Common Molding Defects

Common Defects of Molding

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), or even by design.

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.

Molding Defects	Alternative Name	Descriptions	Causes	Images
Blister	Blistering	Raised or layered zone on surface of the plastic part	Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater.
Burn marks	Air burn/gas burn	Black or brown burnt areas on the plastic part located at furthest points from gate	Tool lacks venting, injection speed is too high.
Color streaks (US)		Localized change of color	Plastic 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 wall	Contamination of the material e.g. PP mixed with ABS; very dangerous if the part is being used for a safety-critical application. The material has very little strength when delaminated as the materials cannot bond.
Flash	Burrs	Excess material in thin layer exceeding normal part geometry	Tool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.
Embedded contaminates	Embedded particulates	Foreign particle (burnt material or other) embedded in the 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.
Flow marks	Flow lines	Directionally “off tone” wavy lines or patterns	Injection 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.
Jetting		Deformed part by turbulent flow of material	Poor 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 shot	Non-fill/short mold	Partial part	Lack of material; injection speed or pressure too low.
Splay marks	Splash mark/silver streaks	Circular pattern around gate caused by hot gas	Moisture in the material, usually when resins are dried improperly.
Stringiness	Stringing	String-like remains from previous shot transfer in new shot	Nozzle 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 line	Discolored line where two flow fronts meet	Mold/material temperatures set too low (the material is cold when they meet, so they don’t bond).
Warping	Twisting part	Distorted 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).

How To Get Your Projects Quality Assured?

Entering the global manufacturing industry can be quite intimidating. But don’t let that stop you because you are so close to bringing your idea to life. At HIPOS Shenzhen Technology Co., Ltd, we simplify managing your projects with our excellence and do things in right way. We encourage you to reach out to us if you have any questions because we are always here for you. Our incredible team will help you on your journey from Concept to Reality so that you can be confident that you are providing the best product for your company.