Ribs should follow the proportional thickness guidelines shown below. If the rib is too thick in relation to the part wall, you may experience sinks, voids, warpage, weld lines, and longer cycle times. Position ribs in the line of flow to improve filling and prevent air entrapment.


Molds must be provided with adequate cooling to take advantage of the faster cooling rates of reinforced compounds. Poor cooling results in rising mold temperatures and longer cycle times. Inadequate heating can result in voids, shorts and poor surface finish. Cooling and heating channels should be located directly in the mold inserts and cores if mold design permits.

Ejector Pins

Should be located on the heaviest sections of the part to minimize distortion when it leaves the core. They should be balanced as much as possible over the part’s surface. Reinforced thermoplastics require more pins due to lower mold shrinkage and greater potential for drag during ejection.

Cooling Design

Proper venting of cavities is very important. Inadequate venting can result in gas burns, poor weld line strength and nonfilled parts. Too much venting can result in excessive flash and poor weld lines due to inadequate pressure buildup. Venting should primarily be located at the last point of fill and where weld lines occur. Vent size depends on the viscosity of the polymer and can vary from 0.0005 – 0.0030 in (0.0127 – 0.0762 mm) deep. Venting can also be used around knockout pins, moving cores and mold inserts.

Venting Design