Last updated 23rd January 2002

The vast majority of injection moulding machines are hydraulically operated. Electric motors drive pumps which suck oil up from a sump and feed pressurised oil to the moving parts of the machine.
Under the control of the moulding machine's computer, various 'directional' valves are opened and closed at discrete points during a mould cycle.
These direct the pumped oil into the hydraulic cylinders that cause the various machine movements e.g. clamp closure.
The speeds at which these movements occur are controlled by proportional flow valves. The forces associated with these movements are controlled by proportional pressure valves.
Both types of proportional valve act as energy shunts as can be seen in the figure below:

Here we see a pump running at full speed, delivering its full capacity of oil, at a time when only a 50% flow is required e.g. during injection. The excess oil delivered by the pump is merely 'dumped' over the proportional flow control valve 'Q'.
During most parts of a moulding cycle the control system implements this type of flow control since the rate of oil delivery determines the speed at which things move.
The amount of oil dumped in this way varies throughout the mould cycle depending upon what is moving and how fast it needs to move.
This is a very inefficient way to control oil flow since it creates flows of oil which merely circulate to and from the sump. This process consumes energy without doing any real work for us. The wasted energy largely dissipates into the oil in the form of heat.
This is useful when the machine is first used from cold as it helps the oil reach its optimum working temperature. However once this temperature is reached the waste energy must be removed from the oil to prevent it from overheating.
This is done by passing the oil through a heat exchanger that is fed with water from a 'chiller'. So, the energy wasted in the hydraulic system requires yet more energy to be wasted in removing it from the system!
If that weren't bad enough there is an even more inefficient part of the moulding cycle i.e. 'follow-up'. During 'follow-up' the control system switches from flow control to pressure control.
Once the set pressure has been reached very little oil flow is required. In fact the only flow that is necessary is that which is needed to make up for any slight movement of the injection cylinder caused by the moulded component shrinking slightly as it cools.
Pressure control is achieved by the use of proportional pressure valves. These introduce a controlled leak into the hydraulic system as shown in the illustration below:

Here the pump runs at full speed delivering its full capacity of oil when only a small flow, albeit at high pressure, is actually required. The vast majority of the oil delivered by the pump vents over the pressure relief valve!
During such times as these machine efficiency can fall to below 5% i.e. 95% of the supplied energy is wasted!
Worse still, since the escaping oil is vented under relatively high pressure, the amount of energy being dissipated is also very high.
In fact most machines use high pressure oil dumping like this to implement 'oil preheat'.
Let us take a look at the root cause of all these problems i.e. the pump system. Inefficiencies arise purely because the pump runs at a fixed speed, has a fixed capacity and therefore delivers a fixed amount of oil, all of the time.
The machine rarely needs all of the oil delivery and the excess is simply dumped to tank.
The problem is slightly alleviated on most machines by the use of more than one pump. Each pump is fitted with an 'unload' valve that allows it to discharge its output straight back to the sump at relatively low pressure.
This is an improvement, but energy is still lost pumping oil back and forward to tank. This could be improved upon if it were possible to mechanically de-couple pumps at parts in the cycle when their output was not required, though in reality this isn't too practical.
The best way to dramatically improve efficiency is to use all of the pumps all of the time, at least where conditions permit, and vary the oil delivered by varying the speed of rotation of the pumps.
This is made possible by electronically varying the speed of the electric motors that drive the pumps.
Using this arrangement we can vary the speed of the motors, and hence the pumps, so that at each instant in a mould cycle we are pumping just as much oil as we really need.
When oil demand increases, such as when the clamp needs to open, the motor speed is increased. When demand falls, such as during cooling, the motor speed is reduced.
This is exactly how the Powermiser system works.
We have an interface unit that monitors all of the machines directional and proportional valves, calculates in real-time how much oil is actually required, and sets the proportional valves and motor speeds accordingly.
The benefits are obvious as can be seen in the illustration below:

In this example the press needs an oil output equivalent to 50% of maximum. The machine on the left achieves this by dumping half of the pump delivery over a proportional flow valve.
The machine on the right, however, is fitted with a Powermiser system so achieves 50% delivery by reducing the motor speed, and hence the energy consumption, to one half!
Notice that the only difference between both machines is the amount of oil delivery that goes to waste. The actual oil delivered to the press is identical.
The machine on the left shows an output hose half full of fast moving oil to make it easier to visualise the dumping process. In reality the hose would be full of oil flowing at half the speed i.e. as shown for the machine on the right.
Since the Powermiser system constantly provides exactly the same delivery of oil to the press as is actually required the system is totally transparent to the end user.
The oil demand calculations performed by the Powermiser interface unit are based upon the proportional and directional valve setting signals produced by the machines existing control system.
This means any adjustments in machine settings, either minor adjustments or complete changes as might occur after a tool change, are automatically accounted for.
So you see once the Powermiser system has been configured for a particular machine it needs no further calibration or adjustment and simply adapts itself to the press setup.
Now let's take a look at what happens with the Powermiser system during 'follow-up'. Remember that during 'follow-up' only a small amount of high-pressure oil is actually required by the press.

The unmodified machine on the left is wasting a tremendous amount of energy discharging high flow at high pressure. Compare this with the machine on the right that has been fitted with a Powermiser system.
Since very little flow is required the pump speed has automatically reduced to a 'tick-over'. The quantity of oil and the pressure at which it is delivered to the machine remain the same. However, the energy required does not!
Experimental readings have shown that during this phase of moulding the Powermiser system allows transparent operation whilst saving in excess of 80% of the energy normally required!
Factors that affect the amount of overall saving that can be achieved with a Powermiser system are:

• Machine type
• Manufacturer
• Shot size
• Cycle time
• Material

However by far the most important factor is wall thickness. A thick wall means long cooling times and long 'follow-up' periods.
The best result achieved to date with a Powermiser system was a saving of 73% which was largely possible since the component in question had a cycle time of about 4 minutes and a wall thickness of around 15mm!
Unfortunately the corollary to this is the Powermiser system will not produce very good savings for fast cycling thin walled components such as disposable packaging.
Also, whilst savings can be achieved for machines fitted with hydraulic accumulators, the level of saving is generally reduced.