gate positioning and molding strategy

4.2 Design and Process Strategies for Injection Molding 39

4.2 Design and Process Strategies for Injection Molding

This section provides a series of 16 design and process strategies based onwhat was discussed in the previous three chapters. These strategies willsummarize information and focus on design and process issues that willhelp minimize injection molding problems. Both injection moldinganalyses and actual case studies of moldings are utilized to explain thesestrategies. The optimum application of many of these strategies requiresthe use of mold filling simulation.

4.2.1 Maintain Uniform Wall Thicknesses in a Part

Uniform wall thickness in an injection molded plastic part is critical tominimize both shrinkage and mold filling related problems. Non-uniformwall thicknesses will result in both volumetric and directional shrinkagevariations. It is these variations that are often at the heart of warpageproblems and other defects, such as sinks and voids. In addition, non-uniform wall thickness will disrupt filling patterns, potentially causing racetracking and hesitation related problems.

Resulting problem and explanations:

1. The thick perimeter is shrinking more than the thinner center region.This is causing the center region of the part to pop up. The radial flowleaders disrupt this effect, causing some irregularity in the warpage.Figure 4.1 is a side view of the part showing the warpage resulting fromthe variations in wall thickness.

2. The thin center region not only creates a problem of differentialshrinkage between the center and the perimeter, but is also verydifficult to fill. The thickened radial spokes help the melt get to theperimeter, but they also create gas traps as the melt races down thespokes and around the thicker perimeter, while hesitating in thethinner center regions (see Fig. 4.2). Figure 4.3 shows the result of a

Example 1

The center region of the defective part shown in Figs. 4.1 and 4.2 has awall thickness of 1 mm, while the perimeter was designed with a wall,which is approximately 2 mm thick. Eight radial flow leaders (thickenedregions radiating from the center of the part) were added to the thincenter region in an attempt to help with production problems. This is alow production part, to be molded in a single-cavity three-plate coldrunner mold. The part shown is molded with eight gates, each feedinginto one of the eight radially positioned flow leaders.

Figure 4.1 Part showing deformation caused by variation in wall thickness

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Figure 4.3 Melt front temperatures from a mold filling simulation, showing effects of the melt hesitating in the thin region of the cavity

Figure 4.2 Gas traps resulting from the addition of flow leaders to the thin center region of the part

04.fm Seite 39 Montag, 6. September 2004 6:22 18

4 Gate Positioning and Molding Strategies40

mold filling simulation for this part showing the melt flow fronttemperature. Note, how the melt cools as it hesitates in the thin region,while maintaining a relatively high temperature in the ribs and at theperimeter, where it is race tracking around the thinner region.

There is no good solution to molding this part other than to provide auniform wall thickness. With the current design, a single center gate wouldresult in even worse gas traps in the same places as with the current gating.Without the radial flow leaders, the warping problem would be moreextreme. Gating into the perimeter would result in the material racingaround the entire perimeter, while experiencing severe hesitation near thegates, resulting in little chance of filling the part at all.

4.2.2 Use Common Design Guidelines for Injection Molded Plastic Parts with Caution

There are many sources offering general guidelines for designing plasticparts. Using these can often avoid problems that might otherwise develop.However, it should be realized that many of these are very general and they

Example 2

The part in Fig. 4.4 has a thick cylindrical region, which runs along athinner rectangular region. Regardless of gating, the thicker region willwant to shrink more than the thinner rectangular region. As the thinnerregion freezes first, it is forced to warp as the thicker cylindrical regioncontinues to shrink inward. In this case, there is a possibility to reducethe problem by coring the thicker cylindrical region.

Example 3

The Port Erie plastic pallet – Skidmarx® shown in Fig. 4.5 is the firstknown pallet to be fully injection molded without the use of structuralfoam. The part was initially designed with a solids modeling program. Amid plane mesh was created and a series of injection molding, moldcooling, and structural analyses were performed. The part has a nearconstant wall thickness of about 2.8 mm. For ejection, the partincorporates alternating angled ribs crossed with alternating taperedribs with wall thicknesses averaging 2.8 mm. The alternating ribsfacilitate ejection. As a result of utilizing CAE to help optimize thedesign and evaluate manufacturability prior to mold build, this partstarted up with virtually no molding problem. Good parts wereproduced within 10 shots with a cycle time of just over a minute. With aweight of only 6.8 kg (15 pounds) when molded of HDPE, this part isshown holding 1,361 kg (3,000 pounds) in Fig. 4.6.

