The shear sensitive polymers like CA, POM, PMMA, PPO, PA, PBT, RPVC,, SAN etc have limited permissible residence time. It may be observed that

Higher  percentage utilisation of  injection capacity  can reduce the residence time thereby maintaining it well below the permissible level.

Therefore  actual  %age utilisation of  injection capacity should be more than minimum % utilisation.

Permissible residence time has to be lower than thermal stability time.

METERING STROKE

Maximum metering stroke available on the machine and percentage utilisation of maximum shot volume of the machine has important influence on processing and quality of moulded parts.

In 1980s the metering stroke used to be around 3.5 to 3.75 of screw diameter i.e s:D ratio. By 1987 the s:D ratio increased to 4.2. Now Italian machines have even higher ratio of over 4.5. Even higher ratio are also available. Higher ratio is not helpful in processing. Streaks and air bubbles appear on the moulding on account of excess s:D ratio.

It can be observed that effective screw length - from non return valve to the feed throat - decreases as the metering stroke increases. It is maximum (- as specified L/D ratio of 18 to 23 -) when screw tip is at fully forward position ( metering stroke is zero) and it becomes minimum when maximum metering stroke is reached.

It means that amount of heat absorbed by the material due to conduction from heater and shearing in compression zone will not be constant but it will be varying from maximum to minimum. Therefore a temperature & viscosity gradient is introduced along the axis of the screw. This results in inhomogeneities in melt causing quality problem in moulded parts when metering stroke is more than 4D.

Applicable shot weight = ( 3 / s:D ) . Stated shot weight

FREEZING TIME & INJECTION TIME

Injection time is related to injection rate (cc/sec.) and injection rate should be high enough to avoid freezing of melt during filling phase. Higher injection rate does not affect the thermally stable commodity polymers like HDPE, LDPE, LLDPE, PP, PS ect. The higher injection rate can be limited on account of sensitivity of polymer to shearing while passing through narrow passages (especially for engineering polymers). Freezing time is proportional to cube of minimum wall thickness. Generally injection time is also proportional to square of wall thickness.

MAXIMUM INJECTION SPEED

Modgrn injection moulding machines are equipped  with variable delivery or multiple pumps which are capable of delivering sufficient oil to injection hydraulic cylinder to give high enough injection speed for filling mould cavities rapidly.

Further,   higher injection speed can be achieved by the  use of hydraulic accumulator (normally offered  as  optional). Higher injection speed can push the melt to furthest  part of mould  at shorter time before  the freezing of  melt (increase in melt viscosity) on account of lower mould temperature.

MAXIMUM INJECTION PRESSURE

The injection  pressure is required to overcome the resistance to flow of melt in the mould. It depends on melt viscosity,flow ratio and mould temperature. Set pressure is higher than actual pressure during filling phase. The relief valve is actuated on reaching set pressure during pressure phase. Higher injection pressure is required for processing high viscosity materials like PC, RPVC, TPU ect. At  present new  generation machines are  capable  of  giving  maximum injection pressure of 2200 bar or even 2500 bar.

Injection  units  are normally offered with  the  option  of three screws (A, B & C) of different diameter.

A  screw  (lower dia) gives maximum injection pressure of around 1800 - 2400 bar.

B  screw  (medium dia) gives maximum injection pressure of around 1800 - 1500 bar with max. shot weight/volume higher than that of A.

C  screw  (higher dia) gives maximum injection  pressure of around 1500 - 1300  bar with max. shot weight/volume  higher  than that of B.

Screw A is recommended for moulding

• engineering parts of higher flow ratio with high viscous polymer.
• commodity items of low flow ratio with commodity polymers.

Screw B is recommended for moulding

Cooling time = Cc x (max. wall thickness mm)2

Cc = 3.5 to 5.5 for PS

1.9 to 4 for ABS

3.05 to 5 for POM

1.4 to 3.5 for PA6/6

2.0 to 4 for LDPE

The constant will also dependent on design of cooling channels in mould and it's effectiveness. In actual practice it will be more.

Latest machines provide screw rpm and reaction pressure settings (both) in 3 to 5 stages. It helps in improving precision of moulding.

CLEARANCE BETWEEN TIE BARS AND PLATEN SIZE

The  dimensions of shut mould should be such that it  should be possible to pass through the clearance  between  tiebars and it should be possible to accommodate the mould on platen of  the machine. Therefore, one dimension should  be  less than  the clearance between tiebars and another dimension should be less than the platen size. Enough space should be made  available  to accommodate the clamps if direct bolting of mould on the platen is not provided.

The  moulds  should  not  be  smaller  than  0.7  times  the clearance between tiebars. Smaller moulds if used at  full clamping  force  are likely to  give unfavorable  platen deflection harmful for platen and tiebar life.

