A manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed & forced into a mold cavity where it cools and hardens to the configuration of the mold cavity. Injection molding is widely used for manufacturing a variety of parts, from the smallest component like small Lego accessories, to entire body panels of cars.
Injection molding is used to create many things such as wire spools, packaging, electrical connectors, bottle caps, pocket combs, and most other plastic products available today. Injection molding is the most common method of manufacturing parts. It is ideal for producing high volumes of the same object.
Some advantages of injection molding are high production rates, repeatable high tolerances, the ability to use a wide range of materials, low labor cost, minimal scrap losses & little need to finish parts after molding.
- Utilizes a screw-type plunger to force molten plastic material into a mold cavity
- Produces a solid or open-ended shape that has conformed to the contour of the mold
- Uses thermoplastic or thermoset materials
- Produces apartitng line, sprue & gate marks
- Ejector pin marks are usually present
Tolerances & 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 & maximum limit on thickness, based on the process used. Surface finishes of two to four microinches or better can be obtained. Rough or pebbled surfaces are also possible.
Materials that are typically manufactured through compression molding include: Polyester fiberglass resin systems (SMC/BMC), DAP (short and long glass-filled), Epoxy (short & long glass-filled), & many grades of Phenolic.
In compression molding there are six important considerations that an engineer should keep in mind:
- Determining the proper amount of material.
- Determining the minimum amount of energy required to heat the material.
- Determining the minimum time required to heat the material.
- Determining the appropriate heating technique.
- Predicting the required force, to ensure that shot attains the proper shape.
- Designing the mold for rapid cooling after the material has been compressed into the mold
The use of thermoset plastic compounds characterizes this molding process from many of the other molding processes. These thermosets can be in either preform or granule shapes. Unlike some of the other processes, the materials are usually preheated & measured before molding. Inserts, usually metallic threaded studs or bushings, can also be molded with the plastic.
Transfer molding, like compression molding, is a process where the amount of molding material (thermoset plastic) is measured and inserted before the molding takes place. The molding material is preheated & loaded into a chamber known as the pot. A plunger is then used to force the material from the pot through channels known as a sprue and runner system into the mold cavities.
The mold remains closed as the material is inserted and is opened to release the part from the sprue and runner. The mold walls are heated to a temperature above the melting point of the mold material; this allows a faster flow of material through the cavities. The molds in both compression & transfer molding remain closed until the curing reaction within the material is complete. Ejector pins are usually incorporated into the design of the molding tool and are used to push the part from the mold once it has hardened. These types of molding are ideal for high production runs as they have short production cycles. Transfer molding, unlike compression molding uses a closed mold, so smaller tolerances and more intricate parts can be achieved. The fixed cost of the tooling in transfer molding is greater than in compression molding and as both methods produce waste material, whether it be flash or the material remaining in the sprue & runners, transfer molding is the more expensive process.
Transfer molding differs from compression molding in that in Transfer process the resin is inserted into the “closed” clamped mold (or tool) and the resin pressed into place by a transfer ram (plunger). In compression molding the compounds are placed into an “open” mold which is then closed to force the material into the cavity.