The quality and precision of an injection molded part rely heavily on the part design and the quality of the mold. A well-designed part, made in a subpar mold, will produce a part that won’t consistently meet quality specifications, if at all. Conversely, a well-designed part in a well-maintained, quality mold will continuously deliver parts that meet or exceed expectations. Below we break it down, explaining all the parts of an injection mold.
The Many Parts of an Injection Mold
The mold is where the part is formed in the injection molding process. The complexity of the mold will depend on the part. Simple parts will have a straight pull injection mold, while more complex parts with undercuts may require a side-action cam. Undercuts prevent the mold from cleanly pulling apart. A cam device in the mold pulls side action surfaces away from the undercut feature so the part can be removed.
A plastic injection mold comprises several parts, each serving a specific purpose.
Here are the primary parts of an injection mold and their roles.
- Mold Base – This is the main structure holding all other parts of the mold. It’s essentially a shell that provides support and stability for the whole assembly.
- Cavity and Core – These form the actual shape of the plastic part. The cavity is the hollowed-out part of the mold that receives the molten plastic. The core is the part that shapes the inside of the molded plastic part.
- Sprue – This is the channel through which the molten plastic flows from the injection machine into the mold. It connects to the runners.
- Runner – It’s a system of channels that distribute the molten plastic from the sprue to the cavities.
- Gate – This small opening allows the molten plastic to enter the cavity from the runner. The gate’s size, type, and location can significantly affect the quality of the final product.
- Ejector Pins – These rods, which are mounted in an ejector plate, push the cooled, solidified plastic part out of the mold after it’s been formed. The ejection system is essential for quick and efficient production.
- Venting – This is a feature designed into the mold to allow trapped air and gases to escape when the molten plastic fills the mold. Without proper venting, the final part can have defects.
- Cooling Channels – These are passages in the mold base through which a cooling medium (usually water) is circulated to control the mold temperature and help the plastic part cool and solidify faster.
- Guide Pins and Bushings – These ensure that the two halves of the mold correctly align when they close. This helps maintain accuracy and precision in the final product.
- Mold Plate – These are the parts of the mold that move and interact with the molding machine to operate the mold. They often include the ejector plate, which moves to eject the part out of the mold.
- Interchangeable insert – a removable part containing the cavity and core that can allow different but similar parts to be produced from a single mold body. These are better suited for low- to medium-volume parts.
Each part is critical in ensuring the plastic injection molding process is successful and the final product is high quality.
More About Molds
Molds can be built using several common grades of metals, including aluminum, soft and semi-hardened steel, and hardened steel. When the metal is chosen, the product development stage and validation, the cavity number, the part size, and the number of shots the mold is expected to produce will all be considered.
Molds are designated by SPI class, standardized by the Plastics Industry Association, and define the physical tool construction and the mold’s life expectancy. The mold classes range from Class 101, the most robust, through Class 105, intended for prototype parts only. Your injection molder will choose the most appropriate mold to optimize cost and efficiency.
Molds are classified as cold runner or hot runner molds. Which is used is dependent on part geometry, expected part quality, and volume requirements. As mentioned above, the runner is the channel in the mold that distributes the molten plastic.
These channels are not heated in a cold runner, so the plastic in them solidifies attached to the part. In a two-plate injection mold, an additional step is required to remove the runner from the part. In a three-plate mold, the runner separates from the part when the mold opens. Conversely, these channels are heated in a hot runner, and the plastic remains molten.
Although hot runners have reduced cycle time, lower part manufacturing costs, and high-quality parts, the molds are more expensive to manufacture and maintain. Cold runners have less expensive molds, and changing materials and colors for the parts are easier.
Exceptional Molds Create Exceptional Parts
With our own tool and die shop, we have extensive experience with new builds, insert changes, modifications, maintenance, and repairs. We can build molds in-house or source them overseas to ensure the best possible price and quality.
Contact us with your next plastic injection molded project, or talk to us about an existing challenge. Learn how working with SEA_LECT plastics can make a difference.