Figure 4.6 The lightweight Skidmarx®plastic pallet shown in industrial application carrying over 2000 lbs

Figure 4.5 Non-foamed injection molded plastic pallet (Skidmarx®) developed with the aid of solids modeling, injectionmolding simulation and structural analysis

Figure 4.4 The thick cylinder along the thin rectangular region of the part initiates warpage because of differential shrinkage



commonly focus on local problems, while ignoring negative global effectson the part. These general guidelines also often do not consider thecharacteristics of the specific material being molded.

Of particular concern is the common practice of thinning ribs that areattached to the primary wall of a molded part. Thinned ribs, which willreduce the potential for local sinks and voids at their intersection with theprimary wall, can result in warpage, residual stresses, and numerous mold-filling problems. If cosmetics are a concern at the location of the rib, athinned rib may have to be used. Voids at this location are usually not a realconcern as the highest stress in a rib under a flexural load is normally at itstip rather than at its intersection with the primary wall.

The part shown in Fig. 4.7 is from a test mold and is molded from a neatnylon. It has a primary wall of 3 mm with a 1.5 mm rib, which is 6 mm tall.This design is based on common design guidelines for injection molding ofhigh-shrink materials. The mold cavity is fed by a wide fan gate, whichcreates a near ideal linear flow. Note how the variations in wall thickness,resulting from following the guideline, are causing the part to warp. Thethinner rib freezes first, while the thicker primary wall will continue to cooland shrink causing the part to warp as shown.

4.2.3 Avoid Flowing from Thin to Thick

Whenever a part must have variations in wall thickness, it is highlydesirable to gate into the thicker region so that plastic flows from thick tothin. This will minimize the potential for sinks and voids in the thickerregion as well as the risk of uncontrolled shrinkage.

Whether gating from thick to thin or from thin to thick makes littledifference during initial mold filling. However, during the compensationphase (packing phase), a thin wall can be expected to freeze-off prior to athicker wall. If the part is gated into the thin wall, which will freeze duringthe compensation phase, all flow to the thicker region will be blocked.The thicker region will continue cooling and shrinking without anycompensating flow. The molder will have lost control of the shrinkage ofthis portion of the part. The resulting high shrinkage in the thicker regioncould result in sinks, voids, and a stress, relative to the thinner region.This residual stress is what leads to warpage in plastic parts. Figure 4.8illustrates the progression of a developing frozen layer during thecompensation phase and the resulting loss of shrinkage control in thethicker region.

One should not only understand the benefits of using a particulardesign guideline, but should understand the potential negativeconsequences of applying the guideline.

Figure 4.7 Molded test plaque showing warpage resulting from the addition of a rib that was designed according to common design guidelines for ribs

Figure 4.8 FEA analysis shows the thinregion of a part freezing off prior to the thickregion. Gating in the thin region will result in a loss of shrinkage control in the thicker region

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4.2.4 Establish a Simple Strategic Flow Pattern within a Cavity

As plastic is forced through a mold cavity under high pressure, shear andextensional flow forces act on the polymer and any fillers or reinforcementsit is carrying. These forces cause the polymer molecules, and anyasymmetric additive, to become oriented in the direction of the principlestrains developed from these forces. This orientation in turn results inanisotropic residual strains, shrinkage, and changes in mechanicalproperties. The effect is most dramatic with fiber-filled materials.Therefore, when positioning the gates of a mold a designer should considerthe following:

• In parts with elongated shapes, it is generally preferred for the plastic toflow in one direction when filling the cavity. This will result in fewerconflicts in residual strains and shrinkage and thereby reduce stressesand tendencies for the part to warp. Therefore it is generally preferredthat a designer attempt to gate from one end of a cavity resulting in thematerial flowing across its length. This simple flow pattern is particularlyimportant with fiber-filled materials, where warp developed fromanisotropic shrinkages can be severe. The ideal position for a fan gate isalong one edge of a part so that the plastic melt is flowing across itslength. It should be noted that gating from one end of a part also creates thebiggest variation in melt pressure across the part during filling andpacking. Without use of packing profiles the regions near the gate will bepacked out better than regions away from the gate. However, thereduction of conflicts in orientation-induced strains normally has amore significant impact on reducing warpage than the effects ofdifferential pressures.

• A circular shaped part should be gated in its center, thus providing auniform flow in all directions throughout filling. Circular geometriesare not conducive to a linear flow path. Gating from one side to developa linear flow would cause the part to become more oval in shape.However, if the part includes regions that are not linear, another gatinglocation might be considered. A preferred position might again be fromone end or centrally positioned to provide a balanced fill between thetwo most extreme locations of the part.

