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UNDERSTANDING ENERGY CONSUMPTION IN
INJECTION MOULDING MACHINE

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.

POWER CONSUMPTION

The Injection moulding process involves

• Melting of solid and shear sensitive polymer granules
• Injecting the polymer melt in to the closed and clamped mould.
• Cooling of melt inside the mould through circulation of cold water.

The polymer demands specific Kilo-Calories of heat for changing its state from solid to melt.

The required heat comes from

• shear action in the compression zone of the screw and
• the Heat in-put through pre-heating and
• the heat in-put through heaters on the barrel.

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 -

• Pressure and
• Flow rate.

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.

• mould open/close and clamp.
• injection unit movement.
• screw rotation.

PRODUCT COST ANALYSIS

On analysing cost of moulding-production of injection moulded parts it is observed that- ( see fig. 1)

• Raw material-polymer takes up the largest share.(50 / 75%)
• Cost of machine takes up second highest share.(30 / 15 %)
• Cost of mould is lower than that of machine.(10 / 5 %)
• Cost of personnel takes up the least share.( 10 / 5 %)

On further analysing the cost of machine which is only 30% / 25% of the total cost of production, it is observed that-

• 60 / 45 % towards fixed cost
• 30 / 38 % towards power
• 7 / 12 % towards water
• 3 / 5 % towards maintenance

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-

• Efficiency of Power unit consisting of electric motor and pump or pumps,
• Selection and utilisation level of Pump.
• Economy of power consumption by Clamp system.
• Efficiency of Screw drive
• Economy in Plasticising of polymer

POWER UNIT

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

• One or more constant delivery pump.
• Constant delivery pump with pressure accumulator to meet peak pressure demand.
• Constant delivery pump and Pressure accumulator for all actuators.
• Regulated variable delivery pump.
• Regulated variable delivery pump with pressure accumulator to meet the peak demands of flow.

PUMP

The energy efficiency depends on the

• selection of pump's flow rating
• type of pump
• use of accumulator
• operating parameters - P and V

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)

• The best efficiency is achieved with flow and pressure controlled regulation of variable delivery pump. There is least pressure loss in the valves with this type of power unit. The pump delivers the exact set value of volumetric flow of oil. Loss of energy = Q(req.) x p(dr.)
• The worst efficiency is achieved with simple throttling on the fixed delivery pump. Loss of energy = Q(max.) x p(max.) - actual required energy.
• The accumulator operation is most favorable when the ratio

p(max.)/p(req.)= 1

• When p(max.)/p(req.)=1 and also Q(max.)/Q(req.)=1 then the constant delivery pump is most favorable.
• Constant delivery pump with pressure accumulator for peak demand of flow for injection boost can be okay provided the accumulator can be charged during remaining period of cycle time.
• All the power units operate at better efficiency while operating at closer to the maximum flow and pressure rating of the pump.

CLAMP SYSTEM

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.

• difficulties of mould setting demanding more skill
• absence of simple clamp force indication resulting in application of higher clamp force on the mould. This reduces the life of mould.
• sensitive to expansion of mould due to it's temperature.
• sensitive to tie bar expansion.

A saving of 10% of energy cost (which -energy cost- in turn is merely 30% of production cost ) works out to hardly 0.9 % of production cost. This is absolutely negligible considering the other aspects of toggle system and advantages of hydraulic system.

Hydraulic clamp system offers further advantages-

• Ease of mould setting,
• Excellent mould safety,
• Safety of tiebars by not allowing them to over stretch,
• Long life to mould by not deforming (compressing) mould plates excessively, Adjustable precise clamping force setting is advantageous in improving the life of the mould.
• Good access around clamp .

SCREW DRIVE

The screw converts granules in to melt and acts like a melt pump. The screw is required to be rotated to carry out it's function. It can be coupled

• to either electric motor through a clutch or
• to hydraulic motor.

Since electric motor can not be switched ON and OFF frequently at the interval of a cycle time it requires a clutch which engage and disengage during the moulding cycle. Hence electric motor would be running idle during non-plasticising part of the cycle. Therefore the advantage of efficiency of electric motor is lost.

Hydraulic motor drive for screw is most suitable because of its compactness and ease of RPM regulation. This advantage overcomes the disadvantage of less efficiency on account of leakage losses in the motor.

PLASTICISING OF POLYMER

The heat required to melt the polymer depends on its specific heat property. The heat is also produced by shearing action in the compression region of the screw.

Screw design could be optimised to get homogeneous melt at lowest possible uniform melt temperature.

Considerable heat loss could be prevented by proper insulating the heaters ( ceramic insulation). In addition to this screw- barrel assembly can be covered to keep the radiation losses as low as possible.

Specific energy consumption (Watt-sec/Gram) is the ratio = energy consumed per moulding cycle / weight of moulded part in grams. This ratio could be maintained as low as possible provided efficiency aspects of machine selection, pump and controls are understood while setting the machine.

CLASSIFICATION OF PARTS TO BE MOULDED

Wall thickness and flow ratio are very are important while evaluating the machine specification and cycle time. Therefore we can classify the parts -to be moulded - on the basis of wall thickness.

• Thin walled parts with maximum flow ratio. Wall thicknessequal to 1mm and less than 1 mm.

• Medium wall parts with wall thickness between 1 mm and 2 mm.

• Large wall parts with wall thickness larger than 2 mm.