MAXIMUM DAYLIGHT

The shut mould height of the mould should be less than 1/3rd of maximum daylight so that there should be enough room  for the ejection of part when mould platen is fully opened.   If it is closer to 1/3rd of maximum daylight then the clearance between  core  and cavity of mould in  fully  open  position should be verified on the drawing board.

With Hydraulic clamp machine, Maximum daylight = Minimum mould height + mould open stroke. If shut mould height is more than minimum mould height of machine then the mould open stroke is reduced to the extent of difference in shut mould height of mould and minimum mould height of machine. In case of Toggle machine, Maximum daylight = Maximum mould height + Mould open stroke. Mould open stroke remains constant in toggle machine.

MOULD OPEN STROKE

MINIMUM MOULD HEIGHT

With hydraulic clamp machine there can be only one specification of mimimum mould height. However, with toggle clamp machine, the shut mould height has to be between minimum mould height and maximum mould height of machine.

If  the shut mould height is lower than the  minimum  mould height of the machine then the additional  back  plate  of suitable thickness should be provided on the mould to  build up  the  shut mould height equal to more than  the  minimum mould height of the machine.

CLAMPING FORCE

The clamping force should be greater than the cavity force in the mould, i.e., mould opening force exerted by the melt in cavity. The cavity force depends on injection  pressure and projected area of moulded part.

Injection pressure depends on maximum flow ratio of part and melt viscosity. The flow ratio of part depends on geometry of part and location of gate. Viscosity depends on characteristic of polymer.

It is necessary to calculate the following to determine cavity pressure.

Multiply the cavity pressure obtained from the above table by the following factors for different thermoplastics.

It can now be observed that:

• If 6 pole motor is used RPM is lowered and hence noise level is substantially lowered. However flow rate of pump is also lowered thereby affecting speeds.
• To get maximum advantage of flow rate influencing speed of operation, 4 pole motors are used by Indian machine manufacturers.
• The change in supply frequency from 60 Hz(Japan / Taiwan / Hongkong) to 50 Hz. results in 16% to 20 % lower flow rate thereby reducing speed of operations to that extent

DRIVE POWER

The size of the power unit is decided by plasticising rate and injection rate both having influence on quality and production rate. Fast cycling machine requires higher drive power.

Accumulator  is used to increase the injection speed  (rate) to enable parallel operations of ejector and core pull .

The option of three different drive power is possible

• for  moulding  thin walled, faster cycle (upto 10 sec) items it will require higher drive power.
• for  moulding engineering parts, medium cycle  (upto 70 sec) medium power drive is required.
• for moulding thicker walled special moulding with larger cycle time, lower drive power is required.

CONNECTED POWER & POWER CONSUMPTION.

The connected power is decided by the power requirement for plasticising and injection. Moulding cycle has consist of 70% cooling time. 15 to 20% injection time and remaining 10 to 15% is taken up by mould close / open and ejection. During cooling time after dosing is over the machine is idling.

Low powered machine can enhance the injection power by incorporating hydraulic accumulator. This increases the cost of machine and also hassles of maintenance of accumulator.

MOULD CLAMP SYSTEMS

The injections moulding machines with hydraulic clamp as well as toggle clamps are being manufactured in Europe, U.S., and Japan for last few decades.

MOULD SETTING

Mould setting is quit simple in hydraulic clamp machine whereas in toggle clamp machine toggle mechanism is required to be shifted on the tie bars. Proper clamp adjustment demands more skill and judgment on the part of workmen. This is a very favourable advantage which user of both types of machines can confirm.

MAXIMUM DAYLIGHT AND MOULD OPEN STROKE

Maximum daylight as well as mould open stroke depends on toggle geometry whereas hydraulic clamp machine can be built easily for larger daylight by simply providing longer cylinder and longer tie bars. The longer mould open stroke can be provided by using longer clamp cylinder and ram. Therefor it is easier to satisfy special requirements of customers.

In hydraulic clamp machine

Mould open stroke = Maximum day-light minus Closed mould height.

Hydraulic clamp machines have enabled the moulders to set the mould easily by shifting the limit-switches. This feature has been liked by the users of machines.

FRICTIONAL RESISTANCE TO MOVE THE MOULD

In hydraulic clamp machine frictional resistance are encountered at

• Tie bar bushes fitted on moving platen while sliding on the polished tie bars.
• oil-seals and polished ram surface of clamp cylinder. This does not offer much frictional resistance.

Therefore these seals have long operational life. It does not require any special lubricants. In-fact the hydraulic clamp does not require any attention and has always been proved more reliable.

Since there are many machined parts in toggle any inaccuracies in these parts would result in mismatch of cylinder piston with cross head bore. This would result in unequal wear-tear of toggle pin / bush joints. It can also result in loss of parallelity between movable and stationary platens.

SPEED

Mould close operation is carried out through small diameter bore cylinder and mould open operation is also carried out through the ram end of clamp cylinder. Equivalent cylinder is used in toggle machines to move the mould. Therefor to perform mould close / open operations oil requirement would be similar. The speed control is provided by hydraulic valves i.e. proportional valve or pressure compensated flow control valve in both types of machines. In spite of natural kinetics of toggle mechanism speed control in hydraulic system can not be eliminated.

