Injection Moulding Quality Control

At B&C Plastics we invest a lot of our resources in creating repeatable systems of quality control for injection moulding.  We have dedicated staff whose role is to ensure that every stage of our processes lead to quality products for our customers. 

Injection moulding is a complex technology with possible production problems. They can be caused either by defects in the molds, or more often by the moulding process itself.  This is why B&C Plastics have been so successful over the years. Our expert manufacturing reduces or eliminates variation to make consistent product time after time.

As such we thought we could give you an insight in to just some of the areas that we examine and check before any product is dispatched.

 

Potential Moulding Defects

Moulding 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

 

Colour streaks

Colour streaks

Localized 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.

 

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

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)

 

Jetting

 

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 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 mould-flow study when the mould is in design phase. Once the mould is made and the gate is placed, one can minimize 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]

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 mould

Partial part

Lack of material, injection speed or pressure too low, mould 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 mould 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 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 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

Discoloured line where two flow fronts meet

Mould/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

 

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