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Injection Molding: Just Go With The Flow

By Bob Alvarez

 Anyone who has studied fuid dynamics will remember that as a fuidfows through a pipe the pressure changes as the fuid goes rom onediameter pipe to another. When it moves rom a small pipe to a largepipe the pressure drops. When the fuid goes rom a large pipe into asmall pipe, the pressure increases to push the fuid into the smallerpipe. Plastics are fuids, and are ruled by the laws o fuid dynamics.

No designer wants to interrupt wor on a pro!ect to spend an entireday "guring out the dynamics o plastic fow. But, as the saying goes,#$ you don%t court trouble, it won%t come calling&. 'sing simpledesigns reduces the lielihood that fow dynamics will be a problem,and can help avoid bothersome calculations.

(hanges in pressure, velocity, and plastic viscosity ) especially as theplastic cools ) all result in a variety o manuacturing problems. Avoiding dramatic changes in these parameters will obviously eepmanuacturing issues to a minimum. *he ollowing rules will assist inavoiding the common pitalls encountered when designing in!ectionmolded plastic parts.

RULE 1 – Unifor W!ll Thic"ness +eep plastic part walls uniorm. ost problems with part warpage, varying shrinage, and part "ll are the result o non-uniorm wall

thicness. *his is the most basic o all the rules and will dodge manyo the problems associated with manuacturability o plastic parts.

 +eep the walls o the plastic part uniorm and the part won%t warp, itwill "ll properly, and parts will "t together because the shrinage isunder control. $t might sound lie an oversimpli"cation o the verycomple science o fuid dynamics and stress, but it wors. $n act,that is eactly what should be done in plastic part design. As soon asthis most basic design parameter is violated, the pro!ect is headed ortrouble. With high mold shrinage plastics, wall thicness variationsshould be limited to /01. 2ven this relatively small variation can be

enough to cause processing and 3uality issues. RULE # – $ro%er G!te Loc!tion 4aced with the need to vary wall thicness, provide or proper gatelocation. 5ate location is best when the melt enters the cavity in thethicest area and fows to the thinner areas. $ this is not done, it willbe virtually impossible to achieve uniorm pac o the molded part.

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 RULE & – 'oin!l W!ll Thic"ness $n!ection molding is a melt-fow process that has limitations on howar the molten plastics will fow. *heoretically, in!ection molded partshave no maimum wall thicness. But almost no one sets out to mae

a part as big as possible, when both trends and cost constraintsdictate that smaller is better.

 *he cost o a molded part is dictated by its size, shape, and the indo plastics speci"ed. *hicer parts use more raw plastic material.*hey also re3uire longer cooling cycles. *hese two actors contributesigni"cantly to the cost o an in!ection molded plastic part, so thinneris almost always less epensive. *here are, however, de"nite limits tohow thin a wall can be.

*he criteria or selecting a wall thicness have to be based on there3uirements o the product. 'sually, strength determines the wall

thicness. 4rom a unctional perspective, interpretation o a "niteanalysis can be relied upon to optimize selection o suitable wallthicness.

$n medical plastics parts, thinner is always better in un"lled plasticsthat don%t re3uire glass or carbon "ber reinorcement. $ a medicaldesigner is aced with a "lled plastic, which does re3uirereinorcement by glass or carbon "ber, the "ber alignment can be aproblem. Wall thicnesses below 6mm do re3uire "ber alignmentreview.

 RULE ( – R!dius )orners 7eviewing that fuid dynamics course again, when a fuid fowsthrough a maze and reaches a turn or corner, it needs to go aroundthe corner. 7ounded corners improve the fow o the plastics, soradius the corners o all parts generously.

*he in!ection molding process is unorgiving o sharp inside corners.8uring the cooling portion o the molding cycle, the top o the partattempts to become smaller. *he steel core o the mold controls theinside diameter, so this cooling process can create a high level o

molded-in stress, as the material pulls against a sharp corner.

$ a part is designed so the inside and outside radii are each e3ual tohal o the nominal wall thicness, a uniorm wall results around thecorner. 9hrinage will be the same on both sides o the corner andsin mars are eliminated. And, designing corners this way maintainsthe "rst golden rule o plastics ) uniorm wall thicness.

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 As the melt fows around a properly proportioned corner the areadoes not increase and no abrupt changes in direction appear. (avitypacing pressure remains which results in a strong corner with areduced tendency or post-mold warpage. A corner o this type will bedimensionally stable.

RULE * – +r!ft ,ngles *he !usti"cation or drat angles comes rom the nature o thein!ection molding process and mold shrinage, the ever-presentnemesis o a wea design. $n!ection molding is a high-pressureprocess.

:igh pressures are needed to orce the plastics into intimate contactwith all suraces o a mold%s cores and cavities, but this pacing o thecavity maes it di;icult to demold a part. $n some lucy cases,shrinage o the plastic material will pull the part away rom the

cavity, maing the demolding easier. $n other instances, shrinagecauses the plastic material to tightly grip the cores that orm theinside suraces o the part. *his natural occurrence re3uires dratangles to remove the plastic part without distortion or damage.

No single drat angle eists that is ade3uate or all in!ection moldedparts. 2ach individual part has its own uni3ue drat re3uirements.<arge parts re3uire more drat than small parts. *hin-walled partsthat are molded at high pressure re3uire more drat than partsmolded at lower in!ection pressures. *heoretically, core pins re3uiremore

drat than cavities.

*he amount that a plastic material shrins and its physical propertiesmust also be considered in selecting suitable drat angles. <arge dratangles and smooth polish are re3uired or parts molded in strong,brittle, abrasive and sticy materials. 9maller drat angles canbe used on sot, ductile, and slippery materials.

*he ideal drat angle, rom a cost and manuacturabilityperspective, is the largest angle that will not detract rom thecustomer%s acceptance o the product. *he minimum allowable drat

angle is more di;icult to de"ne. *he hands-on eperience gained byplastic material suppliers and molders maes them the ultimateepert on the minimalacceptable drat.

$n most cases, /= per side will be ade3uate, but between 6= and >=per side would be better. $ the design cannot tolerate /=, then speciy

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0.>= on each side. 2ven a minimal drat angle, such as 0.6>= is betterthan no drat angle at all.

:owever, the important cost savings bene"ts o drat angles will belost by a hal-done !ob. 4ailing to provide a drat angle on a long side-

acting core pin that orms the hole in a threaded pro!ection would bea serious mistae. *he orce re3uired to pull that core pin couldcreate the need or a longer cooling cycle. $t%s common nowledgethat drat angles must be speci"ed on the sidewalls o a part that areperpendicular to the mold%s parting line, but it%s also important not tooverloo drating the other details o the part. 8rat angles onmounting fanges, gussets, holes, hollow bosses, louvers, and otherholes will all improve the manuacturability o the part.

 When designing a part, the location o the mold%s parting line issometimes unnown. Without that inormation, it is impossible todetermine whether the part should have positive or negative dratangles. *his lac o nowledge has resulted in the common practiceo drawing the part without drat angles and speciying the drawingas a drawing note, such as #allowable drat /=,&. *his techni3uesimpli"es the part design process, but it is an undesirable drat anglespeci"cation that leads to misinterpretation. 2ach angle should bespeci"edindividually.