The Plastic Injection Mold Cavity
The mold is where parts are actually made during the plastic injection process. Every mold is custom-made for a specific part.
A plastic injection mold consists of:
One or more mold cavities
Gate(s)
Sprue
Runner system
Cooling system (if thermoplastics are used)
Ejector system.
Using the above elements, the mold performs the following steps in order to make parts:
Channels melted polymer into the cavities
Shapes the part
Cools the part
Ejects the finished product
More About Runners….
Symmetric vs. Asymmetric Runner Systems
This runner system is symmetric. All cavities are filled at the same time, and will have the same filling pattern. However, this symmetric runner system has three problems: a) a long flow path, b.) a high rate of material consumption, and b.) high pressure requirements.
This runner system is asymmetric. Some cavities will be filled more quickly than others. While this runner arrangement uses less material and has lower pressure requirements, it may produce parts of varying quality. (This problem may be countermeasured by varying the lengths of the runner diameters, so that the cavities farthest away from the gate are fed by larger flows.)
Hot and Cold Runner Systems
Runner systems are categorized as either “hot” or “cold.” Cold runners are ejected with the part and trimmed off after the molding process. The chief advantage of cold runners is that they facilitate lower mold costs.
Hot runners maintain the molten polymer at melt temperature. The polymer stays in the runners, and is injected into the cavity during the next cycle.
Hot runner systems increase the cost of a mold, but this may be offset by two advantages: a.) the elimination of a trim process, and b.) lower injection pressure requirements.
Gate Design and Trimming Requirements
The design of the gate is a major determinant of post-molding trim requirements. If a direct (sprue) gate is used--- as is often the case with large parts---- significant trimming and surface finishing operations might be required. By contrast, a pin gate is easily detached from the part--- it leaves only a small trace that may not require any trimming/finishing operation.
(For more information about gates, see the Gating Design and Gate Types pages.)