Producing parts with injection molding requires the creation of a mold of the part that will be produced. Each mold will be custom and some could be quite complex. A mold basically consists of a cavity, a way to feed the molding material into the cavity, a way to cool once material has been injected into it, and a way to eject the part.
Molds have two halves, usually known as “A” and “B” or “front half” and “back half.” Typically, the A-side is the “show side” of the part. So if a part needs to look good on at least one side, it will be the A-side of the mold.
Separation of the mold usually occurs where the A-side and the B-side meet. This meeting point between the halves is often known as the parting line. The “line of draw” is used to describe the direction these halves move relative to the part.
Some features of a part, such as undercuts, will not be in the line of draw, which will impede the ability to eject the part from the mold. When this situation arises, other solutions for ejection are needed, such as a cam or a lifter to remove the part. Cams and lifters add cost to a mold, but they are effective ways of producing undercuts.
Feeding the molding material into the mold cavity is usually done with a feed system that includes a sprue, runner, and gate. (The terms sprue and runner can be used interchangeably.)
The sprue is where the molding material enters the mold. The sprue will move through a series of runner channels, gates, and then into the cavities. The molding material is not always molten as it enters the mold cavity. Depending on the application, some sections of the feed system keep the plastic molten at all times; known as a Hot Runner system. These systems can offer better control of the process and faster cycle times. Applications involving high volume and large parts often use Hot Runner systems.
Many traditional feed systems have the sprue and runner cool; known as Cold Runner systems. Other molding systems are hybrids of these versions.
Once the molten plastic is inside the mold and in the cavities, it needs to cool as quickly as possible. The mold transfers heat from the plastic into a cooling system, which can consist of a series of water or oil channels. These channels help maintain mold temperature and remove heat from the plastic.
For these reasons, even a basic simple mold can be complex to design and build to accommodate the needs of the part and the injection molding system.
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