Injection Molding - Process Troubleshooting

Process Troubleshooting

Optimal process settings are critical to influencing the cost, quality, and productivity of plastic injection molding. The main trouble in injection molding is to have a box of good plastics parts contaminated with scrap. For that reason process optimization studies have to be done and process monitoring has to take place. First article inspection of internal and external geometry including imperfections such as porosity can be completed using Industrial CT Scanning, a 3D x-ray technology. For external geometry verification only a Coordinate-measuring machine or white light scanner can be used.

To have a constant filling rate in the cavity, the switch over from injection phase to the holding phase can be made based on cavity pressure level.
Having a stable production window, the following issues are worth investigating:

• The Metering phase can be optimized by varying screw turns per minute and back pressure. Variation of time needed to reload the screw gives an indication of the stability of this phase.
• Injection speed can be optimized by pressure drop studies between pressure measured in the Nozzle (alternatively hydraulic pressure) and pressure measured in the cavity. Melted material with a lower viscosity has less pressure loss from nozzle to cavity than material with a higher viscosity. Varying the Injection speed changes the shear rate. Higher speed gives a higher shear rate and lower viscosity.

• Gate seal or gate freeze / sink mark / weight and geometry studies prevent sink marks and geometrical faults. Optimizing the height and duration of applied holding pressure based on cavity pressure curves is the appropriate way to go. The thicker the part, the longer the holding pressure should be applied.
• Cooling time starts once the injection phase is finished. The hotter the melted plastics, the longer the cooling time, and the thicker the part produced, the longer the cooling time.