Question: I am running Nylene 494 IM BLK. The spec shows 1.5% to 2.5% shrink rate. I am getting 5.1% shrink on a new mold. Any idea what would be contributing?
Dr. Nick: The shrinkage range shown on the spec sheet relates to a measurement of linear shrinkage, ie shrinkage in one dimension. On a real, three-dimensional part, you will probably be measuring the combined effects of shrinkage and warpage. Either way, this is having an undesirable effect for you.
Polymers like nylon and polyethylene are what we call “semi-crystalline”. This means that, as the molten polymer cools and starts to solidify, some of the individual molecules form themselves into regular structures; this is what we call the crystalline phase. Other molecules don’t do this and remain in a disorganized structure; we call this the amorphous phase. Molecules in the crystalline phase pack together more tightly and crystalline areas are therefore more dense than amorphous areas (see Table). As you can see, the difference in density isn’t huge, but it’s significant enough to mean that, overall, the solid polymer will take up less space than molten polymer; this is what creates the shrinkage effect that we experience.
|Polymer||Density of Crystalline Phase(g/cm3)||Density of Crystalline Phase(g/cm3)|
|Polyamide (“nylon”) 6||1.090||1.190|
Control of shrinkage and warpage in rotomolding depends on several circumstantial factors, but some general rules apply.
The main thing to aim for is to delay the point at which the part releases from the tool surface. Once the temperature of the molten polymer has dropped below its crystallization point, the part will have the potential to shrink. However, if the part remains stuck to the mold surface, this shrinkage will be retarded to some extent. If you ever have the unfortunate experience of a total “stick-up”, you will notice that when you finally get the part out of the mold, it has shrunk and warped less than normal.
By controlling the release temperature (ie the temperature at which the release of the part from the mold actually happens), you can reduce the amount of shrinkage and warpage that occurs.
If you reduce your cooling rate around the point where crystallization is occurring, you should have a beneficial effect on shrinkage. Once release has occurred, you can increase the cooling rate again. This point will be roughly one-third into the cooling cycle.
More practically, I would advise you to consider the release characteristics of the tool. You mentioned that the problem was happening with a new mold, so I’m guessing that, at the moment, the mold surface is a bit too “releasey”.
If you’re using a semi-permanent mold release agent (mra), remove what’s there and replace it with a mra that has slower release characteristics. Your mra supplier should be able to advise you.
If you have a permanent release surface (eg Teflon coated), consider slightly abrading the surface with a “scotch” pad. Generally fresh Teflon surfaces are excessively slippy, but they lose this property after 10-20 parts have been made. In extremis, you could consider getting the existing Teflon layer stripped off and replaced with a slower-release version. However, this would be a pretty drastic action and I would try other things first.
Some materials suppliers incorporate lubricant-type additives in the polymer that act as an “internal” release agent. It might be worth checking with your supplier whether this is the case.
In summary, it sounds like your best bet is to look at the release surface of the mold.
Dr. Nick Henwood is the Technical Director for the Association of Rotational Molders. He has 30+ years of experience in rotomolding, specializing in materials development and process control. In 2022 he was inducted into the Rotational Molding Hall of Fame. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.