Moulding Defects 4d5d20

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 part processing (molding) Molding Defects

Alternative name

Descriptions

Causes

Raised or layered zone Tool or material is too hot, often caused by a lack of on surface of the part cooling around the tool or a faulty heater Black or brown burnt areas on the part located Air burn/gas Burn marks at furthest points from Tool lacks venting, injection speed is too high burn/dieseling gate or where air is trapped Masterbatch isn't mixing properly, or the material has run Color streaks Colour streaks Localized change of out and it's starting to come through as natural only. (US) (UK) color/colour Previous colored material "dragging" in nozzle or check valve. Contamination of the material e.g. PP mixed with ABS, Thin mica like layers very dangerous if the part is being used for a safety Delamination formed in part wall critical application as the material has very little strength when delaminated as the materials cannot bond Mold is over packed or parting line on the tool is Excess material in thin damaged, too much injection speed/material injected, Flash Burrs layer exceeding normal clamping force too low. Can also be caused by dirt and part geometry contaminants around tooling surfaces. Foreign particle (burnt Particles on the tool surface, contaminated material or Embedded Embedded material or other) foreign debris in the barrel, or too much shear heat contaminates particulates embedded in the part burning the material prior to injection Injection speeds too slow (the plastic has cooled down Directionally "off tone" Flow marks Flow lines too much during injection, injection speeds must be set as wavy lines or patterns fast as you can get away with at all times) Deformed part by Poor tool design, gate position or runner. Injection speed Jetting turbulent flow of set too high. material Caused by the melt-front flowing around an object standing proud in a plastic part as well as at the end of Small lines on the fill where the melt-front comes together again. Can be backside of core pins or Knit lines Weld lines minimized or eliminated with a mold-flow study when windows in parts that the mold is in design phase. Once the mold is made and look like just lines. the gate is placed, one can minimize this flaw only by changing the melt and the mold temperature. Polymer breakdown Polymer Excess water in the granules, excessive temperatures in from hydrolysis, degradation barrel oxidation etc. Holding time/pressure too low, cooling time too short, Localized depression (In with sprueless hot runners this can also be caused by the Sink marks [sinks] thicker zones) gate temperature being set too high. Excessive material or thick wall thickness. Non-fill / Short Lack of material, injection speed or pressure too low, Short shot Partial part mold mold too cold Splay marks Splash mark / Circular pattern around Moisture in the material, usually when hygroscopic Silver streaks gate caused by hot gas resins are dried improperly. Trapping of gas in "rib" Blister

Blistering

areas due to excessive injection velocity in these areas. Material too hot. Stringiness

Voids

Weld line

Warping

String like remain from Stringing previous shot transfer in Nozzle temperature too high. Gate hasn't frozen off new shot Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Filling to fast, Empty space within part not allowing the edges of the part to set up. Also mold (Air pocket) may be out of registration (when the two halves don't center properly and part walls are not the same thickness). Knit line / Meld Mold/material temperatures set too low (the material is Discolored line where line / Transfer cold when they meet, so they don't bond). Point between two flow fronts meet line injection and transfer (to packing and holding) too early. Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts Twisting Distorted part bow inwards towards the hot side of the tool) Uneven shrinking between areas of the part