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Common Molding Defects & Easy Solutions

Common Molding Defects & Easy Solutions


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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

With so many different variables, conditions, settings, etc. causing problems within your plastic processing equipment it’s vital to control what we can. One major component and many times overlooked is a quality purge compound. Keeping your equipment purged and cleaned on a regular basis will eliminate time troubleshooting later. Purging your equipment is absolutely one variable you have complete control over. Stop wasting money and start using Asaclean!

Want to save a little more money?  Try purging with Asaclean today by requesting your free sample below.

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