How to calculate injection pressure,injection time

This seems a lonely thread. Been here two months and no-one has responded.

You have asked a very complex question without sufficient detail, but assuming that you have access to a machine and want some "rough cut" information I'll chip in.

Injection time:

Your cavity volume (including runners and scrap) relative to your injection unit (screw) size and pitch will determine possible injection time. Often you will not run at maximum speed as there needs to be time for the gas inside the cavity to leave the cavity through vents that are present on the parting line of the tool. (If you inject too fast, you can get gas trapped inside the part (bubbles) or trapped inside the cavity and as this gas is compressed it gets VERY HOT (Just like inside a diesel engine) and will burn the surface of your plastic part.

We would instead consider shot size, injection speed, pack pressure etc. and treat injection time as an outcome of other parameters that are more important to control.

Injection pressure:

This is the pressure necessary to move the plastic into the cavity fast enough that it fills before the plastic freezes and in some way represents the viscosity of the plastic as it flows into an incompletely filled cavity. There is also another parameter that we call "pack pressure" which is a final "bump" of pressure when the cavity is filled but before the gate freezes off to compensate for the shrinkage as the material cools. Sometimes this is measured as the pressure in the hydraulic system pushing the screw forward and sometimes it's the pressure measured in the material.

Refilling time:

I think this is what we call "recovery time". That is the time it takes for the screw to return to the pre-injection position ready for the next shot. This is again an outcome of the shot size and the ability of your material feed system to refill the barrel.

Cooling time:

For us, this represents the time from when pack pressure is removed (Screw begins refilling) till the part is sufficiently solid to be ejected from the die. At the end of that time, the tool can be opened and the part ejected. If you open the tool before that time, it gets real messy. The cooling time is again an outcome of factors that can be influenced. Do you have coolant flowing in the tool? What is the thickest developed wall section of the material? How far above HDT are you moulding the material? How quickly does heat transfer through the material? and so on.

If you ask GE, DUPONT, ICI, or the other plastics suppliers, they still have a small handbook that will show common problems in injection moulding and also helps with some terms used.

Hope this has helped.