The Injection Molding Process

At the start of the cycle, the mold is closed by actuating the press, which on aninjection molding machine is called the clamping unit.. Before the melt, which isgenerated in and supplie

The injection molding process is one of the key production methods for processingplastics It is used to produce molded parts of almost any complexity that are to be made

in medium to large numbers in the same design There are major restrictions on wallthickness, which generally should not exceed a few millimeters, and on shape - it must

be possible to demold the part This will be discussed later

The advantages of this process are:

- direct route from raw material to finished part,

- very little finishing, or none at all, of molded parts,

The raw material, usually in the form of pellets, is fed into the plasticating unit where

it will be melted The plasticating unit is generally a single-screw extruder in which thescrew reciprocates coaxially against a hydraulically actuated cylinder The continuallyrotating screw plasticates the pellets to form a melt that is transported forward by therotation Because the injection nozzle is still closed during plastication, the melt ispushed to the front of the screw As a result, the screw is pushed to the right against theresistance of the barrel, which is called the back pressure

At the start of the cycle, the mold is closed by actuating the press, which on aninjection molding machine is called the clamping unit Before the melt, which isgenerated in and supplied by the plasticating unit (in a precise, metered quantity), isinjected into the closed mold, the plasticating unit traverses against the mold, causing theinjection nozzle of the plasticating unit to press against the sprue bushing of the mold.The pressure with which the nozzle is pressed against the sprue bushing must be adjusted

in such a way that the joint remains sealed when the melt is injected afterwards At thesame time, the nozzle is opened and the melt can be pushed from the front of the barrelinto the cavity of the mold

As the cavity is filled, pressure builds up inside This is counteracted by pressing theclamping unit against the mold under as much clamping force as possible to prevent meltfrom escaping out of the cavity through the mold parting lines

The connection between the mold and plasticating unit is maintained until the fillingprocess is complete Generally, however, filling of the cavity does not mean the end ofthe process because the melt changes its volume on solidifying (freezing) In order that

4 T h e I n j e c t i o n M o l d i n g P r o c e s s

Mold clamped Heater band

Mold partially filled

Stage 2: Holding pressure and plastication

Screw rotates for plastication

either more melt may be forced in to make up the difference in volume or to preventthe melt from running out of the mold, the connection must be maintained until themelt has frozen in the gate The connection is broken by screwing back the plasticatingunit, and closing the injection nozzle Detaching of the nozzle causes thermal isolationbetween mold and plasticating unit because these are at totally different temperatures.Since the plasticating process requires a certain amount of time, as soon as the nozzle

is detached and closed, the plasticating unit usually starts rotating, drawing in metering - more material, melting it and moving it to the front

-When the molding (molded part) has solidified to the extent that it can retain its shapewithout external support, the clamping unit opens the mold and the molding is pushedout of the cavity by ejectors

The cycle then repeats Figure 4.1 shows the order in which the processes occur Thebasic cycle described here may vary for other materials and processes

4.1.1 Injection M o l d i n g of T h e r m o p l a s t i c s

When thermoplastics are heated, they experience a change of state; they turn soft andmelt, becoming flowable When cooled down, they solidify again This is the reason thatplasticating units are operated hot and molds are operated cold when working withthermoplastics Generally, the temperature difference is more than 100 0C Thethermoplastic materials developed for injection molding generally constitute relativelylow-viscosity melts with the result that injection times are short and low clamping forcesare needed

The injection mold should remove the heat from the material fast and steadily.Therefore, the cooling system has to be carefully designed The coolant - usually water,provided the mold temperature is below 100 0C - flows through channels around thecavity For reasons of economics, such as the quality of the molded parts, which dependsheavily on uniform heat flow in the mold, the cooling circuit is monitored very preciselyand cooling equipment is used to ensure that the coolant is always at the sametemperature

