Calculating tonnage for preload of cores and projected area
By S. Kern
Published: February 18th, 2010
Published: February 18th, 2010
Time to humble myself. I've always used part projected area to calculate the size press needed to mold a given part. I have one that I could use some assistance in understanding.
The part is a small cylinder of about 1.0" long X .700" diameter with a wall stock of .070" Tunnel gated with runner cut in 'B' half. Gate (.030") is located about .200" from end of part.
Resin is 12 MFI polypro
Mold is 32 cavity with 2 stripper plates 7 X 7" proud by .005"
I think the preload on the core shutoff is about .0008"
I calculated runner and part projected area of 14 square inches.
This mold will run in a 200 T (metric) netstal with zero issues but not in a 165 T JSW.
In the JSW it blows the parting line and the core shutoffs and runner
I discussed with a local toolmaker and he said that the preload and stripper blocks were adding to tonnage required. His explanation was the 98 square inches of mold contact that distributed the tonnage over that area not the parts projected area that causes extra tonnage needed.
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Kern There is one easy way to
Kern
There is one easy way to check the preload.
Use shim.
First clamp the mold at zero ton and check the gap between the inserts with shim.
Next increase the clamp to 10 ton then check the gap again. If there is no gap then the preload is somewhere between 0 and 10 ton. If you can still fit shim in increase the clamp to 20 ton and check the gap again. Repeat this process until you get zero gap.
The smallest shim that I know is 0.01mm and this is aluminium foil used for food packaging. So technically the gap may not be zero but as long as 0.01 shim doesn't fit then it is close enough to zero.
Another way to check the preload is using bearing blue.
Another thing to consider is the injection rate of both machines. Are they the same? Or does the JSW have a slower rate? If the injection rate is too slow the molten plastic will start to solidify in the cavity which will increase the injection pressure and the clamp tonnage required.
Hope this helps
Paul.
Also keep in mind the support
Also keep in mind the support required in the ejector half, not enough support and the PL will flex, the locating ring diameter normally is 4", this can also be of concern, there is no support within the 4" diameter, the mold plate thickness on the sprue side will have to overcome the molding pressure also.
Dennis
www.burco-molding.com
Also keep in mind the support
Also keep in mind the support required in the ejector half, not enough support and the PL will flex, the locating ring diameter normally is 4", this can also be of concern, there is no support within the 4" diameter, the mold plate thickness on the sprue side will have to overcome the molding pressure also.
Hope this helps
Hans
www.matrixtooling.com
Sam, I assume you are running
Sam, I assume you are running both presses at max tonnage.
I wonder what would happen if you run the mold in the 200 tonner at 165? I'd try this before cutting steel. not 100% sure about Netstal andJSW presses, but am thinking toggle presses in both cases.
brent
www.matrixtooling.com
PS Are you sure that the nozzle tip is ELIMINATOR on the flashing press?
Mold acreage and preload
Mold acreage and preload can't exert any force against the clamp - unless the preload is in the wrong place, i.e. somewhere other than the parting line shutoffs.
You say that "preload on the core shutoff is about .0008"". If that means the part ID cavities are 1.0000" deep (below parting line) and the part ID cores are 1.0008" tall (above P/L) then, if my arithmetic is correct, it's taking you around 145 - 150 US tons to close up the parting line, before you start shooting plastic. If the press is 165 metric tons, that leaves you 2 or 2 1/2 tons per square inch for the shot. Looking at your peak injection pressure might give you an idea if that should be enough.
If that really is the issue then the difference between a 200 (metric) ton press and a 165 (metric) ton press is about 40 US tons. You could get that back (again, if you trust my arithmetic) by relieving the preload in the middle of the cores with a 3/8 end mill.