Shrinkage – the barrier to tight tolerances or even meeting tolerances

Published On: August 29th, 2018|Categories: Blog|

Shrinkage in the molding sense, (with all due respect to George from The Jerry Seinfeld show), is referring to the contraction of a plastic/composite after it has been melted, shot into a mold and is in the process of cooling.

The majority of the shrinkage occurs while the part is still in the mold. The mold is kept shut to allow the part to solidify enough that the shape will be retained.  The balance is always how long to keep the mold shut. Longer close time assures more stability and higher likelihood of maintaining dimensional requirements, but that means longer process cycle and added cost. A shorter mold close time results in the opposite effect, lower cost but more dimensional variability.

Then there is that portion of shrink that occurs out of the mold. Again, dependent on how long the part is in the mold before ejection, the part will continue to shrink, sometimes minutes, sometime hours, possibly even days depending on part design, retained heat and moisture content/absorption.

All thermoplastics will exhibit shrinkage to varying degrees. Shrinkage is specified either dimensionally or as a percentage, possibly both. In either case, typically the shrinkage is specified fairly broadly. For example, a resin specification would establish that a material is prone to shrink 2-10 thousandths of an inch per inch of surface area. It may be expressed as an approximate or “typical” percentage like 0.1% to as high as 3%.

So if your parts are looking to depend on a tolerance of +/- 0.005 or less, or if you are looking for features that will allow a snap together, a press fit or interweaving with other parts, talk to us.  We can help guide with design and polymer selection input and possibly save you precious design time.

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