Matching Screw Size to Part Weight on Rotary Machines

Screw size determines how much material a rotary vertical machine can inject per shot, so choosing the right screw directly affects part quality.

• Screw too large for a light part: excessive melt volume and residence time tend to burn or yellow the part.
• Screw too small for a heavy part: the part is likely to be short-shot and fail to meet design requirements.

Mold design causes of incomplete filling

• Over-complex molds with many turns, poorly placed gates, narrow runners, too few gates or the wrong gate type.
• Very thin local sections: increase wall thickness overall or locally, or add an auxiliary runner or gate near the unfilled area.
• Inadequate venting: short shots often occur at corners, deep recesses, thin sections surrounded by thick walls, and the bottoms of thin housings molded with side gates. Add effective vents, position gates so air is pushed out first, and where necessary make the trapped-air area a separate insert so air can escape through the gap.
• Unbalanced filling in multi-cavity molds: reduce the number of cavities if needed to keep the remaining parts within specification.