Plastic Injection Mold Design

Plastic Injection Mold Design

Every mold we make is different from top to bottom, and each choice we make in designing it affects its capability, cost, life cycle, and more. Here are some of the things we consider before designing the mold that will make a new product.

The cavitation of a mold is the number of cavities, or spaces that create the molded object, in the tool. It's measured in shot quantity, meaning the number of plastic parts produced each time the mold is run. It's different for every product we make, and here's how we figure it out. We divide the number of molded pieces needed each year by the shot quantity, or number of pieces per mold, and an estimated cycle time of the tool to get a cost for each piece. Then we change the shot quantity variable, or cavitation, in that equation to find the perfect balance of production capability, cost, and speed for that product.

Mold material
What will we use to make the mold? The biggest determinant is it's needed longevity, or the life of the tool. For short-run or prototype tooling (doesn't need to last as long), we can use cost-saving aluminum plates. If a product is going to be made in our machines for a longer period of time and in greater numbers, we'll use costlier - but much harder - tool steels.

Shot material
We make products out of many different plastics, and we have to know what material is going in before we build any mold. Different molding materials have different properties and limitations, like part shrinkage, required wall thicknesses, piece-per-part costs, and more. If the material is changed after the mold is made, we'll most likely have to rebuild the tool to get it right.

Mold base size
With a huge number of mold bases available, we determine the best for each product - taking cavitation, minimum stack height (the size of the mold when fully closed), cooling line placement, and the press or presses in which it will run into careful consideration.

Hot runner systems
The hot runner system is what delivers the molten plastic into the cavities in our mold. It consists of a heated runner, or manifold, and a gate that opens and closes. Hot runner is contained in one half of the mold tool, and allows us to mold many shots with one runner. For comparison, a cold runner mold simply has a channel formed between the two mold halves that delivers plastic to the cavities. With a cold runner, each time the mold opens to eject parts, material in the runner is also ejected.

Since a hot runner system is integral to the tool design, we make the determination to use one right away. The are two great advantages to choosing a hot runner - decreased material costs with no runners to scrap, and shortened cycle times without waiting for runners to cool. For each individual tool we create, we'll weigh these against the increased cost and complications and determine what's best.