Burn marks appearing on injection moulded parts is one of the most common defects moulders have to deal with from a quality standpoint. They can show up at any time during the moulding process and determining quickly the root cause is extremely important in reducing machine downtime and product loss due to quality concerns.
I will highlight some causes stemming from three different areas which are machine, mould, and material and give some suggested remedies to help deal with this common defect.
Machine: Several different things can cause burn marks associated with the machine or the process. The material could simply be too hot in which case the melt temperature should be reduced. Burns can also be caused by the mould being filled too quickly and you would need to reduce the injection speed. If the backpressure is too high that can also cause burn marks. Improper purging of the previous resin can show up as burns in the current run where the previous material was left to cook in the barrel causing burnt material to hinder the current run. A proper purging procedure should always be put into place when changing materials or performing colour changes.
Mould: If the mould is not properly vented or the clamping pressure is too high then air trapped in the cavity can cause the edge of the part to burn where the improper venting is taking place. In this case, the mould should be inspected and the venting fixed to where proper gas release can be accomplished when moulding.
Material: If the material or resin being run is wet or has an excessive moisture content, then that can cause burn marks to be present in the finished product. If this is the case, the moulder should look at drying the material to proper specs before continuing moulding applications.