nylon

Acrylic

ABS

PS

HDPE

PP

LDPE

(cm)

Maximum Flow Length in a 2.54mm(0.1in.) thick part

The flow of the plastic melt in the mold depends on various factors, such as the

plastic used, melt temperature, mold temperature, length and diameter of sprue

and runners, gate type, etc. In determining the minimum wall thickness of thepart, all these factors have to be considered.

The L/t ratio achieveable depends heavily on the type of plastic to be processed.

A high viscosity (low melt index) plastic such as polycarbonate (PC),

polysulfone (PSU), acrylic,etc., has a higher resistance to flow because of its

microstructure (cross linking, high molecular weight) and thus has a shorter

maximum flow length. It requires higher injection pressure to fill the mold cavity

with sufficient filling speed. For example, in a testing mold with a thickness of

2.54mm (0.1in.), the maximum flow length of PC is 25cm.

On the other hand, for easy-flow, low-viscosity plastics such as poly-propylene

(PP), polyethylene (PE), the maximum flow length is longer and the minimum

wall thickness that can be filled is smaller than for stiff-flowing materials.

Typical maximum flow length of general purpose grades of thermoplastics,

based on a cavity thickness of 2.54mm (0.1 in.) and conventional molding

techniques, are provided here to illustrate their processing properties. These data

are obtained from the spiral flow length experment and can be used as a reference

of moldability of various resin grades.

The actual maximum flow length of a plastic material depends on part design,

mold design, as well as the process variables.


38/43

38


Mold.ppt

Maximum Flow Length ofMaximum Flow Length of

Plastic MaterialsPlastic Materials

MaximumF

low

Length

Part Thickness

increasinginjection pressure

@constant injection speed,

mold/melt temperature

MaximumF

low

Length

Part Thickness

increasingmelt/mold temperature

@constant injection pressure

injection speed

The maximum flow length achieveable for a particular plastic grade depends on

molding conditions of the experiments.

For instance, under a constant injection speed/mold temperature/melttemperature condition, the flow length increases as the applied injection pressure

is increased because of the increasing driving force for mold filling. Thus easy-

to-flow materials require a lower injection pressure to fill the mold cavity with

sufficient filling speed.

Under a constant injection speed/injection pressure condition, the maximum

flow length of a given material increases as the mold temperature and/or the melt

temperature is raised. A plastic material has a longer flow length at higher

temperature because of its thermal-reduced melt viscosity.

These flow length data of plastic materials provide valuable information abouttheir flow behavior and processing properties. They are available from material

suppliers.


39/43

39


Mold.ppt

Wall Thickness of a PartWall Thickness of a Part

Plastics Min. Wall Thickness (mm

Max. Wall Thickness (mm)

Suggested WallThickness (mm)

PO M 0.4 3.0 1.6

AB S 0.75 3.0 2.3

Acrylic/PMMA 0.6 6.4 2.4

Cellulose 0.6 4.7 1.9

Teflon 0.25 12.7 0.9

Nylon 0.4 3.0 1.6

PC 1.0 9.5 2.4

Polyester 0.6 12.7 1.6

LDPE 0.5 6.0 1.6

HDPE 0.9 6.0 1.6

EV A 0.5 3.0 1.6

PP 0.6 7.6 2.0

PS U 1.0 9.5 2.5

PP O 0.75 9.5 2.0PP S 0.75 3.8 2.3

PS 0.75 6.4 1.6

SA N 0.75 6.4 1.6

PVC-Rigid 1.0 9.5 2.4

PU 0.6 38.0 12.7

Surlyn 0.6 19.0 1.6

The nominal minimum, maximum, and suggested wall thickness for various

plastic materials is listed here. The essential issue in determining the wall

thickness of a part is the flowability of polymer melt. The wall of a part shouldallows plastic melt to flow properly under appropriate injection pressure. The

wall should permits effective transmission of packing/holding pressure during the

holding stage. Finally, the wall should withstand the internal/external loading

after the part is ejected from the mold cavity.

The allowable minimum wall thickness is smaller for easy-flow, low-viscosity

plastics such as polyethylene (PE) and polypropylene (PP). This value is larger

for polycarbonate (PC) and polysulfone (PSU) that are more viscous and stiff-

flow.

Typically, a thin-walled partcan be arbitrarily defined as a part with a L/t ratio

greater than 200 or with wall thickness less than 1mm (t2mm). Filling is not a problem in a heavy

wall and the injection pressure needed is lower than that of the thin wall. Cycle

time is long, often longer than 20 sec.

Determining the proper part thickness is important to facilitate the processing

and ensure product strength.


