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Common Molding Defects
- Mar 08, 2018 -

Common Molding Defects

Injection molding is a complex technology with possible production problems. They can either be caused by defects in the molds or more often by part processing (molding)

Molding DefectsAlternative NameDescriptionsCauses

BlisterBlisteringRaised or layered zone on surface of the Plastic partTool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater

Burn marksAir Burn/Gas BurnBlack or brown burnt areas on the plastic part located at furthest points from gateTool lacks venting, injection speed is too high

Color streaks (US) Localized change of colorPlastic material and colorant isn't mixing properly, or the material has run out and it's starting to come through as natural only

Delamination Thin mica like layers formed in part wallContamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond

FlashBurrsExcess material in thin layer exceeding normal part geometryTool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.

Embedded contaminatesEmbedded particulatesForeign particle (burnt material or other) embedded in the partParticles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection

Flow marksFlow linesDirectionally "off tone" wavy lines or patternsInjection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times)


 Deformed part by turbulent flow of materialPoor tool design, gate position or runner. Injection speed set too high.

Polymer degradation polymer breakdown from oxidation, etc.Excess water in the granules, excessive temperatures in barrel

Sink marks Localized depression 
(In thicker zones)Holding time/pressure too low, cooling time too short, with sprueless hot runners this can also be caused by the gate temperature being set too high

Short shotNon-Fill/Short MoldPartial partLack of material, injection speed or pressure too low

Splay marksSplash Mark/Silver StreaksCircular pattern around gate caused by hot gasMoisture in the material, usually when resins are dried improperly

StringinessStringingString like remain from previous shot transfer in new shotNozzle temperature too high. Gate hasn't frozen off

Voids Empty space within part 
(Air pocket)Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness).

Weld line

Knit Line/Meld LineDiscolored line where two flow fronts meetMold/material temperatures set too low (the material is cold when they meet, so they don't bond)

WarpingTwisting PartDistorted partCooling 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) 

Keep these factors in mind when designing your injection molded part, and remember that it is easier to avoid problems in the beginning than change your design down the line.