Molding Conditions

Drying

Successful molding of reinforced thermoplastics requires adequate drying. Inadequate drying can cause extremely erratic molding conditions and less than perfect molded parts. Excessively wet materials outgas and can undergo a viscosity change during processing. This may cause brittleness, blisters, voids, silver streaking and poor surface finish. RTP Company materials are dried prior to packaging in moisture resistant containers. However, we recommend thoroughly drying the materials in a dehumidifying type dryer. This is important with hygroscopic materials but can also be essential for non-hygroscopic materials. Condensed surface moisture can dramatically affect high temperature molded parts. The recommended drying times in Processing Conditions section of this web site are provided as guidelines; however, an actual moisture check is necessary.

Barrel Temperature

Refer to Processing Conditions section of this web site for recommended starting temperatures. Typically the rear zone/zones are set 10-20 degrees F (6-12 degrees C) cooler than the front zone and nozzle. Some modifications may be needed depending on part size and configuration.

Melt Temperature

Refer Processing Conditions section of this web site for recommended starting temperatures.

Mold Temperature

Refer Processing Conditions section of this web site for recommended starting temperatures. Normally, reinforced materials require higher mold temperatures than nonreinforced materials. Higher mold temperatures will achieve a smoother, more blemish-free surface by providing a resin rich skin on reinforced materials.

Injection Pressure

Injection pressure should be set low initially and increased to the point of filling the part just short of causing flash. Maximum pressure without flash generates optimum physical properties for your RTP Company material.

Back Pressure

Low back pressure (approx. 50 psi or 0.34 MPa) minimizes fiber breakage and property deterioration.

Injection Speed

Generally, the fastest possible cavity fill time is best. This minimizes glass orientation and maximizes weld line integrity.

Screw RPM

The lowest possible rpm is recommended to minimize fiber breakage and screw recovery should be set accordingly. Slower rpm’s result in a more uniform melt by minimizing shear heat buildup.