Quality Defects of Molding Plastic Products

Common Quality Defects of Molded Products

Molding plastic parts is a complex process, and various quality defects can arise during production. Addressing these issues is crucial for ensuring the final products meet the required standards. Here are some common quality defects observed in molded plastic parts:

Defects	Alternative name	Descriptions	Possible causes
Blister	Blistering	Raised or layered zone on surface of the 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/dieseling/gas marks/Blow marks	Black or brown burnt areas on the part located at furthest points from gate or where air is trapped	Tool lacks venting, injection speed is too high.
Colour streaks		Localised change of colour	Masterbatch isn’t mixing properly, or the material has run out and it’s starting to come through as natural only. Previous coloured material “dragging” in nozzle or check valve.
Contamination	Unwanted or foreign material	Different colour matter seen in product, weakening the product	Poor material introduced by bad recycling or regrind policy; may include floor sweepings, dust and debris.
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 as the material has very little strength when delaminated as the materials cannot bond.
Flash		Excess material in thin layer exceeding normal part geometry	Mould is over packed or parting line on the tool is damaged, 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 should be set as fast as is appropriate for the process and material used).
Gate Blush	Halo or Blush Marks	Circular pattern around gate, normally only an issue on hot runner molds	Injection speed is too fast, gate/sprue/runner size is too small, or the melt/mold temp is too low.
Jetting		Jetting is a snake-like stream which occurs when polymer melt is pushed at a high velocity through restrictive areas.	Poor tool design, gate position or runner. Injection speed set too high. Poor design of gates, which causes too little die swell and result jetting.
Knit lines	Weld lines	Small lines on the backside of core pins or windows in parts that look like just lines.	Caused by the melt-front flowing around an object standing proud in a plastic part as well as at the end of fill where the melt-front comes together again. Can be minimised or eliminated with a mould-flow study when the mould is in design phase. Once the mould is made and the gate is placed, one can minimise this flaw only by changing the melt and the mould temperature.
Polymer degradation		Polymer breakdown from hydrolysis, oxidation etc.	Excess water in the granules, excessive temperatures in barrel, excessive screw speeds causing high shear heat, material being allowed to sit in the barrel for too long, too much regrind being used.
Sink marks	Sinks	Localised 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. Excessive material or walls too thick.
Short shot	Short fill, nonfill, or short mould	Partial part	Lack of material, injection speed or pressure too low, mould too cold, lack of gas vents.
Splay marks	Splash mark or silver streaks	Usually appears as silver streaks along the flow pattern, however depending on the type and colour of material it may represent as small bubbles caused by trapped moisture.	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. Material too hot, or is being sheared too much.
Stringiness	Stringing or long-gate	String like remnant from previous shot transfer in new shot	Nozzle temperature too high. Gate hasn’t frozen off, no decompression of the screw, no sprue break, poor placement of the heater bands inside the tool.
Voids		Empty space within part (air pocket is commonly used)	The Lack of pack (not holding) pressure (pack pressure is used to pack out even though is the part during the holding time). Additional root causes for voids are un-melt on the melt pool.
Weld line	Knit line / Meld line / Transfer line	Discolored line where two flow fronts meet	Mold or material temperatures set too low (the material is cold when they meet, so they don’t bond). Time for transition between injection and transfer (to packing and holding) is too early.
Warping	Twisting	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) Uneven shrinking between areas of the part.
Cracks	Crazing	Improper fusion of two fluid flows, a state before weld line.	Threadline gap in-between part due to improper gate location in complex design parts including excess of holes (multipoint gates to be provided), process optimization, proper air venting.

resource: wikipedia

Addressing these quality defects often involves a combination of adjusting processing parameters, optimizing tooling and mold design, and selecting appropriate materials. Regular monitoring and quality control measures can help identify and rectify issues during production, ensuring the production of high-quality molded plastic parts.

VolksMolds is a seasoned expert in plastic mold making and product molding. Known for precision and innovation, VolksMolds consistently delivers high-quality solutions tailored to modern needs of packaging, household plastic appliances, and retail display tools.