There are several custom injection molding methods used to manufacture molded plastic products. Traditional injection molding injects the liquid plastic into a mold to create the product. Multi-shot molding and over-molding are variations on the basic process that are used to create different products.
With injection molding, melted plastic is injected into a mold via a machine. When the material cools, it solidifies as one solid part that is then ejected by the machine. Injection molding is easily repeatable with the same results each time once it’s set up properly. This makes it a great choice for mass-produced plastic products. However, it is often used to create prototypes or small batch orders as well.
Multi-shot molding is often used in high volume production when two or materials need to be used with one mold. The process may use the same material of different colors, or different materials. One material is shot into the mold, followed by another. The materials create a molecular bond. The product is then cooled and ejected as one piece.
Multi-shot molding is often seen as a subtype of over-molding, since one material is often molded over another during the process. However, anytime two or more materials are injected into the same mold, it is considered multi-shot molding. Over molding only occurs when one substance partially or completely covers another.
Multi-shot molding is cost-effective. It can eliminate a step from the manufacturing process, which saves production time and labor costs. It also results in reduced waste.
It is often used to create:
- a soft grip
- molded seals and gaskets
- buttons or hinges
- multi-color products
With over-molding, the base material is called the substrate. The substrate can be made of many materials, including metal and plastic. The substrate is placed in the mold, then the piece is injection molded so that the material covers the substrate. This causes the two materials to bond, similar to multi-shot molding.
Over molding can be performed with different combinations of materials, including:
- Plastic over plastic
- Rubber over plastic
- Rubber over metal
- Plastic over metal
One of the most common uses of over-molding is creating soft and comfortable handgrips. Common items produced with this process include tools like screwdrivers, household items like scissors, and toiletries like razors.
The benefits of over-molding include:
- Shortened production time and associated labor costs
- Greater durability and strength
- Design flexibility
Whether you choose traditional injection molding, multi-shot molding, or over-molding will depend largely on the product you wish to create. They all offer reduced production costs, reduced waste, and higher product quality than other production methods.