40/43

40


Mold.ppt

Wall Thickness of a PartWall Thickness of a Part

q Empirical Equation t,L in mm

for easy - flow plastics: t = 0.6L

1000.5

for fair - flow plastics: t = 0.7L

1000.8

for stiff - flow plastics: t = 0.9L

1001.2

+

+

+

e.g.,PP,PE,Nylon

e.g.,POM,PMMA

e.g.,PC,PSU

An empirical equation is presented here to give an rough estimate of wall

thickness for a plastic part. For example, if polypropylene (PP) is used as the

molding compound, the wall thickness of a 50-cm long part will be:

wile for the stiff-flow polycarbonate (PC) the required wall thickness is:

the cooling time is about four times that of PP.

t mm= +

=0 6

500

1000 5 3 3. . .

t mm= +

=0 9

500

10012 5 6. . .


41/43

41


Mold.ppt

Part Wall TransitionPart Wall Transition

flowt Sharp/Stepped Transition:

poor design

flow

3t

Gradual Transition:better designthick-to-thin gating

flow

3t

Gradual Transition:thin-to-thick gating(not recommended)

flow

3t

Smooth/Tapered Transition:best design

For a variable wall thickness part, the wall transition should be gradual to

ensure proper mold filling and part strength.

Consider the sharp or stepped transition case, the wall thickness undergoes astep change in the part. During the filling stage the melt front chages its filling

velocity suddenly in the wall thickness transition region and a pressure loss is

caused by theflow contraction effect. The filling pattern in this design may result

in air entrapment and stress concentration problems.

A better design is to modify the stepped transition into a gradual transition

(usually tapered a transition length equal to three times the difference in

thickness). The melt velocity undergoes a gradual change as the cross section

contracts gradually. Pressure loss due to the gradual contraction is lower than that

of the stepped transition. High stress concentration around the transition region

can be avoided.

The best design is to vary the wall thickness as smooth as possible, usually a

tapered transition is adopted. Pressure loss and stress concentration can be

minimized in this design.

Note that the melt flow should be directed in the direction from thick-to-thin

whenever posible. The thicker section requires more packing/holding to

compensate for volume contraction and should be located closest to the gate. If

the flow direction is from thin section to thick section, the thinner section may

freeze off faster and hinder the packing of the thicker section, poor surface

finishes and sink mark/warpage problems may be caused.


42/43

42


Mold.ppt

Wall Thickness and ShrinkageWall Thickness and Shrinkage

local heavy section:poor

sink mark

Shrinkage voidsUse two short thick ribs:good

Use a long thin ribs:betterCore out the heavy section:better

Thin wall parts with heavy boss, ribs, rims, and/or other local heavy cross

sections usually is difficult to molding. Usually the poorly cooled heavy sections

will shrink more because the holding pressure will be ineffective after the thinwalls freeze and block the melt flow to these heavy sections. This can be often

seen by the sink marks on the surface behind these local heavy sections. Also, the

differential cooling and shrinkage of the thin and thick sections lead to warpage

of the molded part. When the cooled outer surface of the part is strong to resist

sinking and the inner hot melt cools and shrinks, shrinkage holes/voids will be

created within the plastic wall.

Thick ribs provide improved structural benefits and are easier to fill, however,

the level of sink associated with the thick ribs can be excessive. The sink mark

and internal shrinkage voids problems are significant if the rib wall thickness is

too heavy and/or if the rib base is wide.

Adopt a long but thin rib is a good strategy to improve the design. In practice,

rib wall thicknesses are typically 40%-80% as great as the wall from which they

extended, with a base radius values from 25%-40% of the wall thickness. The

specific rib designs are material dependent, and are influenced primarily by the

shrinkage behavior of the plastic material.

Alternative better design is to core out the heavy section, uniform wall thickness

can be obtained in this case. This results in cycle time reduction along with an

overall quality improvement.


43/43


Mold.ppt

Wall Thickness and ShrinkageWall Thickness and Shrinkage

original design

thick section thick sectionthin section

rib

sink mark

sink marksink mark

sink mark

part warpage

voidsstress concentrationbetter design

Thick walls in a part will fill easily, with less pressure, but will take a longer

time to cool and shrink more; on the other hand, thin walls require much higher

pressure to fill the cavity space at high speed and will not shrink as much asheavy walls.

Thin wall parts with heavy boss, ribs, rims, and/or other local heavy cross

sections usually is difficult to molding. Problems such as sink marks, warpage,

and shrinkage voids may be caused if the part wall is not properly desinged.

When parts have both thick and thin sections, gating into the thick section is

preferred because it enables packing/holding of the heavy section, even if the

thinner sections have frozen off. The design can be further improved by coring

out heavy bosses and heavy sections, and by using ribs and edge stiffeners to

compensate for the loss in stiffness of a thinner section. A cored out section not

only shrinks less but also takes a shorter cooling time.

A properly design part, with even wall thickness and adequate ribbing, is

usually stronger and stiffer than a part with thicker and/or uneven walls. Saving

of material, reduction in part weight and cycle time, improvement in part quality

, etc., are the advantages obtained if we design the part carefully.

thiet ke khuon mau

Documents