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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.
It is observed that many machine manufacturers in Japan, Taiwan and Hongkong offer larger platen area for a given clamp force but offer low powered injection unit when compared to SP series of DGP Windsor. With such specification, it can not be taken for granted that machine can mould any item whose mould can be installed on the platen of the machine , even though there is enough maximum shot weight available. It means that there is mismatch of platen size and maximum Injection power. Large size of platen becomes superfluous and misleading.
With out realising significance of Injection Power, the buyer tends to compare two different makes of machines on the basis of clamp force alone. This is not correct. The comparison should be on the basis of moulding capability - maximum Injection Power.
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.
Such machines are to be DE- RATED while comparing the price or cost benefit ratio. Maximum Injection Power of the machines under consideration can be computed and its ratio with the existing machines - SP series - maximum Injection Power can be found out. If ratio is lower than 1. then it should be DE- RATED by that factor. It means that lower powered machine can mould largest possible part smaller by that ratio factor.
Many Italian machine manufacturers offer larger maximum injection stroke up to 7 times diameter of screw to increase the maximum shot weight of the machine. As explained earlier, maximum injection stroke can not be more than 4 times diameter of screw for screws of L / D ratio up to 21. If L/D ratio is more than maximum injection stroke can be up to 4.5 times the diameter of screw. The maximum swept volume of the machine with larger injection stroke should be DE- RATED to the value corresponding to 4 times diameter of the screw while comparing with other machines. This is because such larger stroke can not produce uniform and consistent quality of melt resulting in poor consistency of quality of moulded parts.
Utilisation of shot capacity of the machine depends on cycle time, residence time and the thermal stability of polymer used.
DE-RATING of the speeds up to 20% are required if the specification is based on 60 Hz power supply.
Against the advantage of harmonic movement of toggles the following disadvantages of toggles OFF-SETS the balance and HYDRAULIC clamp becomes more favourable.
Energy efficiency deals with reduction in wastage of power by using superior hydraulic system.
The power wastage during idling is reduced by usage of multiple pumps which can be unloaded or loaded depending on the demand for power. The sequencing of loading and unloading is programmed in the software of microprocessor controls. The best energy efficiency is obtained by using variable delivery pump. Thus energy efficiency or less wastage of power depends on hydraulic system of the machine. This calls for evaluating hydraulic system.
The power consumed depends on product of actual pressure (read on pressure gauge and not set pressure) and injection speed. These parameters depend on geometry of part and viscosity of melt. If the connected power is lower than it may not be enough to fill larger parts of thin wall thickness (long flow ratio) even though the machine may have enough shot capacity. The lower connected power does not indicate lower consumption of power. It indicates lower capability for moulding bigger size of parts with large flow ratio. It defines the limitation of machine.
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.
CUSTOMERS NEEDS AND VARIOUS DESIGN OPTION AVAILABLE IN THE MARKET
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CUSTOMER'S |
DESIGN OPTIONS |
REMARKS |
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REQUIREMENTS |
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Easy to set mould. Energy saving in clamp or Convenience of mould setting ? |
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Easy to set process ∓ user friendly control. |
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Scope for minimising sink marks. |
Multi-steps Injection speed settings |
Part design should avoid sink mark |
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Scope for minimising weld line or shift of weld line? |
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Part design should consider the question of weld line position. |
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Scope for shot weight consistency. Scope for precise dosing. |
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Not to use more than 3D stroke for injection. |
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Graphics of process settings |
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Useful only for those who understand it. |
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Sufficient Injection rate |
Flow rate determines flow ratio. i.e size of the possible moldable part. |
High injection rate enables to mould large parts. |
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High plasticising rate |
This decides the minimum cooling time for commodity polymers. |
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Life of non return valve on screw |
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Largest moulding possible |
Usable stroke can not be more than 3D. Largest flow ratio depends on injection rate. |
Maximum shot weight of machine = Swept volume x Density at moulding temperature. |
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Scope for Shortest cycle time. |
Dry cycle per hour indicates the fastest possible mould close / open time. Actual cycle time depends on cooling time which is largest in any cycle time. |
Cooling time depends on design of cooling circuit of mould and flow rate of cooling medium. |
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Open loop or closed loop controls. Open loop can give good results. (Actually it is Part design, mould design which is responsible for quality.) |
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Parameters under closed loop should be listed. |
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Reliability of mechanical parts |
Tie bars / Toggle pins,/ Gear box / Hydraulic monoblock / Non return valve of screw. |
How reliable they are? |
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Reliability of Electrical parts |
Contactors, relays, pressure switch, transducer, |
How reliable they are? |
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Reliability of Electronic systems |
All PCBs, Temperature controllers, |
How reliable they are? |
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Reliability of Hydraulic system ∓ energy efficiency. |
Fixed delivery or variable delivery pump, no. of proportional valves, Cartridge valves, Hydro motor |
How reliable they are? |
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The machine has to be totally leak-free of hydraulic oil. |
This depends on manufacturing system, procedures, Company standards of precision, design features etc. |
How reliable they are? |
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After sales services |
Well trained service engineers with knowledge and experience in mechanical / hydraulic / Electronics should be stationed / available. |
This is very essential and very important. |
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Technical Papers by Prabodh C. Bolur
Technological Solutions for Quality in Injection Moulding of Plastics(1998)
Technological Tools for Part Design, Mould Design ∓ Mould Fabrication.(1999)
Understanding Selection of Injection Moulding Machine.This paper was part of authors lectures at CIPET since 1980. It has been regularly updated
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