My interests is in keeping molds in "ready to run" condition. The issues I deal with regularly are "drags" "PL flash" and "Hobbed (or Coined) parting lines" Obviously clamp tonnage is a requisite for making good parts. I see conversations (here and elsewhere) about fill & pack studies, gate freeze studies etc, how much attention is paid to sufficient but not excessive clamp tonnage? What precautions are being observed to protect expensive tooling from unneccesary damage? All the defects listed above are either directly or indirectly the result of excess tonnage.
You are correct. Sometimes people use excessive tonnage or more than what is required. A little bit of a catch22 here but there is a solution. One would start off with doing a quick calculation on the projected area and using that tonnage value as a starting point. Two things here - one these calculations do not always work and second if you see that at very low packing pressures if you see some flash and if you increase the tonnage and the flash goes away then you need to use the higher tonnage.
After all the scientific molding studies that are done and a process is established, start dropping the tonnage in steps of 10 tons or so and start collecting the parts. Look at the part quality, not just cosmetically for flash but also for dimensions (you may be surprised to see the dimensions also change). Then once you know where the parts are not acceptable, increase the pressure back up with a reasonable % and set this as the tonnage you need.
Hope this helps. i am sure there will be some other people who will give their two cents also.
Regards,
Suhas
Too much clamp tonnage= wasted power consumption,increased machine wear (mold already mentioned) closing(and hobbing) of vents (which then makes filling harder, then the vicious circle to overcome).
No excuse for over-setting clamp tonnage on modern machines. On old toggle machines, one had to use stretch indicators and time consuming calculations to set, and then never knew if setting had been changed.
Too much clamp is nasty!
It's a good idea to check your tie bar stretch to make sure that all four tie bars are stretching the same amount. Make sure to do this with a mold that is centered (no offset locating ring). Stretch strain gauges can be bought from PPE. There are two sizes. A one piece gauge for smaller tonnage presses (25-100) and a two piece gauge for the larger press sizes (100+). We check tie bare stretch as part of our quarterly mantainance.
Effects of process parameters on tooling.
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