What Is the Process of Each Cycle of Injection Molding?

You might see a million plastic parts every day, but have you ever considered the fast-paced action happening inside the machine? The whole process, from start to finish, is a high-speed ballet. Trying to troubleshoot a part or predict a problem without understanding this process is like flying blind.

Our guide breaks down the science and the art of the entire cycle, so you can master each step and stop guessing.

Clamping The Mold: The Iron Handshake

The first step in every injection molding cycle is clamping the mold. Think of it as the iron handshake that holds the two halves of your mold together. This isn't just a simple closing motion. It's a powerful, hydraulic embrace that resists an immense amount of pressure later in the cycle. Without enough force here, your part will be a mess.

When we inject molten plastic into the mold, that liquid pressure tries to pry the mold halves apart. The clamping unit's entire purpose is to prevent this. We have to apply a force greater than the pressure the plastic exerts. The science behind this is pretty straightforward: you need to calculate the clamping force required to keep everything closed.

How to Calculate Your Clamp Force

Getting this number right is one of the most important things you can do. You can find the approximate force needed by multiplying the part’s projected area by the pressure of the injected material. We always add a little extra to the calculation for a safety margin. A good rule of thumb is to add at least a 10% buffer. It's better to have a bit too much force than a leaky mold.

The Consequences of a Bad Clamp

A common mistake is using a clamping force that is too weak. If the mold flexes even a little bit, a thin sheet of plastic, called flash, will leak out along the parting line. This makes a mess and creates a defective part. On the other hand, if you apply too much force, you risk damaging the mold itself. Excessive pressure can also trap air inside the cavity, leaving burn marks on your final product.

Injecting The Molten Plastic

After the mold is securely clamped, we can finally inject the material. This stage is all about moving a precise amount of molten plastic, fast and under intense pressure, from the machine's barrel into the mold cavity. Think of it like a hypodermic needle for plastic. The machine's screw pushes forward like a piston, forcing the material through the nozzle and into the gates of the mold.

The Power of Pressure and Speed

The two most critical variables in this stage are injection speed and injection pressure. The speed determines how quickly the plastic fills the mold. Go too slow, and the plastic might cool and solidify before the cavity is full, giving you a "short shot." Go too fast, and the rapid flow can cause defects like burn marks or even flashing if your clamp force isn't set correctly.

Your pressure settings ensure the material gets into every nook and cranny of the mold. We need just the right amount of force to fill the cavity completely without causing stress on the part or the mold.

Packing It All In

Once the mold is full, the machine doesn't just stop. It switches to what we call packing pressure or holding pressure. This is a sustained, lower pressure that forces more material into the mold as the part begins to cool and shrink. This is a crucial step that prevents major issues. Without proper packing, you will get voids, or even worse, noticeable sink marks on the part's surface.

Letting The Part Cool Off

After the plastic has been injected and packed into the mold, it needs to solidify. This is the cooling phase. It’s a vital step because it gives the part its final shape and structure. Without it, you’d have a soft, gooey mess that can't be removed from the mold.

The mold itself has a network of channels running through it. These channels circulate a coolant, usually water or oil, that pulls the heat away from the plastic. This is the main reason a part can be made in just seconds. We need to cool the plastic quickly and evenly so it holds its shape.

Cooling Time is Everything

The cooling time is often the longest part of the entire cycle. It depends on several factors, including the type of plastic you're using, the thickness of the part, and your mold's design. If we don’t let the part cool long enough, it will be too soft. Trying to eject a part that isn't fully solidified will cause it to warp and deform.

On the other hand, if we leave it to cool for too long, we are wasting valuable time. Finding the perfect cooling time is a balance between a solid part and a fast cycle.

Common Cooling Problems and Fixes

● Warping: If a part cools unevenly, it will distort. This often happens with thin-walled parts. Adjusting the coolant temperature or the flow rate can help you achieve a more uniform cool.

● Sink Marks: These are small indentations on the part's surface. They often happen in thicker sections where the outer skin cools faster than the inside. You can fix this by increasing your packing pressure or adding a bit more cooling time to allow the inner material to shrink properly.

Ejecting The Finished Part

Once the plastic has cooled and solidified, the mold opens. Now, it's time to get our part out. This ejection stage is the final step in the cycle. It needs to be a clean, simple, and gentle process. The goal is to get the part out without any damage.

The most common way to do this is with ejector pins. These small rods are located in the mold and push the part out. We use air blasts or stripper plates for more complex parts, but most of the time, it's the pins that do the heavy lifting. The pins should be placed on a rigid, flat surface of the part to ensure an even push.

Common Ejection Problems and Fixes

If your part isn't coming out cleanly, you have a problem. Here's a quick rundown of some issues you might see:

● Part Sticking: This happens when the part is difficult to remove from the mold. It could be due to a vacuum forming or because the part has shrunk onto the core too tightly. A bit more cooling time can help, or you can try a small amount of mold release spray.

● Pin Marks: If the ejector pins leave behind ugly marks or white spots on the part, your settings are likely off. Those white spots are called stress whitening, and they show you where the plastic was stressed during ejection. Try using more pins to distribute the force, or adjust their location.

● Deformed or Cracked Parts: This is a clear sign that the part was not fully cooled before ejection. The machine is trying to push on a part that's still too soft, causing it to warp or break. Just let it cool for another second or two.

The Cycle Comes Full Circle

Every single plastic part you see starts with the same four-stage cycle. We see it all the time: clamping, injecting, cooling, and ejecting. Mastering each step gives you full control over the final product. It’s the difference between a good part and a truly great one.

Understanding this process helps you get the results you want. It also helps you troubleshoot problems when they come up. If you have questions about a project or need a part made right, our team is ready to help.

Find out more about what we can do for you on our website. We are ready to help you with your next custom plastic part project. You can find us at Mulan Manufacturing Group.