Compression moulds. Production and design of compression moulds

Plastic injection molding is the preferred process for manufacturing plastic parts. Injection molding is used to create many things such as electronic housings, containers, bottle caps, automotive interiors, combs, and most other plastic products available today. It is ideal for producing high volumes of plastic parts due to the fact that several parts can be produced in each cycle by using multi-cavity injection molds. Some advantages of injection molding are high tolerance precision, repeatability, large material selection, low labor cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process are expensive upfront tooling investment and process limitations.

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

Co-injection(sandwich)molding
Fusible(lost, soluble)core injection molding
Gas-assisted injection molding
In-mold decoration and in mold lamination
Injection-compression molding
Injection molding of liquid silicone rubber
Insert and outsert molding
Lamellar (microlayer) injection molding
Low-pressure injection molding
Microinjection molding
Microcellular molding
Multicomponent injection molding(overmolding)
Multiple live-feed injection molding
Powder injection molding
Push-Pull injection molding
Reaction injection molding
Resin transfer molding
Rheomolding
Structural foam injection molding
Structural reaction injection molding
Thin-wall molding
Vibration gas injection molding
Water assisted injection molding
Rubber injection

Each injection mold design must have a gate, or an opening that allows the molten plastic to be injected into the cavity of the mold. Gate type, design and location can have effects on the part such as part packing, gate removal or vestige, cosmetic appearance of the part, and part dimensions & warping.

Gate Types
There are two types of gates available for injection molding; manually trimmed and automatically trimmed gates.

Manually Trimmed Gates

These type of gates require an operator to separate the parts from the runners manually after each cycle. Manually trimmed gates are chosen for several reasons:

The gate is too bulky to be automatically sheared by the machine
Shear-sensitive materials such as PVC cannot be exposed to high shear rates
Flow distribution for certain designs that require simultaneous flow distribution across a wide front

Automatically Trimmed Gates

These type of gates incorporate features in the tool to break or shear the gates when the tool opens to eject the part. Automatically trimmed gates are used for several reasons:

Avoiding gate removal as a secondary operation, reducing cost
Maintaining consistent cycle times for all parts
Minimizing gate scars on parts