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Injection Mold cooling

Most of my injection experience is in optical lens, so stress (optical birefringence) is visible and to be avoided. We molded opthalmic lenses and magnifiers so we had high aspect ratio parts. We measured part deformation with a Wyco. We also added dyes to the products, mixing in the injection molding extruder only.

Typically we ran the injection mold mold near the Tg of the plastic, had long mold cooling times and pulled the part hot and then air cooled slowly. Naturally these were high $ parts so.....

To me the key is getting the resin into the injection mold quickly, slowing cooling to the Tg all the part and then cooling it slowly down to room temperature. If there was a way to injection a warm injection mold and then bring the temperature of the part down slowly and evenly at first and then speed it up this would be an asset. Naturally I have considered other ideas over the years. Air cooling could allow an on off approach to cooling, but I have set up a PET tape process with just air drying after the bath and the volume of high pressure air required to get significant cooling is costly.

Way back in 1990s, the suitcase shells were injected and kept in wooden fixtures. Thick section of PP and HDPE is immersed in warm water then in cold water. PP cane strapping were extruded into 70C water bath, we used to add soap solutions to keep the temperature maintained at 70C.

It is always improper to cool polymer drastically, there should be exponential cooling. Every cooling mechanism cools the surface of the component, and this in turn insulates the heat inside the component, this residue heat causes shrinkage and warpage.

Polypropylene Talc and glass fiber, Talc being the most notorious one. PP + 40% loaded talc with irregular sections where the thickest section is a 200mm, check it after 72hrs, the center would turn out to be brown and still burning.

I was doing a project for round of 750mm dia and 2500mm cylindrical piece of PP, we tried casting and extrusion processes. In extrusion process it came good where we were heating the surface of the extrudate at about 70 to 80 C, till the core of the PP solidifies. With the product we made washers, screws, bolts and they were used for conveyors, somehow it never worked with the too jealous people who thought that was impossible.

There are various methods followed by industry, depending on product, mold design, etc., but at the end what is the return of profit is considered.

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