How to select a plastic injection molding machine.


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Buying a PIMM is not a small investment. Too much machine for the job at hand is wasteful. Too little machine does not get job done. Careful matching of the jobs needs and the attributes of a PIMM is well worth the effort.

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How to select a plastic injection molding machine.

  1. 1. How to select a pimm. Naik devang 12/01/2012
  2. 2. introduction • Buying a PIMM is not a small investment. Too much machine for the job at hand is wasteful. Too little machine does not get job done. Careful matching of the jobs needs and the attributes of a PIMM is well worth the effort.
  3. 3. sHot weigHt • The shot weight is measured weight of the plastic injected when the nozzle is free standing. the plastic used is usually polystyrene with a specific gravity (SG) of 1.05. this is specified in the specification as PS. • Shot weight in terms of a resin= c*b/1.05 where b=SG of resign & c = shot wt in terms of PS.
  4. 4. selecting macHine • A PIMM of a specified shot weight can be used to mold articles including the runner weighing from 35% to 85 % of the shot weight. The lower limits comes from bending on the platens, barrel resident time of the resign and electric power consumption per kg of the processed material. • BRT=wt of melt in barrel*ct/actual shot wt.
  5. 5. clamping force • Clamping force is an important attribute of the clamping unit of a PIMM. It is the maximum force the machine is capable of to keep the mold close against cavity pressure during injection. Insufficient clamping force gives rise to flash at the mold joint.
  6. 6. GPPS cup has flow path length of 104mm. Flow path to thickness ratio=104/0.6=173.from fig at 0.6mm wall thickness the CP is550bar. From conversion table 1bar=1.02kg/cm2. the CF=550*1.02*49=27500kg=27.5tonnes • CP as a function of wall thickness and flow path length
  7. 7. screw diameter • L/D ratio. A high L/D ratio of 22:1 or above provides better mixing and more uniform heating due to compression in transition section of the screw. • A medium L/D ratio of 20:1 is used for general application with medium requirement. • A low L/D ratio of 18:1 is used for low requirement where shot weight is more dominant selection criterion. The injection
  8. 8. InjectIon pressure • Injection pressure means the maximum pressure in the barrel during injection, not the max hydraulic pr. The two are related by the ratio of the screw cross section area of the injection cylinder area. Usually, injection pr. Is higher than the max hydraulic pr by about 10 times.
  9. 9. InjectIon speed • Inj speed is the max speed of the screw the m/c is capable of during inj. • Inj speed effects the inj time. Moulding thin wall articles require high inj speed so that melt does not solidify before the cavity is completely filled. • Machine should have multiply
  10. 10. accumulator • It is a option to increase inj speed. An accumulator is an energy storing device that stores up pressurized hydraulic oil in a phase of low demand to be used in inj ( high demand) phase. While increasing the electric motor and hydraulic pump sizes does increase the inj speed by 25% an accumulator does
  11. 11. InjectIon rate • As an alternative to inj speed some m/c use inj rate. Inj rate is the max volume swept out by the screw per sec during inj. It is expressed in cm 3 /s. • Inj rate =inj speed*3.14*(d/2) 2 . where d=screw dia in cm. • Note that inj speed is not independent of screw dia, but inj rate is.
  12. 12. screw surface speed • Screw rotary speed is specified as a range in rpm. Screw rotary speed by itself is not critical as screw surface speed. The two are related by screw dia. • SSS=0.052*screw dia*rpm. • Each plastic material has a recommended maximum sss which must not exceeded. For example PP and HDPE should not experience a sss of higher than 800 mm/s .
  13. 13. plastIcIzIng capacIty • It is the amount of PS that a PIMM can uniformly plasticize in one hour at max screw rotary speed with 0 back pr. • To check if the plasticizing capacity of the PIMM is not being exceeded calculate the wt of the component and Sprue per shot W 9g) divided by screw rotation time t (s) and convert the quotient to kg/hour. • W*3600(t*1000) this must be less
  14. 14. plastIcIzIng capacIty • Since ct is longer than screw rotation time the shot wt S (g) of a m/c and its plasticizing capacity G (kg/hr) set a lower limit on cycle time t min (s) as follows. • T min =S*3600(G*1000). • It is particularly important to match shot wt and plasticizing capacity in the case of fast cycling m/cs producing thin walled or close tolerance
  15. 15. Mold opening stroke • It is the displacement of the moving platen from mold close to mold open. Mold opening stroke determines the max height H of the mold part the m/c is capable of. The relationship is • Mold opening stroke>=2H+sprue length L • In hot runner system L=0
  16. 16. Mold height (thickness ) • Mold height is left over from the days when press are vertical. In horizontal press, a more appropriate description is mold thickness. • In toggle clam PIMM specification, mold height is expressed as a range, from the minimum to maximum to the height the machine could accommodate. The difference is
  17. 17. MaxiMuM daylight • The maximum opening between the fixed and moving platens when the clamp is wide open. It is related to mold opening stroke and minimum/maximum mold height as follows. • For a toggle clamp m/c. • Max daylight=mold opening stroke + max mold ht. • For direct hydraulic clamp machine • Max daylight=mold opening stroke
  18. 18. space between tie bar • Mold width must fit within the horizontal space between tie bars if the mold is lowered from above. The mold length must fit within the vertical space between tie bars if the mold is slit in from the side. It is advise that there is a clearance of 25mm on each side for small mold and 50 mm for big mold.
  19. 19. tie bar diaMeter • If the tie bar tension are even, the stress in each of them is given by • Stress= clamping force +1000/ (3.14* d 2 ) where stress is in kg/mm 2 clamping force is in tones and dia d is in mm. • High tensile has a breaking stress of more than 90kg/mm 2 mild steel 20/ kg 2 . • Non parcel mold faces, onsymmetrical cavity with respect to Sprue and maladjustment of mold height mechanism of a toggle clamp m/c.
  20. 20. platen thickness • The moving platen and fixed platen must have sufficient stiffness to transmit the force of tie bar to the mold with minimum deflection. For a given geometry a flat platens deflection is proportional to the cube of its thickness. Especially for the moving platen, a compromise has to be struck between wt and
  21. 21. platen deflection • ribbed stationary platen
  22. 22. Dry cycle time • It is the mold closing time plus mold opening time plus ideal time. Dry cycle time is the ultimate cycle time as there is no cooling period. • Running a m/c at max possible ct is not desirable if the m/c is not running smooth and stable.
  23. 23. electric motor ratting • The current per phase drawn by a three phase motor at its rated power is • i m (A)=motor power rating (kw) * 1000/ (3*single phase power voltage (V) *efficiency * power factor) • For most three phase motors efficiency=0.88to 0.91 and power factor=0.84 to 0.88
  24. 24. Power DemanD During the molDing cycle
  25. 25. electric heater rating • Electric band heater along the barrel provides heat up to the resin at start up. It also supplements the heating by plasticizing (when screw rotates) during the molding cycle. A higher rating per heater has the advantage of shorting the initial heat up time. • i h (A) = electric heater rating (kw) * 1000/ (3* single phase voltage (v)
  26. 26. SyStem PreSSure • The most common hydraulic system pressure used in PIMM is 140 bars, which apx equals to 140 kg/cm2. this is limited by vane pump. • A higher system pr. Of 170 bar to 240 bar are used with piston pump which demands cleaner oil to work with.
  27. 27. Final concluSion Select Pimm DePenDing on article to be molDeD. • Thank you • naik devang