Injection Molding defect:molding burn Solving molding burn problemsDefinition Burn marks are small, dark or black spots on the part surface. This phenomena is also often referred to as dark streaks or specks.Causes Adiabatically heated trapped air (see Air Traps) Air trapped in pockets may compress, heat up and cause burn marks. (Animations not available More...) Material degradation This can be caused by excessive injection speed, residence time or melt temperature. Improper screw or runner system design may also lead to material degradation. Remedies Eliminate air traps To prevent burn marks, you should move air traps to places which can be vented, or where ejector pins can be added. Optimize the runner system design Restrictive sprue, runner, gate, or even part design could cause excessive shear heating that aggravates an already overheated material, causing material degradation. Modify screw design Contact material/machine suppliers to get the right screw design information to avoid improper melt mix or overheating that leads to material degradation. Select machine with smaller shot size Optimize melt temperature Reduce temperature to avoid material degradation from overheating, or increase it to limit residual stress. Optimize back pressure, screw rotation speed, or injection speed Balance shear heat against residual stress.
