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Common Defects





Possible Solutions


Embedded contaminates

Black specs
White specs

Foreign particles (burnt material or other) embedded in the part.

Anywhere on part.

Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection. Contamination from previous run. 

Replace with new material. Check for over run/over heating.  Clean screw and barrel.  Lower melt temperature, lower back pressure, lower screw RPM.


Burn Marks

Air burn
Gas burn

White, black or brown burnt areas on the part.

Located at the furthest points from gate or where air is trapped.

Tool lacks venting, injection speed is too high.  Melt temperature of material too high.

Check for heater malfunction. Decrease injection pressure, injection speed. Decrease melt and/or mold temperature. Reduce clamp force to improve venting. Alter gate.



Short shot

Partial part

Anywhere on part.

Lack of material, injection speed or pressure too low, mold too cold, lack of gas vents.

Correct melt and/or mold temperature. Increase shot size. Inspect non-return valve. Modify mold–vents, runner, gate. Use larger orifice nozzle.



Parting line flash

Excess plastic squeezing out perpendicular to the part at parting line.

Along any parting line.

Mold is over packed or parting line on the tool is damaged. Too much injection speed/material injected, clamping force too low.   Or dirt and contaminants around tooling surfaces.

Relieve areas of the parting surfaces that are not immediately adjacent to the part. Reduce PSI/speed of plastic on machine.


Knit Line

Weld line
Meld line
Transfer line

Weld lines are created when two or more melt flow fronts meet possibly causing a cosmetically visible line. 


Mold/material temperatures set too low.  The material is cold when they meet, so they don’t bond.  May also weaken area in molded part.

Increase injection pressure, speed or hold.  Increase melt and/or mold temperature. Vent cavity in weld  area.


Pin Push

Circular or semicircular stress rings opposite an ejector pin. May even be raised circular bumps. May push through the part.

On the cosmetic side of the part opposite an ejector pin.

Over packing, sticking on the core, inadequate ejection.

Solve over packing problem. Polish core or increase draft on core. Add more ejector pins. More small pins are better than a few big ones.

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Depressions or dimples in the part that are usually adjacent to thick areas. In clear parts, bubbles can be seen in thick areas. These bubbles can be the precursors of shrink.

Inconsistent wall thickness.

Thick areas adjacent to thin areas.

Holding time/pressure too low, cooling time too short. Non-uniform cooling rate.   Too much moisture in material. Excessive material or wall thickness.

With sprueless hot runners this can also be caused by the gate temperature being set too high.

Maintain constant wall thickness by coring out. If you must have thick areas lead gradually into them. Follow the 66% rule for wall thickness. Keep it down to 60% or less if you can.
Frequently the solution to sink is to pack the part out tighter. Over packing can then cause other problems. The best solution is to avoid it with part design.



Splay mark
Silver streak

Silver or whitish streaks


Most predominant near gate.

Overheated or too hot material. Moisture in the material, usually when hygroscopic resins are dried improperly. Trapping of gas in "rib" areas due to excessive injection velocity in these areas.

Check for contamination. Dry resin according to manufacturer’s recommendations. Raise mold temperature. Shorten overall cycle.  Decrease melt or nozzle temperature.

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Distorted part.
The failure to maintain flatness of a plastic part that was intended to be flat. Distortion from the intended shape of the plastic part.

Anywhere on part.

Cooling is too short. Material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool) Uneven shrinking between areas of the part.  Overpacking can induce warp.   The underlying cause of most part warpage is the shape of the part itself.

There is no cure for this one, only control. Differential mold cooling can get you parts that are flatter.  A cooling fixture that the part is placed into immediately after ejection can also straighten the part.  Flat parts are more susceptible to warpage than curved parts.

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Scratches on the surface of the part

Anywhere on part.

Improper ejection from mold or improper handling. Part hitting ejector pins, conveyer or anything that causes a scratch. Or scratch on the surface of the mold.

Ensure that the part is not hitting any surface before it is properly cooled. Operators should handle parts by the outside edges only.
Repair mold if scratched.


Dimensional Irregularities

Oversized part
Undersized part

Part dimensions are not consistent with print or pervious production run.

Anywhere on part.

Two common causes are 1) problems with the process for jobs that have previously produced good parts or new molds or 2) mold and print not consistent for new molds.  Improper process could be wrong pressure, cooling time, holding pressure, injection pressure, mold temperature, size of gates, size of runners.

Undersized parts need increase in molding parameters. Oversized parts need decrease in molding parameters.  Gates and runners may need to be modified.
New molds that aren’t consistent with prints need an evaluation for potential corrections or print modifications.


Mold Damage



Part has damage or is inconsistent with print or requirements.



Anywhere on part.

A mold can be easily damaged during production or handling.  Damage may not be evident until future production runs or a decision may be made to run part with operator correcting the problem or shut-off cavity until damage fixed.


Repair by an appropriately trained person is required.



Part has surface contamination that can’t be removed,

Improper color or finish or degradation.

Anywhere on part.

Surface contamination is likely from an unclean environment. Improper color is wrong colorant, wrong mix ratio or insufficient mixing. Degradation is a chemical change in physical properties or appearance of a plastic caused by exposure to heat, light, oxygen, weathering, or external influence.

Clean environment or use clean room if needed. Fix color mix. Degradation can not be repaired, material must be replaced.



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