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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 ]
Toggle (over-center clamps) - [ Found on many entry or hobby type injection molders ] Clamp pressures usually range from only a few hundred pounds to around 1 ton depending on toggle size. Very limited use in injection molding but can be suitable for fast production of very small parts with very easy to fill cavities and easy to inject plastic. Part sizes can approach 2 sq inches ( 13 sq cm) of parting line area if the mold is easy to fill, but can have problems in parts even as small as 1 sq inch (6.5 sq cm ) of parting line area when using even slightly viscous resins or molds with difficult to fill details. Mostly suitable for educational or lite manufacturing where the parts are suitable for these limitations.
Small vises - [ Also found on many entry or hobby type injection molders ] Clamp pressure range of about 1 to 2 tons. Somewhat more capable than toggle clamps as far as clamp pressure goes, but their low jaw profile ( around 3/4" to 1" ) allow even small molds to extend out of the top of the vise which allows the mold to spread open at the top during injection causing flashing of the part at the top. These clamps are also only best suited for small parts that can stay mostly below the tops of the vise jaws and use of high flow resins.
Medium vises - [ Powerlab/EMCO machines ] Max clamp pressures of around 3 to 4 tons. More capable still than small vises mostly due to the fact that they are taller and will encompass the entire mold keeping it closed over the entire area of the mold. The added height of the jaws ( around 2+ " ) allow these type clamps to produce parts of as big as 3 sq inches ( 19.4 sq cm ) of parting line area and sometimes even slightly larger depending upon resins and part cavity attributes. This type of clamp is often found on manual injection ( lever type ) as well as air injection machines as this is the point where the added injection pressure that air can provide becomes more useful.
Large vises - [ Proto-Ject machines ] Max clamp pressures start around 3 - 4 tons ( left vise ) and can reach as much as 10 tons or more ( right vise ) depending on model. More capable still than medium vises due both to stronger frames but also larger jaw areas.
Vises with tie bars along the top ( right vise ) allow the vise jaws to remain parallel and keep the mold from opening at the top. This type of clamp can produce parts approaching 6 sq inches ( 38+ sq cm ) of parting line area with medium to high flow resins and an accommodating mold cavity. The limiting factors with this type of clamp start to become the injection pressure and volumes of the injection unit, which generally makes manually ( lever operated ) injection machines inadequate.
Hydraulic clamps - [ Proto-Ject machines ] Max clamp pressures start around 15 tons and are only really limited by design. Benchtop machines can easily have hydraulic clamps capable of 30 ton or even higher. Such pressures allow the use of the more difficult to inject medium to low flow plastics and even extrusion grade materials or the production very large parts with medium to high flow plastics. In such cases the shot size of the injection unit typically becomes the limiting factor as to how big of a part the machine can produce. This type of machine must have powered plastic injection of some type ( air, hydraulic, electric, etc. ).
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.