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Injection moulding

Updated Monday 23rd October 2017

A process where polymer pellets are melted, then hydraulically injected into a mould to solidify into 3D thermoplastic components.

Diagram of electric motor, pump, valve mechanism Copyright free image Icon Copyright free: OU

Manufacture:

  1. Process normally operates at pressures up to 200 MPa, although “high pressure” injection moulding machines, operating at 500 MPa, are currently under investigation (cf. low pressure die casting, 0.021 MPa; high pressure die casting, 3200 MPa).
  2. Machines are classified according to the locking force of the platens and can vary from 13000 tonnes. Most machines lie in the range of 50250 tonnes. The higher the locking force, the larger the projected area of the component that can be produced. Locking can be either mechanical or hydraulic, although mechanical locking is considered to give faster cycle times.
  3. A typical injection moulding cycle is shown below. It has been claimed that by reducing the floor cycle time from 25 to 24 s, 2 weeks of production time per year can be saved.

    Diagram of an injection moulding cycle Copyrighted image Icon Copyright: OU
     

  4. Cycle times for reinforced thermosets are usually longer than for thermopolymers, although 30% better than in compression moulding and result in less porosity.
  5. Yields are high, varying from 9599%. although the process is only 2540% energy efficient.
  6. Due to the high capital cost of machines and dies, the process is only suitable for long production runs.

Materials:

  1. Thermopolymers are the most commonly used materials although the process is being increasingly used on thermosets (such as polyesters and melamines), rubbers, reinforced polymers (glass and carbon fibre) and to produce both structural and sandwich foams.
  2. Material is in the form of powder or granules (thermopolymers), strips (rubbers) and dough moulding compounds (DMC) for reinforced thermosets. Fibre lengths in DMC vary from 550 mm depending on the type of DMC, and the injection moulding method.

     

    Polymer

    Moulding temperature

    To (°C)

    Freeze-off temperature

    TF (°C)

    Freeze-off temperature

    TE (°C)

    Demoulding temperature

    TD (°C)

    LDPE 220 90 50 60
    PVC 170 140 50 110
    PP 250 135 50 105
    POM 220 135 50 105
    PS 230 130 50 100
    ABS 230 140 50 110
    PMMA 260 160 50 130
    PPO 280 180 50 150
    PC 310 200 50 170
    Nylon 66 290 240 50 210
    PES 360 250 70 220
    PEEK 390 310 90 280

Design:

  1. Features such as moulded-in screw threads and undercuts (particularly in the draw-direction) should be avoided if at all possible, since they involve metal inserts or side-cores, which add significantly to the mould costs.
  2. Sprues should have a taper of 10°. There should be draw angles of 2.5° on the sides of the mouldings.
  3. Allowances must be made for material shrinkage, which is greatest with crystalline polymers.
     
    Polymer Linear shrinkage (%)

    Amorphous polymers

     

    PVC

    0.2 - 0.5

    PS

    0.4 - 0.7

    ABS

    0.4 - 0.8

    PMMA

    0.4 - 0.8

    PPO

    0.5 - 0.7

    PC

    0.5 - 0.7

    PES

    ~ 0.7

    Crystalline polymers

     

    LDPE

    1.5 - 2.5

    HDPE

    1.5 - 2.5

    PP

    1.0 - 2.0

    POM

    ~ 2.0

    Nylon 66

    0.8 - 1.5

    PEEK ~ 1.0

     

  4. Components produced by injection moulding vary widely, from spoons, to hollow carbon reinforced tennis rackets, to small sailing dinghies with a projected surface area of more than 3.5 m2.

See Also: Low pressure die casting and High pressure die castingVacuum forming, Compression moulding and Superplastic forming.

This article is a part of Manupedia, a collection of information about some of the processes used to convert materials into useful objects.

 

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