Pressure Requirement

15 to 2000 tons I would guess.....

http://www.wabashmpi.com/compression_molding.html

Why not 5 to 500000 tons. Engineers don't use guess work.

Sorta depends on the material, but a good rule of thumb is 2,000/2,500 PSI pressure is required for compression molding.

I assume this is a thermosetting material, so cure time will depend on mold temperature and the material itself. (Thermoplastic materials do not lend themselves to compression molding)

Rate of mold closing is important, too.

Tell us, are you new to molding, curious or what. There's a lot to processing thermosat materials besides clamp pressure.

Let me elaborate.

Pressure is across the surface area of the part, which is essentially the bottom, plus the draft angle of the sides and top lip, if any.

Pressure is not critical, as long as the mold will close. Once the mold reaches the stops, pressure becomes static and very low.

Pressure is required to fill the mold and squeeze the excess material out the vents, which you will always have with this process.

You've lead to my question for the OP. I thought that a ton was a unit of force or mass, or in the case of refrigeration power. I now see an understandable way that a ton can be a volume, too. A ton is the wrong unit for pressure.

Injection, transfer and compression molding machine's capacitites are commonly referred to in tons.

Ton's of force applied to the hydraulic rams that close and clamp the molds.

100% correct.

Would you know what tonnage press is required for my earlier enquiry for the manufacture of builders buckets

Not new to injection and blow molding, using virgin material.

The bucket we are wanting to manufacture will be of a mixture of recycled HD and LD material.

Opposition are using a machine with a clamping force of 10 tons to mold a similar product.

Then go them one better with 11 tons.

Confusion Here:

Clamping Force is the pressure x surface area here. Therefore, depending on the apparent total surface area of the molding tool, and the required pressure developped at the injection device (Screw), at the temperature of the molten plastic, to create the flow into the mould.

If the competition are using a machine with only 10 tons clamping force, then you need at least that much force. But, all depends on the bucket mould being used, and the time allocated (how fast the cycle is going to be), at the melting temperature (flow caracteristics (thickness of the bucket wall...).

The clamping force is the final force applied to maintain the mould closed.

Now, You are talking about an open edged mould: you are practically extruding the molten plastic loaded at the bottom, to forme the bucket. The mould is not closed such that the plastic is forced to remain inside the mould, but the excess is allowed to flow over the edge. This is why a 10 tons force might be enough, but you need to wait for the plastic to flow slowly and form the bucket. If it was an injection moulding type, with the mould completely closing, you would need much more than 10 tons... ~>50 tons.

You have no understanding of "compression molding".

You cannot "compression mold "HD' and "LD". These are thermoplastic materials and must be injected into a cold, or cool mold to solidify.

You must be injection molding, and the clamp pressure has been explained already. 2,000 PSI of projected surface area.

The tonnage capacity of the press is not important, if it is enough.

The shot capacity of the injection screw is critical, if it isn't large enough.

You obviously have no clue.

If you put polyethylene into a compression mold it will have to be cooled before you can open the mold.

Do some research on the web. Thermoset materials are compression molded.

Unless you are dropping a pre-melted charge of plastic into the mold and closing it, which seems very slow.

What are you doing?

I bet it depends on how soft the plastic is when you try to squeeze it.

Doesn't it rather depend upon the type of plastic and its temperature?

Yes it does, I mentioned earlier that we would be using a combination of recycled LD & HD material. The MFI being in the region of 130-150 deg celcius.

I hope this helps.