Molded parts requiring no machining can only be produced if all joints and moldparting lines are so well sealed that melt is unable to penetrate and harden there.Otherwise, flash would be formed and machining become necessary To this end, all jointgaps must remain smaller than 0.03 mm even during full injection pressure, until themelt has solidified These requirements are particularly demanding where large moldedparts and large injection molding machines are involved as the molds must be extremelyrigid and the clamping units must function very precisely; the rigidity of the clamp plates

is particularly critical

4.1.2 Injection M o l d i n g of Crosslinkable Plastics

These plastics only attain their final molecular structure by crosslinking under heat Forthis reason, they must be kept at as low a warm temperature as possible in the plasti-cating unit, i.e the viscosity must be just low enough that the cavity is filled Thisprevents premature crosslinking from interfering with complete formation of the moldedpart The plasticating unit is therefore usually kept below 100 0C and frictional heat isminimized through the use of compressionless screws

Figure 4.2 Viscosity function of cross-linking molding compounds - processing limits

The mold, on the other hand, is at such a high temperature that a reaction, and thereforecrosslinking, occurs rapidly There is a limit on the upper temperature because nothermal damage may be done to the surface of the molded parts

Figure 4.2 shows the change in viscosity of such plastic materials and which cyclesthey occur in

4.1.2.1 Injection Molding of Elastomers

Elastomeric materials, such as rubber, have virtually the same molecular structure whensupplied as when they are in their final state The only effect of heat is to generate awide-meshed lattice in which adjacent molecules are chemically bound to each other.Consequently, the change in volume that accompanies crosslinking is slight

To prevent elastomers from crosslinking before entering the mold, the plasticatingunits are generally kept at below 100 0C

A number of modern, synthetic elastomers are an exception here, however, e.g liquidrubbers

Because the elastomer is heated up by more than 60 0C in the cavity, its volumeincreases despite the fact that crosslinking is occurring at the same time; the result is thathigh cavity pressures are generated Since, on contact with the hot wall of the cavity, thematerials undergo a decrease in viscosity before they crosslink, the gaps of the partinglines must be smaller than 0.02 mm if flash is to be avoided This is generally beyondthe realms of possibility, especially with large molds, and so flash is often unavoidable

4.1.2.2 Injection Molding of Thermosets

These materials are supplied in a low-molecular state for injection molding They are, inaddition, mostly filled with mineral or wood powder, or fibers and have a relatively highviscosity in the injection unit at the low temperatures permitted there (< 120 0C)

Here, too, the temperature of the molds is about 100 0C higher than that of theplasticating unit Narrow-meshed crosslinking results in rapid solidification Thecrosslinking process releases heat that has to be dissipated These materials becomeparticularly fluid when they come into contact with the hot cavity wall Therefore, gapsalong the parting lines have to be less than 0.15 mm wide if flash is to be avoided.

4 2 T e r m s U s e d i n C o n n e c t i o n

w i t h I n j e c t i o n M o l d s

The terminology used in this book corresponds largely to that shown in Figure 4.3 Theseterms are established in practice There also exists an ISTA booklet (International SpecialTooling Association) which deals with the terminology of components of injectionmolds

Figure 4.3 Designations for

components of an injection mold (typical

European design) [4.4]

1 Compression spring, 2 Ejector bolt,

3 Movable clamping plate, 4 Ejector and

ejector retainer plates, 5 Ejector pin,

6 Central sprue ejector, 7 Support plate,

8 Straight bushing, 9 Cavity retainer

plate, 10 Leader pin, 11 Shoulder

bushing, 12 Parting line, 13 Cavity

retainer plate, 14 Stationary clamping

plate, 15 Plug for cooling line

connection, 16 Locating ring, 17 Sprue

bushing, 18 Cavity insert, 19 Cooling

line, 20 Cavity insert, 21 Support pillar

4 3 C l a s s i f i c a t i o n o f M o l d s

Depending on the material to be processed one frequently talks about

- injection molds (for thermoplastics),

4 4 F u n c t i o n s o f t h e I n j e c t i o n M o l d

For the production of more or less complicated parts (moldings) in one cycle, a moldcontaining one or several cavities is needed The mold has to be made individually ineach case The basic tasks of a mold are accommodation and distribution of the melt,shaping and cooling of the material (or adding activating heat for thermosets andelastomers), solidification of the melt, and ejection of the molding All these tasks of amold can be accomplished with the following functional systems:

