RTP Company’s foaming agent masterbatches provide several benefits to injection molded plastic articles:
Reductions of plastic part weight by up to 20% for density critical applications such as fuel floats and energy saving transportation equipment.
Elimination of sink marks on injection molded parts — sink marks are shallow depressions on a part surface and are the result excessive shrinkage in thicker areas of the part — they can be minimized by adding a small amount of foaming agent which creates just enough internal pressure within the molded part to offset excessive shrinkage and reduce sink marks.
Foaming agent masterbatches are available for most thermoplastics. Choosing the proper masterbatch grade is important:
It should activate and foam at a temperature slightly below the normal processing temperature of the host polymer.
Masterbatch carriers must also be compatible with the host polymer to avoid blistering and delamination during molding.
RTP Company has developed specific foaming agent masterbatches that have excellent activation and compatibility with intended polymer systems.
1.0 to 5.0% depending on the activation level of the product, accuracy of the feeder, and the objective.
Foam Molding with FCX Foaming Agent Masterbatches
Maximum foaming of a material should never exceed a 20% density reduction; 10-15% reductions are preferred. The recommended level of foaming agent is 1-5% FCX.
For a foamed part, a clamp requirement of approximately one ton/in2 (14 N/mm2) is adequate.
Wall thickness for a foamed part should have an absolute minimum of 0.187 in (5 mm).
When using an exothermic CFA, the surface of a foamed part typically contains many swirls and flow lines compared to standard injection molding. When using an endothermic CFA, a good surface appearance is possible with a slightly less density reduction.
Injection speeds should be set as fast as possible so the material entering the cavity is hot and has a low viscosity. This way, the foaming agent expands and fills the cavity before solidification occurs. Such temperatures, plus internal pressure from the foaming agent, mean the part must cool long enough to form a rigid skin that is thick enough to resist deformation upon demolding. With maximum foaming, the mold closed time may need to be increased.
Use low holding pressure and zero cushion.
A foamed part is generally more stress free and, therefore, less likely to distort or warp.
Shut-off nozzle is required.
Sink Control Molding with FCX Foaming Agent Masterbatches
Molding conditions and requirements are similar to normal injection molding parameters.
Shut-off nozzle is recommended.
To eliminate sinks, start with a low activation level CFA at 1.0%.
Press requirement is approximately 4 tons/in2 (55 N/mm2).
Use low holding pressure and minimum cushion.
Mold Design Recommendations
Gate placement should be in a thin area.
Runner size should be 0.375 in x 0.250 in (10 mm x 6 mm) trapezoidal to allow as much flow as possible.
Venting is required around each cavity and additional vents near the end of the fill may be needed.
Good turbulent flow cooling is needed in the tool to prevent post blow.
Valve gates are preferred to keep the plastic melt under pressure.
Troubleshooting procedures for foamable resins are much the same as for other thermoplastics. This table is presented as a quick reference guide. Only one solution to a given problem should be tried at any one time; never attempt to combine possible remedies.