What is Plastic Injection Molding and How Does it Work

Plastic is one of the most versatile materials in existence.  It can be used for anything from CD cases to car bumpers, and even medical devices. But how does it get there? The answer is plastic injection molding.

This process takes raw plastic pellets, melts them down with heat, then forces them into a mold that has been shaped by computer software or 3D printing to form the final product.

Plastic injection molding machines are some of the fastest production systems around, with many products taking less than 5 minutes to create!  Here’s everything you need to know about what plastics injection molding is and how it works!  Let’s get started…

The plastic injection molding process

First, we need to understand what polyethylene (PE) is. PE is a term used to describe a group of plastics composed of many long chains known as polymer chains. These polymer chains are what give plastic its strength and durability.

There’s no such thing as “pure” PE, but materials close to it include: LDPE, HDPE, MDPE, LLDPE, and PP.  When liquid PE pellets reach the injection molding machine, they go through a heating process that melts them into liquid form before entering the injector portion of the machine which pushes them into the mold cavity where our product gets made!

The first part of this cycle takes place in an open chamber known as the pre-injection unit. This unit is found directly below the injection chamber, where it heats up to around 200˚ Celsius (392˚ Fahrenheit), which is enough to melt the pellets.

The molten plastic drops down into a heated barrel and plunger called the pre-compression cylinder. Once in this machine, we mix in chemicals such as calcium chloride and benzene sodium sulfonate or water-soluble polymers that help modify and control the flow of our polymer chains! 

This injection around the main part helps reduce cooling time and allows for shorter cycle times which means that more parts get created every hour!

In some cases, the pre-injection unit only needs to reach a certain pressure in order to fill a mold cavity. This is because a secondary injection cylinder can apply injections to compensate for any lost pressure.

These secondary injection processes are known as peek injection molding and contribute by adding extra injected molten plastic directly into the middle of your molded product! This is particularly useful for cooling down the plastic faster. 

The next part is known as clamping, during which time our mold closes down onto a fixed plate. Then, with extreme force applied by many hydraulic cylinders, pressure is injected into all three axes of motion: x-, y-, and z-axes. As this pressure is applied, it’s important to make sure that the mold does not exceed the maximum clamp force threshold.

To make sure this doesn’t happen, the mold includes a hydraulic monitoring system consisting of pressure sensors and flow transducers to maintain constant feedback about the injection process! 

This whole time, the ejection unit has been prepared for when our product gets ejected from its mold.

This part consists of an ejection plunger that helps push your part out once the injection process is complete. Sometimes, it’s not as simple as this and we must use a secondary ejector pin to push out harder – or larger – parts! A system of camshafts works to coordinate everything that’s going on inside the machine. 

The mold then opens up and your product gets ejected onto a conveyor belt where it can be moved onto the next stage of production, if necessary. The whole process then starts over again until our injection molding machine creates all of the parts that have been programmed into its computer system! 

Careers in plastics injection molding

Plastic injection molding careers involve anything from working on an assembly line putting together small items such as Christmas lights, to working within an engineering team designing custom products for clients who want their plastic parts.

Many people don’t realize that it takes a lot of time to design and manufacture just one part, and that’s why many plastic injection molding careers revolve around designing new parts for clients. 

It’s also important to know how much energy goes into producing plastics. Since PE is so cheap compared to the labor involved, the process only costs about $1 per pound!

Plastic Injection Molding and 3D printing

This process is often used in conjunction with 3D printing to produce custom products on a smaller scale. In fact, plastic injection molding equipment can be found on many different types of factory floors and might even be located on the same line as a 3D printer! 

It involves taking a block of solid PE pellets and warming it up before injecting it into a mold where our new product gets created. From here, we use a robot to remove the newly formed parts from their molds and inspect them for any defects or irregularities that may have been produced during the process.

These products get sealed within large bags filled with nitrogen gas which prevents them from oxidizing before being moved onto packaging lines where they’re together by type of product.

Cost Efficiency

The plastic injection process is so cost-efficient because it uses so little energy, but is able to produce parts quickly. Here are some of the reasons:

  • Average cycle times are around 5 minutes so there is no time lost between runs. 
  • PE has a relatively low melting point which means that the injection molders don’t have to worry too much about maintaining expensive machinery or high temperatures. 
  • Polyethylene pellets are extremely cheap compared to their metal counterparts. For example, tungsten carbide can cost upwards of $20 per pound while some grades of PE including HD-PE and LLDPE only costs about $1 per pound!
  • Can be used on many different types of material (polyester, nylon…)
  • Can be used on any product shape (round, spherical…)

In this article, we’ve explored the plastic injection molding process for creating high-quality parts. This is a relatively cost-efficient way to produce products because it uses so little energy and can be used on many different types of material. While this process is often used in conjunction with 3D printing to produce custom products, it can also be used on its own. 

If you are looking to get into plastic injection molding, there are many companies out there that work with plastics and can offer their professional services to anyone who needs them!

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