The Basics Of An Injection Mold ~ Where Recycled Plastics Become New Products

Many who are environmentally conscious practice recycling to support better use of our resources. Plastics are one of those frequently recycled products. You may wonder what actually happens to all those jugs, bottles and plastic containers you tote to the curb in your recycle tub. 

One way that waste plastic is recycled back into circulation is through melting it down to produce new products. That process is called injection molding. To help you better appreciate recycling your plastic waste, here are the basics of the injection molding process.

Two Halves to a Mold

Injection molds are made of two metal plates, each making up one-half of a plastic product. These are separated into two pieces at a parting line. The A and B plates make up either the top and bottom of a product or two halves of a solid item. The parting line is important because this is where the mold is broken apart once the plastic has hardened into a new toy, tool, or other plastic product.  

The Injection Mold Process

Fluid plastic resin is poured into the injection molds through a small opening called a sprue. The sprue is located in the A plate.

From the A plate the fluid resin moves throughout the mold through runners. Runners are like little tunnels that distribute the plastic evenly to the different part of the mold.

Within each of these cavities the resin flows around wall-like barriers, or protrusions called cores. The shape of these cores is what makes the shape of the final product.

The resin injected into the sprue, through the runners to fill up the cavities in the injection mold is called a shot. The amount of fluid plastic resin in each shot will equal the amount of final plastic that makes up whatever the new product is going to be.

If there is a hole or another type of opening in the part, there is a block in the cavity to stop the flow of resin. This concept is also used to separate two distinct cavities in the final plastic product.

Air pockets, which are not specific aspects of the product design to produce holes or gaps in the finished item, leak out slowly through little gaps in the plates called vents. While each side of the part is distinct, with a unique shape and specific holes or cavities, the parting line is placed in the best place on the product to simplify the release from the mold. This line is often very evident on household plastic products, along with a hidden little vent hole where the excess air was released.

Ejector pins are another way to remove the final products from the mold. These are usually in the B plate side of the item. Ejector pins are almost always used for any half of the product that moves. These can be spring loaded to essentially pop the bottom half of the mold out of the B plate.

While all this is pretty easy to visualize when you're thinking about small plastic items like toys, tools, or other household things, there are massive injection molds that produce plastic products so large they need a forklift or crane to maneuver. But, even the largest plastic products use the same basic concept of injection molding we covered here. Only real difference is the size of every component and the amount of plastic resin required in the process.

So, from things as little as a plastic cap for your writing pen to huge wooden planks that are positioned to make docks in a shipyard, injection molds are used to produce all of them. Helps to fuel the motivation to recycle your plastics when you know how understand how vital this waste can be in the production of new things.