Get Your Pressure Right

Hydraulic Pressure vs Plastic Pressure

When we refer to injection pressure, we can be talking about one of two things. We can be talking about hydraulic pressure which refers to the pressure that the hydraulic ram is exerting on the screw or we can be talking about plastic pressure which is the pressure that the screw puts on the melted plastic.

These two are not the same thing and if you specify an injection pressure range on your processing guides, you should only specify hydraulic pressure and you should be very clear about it.

The surface area of the hydraulic piston is larger than the surface area of the front of the screw that is pushing on the melted plastic. Thus, what is called plastic pressure will be quite a bit higher than the hydraulic pressure. It’s simple math. The same force with less surface area equals more pressure. The plastic pressure is typically 8 to 10 times higher as a matter of fact.

The ratio between the hydraulic pressure and plastic pressure is called the intensification ratio. The intensification ratio varies depending on the specific dimensions of the hydraulic ram and screw/barrel set up. They are often 10:1 but can vary widely.

The problem is that measuring the plastic pressure requires either specialty equipment like ejector pin transducers on the tool or it requires the processor to know the intensification ratio which most do not. All machines do have a hydraulic pressure gauge so listing hydraulic pressure on your processing guide just makes sense.

You might say that specifiying hydraulic pressure on a processing guide is imprecise. Because of varying intensification ratios, one cannot be sure that 500 psi hydraulic pressure, for example, is going to lead to the correct plastic pressure. Here’s the problem, the processing guide is just a guideline anyway. It is really designed to give the processor a starting point in processing the material. Specifying a hydraulic pressure is close enough for a starting point.

The actual processing parameters are going to vary depending on lots of factors that you, the material supplier, cannot control for such as gate dimensions, part size, etc. I could make an argument that you should not even list injection or hold pressures on a processing guide. It is much more important to recommend barrel temperatures and drying parameters. Most process techs don’t look at the pressure anyway. They start low and turn up the pressure until they fill the cavity.

Hydraulic pressure recommendations, if included, should range from around 400 psi for high flow polypropylene and most nylons up to 2000 psi for stiff flowing materials like PC/PBT or PPO. Also, I think that it is a good idea to make sure that your processing guides indicate “hydraulic pressure” so as to avoid any confusion.

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