- sprue and runner system,

- cavity (venting),

- heat exchange system,

- ejection system,

- guiding and locating system,

- machine platen mounts,

- accommodation of forces,

- transmission of motions

Figure 4.4 demonstrates these functions with a simple mold for a tumbler

Besides forming the part, the mold has another important function; demolding thepart From an economic viewpoint the cycle should be as short as possible, but from theaspect of quality, ejection, especially of complex moldings, has to be reliable withoutdamage to either part or cavity

The design of an ejection system depends on the configuration of the molding [4.7];one distinguishes parts

- without undercuts,

- with external undercuts,

- with internal undercuts

A number of design possibilities arise from this distinction as well as another importantclassification From the fact that moldings can be pushed out, stripped off, unscrewed,torn off, cut off, one can recognize the demand for a classification with respect to the

Figure 4.4 Breakdown of the

functions of an injection mold [4.5]

Ejection system

and transmission

of movements

Leading and aligment Sprue andrunner system

Heat exchange

system

Mounting and transmission

demolding system This classification is justified because it immediately allows thenecessary amount of work to be recognized, which affects costs It also indicates thefeasible size and number of cavities as a result of space requirements.

Ejection system (partly) Cavity layout

Number of parting lines Sprue and runner system (partly)

Number of floating plates Heat-exchange system

Alignment Slides and lifters

Transmission of forces Ejection system (partly)

Mounting to machine platen

Table 4.2 Distinction of molds according to primary design features [4.5]

Shape of molding Plastic material Processing parameters Lot size

Position of molding relative to parting line Injection molding machine

Cycle time Plastic material Economics Rigidity of mold Geometry of molding Injection pressure (spec.) Plastic material

Design version Two-plate mold Three-plate mold Stripper plate (two parting lines)

Slides Split cavity Unscrewing device Stripper plate

Hot manifold Insulating runner

Split cavity Interlock machined out of the solid material Leader pins

Mold designation Standard mold Mold designed for tearing off molding Stripper mold Stack mold Slide mold Split-cavity mold Unscrewing mold Stripper mold

Hot-runner mold Insulated-runner mold

Split-cavity mold Standard mold

can vary within one group of mold types; design features are invariable within one groupand therefore of general validity for one and the same type.

Another distinction according to primary design features is represented in Table 4.2.This demonstrates how mold types may result from different design criteria and theirassociated effects

Designations of molds are not always uniform in literature and common use They aremostly based on specific components or demolding functions, or indicate the potentialfor a particular application Table 4.3 lists criteria leading to mold designations

Table 4.3 Criteria leading to a characteristic mold designation [4.5]

A classification of molds with regard to the demolding system results in the basic moldtypes shown in Figures 4.5 and 4.6 Molds with a relatively complex design such as cut-off, stack, hot-runner, insulating-runner and other special molds can be integrated intothis system Besides this, a statistical analysis [4.8] has demonstrated that predominantly

"simpler" molds are presently in use

Figures 4.5 and 4.6 clearly summarize what has been described so far The basiccategories are presented in the following sequence:

4 Mold designed for

cutting off molding

5 Split-cavity mold

6 Unscrewing mold

7 Mold designed for

tearing off molding

One parting line; opening motion in main direction and transverse with slide actuated by cam pin

Similar to 1., but demolding with stripper plate Similar to 1., but separation of runner and molding by cutting with additional plate moving transverse (like 3.)

