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Hydraulics in Injection and blow-Moulding Machines

Hydraulic power has been the logical application of Injection Moulding machinery due to the extremely high pressures required during the clamping process, as the melted resin is forced into the moulds.  This is also true in Blow Moulding machines as the hollow parisons are nipped off and compressed air is blown into the closed molds, forcing each tube of softened resin against its inner mold surface.  In machines where the injection process is also hydraulic, the fluid demand peaks as the melt fills the mold and the clamping pressure builds up. At this point an accumulator is sometimes required.   This device is like an emergency power supply where a reserve of pressurized hydraulic fluid is held until the moment it is required, and built up again during the rest of the cycle.

As moulding machines become more complex, the advantages of Hydraulic power become more evident.  Some machine builders have replaced the traditional four ram clamping system on large molds with a very thin cavity at the rear of the mold that mimics the molded part. When this cavity is filled with hydraulic fluid, an extremely even force closes the mold, eliminating bulging in the center.

Multiple cavities can be inserted and retracted hydraulically from between the mold clamping plates to increase the production rate. As the second mold is filling up, the first mold is opened and the parts dropped onto a discharge conveyer, whereas the third mold pair has been closed and waiting its turn to be filled. Even the shot size can be varied between molds. Hydraulic power can be applied both to ram injectors and screw injectors with equal facility.

One drawback to the hydraulic molding machine has been the operating costs of the constant circulation of hot pressurized fluid, and the cost to pass it through chillers.  Whether the machine is idle or working flat out, the pumps keep going and unneeded fluid is dumped back into the fluid reservoir, increasing the demand for chilling water.

This disadvantage has been corrected by at least one company, by adjusting the frequency of the mains supply at the machine.  AC electric motors derive their speed as a function of the frequency of the mains supply, irrespective of voltage.  Clipping the height of the pulses, as happens with light dimmers, does not work with motors.  Recent developments in electronics enables the AC motor speed to be varied by varying the frequency of the supply voltage. Thus, when fluid pressure demand is high, as when the molds are being clamped, the pumps see the normal 50 or 60 Hz. power. During idle conditions, the supply drops down to 20 or30 Hz. at a constant voltage. Pumps slow down and fluid flow to the reservoir drops to a trickle, requiring less chilling.