Mould open.

5.7 kW-sec .

..

Mould close.

7.4 kW-sec.

...

Mould clamp pressure build-up

3.68 kW-sec

...

Total mould movement

16.78. kW sec

12.51%

Injection unit forward

00.67 kW -sec.

00.50%

Injection

10.50 kW sec

07.83%

Follow-up pressure

12.00.kW-sec

08.95%

Plasticising

69.00 kW-sec.

51.50%

Cooling / Inj-unit-retract/idling.

.18.00.kW-sec

13.42%

Ejector forward

05.88 kW-sec

04.40%

Ejector retract

1.20KW-sec

00.90%

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