A certain number of injection molders appear to experience material feeding problems when processing cellulosics. Many newer standard screws are designed to mold engineering plastics, requiring very low compression ratio screws. These screws do not feed cellulosics well.
When customers experience feeding problems, the screw return portion of the cycle can vary significantly. The resulting problems caused by erratic feeding are splay, bubbles, short shots, and burned streaks in the molded parts.
There is one thing a molder experiencing feeding problems can do which almost always solves the problem. The procedure is to tumble zinc stearate powder onto the surface of the pellets before molding. The amount of zinc stearate needed to coat the pellets is somewhat critical. The amount we suggest is 0.03 to 0.05 percent by weight. Note: Too much zinc stearate will cause darkening of the material.
It may be more prudent to purchase a new screw for the molding machine, especially if the machine is only used for cellulosics. This approach has been used successfully by several molders. The cost of a new screw can vary from approximately $1200 to $2800, depending on machine size and L/D ratio.
New Castle Industries have considerable experience with screw designs that feed cellulosics successfully. They would be considered a good source as a supplier of cellulosic screws. Their address is:
New Castle Industries, Inc.
Feed Screws Division
PO Box 7359
New Castle, PA 16107
Phone: 412-656-5600 |