• If gating from one end is impractical, multiple gates might be preferredto develop a simple but balanced flow (see Section 4.2.7 – BalanceFilling Throughout a Mold).

• As mechanical properties are enhanced along the direction of flow, flowshould be directed perpendicular to expected flexural loads and parallelto tensile loads. Again, this is particularly important for fiber-filledmaterials.



Example 7

Radiator end tanks are designed with a thick flange along theirperimeter for mounting purposes. Gates are normally positioned nearthe top center of these parts to minimize race tracking and gas trapsdeveloped as a result of the thickened flange. Without consideringorientation-induced shrinkages it would be expected that the positionof the thickened flange would cause the ends of the part to warp/bowdownward. However, these parts nearly always warp in the opposite

Example 4

The part shown in Fig. 4.9 is molded of a 30% glass-filled nylon 66. Thepart has a 100 mm radius, is 2 mm thick, is tab gated, and develops aradial flow pattern as the melt expands out from the gate. Note thecharacteristic warpage due to the variation in shrinkage from the gate tothe perimeter. A second part with the same 2 mm wall thickness is arectangular 50 × 200 mm plaque, which is gated with a fan gate at oneend and establishes a nearly ideal linear flow across the entire partduring mold filling. Regardless of process variations, this part is verystable and highly resistant to any significant warpage from molding.

Example 5

The part shown in Fig. 4.10 is a wire harness molded of fiber-fillednylon. This part has a complex shape that is assembled with a secondpart. Despite the 600 mm flow length and an average wall thickness ofonly 2.3 mm, the cavity is gated from one end with a fan gate positionedon one end. As a result of the simple linear flow path, the part isproduced with a minimum of distortion problems.

Example 6

The rectangular part at the top of Fig. 4.11 is molded of a neatpolycarbonate and is gated with a tab gate along the edge as shown. In asecondary operation a small metal plaque is mounted to the part with athermally activated adhesive. During application it turned out that themetal plaques were loosening and falling off. Placing the part in an ovenat an elevated temperature revealed the orientation induced residualstresses that were developed in the gate region. The effect of the highshear stress and radial flow pattern on orientation becomes obvious inthis case. At the elevated temperatures the part loses its stiffness and canno longer resist the residual stress developed by the highly strainedpolymers in the gate region. A second mold, which was end gatedeliminated this problem.

Figure 4.9 Warpage resulting from radialflow pattern of a fiber-filled material

Figure 4.10 Wire harness with fan gateresulting in a linear flow pattern, which minimizes the potential for warpage

Figure 4.11 Rectangular plaque as molded (bottom) and after being exposed to heat(top). Note the distortion from residualstresses acting on the part as the polymerchains are allowed to relax at the elevated temperature



direction (see Fig. 4.12). This warp results from the filling pattern that isdeveloped in these parts, combined with the use of a fiber-filledmaterial. The radial flow in the gate region creates both shear andextensional induced fiber orientation. As the melt reaches the flange, itbegins to race down the flange and causes nearly pure linear flow in thisregion. This linear flow orients the fibers along the length of the flangeand significantly reduces its ability to shrink along its length. Flow alongthe upper body is more complex, combining both radial and linearregions. The resulting reduction in fiber orientation along the length ofthe upper body will allow it to shrink more along its length relative tothe thicker flange. This will force the ends of the part to warp upward.To address this warpage, most molders must either build the molds witha counter warp or anneal the parts, while in a fixture, after molding.Flow leaders are also often applied to the top length. These are usedprimarily to avoid gas traps, which are created due to the race trackeffect created by the thicker flanges (Fig. 4.13).

4.2.5 Avoid Picture Framing

There are two different sets of conditions under which “picture framing”can be developed. These are presented in the following examples.

Example 1

Figure 4.15 shows the results of a mold filling analysis of a part that has athinner center wall region surrounded by a thicker perimeter. This“picture frame” design would be typical of a flat part with a thickerflange. This is primarily a part design problem, for which there isgenerally no good solution other than to modify the part design so as to

Example 8

Figure 4.14 shows an exterior automotive body panel with a speciallydesigned fan gate (724 mm wide) attached. The part was moldedoriginally from multiple hot drops placed along where a decorative stripwould be placed (the strip would be able to cover up any local cosmeticflaws at the gate locations). The part required dimensional stability andminimal stresses so as to help withstand elevated temperaturesexperienced during post-mold painting operations. The painted parthad to have a nearly flawless Class A surface. The original multiple gatesresulted in numerous cosmetic and dimensional problems. A single,specially designed, fan gate was able solve all these problems. The fangate encouraged the development of a more linear flow path, therebyproviding a much more dimensionally stable part. In addition, thisapproach eliminated weld lines and other related distortions.

