.

November 1997

Foaming:


Foaming by use of the standard molding machine and not a structural foam machine. Understanding of the basics of the process is needed. When foaming for weight or density reduction, you have to think in terms of a low-pressure system. This relates to lower clamp pressures and no packing pressure, since you want to reduce the amount of material in the cavity and finished part. Also, the real process is in short shooting the material into the cavity. If the addition of the foam is for sink control, than you are using a high-pressure system and would use pack.

The foaming I am talking about is by the addition of a chemical blowing agent. Understanding that you have two types, basically an exothermic or endothermic reaction. The names refer to the reaction. Endo meaning the heat is absorbed and exo meaning the heat is given off. The other factors not discussed are the cell size and shape given by the two types and combinations thereof. (This can be of a benefit and should be discussed with the suppliers of the foaming agent.)

The number one problem with foaming is not enough vents and/or the vent exhaust channels are not large enough. Should you be foaming and notice that the vents are clogging up very quickly and/or build-up in the exhaust channel, check depth and width of these areas. Note that process of foaming is a chemical reaction. The reaction produces a gas, causing the foaming and some other by-products which must be exhausted from the mold along with the air in the cavity. Some of the gases produced have been detrimental to some types of steel. Check the MSDS sheets for the particular grade and type. The vent land should be as short as possible and the exhaust channel as deep and wide as possible to allow easy breathing of the tool.

The number two problem is the activation temperature range. The chemical blowing agent is going to work at a set temperature range. Below that temperature, the chemical reaction is not going to happen. Too hot and you may burn up the reaction or start too soon in the barrel. Each agent has its range and amount for what density reduction you want to have. These are basically starting points for the material you are using. The set points are given by the manufacturers.


Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers