Introduction : The process of making plastic parts with injection molding mainly includes four stages: filling-holding pressure-cooling-demolding. These four stages directly determine the quality of the product, and these four stages are a complete continuous process.

What is Injection Molding?

Injection molding is a process that makes parts out of plastic. You heat up plastic (usually little pieces) in a machine. When it gets all melty, you push it forward with a plunger or screw. and then injected into the mold with lower mold temperature at a very fast speed through the nozzle at the front end of the barrel .

You do it really fast through a hole at the front of the machine. After it cools down and gets shaped, you open the mold and take out the part. You do it over and over again.

What is the Working Principle of Injection Molding?

Injection molding is a process for making plastic stuff. Injection molding machine melt plastic and squirt it into a mold. Then you let it cool and harden. The basic idea is:

Heating and melting: heating plastic particles or powder until they melt, usually in an injection machine. The melted plastic becomes thick and liquid, and can be pushed into the mold by the injection machine.

Filling the mold: Injecting molten plastic into the mold through an injection machine. The injection machine applies high pressure to make the plastic flow and fill the entire mold cavity.

Cooling and solidification: The plastic material in the mold needs to cooling process and solidify. The cooling time and temperature depend on factors such as the type and thickness of the material. The specific design of the mold can also affect the cooling time and quality.

Mold opening and removal: After the mold has cooled and the part has solidified, open the mold and remove the part. There are usually some release agents on the mold to help the part come out of the mold easily.

Deburring, trimming, and processing: After the parts are taken out, the extra plastic and burrs need to be removed, and the parts need to be trimmed and processed. This can be done by hand or by machine.

Inspection and packaging: Finally, we need to inspect the finished product to make sure it’s good enough, then we put it in a box and send it to you.

What are the Injection Moulding Process Step?

Injection molding is a common plastic manufacturing process that is widely used in the manufacture of plastic products. The principle is to inject the heated molten thermoplastic material into the mold through an injection machine, and then cool and mold it to finally obtain a plastic product of the desired shape.

The process flow of injection molding usually goes like this:

Mold preparation: You gotta get the mold ready to make the plastic thing you want. That means you gotta design it, make it, and put it in the machine. When you design the mold, you gotta think about what the thing looks like, how big it is, and what it’s made of.

When you make the mold, you gotta think about what kind of stuff you’re using and how good you want it to be . In addition, when preparing the mold, the design of the clamping unit needs to take the wall thickness into consideration to ensure the stability and molding quality of the mold during the injection molding process.

Plastic material preparation: You need to choose and process plastic materials according to the requirements of the products you want to make. Usually, you heat plastic pellets or powder until they melt, and then you shoot the melted plastic into a mold using a machine.

Injection: injection process is when you shoot hot plastic into a mold using an injection machine. You have to control the injection pressure , speed, and temperature of the injection machine to make sure the injection unit is accurate and good quality.

Cooling: After injection molding, you have to wait for a while for the plastic to cool down and harden. The cooling time and temperature depend on the type and thickness of the material.

Mold opening: After cooling, you have to open the mold and take out the parts. There are many ways to open the mold, such as manual, mechanical and hydraulic.

Deslagging, trimming, and processing: After the parts are taken out of the injection molding machine, they need to be deslagged, trimmed, and processed to remove excess plastic material, flatten the surface, and make the holes required for processing.

Inspection and packaging: Finally, we need to inspect the finished product to make sure it’s good enough, and then put it in a box and send it somewhere.

What are the Stages of the Injection Molding Process of Plastic Parts?

Filling Stage

Filling is the first step in the entire injection molding cycle. The time starts from the mold closing and injection molding until the mold cavity is filled to about 95%. Theoretically, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or injection speed is subject to many conditions.

Holding Pressure Stage

The purpose of the holding pressure stage is to keep the pressure on, pack the melt, make the plastic denser (densification), and make up for the shrinkage of the plastic. During the holding pressure process, the back pressure is high because the mold cavity is already full of plastic.

During the holding pressure packing process, the injection moulding machine screw can only move forward slowly and slightly, and the flow rate of the plastic is also relatively slow. The flow at this time is called holding pressure flow.

During the holding pressure stage, the plastic cools and solidifies against the mold wall faster, and the melt viscosity increases rapidly, so the resistance in the mold cavity is very large.

In the later stage of holding pressure, the material density continues to increase, and the plastic part is gradually formed. The holding pressure stage should continue until the gate is solidified and sealed. At this time, the cavity pressure in the holding pressure stage reaches the highest value.

Cooling Stage

When it comes to injection molding molds, the cooling system design is super important. That’s because the molded plastic parts need to cool down and harden up before they can be taken out of the mold. Otherwise, they’ll get all bent out of shape from any little thing that touches them.

Since the cooling time is about 70% to 80% of the whole molding cycle, a good cooling system design can really cut down on the time it takes to make the parts, make more parts in the same amount of time, and save money.

If the cooling system is not designed properly, it will increase the molding time and cost, and uneven cooling will cause warping and deformation of plastic products.

Demolding Stage

Demolding is the last step in the injection molding cycle. Even though the product has been cooled, demolding still has a big impact on the quality of the product. If you demold the product the wrong way, it can cause uneven force on the product during demolding, which can cause the product to deform during ejection and other defects.

There are two main ways to demold: ejector demolding and stripper demolding. When you design a mold, choose the right way to demold based on the structure of the product to make sure the product is good.

When using molds that require ejector demolding, make sure to set the ejector as evenly as possible. Choose a position where the demolding resistance is the highest and the plastic part is the strongest and most rigid to prevent any deformation or damage to the plastic part.

Conclusion

The plastic injection molding has four main stages: filling, injecting molten plastic into the mold; holding pressure, continuously applying pressure to compact the plastic and compensate for shrinkage; cooling, solidifying the plastic; demoulding, removing the molded part from the mold. These stages determine the quality and production efficiency of the final product.