One of the most common problems meet in injection moulding is the variation of the process.
Many workers have observed variations in moulding weight from shot to shot of 2 per cent with a max spread of as much as 6 per cent.
Obviously，with this amount of variation there must be something badly wrong with the machine and it is a sad commentary on the state of injection machinery in the United Kingdom that it appears to be the norm.
In well-maintained machines，a variation from shot to shot of no more than 0-2 per cent can be expected, any greater changes being caused by actual changes in moulding conditions.
The causes of variations in shot weight and their relationship to the machine and mould operation are shown in Table 8.1. Attention to the matters mentioned in the below column of the table should assist in obtaining better reproducibility and less variation in the process.
|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 of flow|
|Melt temperature||1.||Heater band malfunction|
|3.||Faulty temperature controller|
|4.||Hot spot in cylinder|
|Shot volume||1.||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|
|7.||Leakage at nozzie/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|
|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 below should be consulted.
Weld and flow marks
Poor surface finish
Increase injection rate
Increase cylinder and mould temperature
Improve mould venting
Increase size of gate，sprue, and runner
Reduce effects of mechanical
Increase cooling time
Eject over larger area
stream of melt