what is the plastic injection molding process

injection molding process

Classification of injection molding machines

1.the injection molding machine processing ability

style clamping force(KN) theoretical injection volume(cm3)
mini < 160 <16
small 160〜2000 16〜630
medium 2500〜4000 800〜3150
large 5000〜12500 4000〜10000
super large >16000 >16000

2.the profile characteristics of machine

  • Horizontal injection molding machine: mold clamping device and the injection device is in a horizontal line arrangement.
    Advantages:not high, easy operation and maintenance, high degree automation, stable installment.
    Disadvantages: cover large area
    At present, most of the injection molding machine adopts Horizontal form.
  • Vertical injection molding machine: the arrangement of mold clamping device and injection components  is vertical.
    Advantages: cover small area, easy assembly and disassembly, easy installation, plasticization can be more uniform.
    Disadvantages: the molded product is not easy to automatically remove from the injection machine but need human force, difficult full automation and hard for large parts; high machine, difficult  feeding and maintenance. Shot volume of this type of injection machine is generally below the 60 cm3. (small)

3.Classification by plasticization

plunger style screw style
plasticization the limited plasticizing capacity screw rotation to increase the plasticizing capacity
plasticizing uniform only pushing drive melt, the laminar flow, impossible to get good uniform state good stirring and mixing,fairly plasticizing uniform
Injection pressure due to pressure loss on shunt cone, injection pressure must be  2 ~ 3 times higher than that of screw type small pressure loss, injection pressure just 30% ~ 50% of plunger style, produce larger molding products
Melt retention much on the shunt cone and barrel wall less
Color ring couldn’t be used quick color change
part quality easy deformation,unstable quality stable quality

Structure of screw plastic injection molding machine

injection molding machine-1


  • injection system: Composition: hopper, barrel, heater, metering device, screw (the plunger  for plunger type injection machine), drive device, nozzle and other components.Role: make the polymer pellet or powder  uniform plasticizing and to be molten state , also push the molten into the mold cavity.
  • closing system:Composition: platens, pull rod, mold closing mechanism, ejection device and safety door, etc.. Role:To achieve the mold opening and closing;Provide sufficient clamping force to make the mold close tightly;To eject molded parts after mold opening.
  • hydraulic system:Provide pressure and speed loop for injection molding machine.
  • control system:control  the injection molding machine procedures and actions, as well as the control  the process variables including time, position, pressure, speed and etc…

Injection molding machine main technical parameters

  • injection device parameters
    The specified amount of injection molding (maximum injection volume): injection molding machine can achieve maximum ejection volume to shot in air(polystyrene as the standard)
  • locking parameters:The clamping force, the daylight of the molding machine, the maximum and minimum thickness of the mold.
  • injection pressure


How to select injection molding machine

  • injection volume
    the theoretical injection volume by injection machine each time


V=The maximum theoretical injection volume (cm3)
D= screw diameter, cm;
S=Maximum injection molding travel ,cm

  • Nominal injection volume: In the injection molding process, due to the change of temperature and pressure, the density of plastic will change, plus with the material leakage, the maximum injection capacity of the injection molding machine(Nominal injection capacity)


V=The maximum theoretical injection volume (cm3)
D= screw diameter, cm;
S=Maximum injection molding travel ,cm
K=Injection coefficient(crystallized polymers:0.85;Non-crystallized polymers:0.93)

  • The actual injection shot should be within 80% of the nominal injection volume


MS  =plastic molding weight(including runner)
M=Nominal injection volume weight


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