Plastic injection molding is a commonly used type of molding that is great for the mass production of small and large parts. Injection molding can use various plastic types. But how exactly does this process work, and what products should be used to help make it more efficient?
Today’s blog post will answer these questions as we go through the four phases that make up the total cycle time for injection molding. By using injection molding molds and the right molding products, manufacturers can quickly create thousands of parts. Talk to our experts if you have more questions about the process or need help deciding which products would be best for your operation!
Injection molding molds always come in at least two parts. The first stage of this process is applying your mold release agent and clamping the two halves together using a clamping unit. Both halves are then attached to the injection molding machine. The plastic is injected into the mold while the clamping unit holds both halves of the mold tightly together. Larger molding machines take longer to close and clamp the mold.
Depending on the type of mold release agent you use and the type of plastic materials you’re molding, you may not have to reapply mold release between every cycle. For example, if you use sacrificial mold releases, you will have to reapply mold release. Sacrificial or conventional mold releases do not bond to the mold surface itself. Therefore, these mold releases must be reapplied between each molding cycle.
However, if you choose a semi-permanent mold release, all you generally need is a thin film coating on the mold surface, and it can last between four and 30 cycles. For thermoplastic injection molding, we have silicone-based mold releases and paintable mold releases, which decrease the amount of time between molding and post-production processes. Not sure which mold release would be right for your operation? Talk to one of our experts today!
While the two halves are clamped together, small plastic pellets are fed into a hopper of the injection molding machine. A corkscrew-like barrel rotates and melts the plastic pellets as they move toward the end of the injection unit. Heater bands are wrapped around the barrel to help warm up the plastic and melt it. Once the pellets reach the front of the barrel, they’ve reached their melting point and are entirely molten.
Then, when enough melted plastic reaches the front of the screw, it rams forward like a plunger, and the liquid is injected into the injection molding mold. The volume injected is called the “shot,” and the injection phase ends when 95% to 99% of the mold is filled. Injection times can vary based on the flow and dynamic of the plastic mold materials, injection pressure, power, and shot volume.
The third phase is the cooling phase. As the plastic cools inside the mold, it takes the shape of the cavity injection molding mold. To encourage cooling, a coolant, like water, flows through channels just beneath the surface of the interior of the mold. The part may shrink slightly during the cooling period; an important fact mold designers must keep in mind. Once the part is hardened, the mold can open. The wall thickness and type of plastic used determines how much cooling time is needed.
Finally, the molded part is ejected from the injection molding mold. The mold opens partially, and a burst of air rushes in to break the seal of the completed part from the mold surface. It is then opened all the way, and ejector pins force the part out of the molding machine. In fact, if you look closely at plastic molded parts you have in your home or business, you may see ejector pin “witness marks” that show where the pins met the surface of the molded product.
When the part is released, it will have a sprue attached. The sprue is what connects the mold to the injection unit. Mold operators must manually twist or cut off the part. If multiple parts are made at one time, the sprue is connected to distribution tunnels called runners. If you’ve ever put together a model plane, the parts inside the box typically come still attached to their runners. The tunnels fan out from the sprue and are connected to the cavity of each mold via a smaller tunnel called a gate. You can often see a gate on the ends of plastic cutlery.
To make this process easier, we have ejector pin lubricants that help ejector pins release the parts more easily from metal molds. Depending on the cycle, you may need to clean the mold to eliminate grease, oils, polymers, and contaminants. Then, it’s best to seal the mold with a mold sealer to fill in the microporosity before starting the process again and applying more mold release.
If your plastic injection molds process is following all the required steps and still having issues with sticking parts, defects, and excess build-up, then it’s time to try the best molding products.
At Stoner Molding, we make products for injection molding molds that release parts quickly, reduce build-up, and decreases the amount of scrap part. No matter what injection molding or plastic-type you use, we have a product for you! Get in touch with our team today for advice on releasing more products in less time!