Question: I am looking for technical information on Zinc used as a mold release option. We use cross-linked polyethylene (PE) and understand that spray-on mold release and zinc is effective. What is the science behind how zinc helps the demolding of molding of cross-linked parts? We see coining on some colors and Zinc helps limit the formation of “Swirls.” What is happening on a chemical level?
Dr. Nick: Your reference to the use of “zinc” relates to zinc stearate. This is also known colloquially as “zinc soap.” It is a combination of a metal salt (zinc) and a fatty acid (in this case, stearic acid). It is known to be a powerful release agent used as an additive in a wide range of polymer and rubber processing applications. It can also be used as a dispersing agent for some pigment formulations.
Although zinc stearate can be very effective, it should be used with extreme care because it can have a very powerful effect, even at relatively low concentrations. Calibrating how much to add will be a trial-and-error process and too much additive is likely to create severe warpage effects. A suitable starting point for investigation might be 0.1% by weight. It can work as a dry blend component, but you need to have a thorough mixing process to make sure that it is evenly dispersed. Incorporation by melt compounding in an extruder is a better option, but this will not be possible for a heat-sensitive material like crosslink PE.
Crosslink PE tends to be a “grabby” material and you should expect to see more wear on the release layer on the inside of your mold, compared to standard linear materials. It sounds like this is what you’re experiencing.
Before using zinc stearate, I would strongly advise you to work with your mold release agent (MRA) supplier in order to optimize your process and find the best semi-permanent MRA for your application. Your crosslink PE supplier should also be able to give you some good advice. Once you have established the best MRA type and application method, it’s important to train your operators exactly what to do. A good operator should be able to recognize the signs of an imminent stick-up and take remedial action before it becomes too serious.
You also mentioned “coining,” which refers to the appearance of surface defects, such as small craters (see photograph). This is usually a sign that you have slightly overcooked the crosslink PE material. The first approach is to reduce the cook time or, better still, reduce your oven set point.
To conclude: I would optimize your applied mold release agent before resorting to zinc stearate.
Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.
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