Very basic knowledge of injection molding design (Part 2)

• Injection Moulding Process 

With injection moulding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly pushed forward by a screw-type plunger, the plastic is forced into a heated chamber called the barrel where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that seats against the mould sprue bushing, allowing it to enter the mould cavity through a gate and runner system. The mould remains at a set temperature so the plastic can solidify almost as soon as the mould is filled.

• Injection Moulding Cycle 

The sequence of events during the injection moulding of a plastic part is called the injection moulding cycle. The cycle begins when the mould closes, followed by the injection of the polymer into the mould cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mould opens and the part is ejected.

• Different Types of Injection Moulding Processes 

Although most injection moulding processes are covered by the conventional process description above, there are several important moulding variations including: 

· Co-injection(sandwich) moulding

· Fusible(lost, soluble)core injection moulding

· Gas-assisted injection moulding

· In-mould decoration and in mould lamination

· Injection-compression moulding

· Injection moulding of liquid silicone rubber

· Insert and outsert moulding

· Lamellar (micro-layer) injection moulding

· Low-pressure injection moulding

· Microinjection moulding

· Microcellular moulding

· Multi-component injection moulding (over-moulding)

· Multiple live-feed injection moulding

· Powder injection moulding

· Push-Pull injection moulding

· Reaction injection moulding

· Resin transfer moulding

· Rheo-moulding

· Structural foam injection moulding

· Structural reaction injection moulding

· Thin-wall moulding

· Vibration gas injection moulding

· Water assisted injection moulding

· Rubber injection

• Stress 

The main enemy of any injection moulded plastic part is stress. When a plastic resin (which contains long strains of molecules) is melted in preparation for moulding, the molecular bonds are temporarily broken due to the heat and shear force of the extruder, allowing the molecules to flow into the mould. Using pressure, the resin is forced into the mould filling in every feature, crack and crevice of the mould. As the molecules are pushed through each feature, they are forced to bend, turn and distort to form the shape of the part. Turning hard or sharp corners exerts more stress on the molecule than taking gentle turns with generous radii. Abrupt transitions from one feature to another are also difficult for the molecules to fill and form to.

As the material cools and the molecular bonds re-link the resin into its rigid form, these stresses are in effect locked into the part. Part stresses can cause warpage, sink marks, cracking, premature failure and other problems.

While some stresses in an injection moulded part are to be expected, you should design your parts with as much consideration for stress reduction as possible. Some ways to do this are by adding smooth transitions between features and using rounds and fillets in possible high stress areas.

(Wikipedia.org)