injection molding machine - basics

7/26/2019 Injection Molding Machine - Basics

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Injection Molding Machine:

Injection molding machines, also known as presses, consist of a material hopper, an injection

ram or screw-type plunger, and a heating unit. Molds are clamped to the platen of the molding

machine, where plastic is injected into the mold through the sprue orifice. Presses are rated bytonnage, which is the calculation of the amount of clamping force that the machine can exert.

This force keeps the mold closed during the injection molding process. Tonnage can ary from

less than ! tons to ",### tons, although the higher tonnage presses are rarely used. The totalclamp force needed is determined by the projected area of the custom part being molded. This

projected area is multiplied by a clamp force of from $ to % tons for each s&uare inch of the

projected areas. 's a rule of thumb, ( or ! tons)in can be used for most products. If the plasticmaterial is ery stiff, it will re&uire more injection pressure to fill the mold, thus more clamp

tonnage is needed to hold the mold closed. The re&uired force can also be determined by the

material used and the si*e of the part with larger plastic parts re&uiring higher clamping force.

Mold:

The mold or die refers to the tooling used to produce plastic parts in molding. Traditionallyinjection molds hae been expensie to manufacture and were only used in high-olume

production applications where thousands of parts were produced. Molds are typically constructed

from hardened steel, pre-hardened steel, aluminum, and)or beryllium-copper alloy. The choice ofmaterial to build a mold from is primarily one of economics. +teel molds generally cost more to

construct but offer a longer lifespan that will offset the higher initial cost oer a higher number of

parts made before wearing out. Pre-hardened steel molds are less wear resistant and areprimarilly used for lower olume re&uirements or larger components. The hardness of the pre-

hardened steel measures typically %-(! on the ockwell- scale. /ardened steel molds are heat

treated after machining, making them superior in terms of wear resistance and lifespan. Typicalhardness ranges between !# and "# ockwell- 0/1.

'luminum molds cost substantially less than steel molds, and when higher grade aluminum such

as 2-3 and 2-4# aircraft aluminum is used and machined with modern computeri*ed

e&uipment, they can be economical for molding hundreds of thousands of parts. 'luminum

molds also offer &uick turnaround and faster cycles because of better heat dissipation. They canalso be coated for wear resistance to fiberglass reinforced materials. 5eryllium copper is used in

areas of the mold which re&uire fast heat remoal or areas that see the most shear heat generated.

Injection Molding Process 06 5ack to Top1

7ith injection molding, granular plastic is fed by graity from a hopper into a heated barrel. 'sthe granules are slowly pushed forward by a screw-type plunger, the plastic is forced into a

heated chamber called the barrel where it is melted. 's the plunger adances, the melted plastic

is forced through a no**le that seats against the mold sprue bushing, allowing it to enter the moldcaity through a gate and runner system. The mold remains at a set temperature so the plastic can

solidify almost as soon as the mold is filled.
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Injection Molding Cycle 06 5ack to Top1

The se&uence of eents during the injection molding of a plastic part is called the injection

molding cycle. The cycle begins when the mold closes, followed by the injection of the polymerinto the mold caity. 8nce the caity is filled, a holding pressure is maintained to compensate for

material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. Thiscauses the screw to retract as the next shot is prepared. 8nce the part is sufficiently cool, the

mold opens and the part is ejected.

Different Types of Injection Molding Processes 06 5ack to Top1

'lthough most injection molding processes are coered by the conentional process description

aboe, there are seeral important molding ariations including9

o-injection0sandwich1molding

:usible0lost, soluble1core injection molding

;as-assisted injection molding

In-mold decoration and in mold lamination

Injection-compression molding

Injection molding of li&uid silicone rubber

Insert and outsert molding


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eaction injection molding

esin transfer molding

heomolding

+tructural foam injection molding

+tructural reaction injection molding

Thin-wall molding

=ibration gas injection molding

7ater assisted injection molding

ubber injection

Stress 06 5ack to Top1

The main enemy of any injection molded plastic part is stress. 7hen a plastic resin 0which

contains long strains of molecules1 is melted in preparation for molding, the molecular bonds aretemporarily broken due to the heat and shear force of the extruder, allowing the molecules to

flow into the mold. >sing pressure, the resin is forced into the mold filling in eery feature, crack

and creice of the mold. 's the molecules are pushed through each feature, they are forced tobend, turn and distort to form the shape of the part. Turning hard or sharp corners exerts more

stress on the molecule than taking gentle turns with generous radii. 'brupt transitions from onefeature to another are also difficult for the molecules to fill and form to.

's the material cools and the molecular bonds re-link the resin into its rigid form, these stressesare in effect locked into the part. Part stresses can cause warpage, sink marks, cracking,

premature failure and other problems.

7hile some stresses in an injection molded part are to be expected, you should design your parts

with as much consideration for stress reduction as possible. +ome ways to do this are by addingsmooth transitions between features and using rounds and fillets in possible high stress areas.

