Molding Defects
Injection molding is a complex technology with possible production problems. They can be caused either by defects in the molds, or more often by the molding process itself.
| Molding Defects | Alternative name | Descriptions | 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 | 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 |
| Color streaks (US) | Colour streaks (UK) | Localized change of color/colour | Masterbatch isn't mixing properly, or the material has run out and it's starting to come through as natural only. Previous colored material "dragging" in nozzle or check valve. |
| 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 | Mold 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) |
| Jetting | Part deformed by turbulent flow of material. | Poor tool design, gate position or runner. Injection speed set too high. | |
| 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 minimized or eliminated with a mold-flow study when the mold is in design phase. Once the mold is made and the gate is placed, one can minimize this flaw only by changing the melt and the mold 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 | 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. Excessive material or walls too thick. | |
| Short shot | Non-fill / Short mold | Partial part | Lack of material, injection speed or pressure too low, mold too cold, lack of gas vents |
| Splay marks | Splash mark / Silver streaks | Circular pattern around gate caused by hot gas | 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. |
| Stringiness | Stringing | 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) | Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Filling too fast, not allowing the edges of the part to set up. Also mold may be out of registration (when the two halves don't center properly and part walls are not the same thickness). The provided information is the commond understanding, Correction: The Lack of pack (not holding) pressure (pack pressure is used to pack out even though is the part during the holding time). Filling too fast does not cause this condition, as a void is a sink that did not have a place to happen. In other words as the part shrink the resin separated from it self as there was not sufficient resin in the cavity. The void could happened at any area or the part is not limited by the thickness but by the resin flow and thermal conductivity, but it is more likely to happened at thicker areas like ribs or bosses.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/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 |
Methods such as industrial CT scanning can help with finding these defects externally as well as internally.
Read more about this topic: Injection Molding, Process Troubleshooting
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