IMM 2/00

We would like to know what the industry standard is in reference to who is responsible for providing shrink factor information when a mold is being designed. Is it the responsibility of the customer or the designer? Also, who must absorb the cost of adjustments to the mold resulting from incorrect or unexpected shrinkage?

The SPI guidelines for this are suitably vague: "Accuracy is subject to the commercial variations generally prevailing in the industry." As a program manager at a custom molder that also builds molds, I have found that the responsibility falls upon my company. I typically rely on SPI's Standards and Practices of Plastics Molders, which contains tolerance ranges for various resins for a number of features as a starting point. These ranges theoretically encompass the accuracy involved in moldmaking, shrink variations, and molding variations. The OEM and molder need to take a pragmatic approach to the moldbuild. A part made of açetal or nylon (crystalline polymers with a wide range of shrink) will not have the same accuracy or repeatability as a part made of LCP (low shrink polymer) or glass-filled polycarbonate (predictable amorphous polymer with small shrink range), and the corresponding tool debug will be more involved (read expensive) for the acetal and nylon molds.

As far as the costs for leaving dimensions steel-safe, or the added cost of multiple samples, these costs should be included in the price of the mold. This is probably the most common problem between molders/moldmakers and their customers, which can lead to strained relationships. An educated customer and an enlightened moldbuilder can avoid these situations up front.
-D. Doughty, Tech Inc., Merrimack, NH, (603) 424-4404, ext. 134.

It is the responsibility of the molder to specify shrinkage factors (after you receive input from your resin supplier, processing personnel, and moldmaker). Costs for adjustments must he discussed at the quoting stage and written into the purchase order after both parties agree.
-M. Aufiero, Lombardi Design, Freeport, NY, (516) 546-4400.

Most resin suppliers will he able to provide shrinkage data to the toolmaker. The designer typically will obtain this information from a resin supplier in the early design/material selection stages of a new project. it is important to keep in mind that data supplied by a resin manufacturer are based on ASTM D955 and may only be available for the flow direction as measured on a ?-inch-thick test specimen. This standard shrinkage data will not fit every part geometry, as shrinkage is very much a factor of process conditions, wall thickness, and molecular orientation.

Moldmakers generally apply this guideline well and account for other factors when building the tool, but the ultimate responsibility and costs for adjustments fall on the owner of the tool. If at all possible, analyze existing tools and parts with similar geometry, gate locations, and resins
-D. Burns, Entec Polymers LLC, Maitland, FL., (407) 875-9595.


The question is not about shrink factors. It relates to how one molds to the specified tolerance. Plastic part size is controlled by the mold dimensions, the material, volumetric shrinkage, orientation, crystallinity, and the molding conditions. All except the first are controlled by the molder, who is responsible for the finished part. It is logical for him or her to establish the mold sizes, which are educated guesses.

Regardless of who makes this decision, the cost is a legitimate one that belongs to the owner of the mold. The responsibility for the mold size should be determined before the quotation process so that it can be included in the estimate, or the owner should pay the moldmaker for any needed changes.
-I. Rubin, consultant, Brooklyn, NY, (718) 258-7363.

In our region of the country, the shrink factor is determined by the molder the majority of the time. He/she usually has had prior experience with the material. The molder also has access to the specific processing parameters, as well as behavioral characteristics. The remainder of the time, the part designers specify shrinkage for the same reasons. Often designers have even consulted with the material manufacturers' engineering department for specific information on the application
-J. Buss, Buss Precision Mold Inc., Portland, OR, (503) 652-5804.

The molder is most often responsible for supplying the shrink factor to the tool-builder. The molder should give his customer a tool price that reflects the need to tune the mold after first tooling samples are obtained. If the functionally required dimensional tolerances are really tight, several fine adjustments of the tool steel may be required before sample approval is obtained.
-W. Foster, Tessy Plastics Corp., Elbridge, NY, (315) 689-2077.

Our standard is to get the material specs in writing from our customer. We then will certify that the mold is dimensionally correct. If the shrinkage information was incorrect and the mold needs to he adjusted, we expect the customer to pay for the changes.
-M. Ricciotti, Redoe Mold Ltd., Windsor, ON, (519) 734-6161, ext. 17.