One parting line; opening motion in main direction and transverse; cavity halves slide on inclined planes and can withstand lateral forces Rotational motion for automatically demolding a thread is

mechanically actuated Two parting lines for demolding runner and molding separately after they have been torn apart; one-directional opening motion in two stages

Cavity plates stacked with several parting lines Two parting lines; no conventional runner system but channels with enlarged cross section permitting formation of a hot core insulated by a surrounding frozen skin

Runner is located in an electrically heated manifold Combinations of 2 to 10 for moldings with special requirements which do not permit a simple solution

Most simple sesign;

Two mold halves;

One parting line;

Opening in one direction;

Demolding by gravity,

ejector pins or sleeve

For all kinds of moldings

For cup like shaped moldings without undercut

For parts with cuts or external threads

under-Figure 4.5 Basic categories of injection molds [4.5]

MS = movable side, SS = stationary side, PL = parting line

Design similar to standard

mold but with split cavity

block for moldings with

undercuts or external

threads

For oblong or wide moldings

with undercuts or threads

For moldings with internal

e Sprue and runner

Two parting lines;

Movement of floating plate actuated by latch

or stripper bolt;

Two-step opening movement

Automatic separation

of molding and runner

Figure 4.6 Basic categories of injection molds [4.5]

MS = movable side, SS = stationary side, PL = parting line

The schematic presentation should illustrate the principle of each group.

The row "moldings" (Figures 4.5 and 4.6) provides only an indication of thepossibilities provided with such molds The design examples are taken from thereferences [4.9, 4.10]

The numbers in row "opening path" refer to the sequence and directions of motionsand stand for:

1 Main opening movement: guiding motion

2 Movement between guide and slide: relative motion

3 Movement of slide during demolding: absolute motion

4 Movement of unscrewing core: relative rotation

4.4.2 Basic P r o c e d u r e for M o l d D e s i g n

It is advisable to proceed with any mold design systematically because a mold and itsoperation have to meet a variety of conditions Figure 4.7 demonstrates how interrelatedthe conditions are and which boundary and secondary conditions have to be met by themain function This statement becomes even more evident with an example The path ofdecisions to be made by the designer is exemplified by a flow chart for the design of astandard mold for producing several covers simultaneously (Figure 4.8a-h) It is sug-gested that this path be traced step by step to get a feel for the logic of the procedure

4.4.3 D e t e r m i n a t i o n of M o l d S i z e

The size of a mold depends primarily on the size of the machine Frequently an existingmachine or a certain machine size poses an important limitation, to which the designengineer has to submit

Such limitations are

- shot size, the amount of melt that can be conveyed into the mold with one stroke of thescrew or the plunger,

- plasticating rate, the amount of plasticated material the machine can provide per unittime,

- clamping force, which has to compensate the reactive force from maximum internalcavity pressure,

- maximum area of machine platen given by the distances between tie bars (Figure 4.18)maximum injection pressure

4.4.3.1 Maximum Number of Cavities

At first the maximum theoretical number of cavities is calculated [4.4]

max shot size Sv in cm3N1 = (4.1)volume of part and runner Mv in cm3

This computation assumes utilization of the whole maximum shot size of the machinecomputed from screw diameter and displacement It is not a wise practice, however, forreasons of quality (uniform melt, adequate cushion for holding pressure) to select themaximum quantity

Details of orderGeneral items : size of order, costs, delivery

datesPort : geometry, material, appearancetolerances, strength and other propertiesMachine data

Dimensions: R = 20 mm

h = 8 mms= 2 mm

Material: PS

Machine : Type

Injection pressure Cycle time Molding: Cover

shot weight, economy, delivery dates)

Selected: n=4

Figure 4.8a-b Design example: Standard mold [4.5]

MS = movable side, SS = stationary side

(Continued on next page)

Runner system : Sprue with runner and pinpoint gateCross sections of

runners

* Selected

Figure 4.8b-d Design example: Standard mold (continued)

(Continued on next page)

Figure 4.8d-f Design example: Standard mold (continued)

(Continued on next page)

no

Supports

6 Mounting of ejector

housing

f

Figure 4.8f-g Design example: Standard mold (continued)

The number of cavities for thin-walled parts can furthermore be determined by theplasticating rate of the machine

plasticating rate R in cmVminN2= (4.1a)number of shots Z/min • (part + runner volume in cm3)

