Injection moulding process variables and their causes

[one_half]Variable [/one_half][one_half_last]Possible cause [/one_half_last]

[divider_line]

[one_half]Injection pressure [/one_half][one_half_last]1. Dirty oil; blocked filter
2. Faulty pump; cavitation
3. Faulty relief valve
4. Overheated or cold oil [/one_half_last]

[one_half]Injection rate [/one_half][one_half_last]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 [/one_half_last]

[one_half]Melt temperature[/one_half][one_half_last]1. Heater band malfunction
2. Thermocouple malfunction
3. Faulty temperature controller
4. Hot spot in cylinder [/one_half_last]

[one_half]Shot volume [/one_half][one_half_last]\. 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[/one_half_last]

 [one_half]Mould temperature[/one_half][one_half_last]1. No proper mould temperature control
2. Dirt or rust in cooling channels
3. Cooling channels wrongly connected [/one_half_last]

[one_half]Mould characteristics [/one_half][one_half_last]1. Mould incorrectly set
2. Mould distorted
3. Mould badly vented
4. Mould badly constructed
5. Mould badly designed[/one_half_last]

 

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

[divider_line]

[one_third]Fault [/one_third][one_third]Generalized cause [/one_third][one_third_last] Possible cures[/one_third_last]

[divider_line]

[one_third]Short moulding
Sinks and voids
Weld and flow marks
Poor surface finish [/one_third][one_third]Cavity not filling
properly [/one_third][one_third_last] Increase injection pressure
Increase injection rate
Increase cylinder and mould 
temperature
Increase feed
Improve mould venting
Increase size of gate,sprue, and
runner[/one_third_last]

[one_third]Bum marks [/one_third][one_third]Lack of mould
venting [/one_third][one_third_last] Improve mould venting[/one_third_last] 

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

[one_third]Silvering and
splash marks
Decomposition[/one_third][one_third]Overheating of
material [/one_third][one_third_last] Reduce cylinder temperature 
Reduce effects of mechanical heating[/one_third_last]

[one_third]Distortion [/one_third][one_third]Ejecting too hot[/one_third][one_third_last]Increase cooling time 
Eject over larger area[/one_third_last]

[one_third]Jetting[/one_third][one_third]Linear velocity of
melt too high [/one_third][one_third_last] Reduce velocity or break up stream of melt[/one_third_last] 

[one_third]Lamination, crazing
Surface cracking [/one_third][one_third]Frozen-in stresses [/one_third][one_third_last]Ensure rapid filling at lowest possible pressure[/one_third_last]