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Injection Molding Review | Injection Molding Design Guide
Continued from Injection Molding Process Types
Injection Molding 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. To have a constant filling rate in the cavity the switch over from injection phase to the holding phase can be made based on a cavity pressure level.
Having a stable production window the following issues are worth to investigate:
- 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 sheer rate. Higher speed 3D higher sheer rate 3D lower viscosity. Pay attention increasing the mold and melt temperature lowers the viscosity but lowers the sheer rate too.
- Gate seal or gate freeze sink mark weight and geometry studies have the approach to prevent sink marks and geometrical faults. Optimizing the high 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 applied. The thinner the part the shorter the holding pressure applied.
- Cooling time starts once the injection phase is finished. The hotter the melted plastics the longer the cooling time the thicker the part produced the longer the cooling time.
Molding trial - When filling a new or unfamiliar mold for the first time, where shot size for that mold is unknown, a technician tool setter usually starts with a small shot weight and fills gradually until the mold is 95 to 99% full. Once this is achieved a small amount of holding pressure will be applied and holding time increased until gate freeze off (solidification time) has occurred. Gate solidification time is an important as it determines cycle time, which itself is an important issue in the economics of the production process. Holding pressure is increased until the parts are free of sinks and part weight has been achieved. Once the parts are good enough and have passed any specific criteria, a setting sheet is produced for people to follow in the future. The method to setup an unknown mold the first time can be supported by installing cavity pressure sensors. To see how much the cavities are filled the pressure in the cavity gives a good indication for. Once the mold is set up the first time modern monitoring systems can save a reference curve of the cavity pressure. With that it is possible to reproduce the same part quality on another molding machine within a short setup time.
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