Modern reciprocating-screw injection molding machines have such a high plasticatingrate that the number of cavities N2 should only be checked for thin-walled parts withlarge shot size An empirical rule states

4.4.3.2 Clamping Force

The minimum clamping force is derived from the reactive force of the cavity, whichresults from the projected area of all cavities and runners and the maximum cavitypressure:

F = A - p (4.3)Herein F is the reactive force, A the projected areas of cavities and runner system and pthe cavity pressure Depending on material and part the cavity pressure is between 20 and

100 MPa, proper processing assumed Faulty operation can rise this pressure up to the fullinjection pressure It is advisable, therefore, to calculate with the maximum injectionpressure of the machine and the total projected area that can be covered with melt

4 4 3 3 M a x i m u m C l a m p i n g A r e a

This area is determined by the distances between the tie bars (Figure 4.18) Generally oneavoids the additional trouble of pulling tie bars Therefore between the largest molddimension should be about 10 mm smaller than the distance the corresponding tie bars.Clamping units are built to withstand the maximum cavity pressure that can be expected.Machines for processing foam with low pressure can have light-duty clamping units orlarger clamping platens and wider distances between tie bars Care should be taken thatthe plates do not bend more under loads than several micrometers Otherwise theadmissible gap width of the parting line cannot be maintained even if the molds themselvesare sufficiently rigid In this respect today's machinery is frequently undersized

4.4.3.4 Required Opening Stroke

The opening stroke has to be adequately long to permit troublefree ejection from moldswith very long cores (example: mold for bucket) Minimum requirement is a stroke ofmore than twice the length of the core

On the other hand, a stroke that is longer than needed uses up cycle time, which has

to be kept short for reasons of costs

The opening stroke can certainly be adjusted but the investment for a more thannormal stroke is high Therefore, one has proposed [4.12] to tilt the mold during theopening stroke with an auxiliary equipment and to demold then (Figure 4.9)

Figure 4.9 Device for tilting the mold during demolding [4.12]

4.4.4 T h e F l o w L e n g t h / W a l l T h i c k n e s s Ratio

Another criterion pertaining to the machine is the ratio between flow length and wallthickness According to Hagen-Poiseuille's law the ratio between flow length L and thesquare of the wall thickness of the molding H2 is determined by the injection pressurepinj, a quantity of the machine, and the viscosity of the melt if the velocity of the meltflow is given

For thermoplastics there are certain optimum values for the velocity [4.13], which aredetermined by the orientation, to which the molecules are subjected They are aroundvinj - 3 0 cm/s

Mostly, however, empirical data are used, which are provided by the raw-materialsuppliers for their products in the form of flow-length/wall-thickness diagrams(Figure 4.10) The data for this presentation have been established for each material byexperiment

They present common data for the fabrication of a molding characterized by themaximum flow length of melt in the cavity and the related (thinnest) section thicknessand are purely empirical The flow length/wall thickness ratio derived from Hagen-Poiseuille's law is in accordance with the similarity principle:

L _ Ap

wherein

L Flow length,

H = 2 W • T/(W + T) the hydraulic radius with width (W) and thickness (T),

(p 1.5 for width much larger than thickness (almost always the case),

vF Velocity of the flow front Qualitatively preferable value: ca 30 cm/sec,

Ap Maximum injection pressure; for common machines: ca 120 MPa

For estimates, the "apparent effective viscosity" is

for amorphous materials:

qaeff = 250 to 270 Pa • s (max error + 10%, with a melt temperature TM = TE

(freeze temperature) + 150 0C)

for crystalline materials:

T^eff = 170 Pa • s (max error ±5%, with a melt temperature TM = TE

for crystalline materials

L = 1Q > H = 500 • H2 (cm) (4.7)12-170

Wall thicknessmm

Figure 4.10 Relationship

between wall thickness and

flow length for a number of

PMMA molding resins

(Degussa Corporation) [4.14]

They meet the requirements of

DIN 7745.

There are two series of grades:

Standard grades 6, 7 and 8, and

E-grades with higher molecular

weight.

Flow length mm