Brief Analysis of Ten Common Defects in PP Injection Molding

1. Short Shot

Causes may include improper control of process conditions, insufficient injection capacity of the injection molding machine, too small cross-section of runners and gates, excessively long melt flow path or thin-wall sections inside the mold cavity, poor mold venting, poor melt flowability of raw materials, low barrel temperature or insufficient injection pressure. Solutions: adjust process parameters, replace a suitable injection molding machine, enlarge runner and gate dimensions, set up cold slug wells, optimize the mold venting system, switch to resin with better flowability, increase barrel temperature or injection pressure.

2. Flash

Causes may include insufficient clamping force, severe wear of mold pin holes or guide pins, foreign contaminants on the mold parting surface, excessively high mold temperature or injection pressure during molding. Solutions: increase clamping force, repair or replace worn components, clean up foreign contaminants, reduce mold temperature and injection pressure.

3. Surface Voids

Causes may include small runner and gate dimensions for thick-walled molded parts, excessive part wall thickness, overly high molding temperature or low injection pressure. Solutions: enlarge runner and gate dimensions, reduce wall thickness, lower molding temperature or raise injection pressure.

4. Flow Marks

Causes may include low melt and mold temperatures, slow injection speed, small nozzle orifice, and absence of cold slug wells in the mold. Solutions: raise barrel and mold temperatures, increase injection speed, replace a nozzle with larger orifice, add cold slug wells.

5. Silver Streaks

Causes may include high moisture and volatile content in molding raw materials, poor mold venting, poor contact between nozzle and mold, and blending of different resin grades. Solutions: pre-dry raw materials, add vent slots to improve venting performance, adjust the position and geometry of nozzle and mold, avoid mixing dissimilar resins.

6. Weld Lines

Causes may include low melt and mold temperatures, unreasonable gate location, high volatile content of raw materials or poor mold venting, slow injection speed, lack of cold slug wells, foreign contaminants on mold cavity surface, and improper gating system design. Solutions: raise barrel and mold temperatures, modify gate location, remove volatile substances and improve venting system, speed up injection, add cold slug wells, clean mold cavity surface, optimize gating system design.

7. Black Streaks & Burn Marks

Causes may include oversized injection molding machine, poor resin flowability, high injection pressure, poor mold venting, and unreasonable gate location. Solutions: switch to a smaller-size injection molding machine, use external lubricants to improve flowability, reduce injection pressure, optimize venting system and rationally arrange gate positions.

8. Bubbles

Causes may include small gate and runner dimensions, low injection pressure, high moisture content of raw materials, and drastic wall thickness variation of molded parts. Solutions: enlarge gate and runner dimensions, increase injection pressure, pre-dry raw materials, rationally design part structure to avoid abrupt wall thickness change.

9. Cracking & Whitening

Causes may include low melt and mold temperatures, improper gating system design, short cooling time, unreasonable ejection system design, high injection speed and pressure. Solutions: raise barrel and mold temperatures, optimize gating system design, extend cooling time, adopt pneumatic ejection devices, reduce injection speed and pressure.

10. Warpage

Causes may include high mold temperature or inadequate cooling, uneven cooling, improper gate type selection, and mold core offset. Solutions: reduce mold temperature or extend cooling time, optimize cooling system to ensure uniform cooling, select appropriate gate type, inspect and correct mold core offset.