Mould Locking
Mould locking is a moulding problem when a moulding machine fails to open an injection mould. This is not an uncommon issue while processing polycarbonate resin into a mould which has deep cavities (Fig.:). In a normal moulding cycle, after the cooling part of the cycle is complete, the mould should open to create enough clearance between the two mould halves so that the ejection system built into the mould can be activated and a moulded part is ejected. If a moulders run into such a problem, there is a considerable cost penalty and time lost as the machine has to be stopped and external hydraulic means are required to be set up to make the mould open for a manual part removal. This may take a few hours. The disruption can go on for a few years until the reasons are well understood and preventive measures are in place. I happened to solve such a problem which had been disrupting the production for a few years.
Fig.1:
On my analysis, i decided to focus on three aspects:
1. Mould Venting
2. Runner Balancing & Flow Restrictions
3. Process Optimisation
Mould Venting:
When I slided my fingers on the venting channels, I could feel that the channels had been thinned down. The venting channels had been worsening but the maintenance department addressed the issue by manually creating the inaccurate grooves, which would solve the problem for a couple of days but then again the issue. So, I decided to tig-weld all of the venting channels and create more accurately using a CNC machine. Also, I made sure that all the corners have a vent channel.
Runner Balancing:
It was a 4 cavity 3-plate mould. The gate distance and angle were different for each set of the cavities. Also, there were flow restrictions created in the runner due to the 3-plate nature of the tool. Again, I decided to weld the runner and created a balanced runner where each gate had the same distance and angle. The restrictions of the runner were relieved by redesigning the runner catcher.
Process Optimisation:
The excessive pressure required to overcome the flow restrictions coupled with an imbalanced runner causes some of the cavities to be overpacked, which then leads to vacuum effect due to a minimal shrinkage in these cavities. The process was scientifically set by 98% injection filling followed by holding pressure phase, a gate seal studies, reduction in injection speed and pressure.
Results:
After these remedies, the tool never locked, the process and machine were operated at a much normal setting and cycle time was optimised by 30%.
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