FR series machine has such an excellent mould safety feature that mould close terminates and opens (about-turn) even if a paper is trapped between the mould. Therefore lack of natural kinetics is never felt in SP as well as FR machines.

CLAMP FORCE

In both the machines application of clamp force is against the deformation resistance of tie bars, mould and platen . In hydraulic machine the force is applied by a column of oil under pressure behind the ram of clamp cylinder. The oil is drawn in by gravity flow or sucked in during mould close operation. The oil column is pressurized by intensifier and the clamp force is retained by the non return / check valve with out consuming any additional power. The clamp force can be varied and set precisely at desired value by adjusting pressure switch or pressure control valve to suit the requirement of mould. The pressure intensifier has never posed any maintenance problem even after 7-10 years in SP series machines.

RESPONSE TO PEAK PRESSURE IN CAVITY

As the melt is injected (with speed) in to the cavity the cavity pressure steadily increases. At appropriate instant speed phase is terminated and pressure phase takes over. The injected melt expands in the cavity to reach peak pressure. This peak pressure multiplied by the projected area of mould cavity gives the force opposing the set clamp force. The cavity pressure drops as the melt cools in the cavity.

As long as the clamp force is equal to the peak cavity force both types of clamp system behave in the same manner .

ESTIMATION OF CYCLE TIME

Cycle time consists of

• Mould Close & Clamp
• Injection Unit Forward
• Injection stroke (speed)
• Follow-up pressure
• Cooling time, dozing, injection unit retract
• Mould Open
• Ejection

Mould  open/close time can be determined by dry cycle  time. It could be slowed down if the mould has moving/sliding parts. It should be noted that number of dry cycles per hr could be with or without movement of injection unit.

Injection  time can be estimated by dividing shot weight by injection rate. Injection rate could be slower for shear sensitive material, and higher for commodity polymer.

25%  to 35% of cooling time could be considered as  follow-up pressure time.

Cooling time can be estimated by the following formula.

tc = cc x square of (max. wall thickness mm)

cc = 3.5-5.5 for PS

1.9-4 for ABS

3.05-5 for POM

1.4-3.5 for PA

2.0-4 for LDPE

Ejection time could be 3 - 6% of cycle time if the  ejection is automatic.

For thermally sensitive polymer

                         permissible residence time x % utilisation of injection capacity
cycle time <-------------------------------------------------------------------------------------------------------
                                                        K x 100
where K is a constant dependent on screw geometry

There are no simple generalised formula for determining the injection time, injection speed and cooling time. The complex equations for these parameters can be solved by powerful computer with MOULD FLOW and MOULD COOL program. The constant of proportionality will be different at different set up (mould design) for different polymer. Therefore to predict the power requirement and cycle time we should understand the followings.

• Flow ratio (maximum) of part to be moulded.
• Determination of Cavity pressure with the help of flow ratio..
• Relationship of cooling time with maximum wall thickness.
• Relationship of freezing time with minimum wall thickness.
• Relationship of injection rate & time with minimum wall thickness.
• Relationship of power consumed with connected power.

8. Max.flow ratio of

Part

9. Most prevailing wall in thickness (mm)

10. Estimated cavity

pressure (bar)

11. Estimated Clamping

Force (tons)

12. Recommend IMM Model

a) Clamping Unit

b) Injection Unit

13. Screw size m mm

a) Hydromotor

b) Nozzle

c) Pump Power

 14. %age utilisation of injection shot capacity should be such that Residence time < Thermal stability

SUMMARY

The buyers of INJECTION MOULDING MACHINE normally check Clamp force, Platen size, Clearance between Tie bars, Maximum shot weight and maximum daylight. They do not seem to understand significance of Maximum Injection rate.

If one compares on the basis of Maximum Injection Power, then it will be realised that much larger size (clamp force) machine of Japan / Taiwan/ Hongkong make is required to mould the items routinely moulded on SP series machines at present.

Utilisation of shot capacity of the machine depends on cycle time, residence time and the thermal stability of polymer used.

Energy efficiency deals with reduction in wastage of power by using superior hydraulic system.

Controls should be easily understood and more user friendly. It should produce excellent repeatability of parameters. The operation and setting of controls should be well explained in the manual in ENGLISH. Normally manuals from far east countries are not written properly. It is more misleading and confusing. Before buying the machine one should look at their machine manual. They can also provide Video cassette on the controls and trouble shooting.

With imported machines the Guarantee of replacement of part is not advantageous. Any free replacement of parts attract import duty and it will not be possible to clear the urgent replacement parts with out paying substantial amount and hassles of dealing with custom officials. Hence to avoid such situation, vulnerable parts like set of electronic PCBs should be made available along with the machine.