What causes weld lines in injection molding

What is weld lines in injection molding

 During the manufacturing of injection molding process , the Weld lines , also calles as Knit lines or Meld lines are present on the surface of plastic molded parts where two flow fronts meet and when they are not able to to “knit” together, or “weld” together .These harilines or dark lines usually occur around holes or obstructions and cause locally weak areas in the molded resin part.

Appearance of weld lines in injection moulding

 Notches, hairlines Change in apparent color; especially when inorganic decorative effect pigments are used the weld line appears as a dark line; conspicuous in dark, brilliant or transparent moldings having smooth surfaces polished to a high gloss.

Cause of weld lines injection molding

  1. Appear in multiphase systems such as ABS or ASA. Flow fronts having an already cooled peripheral layer

encounter one another and no longer allow fusion without marks.

  1. Decorative effect pigments and, e.g., glass fibers tend to straighten up in the region of the weld line (adversely affecting appearance and usually mechanical properties also)

 Reduce weld line injection molding

  1. As far as possible, position weld lines where they have no visual or mechanical impact (flow promoters, flow  inhibitors)
  1. Check plastic injection mold  engineering: if necessary enlarge sprue channel, gate and machine nozzle; avoidance of aprupt changes in wall thickness and nonuniform mold filling; rovide effective mold venting.

3.Improve the thermoplastic injection molding process: Optimization of melt temperature, mold surface temperature and injection speed; new color formulation (organic or inorganic pigments, higher pigmentation).

4.Use lower viscosity material.

 As manufacturer of injection molded finished plastics products,  we have accumulated some knowledge of defect of injection molding and how to avoid them can help you cut costs related to unsellable goods and product returns. Contact us for information on injection molding defects and troubleshooting.


Leave a Reply