Melt Pressure

[plasticpeter]

Continuing on from the "Clamp Force Calculation" thread http://www.cnczone.com/forums/showthread.php?t=24719, I am interested in typical values for melt pressure (injection pressure) for common polymers. I want to use these to build a spreadsheet to roughly calculate clamping force requirement. As discussed, the formula is:

Clamping Force = Melt Pressure x Projected Area

Here are some figures that I have come up with:

HIPS 30 MPa (2 Tonnes/sq. inch)
ABS 40 MPa (3 Tonnes/sq. inch)
LDPE 35 MPa (2.5 Tonnes/sq. inch)
PP 30 MPa (2 Tonnes/sq. inch)
PMMA 50 to 60 MPa (3.5 to 4.5 Tonnes/sq. inch)
Polycabonate 50 to 70 MPa (3.5 to 5 Tonnes/sq. inch)

Do other members think these look about right? Do you know where I can get hold of better information? Can you add any other common materials (e.g. PC/ABS, POM, PA66 etc) to my list? I know this is really about having experience in the trade and I'm hoping plenty of you can offer me help!

TIA, plasticpeter

[Wiseco]

I don't catch why you want to know melt pressure of different plastic. What YOU want to know is the cavity pressure to make a good part.

As you start a new topic from the calculation of clamp tonnage topic, I presume this topic is to be in relation with clamp tonnage. So, my opinion and way to work when I want to know clamp tonnage is to presume the max cavity pressure and keep a 20% of safety.

((Melt pressure x project area)/2200) X 20% = clamp tonnage need
ie : ((2000psi X 36SqIn)/2200) X1.20 = 39 tons

But the melt pressure itself can be very tricky to know. It depend at which place you want to know it (in front of the screw, in the runner, in the bottom of the cavity, in the top of the cavity) and what equipement you have (hot runner, cold runner, long deep mold...etc)

If you have a deep tight mold, the pressure will surely not be the same in the bottom of the cavity compare to the top of the cavity.

I think we could debate about this for a year long. The first question I would ask you is what's your needs to know melt pressure for each plastic family?

[plasticpeter]

Hi Wiseco - you are correct, it is about approximating the clamp tonnage. I'm just trying to build a spreadsheet for doing quotes where you input the following information about a proposed mould tool:

Material to be moulded
Total projected area
Total cavity volume (including runners)

The spreadsheet will then lookup the material density and melt pressure and then output the shot weight and required clamp tonnage. I am also going to include a data table that the spreadsheet can compare and suggest the most economical IM machine to do the moulding on (i.e. tonnage, screw size etc).

Of course I realise that melt pressure is a transient thing and the really important question is what is the maximum injection pressure for a particular moulding (of given geometry etc etc) in a given material. But I'm not looking for absolute precision - just a starting point for doing quotes. I have access to Moldflow software so I can calculate precise figures later on in the process.

I suppose what I could do is include approximate melt pressures of different materials for various types of part geometry (e.g. thin wall mouldings, shallow or deep cavities etc etc). Do you think this would give me a reasonably accurate approximation?

[Wiseco]

Yeah I think it will give you accurate approximation. But as I said, keep you a 20% over of clamp tonnage. Anyway, machines are not sell of step of 25 or 50 tons habitually, they are sell in a 100 or 200 tons step.

It's funny, yesterday I was in the way to tell you to use Moldflow but I told myself "he don't need it to calculate clamp tonnage".

Maybe you should search this topic in the plastics.com forum. It's a great forum to talk about plastic related stuff.

Here a link to a good explanation of what you want : http://www.plastics.com/communities/...tonnage#p39174

Hope this help.

[plasticpeter]

Many thanks Wiseco.

And the second link you gave me provides lots of additional help - I recommend it to anybody who has an interest in the subject.

Inicidentally, I've recently looked at the free software from Injecneering that is mentioned but wasn't able to download it. Pity, as it might solve this problem without need to write a spreadsheet