Injection moulding process variables and their causes
Variable
Possible cause
Injection pressure
1. Dirty oil; blocked filter
2. Faulty pump; cavitation
3. Faulty relief valve
4. Overheated or cold oil
2. Faulty pump; cavitation
3. Faulty relief valve
4. Overheated or cold oil
Injection rate
1. Overheated oil
2. Air, water, or sludge in oil
3. Faulty control valve
4. No accumulator, and pump too small for required rate o f flow
2. Air, water, or sludge in oil
3. Faulty control valve
4. No accumulator, and pump too small for required rate o f flow
Melt temperature
1. Heater band malfunction
2. Thermocouple malfunction
3. Faulty temperature controller
4. Hot spot in cylinder
2. Thermocouple malfunction
3. Faulty temperature controller
4. Hot spot in cylinder
Shot volume
\. Faulty switch on feed control
2. Worn cam follower on feed control
3. Worn and leaking check ring (back-flow stop valve)
4. Incorrect screw-back speed
5. Malfunction of pressure reduction switching
6. Nozzle dribble
7. Leakage at nozzle/sprue bush
2. Worn cam follower on feed control
3. Worn and leaking check ring (back-flow stop valve)
4. Incorrect screw-back speed
5. Malfunction of pressure reduction switching
6. Nozzle dribble
7. Leakage at nozzle/sprue bush
Mould temperature
1. No proper mould temperature control
2. Dirt or rust in cooling channels
3. Cooling channels wrongly connected
2. Dirt or rust in cooling channels
3. Cooling channels wrongly connected
Mould characteristics
1. Mould incorrectly set
2. Mould distorted
3. Mould badly vented
4. Mould badly constructed
5. Mould badly designed
2. Mould distorted
3. Mould badly vented
4. Mould badly constructed
5. Mould badly designed
To sum up the discussion of the causes and cures of faults in injection moulded components, Table should be consulted. Here, the main faults are listed on the left. In the second column is a generalized cause，and possible remedies are listed on the right，together with the appropriate text references. Such a table should be used with discretion; nothing is more valuable in the diagnosis and cure of faults than a thorough knowledge of the process and the ability to apply that knowledge to the problem in hand.
Faults in injection moulding
Fault
Generalized cause
Possible cures
Short moulding
Sinks and voids
Weld and flow marks
Poor surface finish
Sinks and voids
Weld and flow marks
Poor surface finish
Cavity not filling
properly
properly
Increase injection pressure
Increase injection rate
Increase cylinder and mould
temperature
Increase feed
Improve mould venting
Increase size of gate，sprue, and
runner
Increase injection rate
Increase cylinder and mould
temperature
Increase feed
Improve mould venting
Increase size of gate，sprue, and
runner
Bum marks
Lack of mould
venting
venting
Improve mould venting
Mechanical sticking
Flash
Warping
Flash
Warping
Over-pressurization
of cavity
of cavity
Reduce injection pressure
Use minimum hold-on pressure
and time
Fill more quickly
Use minimum hold-on pressure
and time
Fill more quickly
Silvering and
splash marks
Decomposition
splash marks
Decomposition
Overheating of
material
material
Reduce cylinder temperature
Reduce effects of mechanical heating
Reduce effects of mechanical heating
Distortion
Ejecting too hot
Increase cooling time
Eject over larger area
Eject over larger area
Jetting
Linear velocity of
melt too high
melt too high
Reduce velocity or break up stream of melt
Lamination, crazing
Surface cracking
Surface cracking
Frozen-in stresses
Ensure rapid filling at lowest possible pressure