Figure 4.12 Typical warpage in most radiator end tanks. Warpage, relative to the thick flange, is opposite to what is expected because of orientation of glass fibers as developed during mold filling

Figure 4.13 Gas trap development in a radiator end tank that is center-gated on its top. During mold filling, material reaches the thick flange near the gate and races down the length of the flange creating a gas trap at the end as shown

Gate LocationRelatively thin

main body

Relatively thick flange

Figure 4.14 Automotive body panel molded with specially designed fan gate toeliminate weld line and minimize potential for warpage and residual stresses



core out the flange. If gated into the thicker region of the part, as shownin the analysis output, the melt will race around the thicker sectionwhile it hesitates trying to fill across the thinner section, thus creatingthe picture frame effect. The melt racing around the perimeter willmeet, trapping air in the center thinner-walled region, thus creating agas trap. If gated in the center region, the melt will flow from thin tothick creating uncontrolled shrinkage in the thicker perimeter. Figures4.1 and 4.2 are examples of a part with a thin center region and thickperimeter. The part is molded with eight gates each feeding midwayalong one of the spoke-patterned flow leaders. The result is a twistedbowl shape as well as formations of gas traps. The bowl shape is evidentin Figure 4.1 and is causing the three vertical features on the left andright sides to bow outward.

4.2.6 Integral Hinges

Integral hinges create a particular problem in injection molding. Mostcommonly formed from polypropylene, these hinges commonly span thewidth of a part connecting a container and its lid. The hinges are muchthinner than the adjoining walls, commonly only about 0.25 mm thick.They get their characteristic high strength and excellent flexural strengthfrom the orientation of the plastic melt as it is flows across the hinge. Toavoid flow hesitation at the hinge, the gate(s) should be placed away fromthe hinge.

Example 1

Figure 4.17 shows a mold filling analysis of two equal halves of arectangular part separated by a thin integral hinge. A poorly placed gateis positioned along one edge near the hinge. Here the melt hesitates as ithits the restrictive hinge. Some of the melt crosses the hinge near thegate into the left side of the part and slowly moves up the left side.Meanwhile, the melt on the right side quickly travels up the length of thepart. The melt traveling along the right side hesitates along the hinge,welding with material meeting it from the left side. When the hotmaterial racing along the right side reaches the far end, it blows acrossthe top hinge region and begins to back-fill while meeting up with theslower moving melt front on the left side of the hinge. Except at the two

Example 2

Another variation of picture framing occurs in a part with a coredsection that is gated along its perimeter. Figure 4.16 is an edge-gatedcup. The flow path from the gate around the perimeter is a shorterdistance than across the top. This results in a gas trap.

Figure 4.17 Mold filling analysis showing the poor flow conditions across an integral hinge that result from a poorly placed gate

Figure 4.15 Injection molding analysisshowing the development of “picture framing” resulting from a part with a thickperimeter and a thin center region

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Figure 4.16 Mold filling analysis showing the development of an air trap resulting from poor gate positioning relative to thepart geometry. Melt flows around theperimeter before it can move over the top ofthe mold core



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extreme points, the hinge has been formed as a weld, with very littleflow across it. This will result in a very weak hinge.

Example 2

It is desirable to gate into a part in such a way that there is aminimum of hesitation at the hinge. Ideally the gating woulddevelop a broad flow front, which hits the width of the hinge at thesame time after all other extremities of the part have been filled.This is not always practical, but conditions close to this can beachieved. Figure 4.18 illustrates mold filling analysis results of fourdifferent gating options. Option A is the least desirable as it repeatsthe conditions discussed in Example 1 earlier. Option B improveson Option A but still results in hesitation and poor hinge strengthat the center region of the hinge closest to the gate. Option Cfurther improves flow across the hinge by using two gates atbalanced locations along the edge of the part. Here hesitation issignificantly reduced at the hinge. Option D shows the nearoptimum condition where four gates provide for a broad flow frontto be developed prior to reaching the hinge. This virtuallyeliminates any hesitation at the hinge, improving orientation andmaximizing hinge strength. The small shallow box in Fig. 4.19 wasoriginally gated near the hinge (as in Fig. 4.18B), which resulted inpremature hinge failure. Two gates were then placed along the edgeof the part as shown. Note the uniform filling pattern on theopposite side of the part from the hinge that resulted from this newgating location.