CLASSIFICATION OF MOULDED PARTS - BASED ON WALL THICKNESS - TABLE

THIN WALLED PARTS

Most common items of thin wall are disposable cup and packaging items and disposable syringe. They are normally moulded from easy flowing PP. Applications in Telecommunication and Computer industry also growing on account of lowering in size and weight of portable items like Cellular phone, Laptop and Note book computer. Materials used in theses items are PC, PC/ABS, Mod.PPO. These materials have high heat resistance, high impact strength and good electrical properties. Hence lower wall thickness resulting in lower weight is advantageous in portable equipments.

The lower than 1 mm wall thickness does not permit the longer flow path. Lower than 1 mm wall also demands fast filling to prevent freezing while passing through the cavity. This is because freezing time is proportional to cube of wall thickness. Therefore high speed (rate) filling during filling phase is required. Since thin walls offer high resistance to flow, filling also takes place at higher pressure. The pressure peak is reached during filling phase when filling rate-speed- is higher. The product of speed and pressure is high during the filling phase.

The pressure during pressure phase is just enough to avoid moulded in strain. The speed at pressure phase should be set low enough just to transmit the pressure. The power consumption during pressure phase is quit normal.

Power consumption is of highest order for parts with thin wall and higher flow ratio. Such items require accumulator assited injection speed. Hence FR or FM or equivalent machines are most suitable.

You might have noticed larger runner thickness than the thin wall thickness of the part. In such cases the cooling time has to accommodate cooling of runner system. If the flow ratio becomes high then multiple gates are to be located so that the longest flow ratio from each gate falls with in the maximum permissible flow ratio of melt. Hot runners can reduce cooling time and hence cycle time.

It is desirable to ensure injection capacity utilisation of less than 50% for fast cycling thin walled parts of commodity plastics. It also becomes necessary to check wether the plastisising capacity is adequate to finish the dosing during cooling time.

In case of engineering materials, percentage utilisation of shot capacity of the machine should be more than 30-35%. Otherwise higher residence time may cause degradation of polymer melt.

PARTS WITH MEDIUM WALL THICKNESS

Most of the injection moulded items have wall thickness between 1 mm and 2 mm. The larger flow length becomes possible with in the limitation imposed by flow ratio characteristics of the given polymer melt. Injection speed required is not as high as that in thin walled parts.

The cavity pressure builds up as the cavity gets filled up to about 80 % or so. Therefore set pressure is not reached during filling phase. The set pressure is reached during pressure phase which is just low enough to avoid moulded in strain. The high speed and pressure do not occur at the same time. Hence actual power consumption is quite lower than the connected power. Medium power unit is to be recommended.

PARTS WITH LARGER WALL THICKNESS

Most common items of large wall thickness are pipe fittings, textile bobbins, etc. Screw driver handle, handles for appliances etc. are also of very large thickness. The melt can be pushed to longer flow path when wall thickness is large. Freezing time is also more and hence high injection speed is not required. The pressure build up in the cavity is slow and set pressure is reached only during pressure phase. The set speed during pressure phase should be low. Therefore speed and pressure never peak simultaneously. Presssure peak is reached during pressure phase when injection rate is low. Hence connected power can be lowest for larger walled parts. Lower connected power should be recommended for such thick walled moulding application.

The large parts with large flow ratio require multiple gates so that the longest flow ratio from each gate is lower than the maximum permissible flow ratio.

INJECTION CAPACITY UTILISATION

Engineering polymers normaly have limited thermal stability whereas comodity plastics like HDPE, LDPE, PP, PS have good thermal stability. While selecting injection unit for engineering polymers it's sucesseptibility to thermal degradation should be copnsidered. Hence in the absence of information about thermal stability of polymer, it is safer to select the injection unit with 35% to 75% utilisation of injection capacity for moulding engineering polymers.

While selecting injection unit for fast cycling parts (< 8 sec) injection capacity utilisation should be lower than 50%. ( or 55%.) This is to allow sufficient residence time for melt to come to desired condition.

SUMMERY

It is now clear that to produce plastic moulded parts with minimum energy consumption the following facts should be understood.

• Since injection moulding process is cyclic with varying load during the cycle time the variable delivery pump with accumulator is the best choice for the Injection moulding machine.
• Best economy of energy is achieved when the pump is utilised at near it's rated capacity. Therefore under-utilising oversized machine consumes more energy to mould a given product.
• Contribution of saving in power by Toggle clamp is very negligible (less than 0.9%) considering its handicaps and advantages of Hydraulic clamp.
• Specific heat and latent heat of polymer demand certain amount of heat energy which is obtained from heaters and also by shearing action in compression zone for melting the polymer.
• The thermoplastic melt follows Vander-Wall equation which defines the relationship among (p) pressure, (v) specific volume and (T) temperature of melt.

( p + a ) ( v - b ) = R T / M

b and M constants.

• It can be seen that reduction in temperature, if quality of end product permits, demands increase in pressure. However reduction in melt temperature can result in reduction in cooling time and hence, increase in productivity.
• Reduction in melt temperature can be achieved by reducing back pressure, with out sacrificing homogeneity, reducing screw RPM and reducing temperature settings of barrel zones.
• Lower MFI polymer requires higher injection pressure. Similarly higher MFI polymer requires lower injection pressure.
• Injection pressure also depends on maximum flow ratio (L/T) of the part to be moulded.
• Thin walled parts require accumulator assisted high injection rate.
• Medium walled parts require high injection rate with out the assistance of accumulator- as available in IM series machine. IM series machine has very powerful injection unit, thereby it ensures mouldability.
• Thick walled parts require lower injection rate. Japan / Taiwan / Hongkong machines have lower injection power. They are good enough for moulding thick-walled parts only. IM series can handle thick-walled parts very easily.

NEXT -Heat Exchange / cooling in moulding

Technical Papers by Prabodh C. Bolur

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