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 Locatio: Home - Mould Knowledge - Injection Moulds

injection moulding of plastics

 

Typical Products Produced
Injection mouldings count for a significant proportion of all plastics products from micro parts to large components such as bumpers and wheelie bins.  Virtually all sectors of manufacturing use injection moulded parts - the flexibility in size and and shape possible through use of this process have consistently extended the boundaries of design in plastics and enabled significant replacement of traditional materials thanks to light weighting and design freedom.  The table below shows the type of products that can be injection moulded, but is by no means comprehensive.

Power-tool housing             Telephone handsets                  Television Cabinets
Electrical Switches               DVDs                                              Automotive bumpers
Automotive dash boards     Battery                                           Casings Syringes
Drug Inhalation Units           Disposable razors                      Washing-up bowls
Wheelie bins                          Crates/Recycling boxes             Bottle Lids/closures


  
The Process
The animation linked from this page provides a strong visual description of the injection moulding process.

The essential elements are as follows:

Material is introduced into the injection moulding machine via a Hopper.  The injection moulding machine consists of a heated barrel equipped with a reciprocating screw (driven by a hydraulic or electric motor), which feeds the molten polymer into a temperature controlled split mould via a channel system of gates and runners.

The screw melts (plasticises) the polymer, and also acts as a ram during the injection phase. The screw action also provides additional heating by virtue of the shearing action on the polymer.

The polymer is injected into a mould tool that defines the shape of the moulded part. 

The pressure of injection is high, dependant on the material being processed; it can be up to one thousand atmospheres.  Tools tend to be manufactured from steels, (which can be hardened and plated), and Aluminium alloys for increased cutting and hand polishing speeds.  The costs associated with tool manufacture means that injection moulding tends to lend itself to high volume manufacture.  Details of process costing can be found at:

 Click Here for an idea of Processing Costs

The tool can be used to manufacture one consistent part in a repeating process or incorporate multi cavities (a multi impression tool), that is many components can be manufactured on the same tool repeatedly with a single injection. 

It should be noted that, whilst in the animation the flute pitch of the screw is shown as constant along its length, in practice it varies considerably dependent upon the polymer being processed. In particular the root diameter increases from hopper to nozzle to provide compression to the melt.

Variants of the injection moulding process include multi-shot (or 2K moulding) (where different materials are injected into the same mould), insert moulding (where metal components are incorporated), structural foam moulding (where the material is foamed to reduce density) and assisted moulding (where gas or water are incorporated to reduce wall thickness).

Size of Machine

On most machines the mould is held closed by a combination of toggle clamping and hydraulic pressure. This is known as the locking force. In a large machine this can be up to thousands of tons and is spread over the area of the mould. To counteract the injection pressure, a moulding with a large surface area will therefore need a machine with a higher locking force than a moulding with smaller surface area.

Computer Controlled

Modern day injection moulding machines are controlled by a built-in computer. Acting on sensor fed information, it controls all the actions of the machine and ensures consistent output and shot to shot quality.