Gates 06 5ack to Top1
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?ach injection mold design must hae a gate, or an opening

that allows the molten plastic to be injected into the caity of the mold. ;ate type, design andlocation can hae effects on the part such as part packing, gate remoal or estige, cosmetic

appearance of the part, and part dimensions @ warping.

Gate Types

There are two types of gates aailable for injection moldingA manually trimmed andautomatically trimmed gates.

Manually Trimmed Gates:

These type of gates re&uire an operator to separate the aprts from the runners manually after each

cycle. Manually trimmed gates are chosen for seeral reasons9

The gate is too bulky to be automatically sheared by the machine

+hear-sensitie materials such as P= cannot be exposed to high shear rates

:low distribution for certain designs that re&uire simultaneous flow distribution across a

wide front

Automatically Trimmed Gates

These type of gates incorporate features in the tool to break or shear the gates when the tool

opens to eject the part. 'utomatically trimmed gates are used for seeral reasons9

'oiding gate remoal as a secondary operation, reducing cost

Maintaining consistent cycle times for all parts

Minimi*ing gate scars on parts

Common Gate Designs 06 5ack to Top1

The largest factor to consider when choosing the proper gate type for your application is the gatedesign. There are many different gate designs aailable based on the si*e and shape of your part.

5elow are four of the most popular gate designs used by 2uickparts customers9


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The Edge Gate is the most common gate design. 's the name indicates, this gate is located on

the edge of the part and is best suited for flat parts. ?dge gates are ideal for medium and thick

sections and can be used on multicaity two plate tools. This gate will leae a scar at the partingline.

The Su Gate is the only automatically trimmed gate on the list. ?jector pins will be necessaryfor automatic trimming of this gate. +ub gates are &uite common and hae seeral ariations

such as banana gate, tunnel gate and smiley gate to name a few. The sub gate allows you to gateaway from the parting line, giing more flexibility to place the gate at an optimum location on

the part. This gate leaes a pin si*ed scar on the part.

The !ot Tip Gate is the most common of all hot runner gates. /ot tip gates are typically locatedat the top of the part rather than on the parting line and are ideal for round or conical shapes

where uniform flow is necessary. This gate leaes a small raised nub on the surface of the part.

/ot tip gates are only used with hot runner molding systems. This means that, unlike cold runner

systems, the plastic is ejected into the mold through a heated no**le and then cooled to the

proper thickness and shape in the mold.

The Direct or Sprue Gate is a manually trimmed gate that is used for single caity molds of

large cylindrical parts that re&uire symmetrical filling. Birect gates are the easiest to design and

hae low cost and maintenance re&uirements. Birect gated parts are typically lower stressed andproide high strength. This gate leaes a large scar on the part at the point of contact.

Gate "ocation 06 5ack to Top1

To aoid problems from your gate location, below are some guidelines for choosing the proper

gate location0s19

Place gates at the heaiest cross section to allow for part packing and minimi*e oids @

sink.

Minimi*e obstructions in the flow path by placing gates away from cores @ pins.

5e sure that stress from the gate is in an area that will not affect part function or

aesthetics.

o If you are using a plastic with a high shrink grade, the part may shrink near the

gate causing Cgate puckerD if there is high molded-in stress at the gate

5e sure to allow for easy manual or automatic degating.

;ate should minimi*e flow path length to aoid cosmetic flow marks.

In some cases, it may be necessary to add a second gate to properly fill the parts.


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If filling problems occur with thin walled parts, add flow channels or make wall thickness

adjustments to correct the flow.

;ates ary in si*e and shape depending upon the type of plastic being molded and the si*e of thepart.


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>niform 7all Thickness9

Thick sections take longer to cool than thin ones. Buring the cooling process, if walls are an

inconsistent thickness, the thinner walls will cool first while the thick walls are still solidifying.'s the thick section cools, it shrinks around the already solid thinner section. This causes

warping, twisting or cracking to occur where the two sections meet. To aoid this problem, try to

design with completely uniform walls throughout the part. 7hen uniform walls are not possible,then the change in thickness should be as gradual as possible. 7all thickness ariations should

not exceed 4#F in high mold shrinkage plastics. Thickness transitions should be made gradually,

on the order of to 4. This gradual transition aoids stress concentrations and abrupt coolingdifferences.

Alternati%es:

If your part is so complex that you need ariations on your wall thickness, look for an

alternatie. Gou may want to use design features such as coring or using ribs. 't the ery least,

try not to make the transitions between thicker and thinner sections too abrupt. Try using agradual transition or chamfered corners to minimi*e the dramatic change in pressures inside the

mold.

Draft 06 5ack to Top1


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Most injection molded plastic parts include features such as outside walls and internal ribs that

are formed by opposing surfaces of tool metal inside a closed mold. To properly release the part

when the mold opens, the side walls of the mold are tapered in the direction that the mold opens.This tapering is referred to as Cdraft in the line of drawD. This draft allows the part to break free

of the mold as soon as the mold opens. The amount of draft re&uired can depend on the surface

finish of the mold. ' smooth, polished tool surface will allow the part to eject with less draft thana standard tool surface.

onsider the fabrication of the hollow plastic box seen to the right. 8nce the plastic has hardened

around the mold, the mold must be remoed. 's the plastic hardens, it will contract slightly. 5y

tapering the sides of the mold by an appropriate Hdraft angleH, the mold will be easier to remoe.