Figure 4.18 Effect of gating locations on mold filling across a thin integral hinge

Figure 4.19 Baby wipe container withoptimal filling pattern across its integralhinge

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This approach cannot be used in a part such as the one shown in Fig. 4.20,because the depth to the box would corrupt the uniform flow front as themelt wrapped around the side walls resulting in a severe weld or gas trap(see Fig. 4.20A). The preferred approach in a case like this is to gate the partapproximately in the center of the main body, i.e., in the center of the baseof the taller box region, so that all extremities of the box, including thehinge, fill at about the same time (see Fig. 4.20B and C). In some cases it isdesirable to position the gate slightly off balance so that the side oppositethe hinge fills first. This will accelerate the melt approaching the hinge andminimize hesitation.

Special Note: In Example 2, as the melt flows across the thin hinges into thethicker lid portions, one of the general guidelines is violated – Avoid flowingfrom thick to thin. In this case, the hinge will quickly freeze and preventcomplete packing of the lid. This creates a challenge in properly sizing thetwo halves, box and lid that would normally require assembly (lid closingonto the container portion of the part). To make matters worse, a warpageissue will arise because the two regions shrink differently. This is less of aproblem in thinner parts. With a thicker part wall, there is a higher risk fora problem to develop.

Gating on either side of the hinge can help control packing, but will resultin a weld line. The position of this weld can be difficult to control, as themelt on each flow front will tend to hesitate at the hinge. This will result inan extremely weak hinge. The problem can be addressed through carefulmodeling and sizing of the runner branches feeding these two halve usinginjection molding simulation software. It can also be addressed usingsequential hot valve gating. One gate could be positioned on the maincontainer portion and opened to fill the entire part. A second gate, located

A B C

Gate Location (Off-Center)

Gate Location

Figure 4.20 Moldflow filling output for a tall box with undesirable gating location on the front edge opposite the hinge (A) and proper gating location (B and C) at the base of the box. Gating on the base, slightly offset away from the hinge, results in a uniform melt front reaching the hinge



on the lid, would open after filling from the first gate simply to assurepacking of the lid portion.

4.2.7 Balanced Filling Throughout a Mold

The volumetric shrinkage of a plastic material can vary by over 20% duringinjection molding. This variation will occur over the range of temperaturesand pressures the plastic material experiences from its molten to itssolidified states. Temperature variations of over 250 °C and pressurevariations of 140 MPa are not unusual.

Variations in pressure will influence how much material is fed into thecavity as the material is shrinking during the packing, or compensation,phase of the molding cycle. The principle of balancing of pressures is notonly important to provide consistency between cavities of multi-cavitymolds, but also to provide balanced conditions within a given cavity tominimize the potential for flashing, residual stresses, and warp. Balancedpressures will increase the process window for making higher quality andmore consistent parts.

4.2.7.1 Gating Position(s) Within a Cavity

Balancing pressures within a cavity should be attempted first by varyinggating location and then by using either flow leaders or deflectors.Unbalanced filling of a cavity develops near hydrostatic pressure regionsand transient flow. Hydrostatic pressure regions are developed in locationsof a cavity that fill early while other locations are continuing to fill. As soonas a location within a cavity is filled, pressure will dramatically increasebetween this location and the gate and will approach the high pressure atthe gate location. In contrast, the pressure at the continuing flow front iszero with a constantly increasing pressure gradient back to the gate. Thereare a number of problems created by this phenomenon:

• The rapid and high pressure development, in the near hydrostatic earlyfilling regions, will unnecessarily increase the force opening the moldand potentially allow the melt to flash into the parting line.

• Due to the stoppage, or decreased flow, in the early filling region, themelt will begin to cool there as other regions are still filling. The meltwill also cool under higher pressures causing it to be packed out betterand shrink less than the later-filled regions. These variations inshrinkage will develop residual stresses, which can cause a part to warp.

• If the early-filled region is not a dead end, the melt will change directionunder the frozen layer and be directed toward the continuing flow front.This will continue through the packing, or compensation phase. Thistransient flow will create variations in orientation through the crosssection of the early filled region, which will develop complex strainsresulting in localized stresses and contribute to warpage of the part.


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