Injection Molding

What Is Injection Molding?

Injection molding (British English: moulding) is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity. After a product is designed, usually by an industrial, mechanical designer or an engineer, molds are made by a mold maker (or toolmaker) from metal, usually either or aluminum, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars.

Process overview

• • Utilizes a ram or screw-type plunger to force molten plastic material into a mold cavity

  • Produces a solid or open-ended shape which has conformed to the contour of the mold

  • Uses thermoplastic or thermoset materials

  • Produces a parting line, sprue, and gate marks

  • Ejector pin marks are usually present

Injection Molding Process Types

Injection Molding Cycle: The sequence of events during the injection mold of a plastic part is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold 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 mold opens and the part is ejected.

Different types of injection molding processes: 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

  • Insert and outsert molding

  • Lamellar (microlayer) injection molding

  • Low-pressure injection molding

  • Metal 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

Injection Molding Mechanical Tolerances and Surfaces:

Molding tolerance is a specified allowance on the deviation in parameters such as dimensions, weights, shapes, or angles, etc. To maximize control in setting tolerances there is usually a minimum and maximum limit on thickness, based on the process used.

Injection molding typically is capable of tolerances equivalent to an IT Grade of about 9-14. The possible tolerance of a thermoplastic or a thermoset is +- 0.008 to +- 0.002 inches. Surface finishes of two to four microinches or better are can be obtained. Rough or pebbled surfaces are also possible.

Injection Molding Geometric / Geometry Possibilities

The most commonly used plastic molding process, injection molding, is used to create a large variety of products with different shapes and sizes. Most importantly, they can create products with complex geometry that many other processes cannot. There are a few precautions when designing something that will be made using this process to reduce the risk of weak spots. First, streamline your product or keep the thickness relatively uniform. Second, try and keep your product between 2 to 20 inches.

The size of a part will depend on a number of factors (material, wall thickness, shape,process etc). The initial raw material required may be measured in the form of granules, pellets or powders. Here are some ranges of the sizes.

Design Considerations for the Process: It is important when designing products for injection molding that you consider how they will be formed in the machine, how they will be taken out of the machine, and the structure of the final product. Some important guidelines are:

Use approximately uniform wall thickness throughout your designs.

Keep walls thin - typically between 132" and 110". This allows for proper cooling and reduces cost by minimizing use of material. Thin walls also reduce problems with material shrinkage. Although some unevenness will occur due to shrinkage, walls as thick as 15" can be used. Keep wall thickness at least wall length 50. Keep 90 deg walls under 0.25" high. Keep thickness of ejection pin surface wall at least .07".

To strengthen parts, instead of using thicker walls, use additional structures such as ribs. Use fillets at the base of ribs.

When using a rib make it about half the main wall thickness.

Round corners and edges wherever possible.

For easy release of the part from the mold, add a slight taper to the sides (typically ~ 2 deg) - especially for textured walls and walls higher than 0.25".

Avoid undercuts that are impossible to remove from the mold.

Lighter colors hide flow patterns better than dark colors.

Where walls meet at a 90 angle, round inside and outside to at least .05" radius - sharper outside corners can create molding problems and sharper inside corners will increase tooling cost.

Keep holes at least .015" from edges.