The amount of draft re&uired 0in degrees1 will ary with geometry and surface texture

re&uirements of the part. 5elow are seeral rules for using draft properly9

5e sure to add draft to your B 'B model before creating radii

>se at least 4 degree of draft on all HerticalH faces

4 degrees of draft is re&uired for light texture

$ degrees of draft works ery well in most situations

degrees of draft is a minimum for a shutoff 0metal sliding on metal1

degrees of draft is re&uired for medium texture

Sin$ Mar$s 06 5ack to Top1

7hen the hot melt flows into the injection mold, the thick sections donJt cool as fast as the rest

of the part because the thicker material becomes insulated by the outside surface of faster coolingplastic. 's the inner core cools, it shrinks at a different rate than the already cooled outer skin.

This difference on cooling rates causes the thick section to draw inward and create a sink mark

on the outside surface of the part, or worse, completely warp the part. In addition to being

unattractie, the mark also represents added stress that is built into the part. 8ther lessconspicuous areas where sink occurs include ribs, bosses and corners. These are often

oerlooked because neither the feature nor the part itself is too thickA howeer, the intersection of

the two can be a problem.


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8ne way to aoid sink marks is to core out the solid sections of the part to reduce thick areas. If

the strength of a solid part is re&uired, try using cross hatched rib patterns inside the cored out

area to increase strength and aoid sink. 's a rule-of-thumb, make sure that all bosses andlocating)support ribs are no more than "#F of the thickness of the nominal wall. 'lso, textures

can be used to hide minor sink marks.

Te&tures 06 5ack to Top1

Texturing is a process used to apply patterns to a mold surface. This process allows flexibility increating the final appearance of your parts. Texturing is an integral piece in oerall product

deelopment and should be considered during the design process to achiee the desired results.

Texture can be a functional component of design as well. Imperfect parts can be camouflaged bythe right texture. Is the part designed for fre&uent handlingK Texture can be used to hide finger

prints and improe the grip for the end user. Texture can also be used to reduce part wear from

friction.

' wide ariety of textures are aailable for injection molded parts such as9

Latural)?xotic

Matte :inishes

Multi-;loss Patterns

:usions

;raphics


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7hen applying a texture to a part, the 'B drawing must be adjusted to accommodate for this

surface ariance. If the texture is on a surface that is perpendicular or angled away from the mold

opening then no draft changes are necessary. If the texture is on a parallel surface with the moldopening, howeer, increased draft is necessary to preent scraping and drag marks that could

occur during part ejection. Bifferent textures hae different impacts on the molded part. The rule-

of-thumb when designing for texture is to hae 4.! degrees of draft for each #.##4D of texturefinish depth.

Parting "ines 06 5ack to Top1

' Cparting lineD is the line of separation on the part where the two hales of the mold meet. The

line actually indicates the parting CplaneD that passes through the part. 7hile on simple parts thisplane can be a simple, flat surface, it is often a complex form that traces the perimeter of the part

around the arious features that make up the partJs outer CsilhouetteD. Part lines can also occur

where any two pieces of a mold meet. This can include side action pins, tool inserts and shutoffs.

Parting lines cannot be aoidedA eery part has them. eep in mind when designing your part,

that the melt will always flow towards the parting line because it is the easiest place for thedisplaced air to escape or CentD.

Common Molding Defects 06 5ack to Top1

Injection molding is a complex technology with possible production problems. They can eitherbe caused by defects in the molds or more often by part processing 0molding1

Molding

Defects

Alternati%e

'ameDescriptions Causes

5lister 5listering

aised or layered

*one on surface ofthe Plastic part

Tool or material is too hot, often

caused by a lack of cooling around thetool or a faulty heater

5urn marks'ir 5urn);as

5urn

5lack or brown

burnt areas on the

plastic part locatedat furthest points

from gate

Tool lacks enting, injection speed is

too high

olor streaks0>+1


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:lash 5urrs

?xcess material in

thin layer exceedingnormal part

geometry

Tool damage, too much injection

speed)material injected, clamping

force too low. an also be caused bydirt and contaminants around tooling

surfaces.

?mbeddedcontaminates

?mbeddedparticulates

:oreign particle

0burnt material orother1 embedded in

the part

Particles on the tool surface,

contaminated material or foreigndebris in the barrel, or too much shear

heat burning the material prior to

injection

:low marks :low lines

Birectionally Hoff

toneH way lines or

patterns

Injection speeds too slow 0the plastichas cooled down too much during

injection, injection speeds must be set

as fast as you can get away with at alltimes1

Oetting

Beformed part by

turbulent flow ofmaterial

Poor tool design, gate position or

runner. Injection speed set too high.

Polymerdegradation

polymer breakdownfrom oxidation, etc.

?xcess water in the granules,excessie temperatures in barrel

+ink marks


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injection molding machine - basics

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