Clamping force calculation

[Wiseco]

Hi,

I'm designing a new 4 cavity mold and I need to know how much ton that I need to hold both halves close to be sure my machine can hold it. Me and a friend are talking each other different : I said that the surface area that must be know to calculate holding pressure is the surface area of the plastic at the parting line only and my friend said we must use the complete surface area of the mold which is the surface of the mold minus the surface of the core.

Which is right??? I think I'm right...

[Pres]

Looking from the part towards the parting line (in a direction perpendicular to the parting line) times the part surface area times the pressure (psi) gives total clamping force needed. Divide by 2000 to get "tonnage".

i.e. you are right!

[Wiseco]

It's funny because when you were replying this thread, my eyes fell on a line in the blow-molding handbook. Here is the quote :

Clamp force on the mold halves required in the IMM also depends on the plastic being processed. A specified pressure is required to retain the pressure in the mold cavity(ies). It is basically the cross sectional area of any melt located on the parting line of the mold that includes the cavities and mold runner(s) located on the mold parting line. ... By multiplying the pressure required on the melt and the melt cross sectional area, the clamping force required is determined

But I'm skeptical again... I think this line is a bit erroneous. I would say :

It is basically the cross sectional area of any melt located INSIDE OF the parting line of the mold that includes the cavities and mold runner(s) located on the mold parting line.

Let say I make caps. The pressure will be apply on the surface which will be equal to the diameter of the caps, right? So I think I was a bit wrong and my friend too.

[Wiseco]

Here is my answer I think :

The clamping force needed can be calculate with the projected area of the cavity times the melt pressure. The projected area of the cavity is the sum of the area of all of the surfaces projected perpendicularly onto the parting plane.

I think it's the best description. What do you think?

P.S. I must stop thinking, my head is getting too hot!

[DJPLAST]

You need 3 - 5 tons per square inch of projected area to keep the mold closed. A tall part ( like a cup or tumbler) could get by with the 3 ton calculation. A fllat part like a dinnner plate, use the 5 ton (or greater) calculation. Again, it depends on the material's flow characteristics.

[Wiseco]

For my application, I don't need that much pressure on mold. Youre talking for injection molding but I'm in injection-blow molding, my mold is keep hot so the melt didn't get cold so it's require less melt pressure to fill all cavities.

By now, if I'm correct with my mathematic, I need 1.68 tons by square inch MAX, I keep a 20% of safety factor. We rarely goes over 5000 psi on the melt pressure as I caluculate with some real data.

DAMN I love my job when I have challenge like that!!!

[DJPLAST]

Oh, sorry, didn't catch the blow part in the first thread. I stand corrected.

[plasticpeter]

Hi I am new to the forum. It is certainly true that the calculation requires only the PROJECTED AREA i.e the area bounded by the silhouette of the cavity if you were to look down the injection nozzle. So, for a square box on a standard 2-plate mould, the projected area is the length x width of the box - it does not matter how deep the box is. At least, this is what my old college tutor (who has written books about the subject) told me!

The problem I have is a definition of the MELT PRESSURE. This is a variable depending on the viscosity of the material, the melt temperature and the geometry of the cavity (e.g. thin wall versus thick wall etc).

For instance, the melt pressure of PS can vary from as low as 15 MPa up to 50 MPa (1 ton/sq. inch up to 3.5 tons/sq. inch).

Does anybody know of a reasonably definitive list of melt pressures that could be used for practical clamp force estimation purposes?

Cheers.

[plasticpeter]

I've continued the debate about MELT PRESSURE in the following thread:

Melt Pressure

Thanks

[tapirus]

hai all...i new here..i got problem with my clamping force(CF)...when i calculated CF for my products(4 cavity)..roughly i can use machine 80T to hold it..but unfortunately it cannot hold my mould..so my sample had flashing at parting line area..what u all think about this??how to show that my clamping force that i calculated not enough for actual condition..

[Wiseco]

Recheck your calculation, maybe youre injection speed is too fast so you have a peak of injection pressure. Show us the specs of your parts with measurement and I will came back to you with my results.