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UNDERSTANDING ENERGY CONSUMPTION IN
Overview of energy consumption becomes necessary so that the moulders can evaluate the real economics of production in a moulding shop.
Injection moulding is a cyclic process with variation of load through-out the cycle. The variation of the load depends on several factors of the part design (to be moulded), polymer used and process parameters set. Therefore presentation of overview becomes quit complex.
The Injection moulding process involves
The polymer demands specific Kilo-Calories of heat for changing its state from solid to melt.
The required heat comes from
The melt is injected in to the mould at a flow rate and at a varied pressure. The power consumed during injection is proportional to -
The heat is drained off through the water circulating in the mould during cooling period.
The power is also required for the various movements in the machine.
PRODUCT COST ANALYSIS
On analysing cost of moulding-production of injection moulded parts it is observed that- ( see fig. 1)
On further analysing the cost of machine which is only 30% / 25% of the total cost of production, it is observed that-
Therefore power cost is hardly between 5% and 9% of total moulding (production) cost whereas the material cost is 50 to 75 % of total cost. It is more economical to reduce the wall thickness in order to reduce the cost of the product. Reducing wall thickness would mean more injection rate and injection pressure and hence increase in power consumption slightly.
Injection moulding machine is actuated by hydraulic system which has electrical three phase A.C induction motor as prime mover. Therefore in the machine electrical energy is transformed in to mechanical energy through hydraulic energy.
The energy reaches the actuators in the form of pressure and volume flow. While transmitting power through hydraulic the loss of energy could be due to flow losses and friction. The compression of hydraulic oil develops frictional heat which has to be controlled by radiation or cooling.
BREAK-UP OF ENERGY CONSUMPTION
If we analyse power consumption in a typical moulding cycle the break up of power consumption would be as follows. (See fig. 2)
This typical energy consumption in a moulding cycle may differ slightly depending up on the following-
An electrical A.C three phase induction motor converts electrical energy in to mechanical energy. This motor drives the pump to provide volume flow to withstand load pressure.
Electrical Motor's efficiency is 0.87 as per the manufacturer.
For a constant delivery pump Q (calculated) = 112.6 l/min.
Q (actual) = 98 l/min.
efficiency = 0.81
Therefore conversion efficiency from electrical motor to hydraulic flow is 0.87 x 0.81 = 0.70
It means that input of 37 kW from electrical motor gives an output of 26 kW at the hydraulic pump.
The following combinations of pump-motor are in use today in hydraulic system of injection moulding machine.
Three phase induction motor with the
The energy efficiency depends on the
Consider a hydraulic cylinder whose piston has to be moved first half of the full stroke (S1) at lower speed and remain stroke (S2) at higher speed. It is well known that flow rate determines the speed and pressure determines the force in hydraulics. (See fig. 3)
Let V1 be the slow speed and P1 be the corresponding pressure and S1 be the corresponding stroke.
Let V2 be the higher speed and P2 be the corresponding pressure and S2 be the corresponding stroke.
We shall analyse the energy consumption separately for slow and high speed movement of piston. It can be seen that the most favorable efficiency is achieved with regulated variable delivery pump while operating at high speed-stroke S2. We shall consider it's efficiency as 1 for comparison purpose. The efficiencies of all other power units will be lower than 1.
We can now observer the following. (See fig. 3)
Toggle , Fully hydraulic and combination of mechanical and hydraulic clamp systems are available.
Toggle clamp, as it is claimed, offers 10% saving in the power consumption as a result of shorter stroke of mould closing cylinder and harmonic movement. This advantage should be weighed against following aspects of the toggle clamp.
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