Injection Moulding

Description

Injection Moulding is the most commonly used manufacturing process for the production of a great variety plastics products and components (inc. parts for plastics model kits for example)

In industry, plastics granules are drawn into an Injection Moulder cylinder, or barrel, via a rotating screw to a point where they are heated and melted. The screw then operates as a piston and forces the melted plastics into a mould to create the products as shown:

Reciprocating screw;
Hopper;
Granules;
Barrel;
Heaters;
Mould.

As a consequence, most injection moulded products can be identified by the presence of a small ‘pip’ or sprue where the melted plastics entered the mould.

Modelling Injection Moulding

The key principles of injection moulding to be demonstrated using a Glue Gun and simply constructed moulds made from MDF or a plastics material such as Tufset Polyurethane. This plastics machines well and can withstand the temperatures involved in the process. In either case, provision needs to be made to ensure the design of the finished product will release from the mould and, in addition, the mould may need coating with a realease agent (e.g. using Silicone Grease spray or by heavily shading over the entire mould surface with very soft pencils to get a Graphite coating).

As an example, the billet shown to make the mould (uncut material we start with) is 100mm wide, 50mm high and 10mm deep. It has 6mm diameter mounting holes along the centreline 82mm apart to allow it quickly to be mounted on a CNC machine and any other injection moulding equipment.

Glue enters the mould as shown in the diagram, and air escapes through a breather hole. The design must allow glue to flow easily to all areas and allow hot air to escape through the breather. The design should not go too close to the mounting holes, as the cutting tool might foul on the bolts which are holding the workpiece to the milling machine. There should be no narrow passages which would constrict the flow of the glue.

In practice, the width of passage which can be cut is limited by the smallest tool available on the CNC machine, in this case 2mm diameter. You should be aware that the CNC machine cannot cut sharp internal corners - the diameter of the radius in the corner again matches that of the smallest tool. Sharp external corners, however, are possible.

Safety Point! Working with hot plastics is always hazardous. Avoid fumes, wear personal protection and ensure the mould is clamped firmly together to prevent any leakage.