Areas on an injection moulding machine that can cause milky mouldings
a) Ledge inside the hopper where BBU can sit
b) Ledge inside the hopper throat
c) Worn screw/barrel or it has a mixing section
d) Ball valve assembly or broken check ring
e) Dissimilar geometry of endcap to screw tip assembly
f) Nozzle not correctly seating
g) Nozzle element too small or at too low a temperature
h) Nozzle too long, internal dimensions too large or it has a filter
If you get milky mouldings you should follow this procedure:
1a) Starve-feed with next material at least 5 times
1b) Screw back and inject and see if the screw rotates on injection (split check ring) or see if it holds a cushion when moulding
2) Ensure the hopper and screw throat are cleaned completely
3) Look down the hopper throat to the screw flights to ensure no powder has pre-melted and stuck to the equipment
4) Visually check the hopper re undercuts or ledges
5) Ask about the age of the screw and barrel, if they use abrasive materials, type of screw tip assembly or if they have mixing sections
6a) Look at the nozzle, if overly large diameter it may have a mixer or filter
6b) Check to see if the element is small compared to the length of the nozzle (should be at least 50% of the length)
6c) Look to see if the hex has bottomed out on the endcap or if there is a leak in this area
6d) Check the temperature of the nozzle on their display panel and with a pyrometer (should be no colder than 10 degrees lower than melt temp)
6e) Remove the nozzle and clean it manually, ensure to check for BBU or other foreign material and that it has been fitted correctly (the seating faces should be clean)
7) Ask about the geometry of the endcap
8) Purge with BBU and fully empty then pull the screw
Notes and Tips
Changing from high temperature materials to lower
It is important to realise that when purging, molten material is always easier to remove than solid. For this reason when changing to a lower temperature polymer you should always purge the higher temperature material at the higher temperature first with Barrel Blitz Universal (BBU). If the higher temperature polymers processing temperature is above 3400C reduce the barrel temperatures to 3400C and then purge with BBU. You should then empty the barrel completely and reduce heats to the processing temperature of the lower following material. Then purge again with BBU at the lower temperature to clear any “sticky” BBU and ensure the equipment is fully clean.
Causes of poor cleaning performance
Degraded material in the feed section of the screw
BBU is a mechanical cleaning purging compound, for this reason it will not clean the feed section of screws and barrels very effectively. The state of your feed section can be checked by looking down a cleaned hopper throat whilst doing suck back or decompression of the screw.
Worn screw or barrel
This is likely to have various effects on the moulding process; increased shear heat, increased screw back time, increased degradation and slower cycle time. It will also affect the performance of BBU which will no longer be self-emptying and may require additional quantity to clean due to poor compression (leakage) in the worn section.
Large nozzle orifice or mismatched endcap and screw tip geometry
As mentioned above, BBU cleans mechanically therefore a large nozzle or mismatch of the angles between the endcap and screw tip effectively reduces compression. This results in poor scrubbing efficiency. Also BBU then becomes more difficult to remove on certain materials such as PC, ABS, Amorphous Nylon etc.