Injection moulding

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, 0.02–1 MPa; high pressure die casting, 3–200 MPa).
2. Machines are classified according to the locking force of the platens and can vary from 1–3000 tonnes. Most machines lie in the range of 50–250 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.
   
   
    
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 95–99%. although the process is only 25–40% 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 5–50 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 casting, Vacuum forming, Compression moulding and Superplastic forming.