In-depth: the injection molding process

Many of the plastic products that customers take for granted in their cars, homes and workplaces will have been created through a single process: injection molding

Although injection molding is often a process of choice for manufacturers due to its cost-effectiveness and high consistency of product, it involves controlling complex systems and parameters.

How is the mould made?

Some mold makers use Computer Numerical Control (CNC) machining. CNC machining uses computers to control the tools so it can make complex molds with accurate details in less time than standard machining. Electrical Discharge Machining (EDM) is another process, which applies a voltage between tool and mould that causes spark erosion of the mold surface.

Each have their own advantages and, really, the best option depends on the mold you’re creating. Generally, EDM is a better choice for a complex mold, while CNC machining offers higher accuracy. A good manufacturer should be able to advise you on which option is the most cost-effective for you, while delivering the best results.

What needs to be considered when creating a mold?

During the injection molding process, the material cools before shrinking from a liquid state to a solid. Your mold will need to accommodate this changed state, which is why cavities are scaled up. This allows the plastic to shrink back to the desired size.

Choosing whether your mould should be made from aluminum or steel is also an important decision. Aluminum costs less, but its production life is shorter. However, if your production run is short – and by short, we’re talking around 10,000 units – then it’s an ideal solution.

Modern hard aluminum can give you a longer production life of around 100,000 units or slightly more but only if the mold design is strong and is properly maintained.

For a production run that will last several years and include millions of parts, steel is your best option. It simply lasts longer and carries none of the risks of softer materials. Before you make that investment though, create a prototype with a softer mold, such as those commonly used in short production runs, to make sure the dimensions are correct.

What will a mold cost?

As the saying goes, how long is a piece of string? In addition to the material used, the number of cavities helps determine the cost. The more cavities, the more complex the mold, the more undercuts needed and so on, the costlier it is to make. However, investing the time and costs now will be sure to pay off in the long run if you get it right.

By talking to an expert from the beginning, you can make sure that the mold you create will meet your needs. We are experts at every stage of the process, from concept to manufacturing and distribution, and we can suggest ideas that will make the mold-making process easier and more cost-effective.

How do injection molding machines work?

There are a variety of injection molding machines used by manufacturers. The main types of machine used are hydraulic, electric and hybrid (a combination of electric and hydraulic) powered, but all use a similar mechanism to create their final product.

Injection molding machines are designed to force molten materials, usually plastic or metal, into a mold that includes both a cavity, which the material is pushed into, and a clamp, which is used to mold and fix the part into its final shape.

Within one machine, the molds can be of a single cavity or multiple cavities, meaning that just one or a number of products can be made during a single process. In addition, the multiple cavity moulds can include identical or unique cavities, so the products created can be the same or individual. Each mold is designed to create more than a million parts across their lifetime, meaning they have to be robust and have low tolerances.

What happens during injection moulding?

When thermoplastics are molded, pelletized raw material is fed through a hopper into a heated barrel with a reciprocating screw. The molten material is then sent down a runner to the gate, where the machine uses a ram or screw-type plunger to shoot it into the cavity.

Sometimes, a two-shot mold process is used to incorporate two separate materials into one part. This is used to add a soft touch to knobs, to provide multiple colors or to produce a part with multiple performance characteristics.

Once the clamp has been pressed down into the cavity to form the part and the material has cooled in the cavity, a pump is used to push the finished part out of the cavity, known as ejection.

Depending on the aesthetic requirements of the product, there may be further processes that help to polish, color or finish the part, but this is the standard injection molding process used to make plastic or metal parts.

Are there different types of injection moulding?

Depending on the characteristics of the part that needs to be created, the injection molding process can be adjusted to create more complex products. Here are a few of the most common processes.

• Overmolding (two-shot molding) is when two different types of material are injected into the same cavity so they combine to create a part with mixed qualities, whether this is two colors, for increased durability or a slightly adjusted performance.
• Insert molding combines two or more different components to create a single part. This may mean putting a finished part into the cavity before refilling it to combine the two together. This can help to make the product stronger or add extra elements to a part without increasing its weight.
• Metal injection molding uses a slightly altered process to turn metal powders into fully molded parts. This is a less common process but can be used to create complex pieces that are more robust or have different aesthetic qualities from plastic.

To make injection molding more efficient, it is now integrating the technologies of Industry 4.0 so the products it creates are of more consistent quality, despite having more complex shapes and characteristics. This means that despite the fact the process has been used by manufacturers for many years, it is highly likely to continue to evolve and be used in the future.