Injection moulding process variables and their causes

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

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

Melt temperature
1. Heater band malfunction
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

 

Mould temperature
1. No proper mould temperature control
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

 

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
Cavity not filling
properly
Increase injection pressure
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
Improve mould venting
 

 

Mechanical sticking
Flash
Warping
Over-pressurization
of cavity
Reduce injection pressure 
Use minimum hold-on pressure
and time 
Fill more quickly

Silvering and
splash marks
Decomposition
Overheating of
material
Reduce cylinder temperature 
Reduce effects of mechanical heating

Distortion
Ejecting too hot
Increase cooling time 
Eject over larger area

Jetting
Linear velocity of
melt too high
Reduce velocity or break up stream of melt
 

Lamination, crazing
Surface cracking
Frozen-in stresses
Ensure rapid filling at lowest possible pressure

 

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