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Common Injection Molding Defects And How To Fix Them

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).Here are some Common Injection Molding Defects :

Molding Defects Alternative Name Descriptions Causes
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 as 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 remain 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)

Keep these factors in mind when designing your plastic injection molded parts, and remember that it is easier to avoid problems in the beginning than change your design down the line.OK industry has served the injection molding industry for over 15 years and has a team dedicated to providing you with the most cost effective solutions for design and manufacturing .To know more on Build a Precision Mold for Custom Plastic Parts,Please visit us at :okmolding.com  or via:info@okmolding.com


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