Plastic Injection Molding – A Beginner's Guide

Plastic injection molding 101. This beginner’s guide will walk you through the injection molding process step by step. Find out how you can make the most of plastic injection molding

Vaughn Cook RockWell Window Wells
Editorial Team
March 16, 2023

Ever wondered how mass production units produce hundreds of thousands of pieces of the same part over and over, with exact precision every time? It is made possible by a technology called plastic injection molding. All those plastic caps, phone cases, keyboards that you use have been manufactured by some type of plastic injection molding. Due to its high efficiency and accuracy, plastic injection molding is one of the vital components of modern industrial production technology. If you are already curious about how all those components, both little and big, are manufactured by the millions with absolute accuracy, the intricacies of the technology will capture you even more. Let’s find out more about how this technology works.

History of plastic injection molding

In 1872, an American inventor named John Wesley Hyatt patented this technology for manufacturing plastic billiard balls instead of Ivory ones. His product did not become a success, but ultimately his technology did. Technology has skyrocketed from that point. Initially, it was used to produce simple plastic equipment like balls and buttons. Today, the same technology powers all fields, from medicine to automobiles to electronics. It would be hard to imagine the things we see around us everyday the same way if this technology did not exist. Before delving into why and how this method became so popular and effective, let’s see how plastic injection molding works.

What is plastic injection molding?

The basic mechanism of the technology is rather simple. At first, the machine injects molten plastic into a metal mold. Inside the mold, the liquid plastic eventually cools and hardens. Once the plastic is no longer molten, the metal mold separates into two, revealing the finished plastic product inside.

Despite this simplicity in the fundamental process, this technology is highly efficient. The probability of error is minimal. Manufacturers and industrialists around the globe rely on injection molding to produce quality goods with consistency.

For a more detailed understanding, let’s see how this injection machine works.

The machine is made up of four main parts: an injection unit, a clamping unit, the control, and the base. To start the process, a barrel containing an internal screw-shaped device is fed with small plastic pellets. The screw-shaped device, known as the auger, feeds the plastic to the barrel by rotating the plastic at a high force. The body of the barrel is fitted with heater bands, which effectively melts the plastic.

The mold closes once the machine cycle starts. Then the auger injects molten plastic in high pressure into the empty space of the mold, called the cavity. It just takes a few seconds to do this. There are coolants present around the mold to speed up the process of solidifying molten plastic. In as little as 60 seconds, the molten plastic obtains the desired shape.

Then the mold opens and the plastic structure is ejected. In just a minute or so, the machine successfully produces one unit of the product and is ready to go again.

Why is plastic injection molding used?

The technology of injecting plastic into a mold is used in every industry to some extent. That leads us to the question of why it got so popular in the first place.

One of the major reasons is the technology’s efficiency. When units can be produced in under 60 seconds, a few hours of application of the machine yields hundreds of units of the product. As more and more units are produced, the initial cost of setting up the machine is met in a very short time. After that point, the profit generation goes up by leaps and bounds. While saving time and money are the primary reasons this technology has garnered widespread popularity and dependency, there are some other reasons as well.

Low scrap rates: A traditional CNC machining would cut off a huge chunk of the original plastic sheet or block fed to it. That leads to the problem of growing scrap rates. All of that waste plastic has to be recycled, reused, or disposed of. Any of these options incur an additional cost, apart from the manufacturing cost itself. Injection mold machines do a great job at reducing scrap. It would in turn save money that would otherwise be spent on proper management of the scrap. Wastage occurs in these machines only in case of malfunction or defects. The gate location, sprue and runners of the machines do produce some scrap, but it is considerably less than what CNC machining would do. Sometimes there is also loss due to leakage from the metal mold’s cavity. Technologies like 3D printing, however, produce even less (or none) scrap.

Scrap also depends on the plastic material being used for production. Thermoset materials like epoxy resin cannot be molten again once it is cured due to exposure to air. Any attempt to melt them will simply cause the plastic to burn. Thermoset material scrap is unfit for recycling or reusing for this reason.

Thermoplastic material, on the other hand, can be recycled and reused. It can be molten again even after it has already been melted and solidified before. Often this recycling takes place within the factory. By scraping waste out of the sprues and runners and injecting it back to the machine, wastage is reduced to a great degree. The recycled material is termed re-grind. It is generally of inferior quality than the plastic that has not been reused. Regrind material is generally used for those units that do not require high-quality, robust material. One production unit can also accumulate a substantial amount of regrind and sell to some other unit that produces goods with inferior material. Overall, plastic injection moulding does a better job than other production technologies in terms of time, money, and resource management.

Plastic injection molding - Beads
Plastic injection molding is fast and works with a wide range of materials

Complex designs: Many complex designs can be produced without any secondary operation in a molding machine. It is achieved by using unscrewing features and core pulls. By eliminating the need for secondary operations, you can save money on the overall production cost. The designs can be parts with threads and side holes, and the molding machine will deliver the exact same product, finished with all the complexities, right out of the machine.

Different finishes: Whether your product needs a smooth finish or a rough one, it can be easily done in an injection molding machine. By default, most machines produce products that have a very smooth finish. The good news is that you can alter the type of finish you desire without any additional operation. These machines also have the ability to produce unique finishes, like engravings or patterns. No matter what type of finish you want: shiny, rough, matte, it can all be done in the same machine, with the same efficiency and accuracy.

Repeatability: Plastic injection molding machines can repeat the production of the same unit as many times as you want. That translates to having your second product identical to your first one, and the third identical to the second, and so on. Every business owner is concerned about brand image and consistency. Injection molding machines take care of that concern with this feature of repeatability. With all products made identical, there is little chance of faulty products going out to the consumers.

Control over color: Injection molding machines can infuse whatever color you want to your product. It is even possible to get two or more colors in a single product. You can achieve this with the use of two-shot or overmolding techniques. For plastic products aimed at children, like toys, having control over color determines the success of the product to a great extent.

Cost of plastic injection molding machines

Getting a perfect design before your machine starts production is very important. A faulty design will result in huge losses and unsatisfactory products. That is the main aspect of injection molding which needs a big investment. Designing and prototyping the part and the mold tool incurs a heavy cost. The Return of Investment, however, covers up for the cost if the prototypes are designed to perfection. Designing the tools and the prototypes requires a considerable amount of time and money, and that keeps some manufacturers away from using these machines. Owing to the complexity of some of these designs, only experienced engineers can manufacture them. Cost of hiring skilled technicians increases in this scenario. If this one time investment is not a hindrance, an injection molding machine makes up for the investment in a very short time.

The average cost of these projects is generally $1,000-$20,000. The ROI also depends on the size of the manufacturing unit. The scale of the production unit and revenue are directly proportional. With a high scale of production, the cost of the machine is covered up in a short time. With low scale production, the time needed increases. The complexity of the design, material used, part size and quantities of production are the factors upon which the initial cost of setting up the machine depends.

To conclude this brief guide on plastic injection molding, we can safely say that it is one of the breakthrough technologies of our generation. It has enabled the mass production of quality goods on an unprecedented scale. Every field of production employs this technology to get their work done effectively and efficiently. So if you are a manufacturer or industrialist wondering how to take your business to the next level, setting up a plastic molding machine is a safe and reliable option. With the development in all fields of technology, it can also be safely assumed that plastic molding machines will only get better and even more efficient with time.

Sourcing Allies is a team of expert China sourcing agents that has helped western customers manufacture and source products from low-cost regions since 2006.

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