Common Injection Molding Defects
Short Shots
Insufficient material fills the cavity. Solutions: Increase injection pressure, raise melt temperature, check for leaks.
Sink Marks
Visible depressions on thick sections. Solutions: Increase packing pressure, adjust cooling time, reduce wall thickness variation.
Flash
Excess material escapes the cavity. Solutions: Reduce injection pressure, repair worn molds, check clamp force.
Warpage
Uneven cooling causes distortion. Solutions: Uniform cooling, symmetric wall design, post-mold annealing.
Surface Streaks
Flow lines visible on surface. Solutions: Increase melt temperature, modify gate location, improve venting.
Frequently Asked Questions
What are the most common injection molding defects?
The most common defects include sink marks (surface depressions at thick sections), flash (material escaping the parting line), short shots (incomplete filling), weld lines (visible seams at flow front meetings), warpage (dimensional distortion), and voids (internal cavities in thick sections). Each defect has multiple potential causes requiring systematic diagnosis.
How do I fix warpage in nylon injection molded parts?
Nylon warpage results from differential shrinkage. Solutions include ensuring uniform wall thickness, increasing cooling time, using glass-filled grades for lower shrinkage, optimizing gate placement for balanced filling, and adjusting mold temperatures differentially between core and cavity. Post-mold annealing at 80-100°C can also relieve internal stress that causes delayed warpage.
What causes silver streaks on injection molded parts?
Silver streaks (splay) are caused by moisture vaporizing in the melt, gas traps from decomposed additives, or entrapped air. For nylon, the most common cause is insufficient drying — PA6 requires drying to below 0.1% moisture and PA66 to below 0.2%. Other solutions include increasing back pressure, reducing screw speed, and ensuring adequate venting in the mold.
How can I prevent flash without increasing clamp force?
Beyond increasing clamp tonnage, flash prevention includes reducing injection pressure and speed, especially at the end of fill using a profiled injection curve. Ensure mold surfaces are clean and properly aligned. Lower melt temperature reduces material flow into the parting line. For worn molds, re-machining the parting line surface may be necessary.
