Practical Processor

Cleaning flat dies; injecting hollow parts.

Q: What is the best procedure to thoroughly clean a flat die?

A: Upon shutdown, the major fasteners, such as end-plate, adapter, and body bolts should be loosened, but not removed. Do this with the die clamped securely to the extruder. Once all major fasteners have been opened, the die can be moved to the cleaning area, where the fasteners are completely removed and the die parts separated. Whenever possible, lip and restrictor-bar settings should remain undisturbed, allowing a faster restart. Since thermoplastics clean up with least difficulty when hot, disassembly should proceed as rapidly as possible to minimize heat loss. It is best to leave the die heating on until the very last possible moment before starting the disassembly process.

After the die is opened and flow surfaces exposed, the cleaning procedure will depend upon the type of material being processed. Many thermoplastics can be readily removed by lifting them away from the die, while at the same time directing a jet of compressed air at the junction of the plastic and the die surfaces. If the molten plastic cannot be removed in this manner, it can be scraped off using scrapers made of brass, copper, or soft aluminum; never use steel. Occasionally, solvents can be utilized to aid removal of stubborn materials. The resin or die supplier can usually offer specific cleaning suggestions.

After the bulk of the plastic has been scraped away, the remaining film can be scrubbed off by scouring with brass wool and dry mild abrasive. Household cleaners make an excellent and economical abrasive for this purpose, although the types designed for very heavy scouring may cause scratching. Test a sample on a non-critical surface before proceeding. Power brushes should not be used as they can easily abrade through the chrome plating and round off sharp edges. Meticulously clean all sealing surfaces to prevent subsequent leakage.
—Sam Iuliano, director of technical sales
EDI LLC, Chippewa Falls, WI.
(715) 726-1201


Q: How does an injection molder create a hollow shape? The section I’m evaluating is a U-shaped hollow yoke with closed ends. I’m unclear on how the air remains inside the central area. I’ve also considered sophisticated rotomolding.
Any thoughts?
—Jim Weaver, Hall-Wade Engineering, Ames, IA

A: There are two technologies I know of for injection molding hollow parts: One is Die Slide molding from JSW; the other is a new Engel technique called joinmelt. They both involve double-shot molding with two shots of the same material so two halves of a hollow chamber can be “assembled” in-mold.
—Matthew Naitove, executive editor
Plastics Technology • (646) 827-4848