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plastic injection mold Process troubleshooting
Sep 08, 2017

Process troubleshooting[edit]

Like all industrial processes, injection moulding can produce flawed parts. In the field of injection moulding, troubleshooting is often performed by examining defective parts for specific defects and addressing these defects with the design of the mould or the characteristics of the process itself. Trials are often performed before full production runs in an effort to predict defects and determine the appropriate specifications to use in the injection process.[3]:180

When filling a new or unfamiliar mould for the first time, where shot size for that mould is unknown, a technician/tool setter may perform a trial run before a full production run. They start with a small shot weight and fills gradually until the mould is 95 to 99% full. Once this is achieved, a small amount of holding pressure will be applied and holding time increased until gate freeze off (solidification time) has occurred. Gate freeze off time can be determined by increasing the hold time, and then weighing the part. When the weight of the part does not change, it is then known that the gate has frozen and no more material is injected into the part. Gate solidification time is important, as it determines cycle time and the quality and consistency of the product, which itself is an important issue in the economics of the production process.[26] Holding pressure is increased until the parts are free of sinks and part weight has been achieved.

Moulding defects[edit]

Injection moulding is a complex technology with possible production problems. They can be caused either by defects in the moulds, or more often by the moulding process itself.[3]:47–85

Moulding defectsAlternative nameDescriptionsCauses
BlisterBlisteringRaised or layered zone on surface of the partTool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater
Burn marksAir burn/gas burn/dieseling/gas marks/Blow marksBlack or brown burnt areas on the part located at furthest points from gate or where air is trappedTool lacks venting, injection speed is too high
Color streaks (US)Colour streaks (UK)Localised change of colourMasterbatch 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.
Thin mica like layers formed in part wallContamination 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
Excess material in thin layer exceeding normal part geometryMould 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 contaminatesEmbedded particulatesForeign particle (burnt material or other) embedded in the partParticles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection
Flow marksFlow linesDirectionally "off tone" wavy lines or patternsInjection 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 BlushHalo or Blush MarksCircular pattern around gate, normally only an issue on hot runner moldsInjection speed is too fast, gate/sprue/runner size is too small, or the melt/mold temp is too low.
Part deformed by turbulent flow of material.Poor tool design, gate position or runner. Injection speed set too high. Poor design of gates which cause too little die swell and result jetting.
Knit linesWeld linesSmall 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 hydrolysisoxidation 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 shotShort fill or short mouldPartial partLack of material, injection speed or pressure too low, mould too cold, lack of gas vents
Splay marksSplash mark or silver streaksUsually 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.
StringinessStringing or long-gateString like remnant from previous shot transfer in new shotNozzle 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.
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 mould may be out of registration (when the two halves don't centre properly and part walls are not the same thickness). The provided information is the common 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 shrinks the resin separated from itself as there was not sufficient resin in the cavity. The void could happen 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 happen at thicker areas like ribs or bosses. Additional root causes for voids are un-melt on the melt pool.
Weld lineKnit line / Meld line / Transfer lineDiscoloured line where two flow fronts meetMould 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.
WarpingTwistingDistorted partCooling 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
CracksCrazingImproper 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

Methods such as industrial CT scanning can help with finding these defects externally as well as internally.