Molding Primer

The topics below are in no certain order. Please scroll down to the topic of interest...

Calculating the Clamp Forces Required

Molds are generally made of at least two halves. Where the halves meet is called the parting line. As plastic is injected into the mold, it can produce enormous forces that try to separate the mold at its parting line. The resulting force (force=F) is a product of the cross sectional area (area=A) of the part at the parting line and the plastic injecting pressure (pressure=P). Though the cross sectional area of the part is fixed, the injecting pressure can vary greatly due to the viscosity (thickness) of the molten plastic being injected and whether the mold has difficult-to-fill features. The resulting force is usually quite large and is usually expressed in tons. This force is calculated as...

F (tons) = A (sq inches) X P (psi) / 2000

If a part has a cross sectional area at the parting line of 3 sq inches and the injecting pressure is 4000psi, the resulting force would be...

F = 3 sq in X 4000psi / 2000 = 6 tons.

For benchtop machines, 4000 psi can do a lot of good with an easy to flow plastic resin, so it can be a fair number to consider. But pressures often need to be significantly higher if the plastic being used is more viscous or the mold has long and/or thin features to fill. Large commercial machines can sometimes produce pressures exceeding 30,000 psi or more, so you can see what this would do to the resulting force. This 30,000psi used in the calculation above would result in a force of 45 tons.

As an example, the small benchtop machines that use a handle to inject might be able to inject with a 2000psi injection pressure at best, so even though the clamp pressure would only have to provide 3 tons of clamp force (typically employing a manual screw clamp), the types of resins and molds that might be used with such a machine are extremely limited. Is it possible to make a 3 square inch part with a machine of this type? Yes, if the part is very thick (which allows the plastic to flow more easily) and/or the plastic used is very runny when melted ( a high flow plastic ) and quite possibly both may be required. Unfortunately, this is not always possible so bigger more powerful machines may be the only option.

Estimated clamp forces for various clamp types [ listed form least to most powerful ]

Other clamp types ...

Air-toggle - [ Morgan Industries and some Simplomatic machines ] where an air cylinder is used as the actuation means of a toggle (over-center) clamp. These can work quite well, but have to be carefully set up for each mold to get the full potential out of the clamp. Changing molds can require these to be readjusted and clamp pressures can be somewhat overstated. Also, these particular Simplomatic machines are quite large and heavy and hardly in the benchtop category. These machines have long enough clamp strokes that they can be used for semi-auto ejection for a suitable mold which can add significantly to the mold costs.

Air over hydraulic - [AB Machinery] Where an air cylinder is used to boost hydraulic pressure feeding a hydraulic clamping cylinder giving a multiplied clamp pressure but a very short stroke of the clamp. Clamp tonnages with these types of clamps are still relatively small so these are still mostly suitable for very small parts with high plastic pressures or somewhat larger parts with lower injection pressures.

Conclusions - More clamp pressure is generally always better as it removes much of the uncertainty as to whether a machine is capable of producing a particular part and will always allow the production of bigger parts or multiple smaller parts in a single mold for a given type of plastic and mold cavity.