SHRINKAGE

Shrinkage as it applies to injection molding is the contraction of the molded part as it cools after injection.

Most of the part shrinkage occurs in the mold while the molds internal cooling systems are solidifying the thermoplastic part. A small amount of shrinkage occurs after ejection as the part continues to cool. After that the part may continue to shrink very slightly for several hours or even days until the temperature and moisture content stabilize. Thus dimensional inspection should wait at least a day.

To determine the proper shrink rate for a given polymer the tooling engineer must consult the manufacturer of the material. Shrinkage varies not only with the polymer used but also with various additives and fillers that have been blended with the polymer.

Part shrinkage in the U.S. is expressed as thousandths of an inch per linear inch. ( 0.00X /in/in ) Typical shrink rates vary between 0.001/in/in and .020/in/in with the most common being around 0.006/in/in.

When calculating shrinkage for your part the tooling engineer simply scales your part by 1.00X. In pre-CAD days the engineer would expand your part by simply multiplying every number on the drawing by 1.00X.

Shrinkage varies with wall thickness also. The material supplier will usually give a range such as 0.005-0.007/in/in. with a 0.100 inch wall. If your wall thickness is 0.100 you would go right in the middle with 0.006.

The molder can fine tune the shrinkage of the parts by adjusting the density of the material i.e. how hard he packs it out, and how long he holds it to cool in the tool.

If your part is large, tolerances are critical, or you are using a new or unusual material. Then it is a good idea to do some test shots. Many molders have a huge rack of obsolete tools. Find one of these that makes a part somewhat similar in size,shape and wall thickness to your part. Then pay your molder to shoot your resin into it and use the parts to calculate a precise shrinkage for your material. The cost of doing this is small compared to that of reworking or scraping a tool.

A note on asymmetrical shrink:

A few plastics shrink differently in one direction than in the other. For example polymers filled with long glass fibers will shrink more in the cross (transverse) direction than the longitudinal (flow) direction. This poses an interesting dilemma for the mold designer. The material supplier will tell you that you have to use a different shrink in the X axis than in the Y. This is fine if you are making popsicle sticks or rulers. But if your part is complex with holes and flow fronts meeting at different angles and running different directions at different places in the part it is impossible to do, and would be outrageously expensive if you could. Even round holes would now become elliptical in the tool so that standard components such as core pins can not be used. Then if it is off, who is responsible? What winds up happening is that the average between longitudinal and cross shrinkage is assumed, everybody buys off on it ( except for the material supplier who is pleased to have absolved themselves of any responsibility ) and then critical features are then altered or added after first shots. Bottom line… Do not use asymmetrical shrinkage resins if close tolerances matter.

Some typical shrink rates:

ABS .......................... 0.005 - 0.007/in/in

Acetal ........................0.018 - 0.025/in/in

Acrylic .......................0.002 - 0.008/in/in

Nylon 66 ....................0.012 - 0.018/in/in

Polycarbonate ............0.005-0.007/in/in

Polypropylene ............0.010 – 0.025/in/in

Disaster recovery:

If a shrinkage mistake has been made on your $200,000.00 tool what do you do? First off there are hundreds of thousands of different polymers. Try to find another resin with similar properties that will shrink to make the part the correct size. If this doesn't work identify the key features in your part that will interface with other parts and components. Can these be adjusted by adding inserts in such a manner as to move these key features? Does your yacht need an anchor? Can you sell your yacht to buy a new tool?

COMING: Links to sites with shrink rates.