Foam molding offers the possibility of increasing the size of a part without increasing weight and reducing the weight of a part with controlled change of properties. Other valuable advantages for foaming are cycle time reduction, improved dimensional uniformity, and increase cavitation in a mold. All of the attributes can be used to increase the productivity of an application. All of EP’s products can be foamed using one or more standard foaming techniques. Common foaming techniques are chemical foaming, MuCell®¹ micro-cellular foaming and structural foaming.
Chemical foaming involves mixing a chemical blowing agent with pellets prior to the pellets being feed into the feed throat of the molding machine. The blowing agent decomposes while the resin is melting, releasing a gas such as nitrogen that is mixed with the polymer. As the polymer is injected into the mold the gas expands. The bubbles formed help pack the resin in the mold. Chemical foaming can be done on any existing molding machine with the only restriction that a blowing agent be used that decomposes at temperature compatible with the processing temperature of the polymer.
Start foaming using standard injection molding conditions for the resin being molded. To adjust the level of foaming, pack pressure and/or packing time will need to be changed. A decrease in packing will increase foaming and an increase in packing will decrease foaming. Setting the stroke to produce a short shot will also increase the amount of foaming. It may also be necessary to adjust the level of foaming agent used, the melt temperature being used or the type of foaming agent. Faster cycle times are achieved by reducing the packing time. This is possible since the gas pressure in the part is actually packing the part and not hydraulic pressure.
The MuCell®¹ process involves metering a gas such as nitrogen into the polymer melt stream as it moves down the barrel. The gas in thoroughly mixed into the polymer creating a single phase solution of polymer and gas. Nucleatation from the pressure drop of the resin being injected into the mold causes gas bubble formation and cell growth occurs while the part cools, packing the resin into the mold like chemical foaming. To practice foaming using the MuCell®¹ process, a license and special gas delivery equipment needs to be purchased from Trexel and the injection molding machine needs to be modified.
The level of foaming is adjusted much like it is in chemical foaming by altering the injection molding process. One difference is that the level of gas in the resin is more controlled in the MuCell®¹ process and one s not limited by finding a blowing agent that decomposes at the right temperature for a given process.
In any foaming process the surface of the part will show a lot of splay due to gas in the resin being smeared between the resin and the surface of the mold. The amount of splay will depend on the level of gas and the level of weight reduction and to some degree the resin being foamed.
MuCell®¹ is a registered trademark of Trexel Inc.