There are many factors and conditions that affect the processing of quality plastic parts. What resin, colorant, temperature, additives, gate size/location, mold design, etc.? This article will discuss symptoms, possible causes, and possible solutions. Defects In injection molding occur during fill time or pack/hold time. Examples of defects caused in the filling process can be flow lines, jetting, gas burn, weld line, etc. Examples of defects during the pack/hold pressure time can be a sink, voids, flash, wrappage, etc.
Flow lines – a ripple-looking pattern on the front of melt flow caused from filling too slowly. Possible molding process issues - low temperature, too low injection speed, insufficient hold pressure, insufficient cushion. Solutions – increase temperature, increase velocity, increase pack pressure and time. Mold could have cooling issues or be too cold.
Sink/Voids- Area(s) of a plastic part not filled out or sunk in on the surface. Many molding processes to check - too short cure time, too slow velocity, too small shot size, pressure loss, insufficient hold time. Possible mold problems – too hot mold temperature, Gates, runner too small, part dimension too thick. Solutions – Increase pack pressure and time, increase injection speed, allow proper cooling time, more up transfer position.
Black specs / Burn – dark spots, streaks, chunks appearing in and on the surface of the parts. Common molding issues are degraded material, excessive injection speed, and pressure, clogged or not enough vents/ gas burn, and residence time of the material. Mold issues could be inadequate vents or plugged/damaged vents causing trapped gas. Solutions – Purge for carbon build-up, inspect vents for damage or blocked, add more venting if needed (common practice is at least 30% of parting line to be vented), decrease the temperature, decrease velocity, back pressure, make sure material is not residing in the barrel too long (residence time).
Warp / Bend – Parts that twist, bend, warp out of shape and dimension after being ejected from the Mold. Pressure and over-packing are very common causes. Mold problems could be unbalanced or not enough cooling/water, mold temperature too high, gate size, part sticking during ejection. Molding issues include too much pack and hold pressure, too low melt temperature, and Injection speed too slow. Solutions- increase velocity, move transfer up, increase mold and melt temp, decrease hold pressure
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