Trouble Shooting

Objectives of Trouble Shooting

Overview

Identify the defect

Determine the cause of the defect

Take appropriate action
Troubles

Air Leak in system

Drool at sprue

Gates freezing

Difficult in Filling

Sprue will not melt

Plastic leakage



Slow startup

Blemish around gate

Burn mark at gate

Gate stringing

High gate vestige

Slow color change



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
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
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 fromhydrolysis,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 or short mould
Partial part
Lack of material, injection speed or pressure too low, mould too cold, lack of gas vents
Splay marks
Splash mark or 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 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 line
Knit line / Meld line / Transfer line
Discolored line where two flow fronts meet
Mould 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.
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