You mean Shrinkage? Yes, Significant shrinkage!
What do Thermoplastics and Seinfeld have in common? You guessed it. Shrinkage!
Shrinkage of thermoplastics has a significant impact on both part design and tool design. A stable geometry in part design will help to ensure the converter’s producability is that much better. Producability is simply the ability to make a part or assembly to print and/or the customer requirements. Shrinkage has to be accounted for in the very beginning of tool design to ensure the part comes out of the mold to the dimensions that are desired.
So what is Shrinkage? Thermoplastics are polymers and polymers are very long molecules. Heat is needed to enable flow. Adding heat makes these long molecules expand/randomize and and during cooling in the mold, they contract or shrink.
How much does the polymer shrink? More than most would expect and that is why I provided the image that I did. That will be explained later. Just as important as total shrinkage is the way polymers shrink. Semi-crystalline resins generally shrink more than Amorphous materials. Because a semi-crystalline resin does have regions of crystalline structure, they often shrink differently in different directions. Polypropylene is a semi-crystalline material and it’s typical shrinkage ratio is 1.5%, which means it will shrink .015 inch for every inch of length. A one inch long part would correspond to the steel or mold increasing to 1.015 inch long. Shrinkage values are often different in the flow direction and in the cross flow direction. This is typically due to orientation of the molecules. Polycarbonate is an amorphous material and it’s typical shrinkage ratio is 0.6 % or a shrinkage of .006 inch per 1 inch of length. Often, shrinkage is either written in ratio, 0.6%, or in/in, .006.
Do fillers affect shrinkage rates? Yes, and the types and amounts can have very different effects. Many fillers have lower thermal expansion than polymers so increasing filler often lowers shrinkage. Fillers that orient may lower shrinkage values but increase the ratio of flow to cross flow variation. Very small fillers may increase nucleation sites to create higher crystallinity and thereby increase overall shrinkage. Unfilled Nylon has a typical shrinkage ratio of 1.5%, but 30% glass filled Nylon has a shrinkage of 0.3% in the flow direction and 1.0% shrinkage in the cross flow direction. The material with the lowest shrinkage that Hi-Tech has run are carbon filled materials. They may be near zero per the datasheet depending on the carbon loading and the substrate resin.
How is shrinkage accounted for when building tooling particularly for differential shrinkage (flow vs cross flow)? Probably one of the best nuggets of knowledge on tool design is this: A designer will take the final model of the part in CAD and simply increase the size by a ratio. If the part is Polycarbonate, the designer would scale the part by 1.006 and start to layout the frame around this slightly larger part model. Based on experience and mold filling simulations, the scaling may be modified to account for expected differential shrinkage.
Why is it good that my converter or molder has experience with a specific material? Material supplier’s datasheet values for shrinkage are generally generated on a plaque with lines for measuring post mold lengths. Most parts are not plaques, so datasheet values are a guideline and sometimes not fully applicable for specific parts. If a molder has built one or several tools for a given material, they will have updated the shrink values each time to hone in on what is the actual shrink in those geometries. Because geometry has a significant impact on shrinkage, having experience both in the material and the given part geometry is a significant advantage.
Why is the picture interesting? Thermoplastics exhibit fountain flow with the greatest flow in the middle of the wall thickness. In this short shot of a thick walled part, the material hasn’t frozen off and higher in the middle. The middle of the part is where all of that shrinkage is occurring and so the material has pulled back into it’s shell. That is significant shrinkage.
If you have any questions on shrinkage or anything else Thermoplastics, please drop us a line at “Contact Us”.
