Injection Molding 101: How It Works

Injection molding is one of the main processes we use. It’s quick and easy, and can produce a huge quantity of similar goods with high accuracy. Injection molding can create super cheap products and luxury ones. The difference often comes down to quality control and finishing work done to the product. High quality mechanical engineering can ensure good parts or cheap parts.

It’s useful for producing smaller parts, but not for bigger ones. Injection molding is a widely used technology in manufacturing. This constant use has created many innovations in pushing the ability for injection molding to make almost anything. Because of this, it offers a lot of freedom in design form and material choice.

Injection molding as a manufacturing method is highly effective and growing rapidly. The global market size of injection-molded plastics was valued at USD 258.2 billion in 2019.

Key Points Of Good Injection Molding

Before you choose injection molding for manufacturing, consider the following points:

  • Financial aspects

Initial setup: Injection molding requires a huge setup cost for manufacturing,

Quantity of products: The number of production products needed to ensure the injection molding is a viable manufacturing method.

  • Design Considerations

Design of parts: You should use simplified geometrical measurements and reduce the number of moving components when possible. This will improve running costs in the future.

Mold tool: Use mold tools that limit defects. You can utilize simulation runs and gate locations to understand how it will pay off.

  • Production Considerations

Production cycle: Choose machines that reduce your cycle time. Hot runner technology, picking the right tools, and cutting even seconds from cycle time can save millions on the parts you are using.

Configuration of parts: Assembly should be minimal to reduce labor costs.

Why Is Injection Molding Important In Product Design?

  • Scalability

The biggest advantage of injection molding is the scaling opportunity. Once you have recovered per unit cost, the manufacturing cost remains pretty low. The more you produce the less the production cost is.

  • Low scrap rates

Injection molding provides very low scrap rates similar to CNC machining. After a cycle has run, the molten plastic will leave solid parts in sprue, gates, cavities, and runners. All of this is done with very little wasted material.

  • Re-grind production

Re-grind is from thermoplastic materials which can be repeatedly melted and solidified. There is little performance reduction when reusing these plastics. Because of this, manufacturers utilize a low percentage of this and can sell this to other factories.

  • Repeatable

The injection molding process is extremely replicable. Because of this, quality control can be very easy. You can also produce the same part a million times almost identically.

Issues With Injection Molding Process – Example

The injection molding process is not without flaws, and some of the problems can be difficult to address. We prevent other flaws through experimenting with the process. This is sometimes possible without entirely replacing the mold tooling or other equipment. Still, it’s important to know what flaws can be experienced in the process.

  • Warping

Uneven shrinking of a plastic part causes what we call warping. This means one section of a part shrinks faster or slower than the rest. Thus, causing it to not keep the shape it should.

  • Flow lines

Flow lines are wavy patterns formed on the part. These flow lines have slightly different coloring than the rest of the part. This happens in the narrowest part of the article and can form ring-like bands. They can be acceptable and negligible in some products but usually frowned upon in high-end products.

  • Air pockets

Air can become trapped in a mold. Because of this, small bubbles can form on the finished product causing pock marks. Air pockets are typically counted as minor defects and can easily be hidden by texturing. Bigger air pockets, however, can compromise the quality of the product.

  • Burn spots

These spots look like discoloration and can be black or brown on the surface of the product. They usually don’t affect the integrity of the design.

Injection Molding Design Process

We start with a heated barrel. This is fed the raw plastic, in pellet form mostly, for the product through a hopper. The hot barrel melts the plastic with heater bands and friction movement of the screw. This liquid plastic is then injected through a nozzle into the mold.

After this, it rests until cooled down and hardens based on the shape of the mold. The mold is kept at a steady temperature during this process. This is to allow the part to cool down as quickly as possible.

The next step, once the plastic has hardened, is to shift the mold to a moving plate. This moving plate opens up the mold. Ejection pins on the plate are then pushed into the mold, pushing the part out of the mold. These pins cause small marks that are tell-tale sign of injection molding.

Materials of Injection Molding Design

When choosing your material for injection molding, your major considerations should be the function and environmental impacts of the material. This includes integrity, strength, lasting time, color, and price. While there are more materials than we would want to list here, there are a few chosen most frequently.

  • Thermoplastic Polyurethane

The nature of this type of material is soft and elastic, with high tensile strength and tear resistance. It is almost close to rubber in consistency and works well at high temperatures. Some common applications are for producing power tools, sports goods, and insulators.

  • Nylon

One of the strongest materials that have many industrial design applications. Nylon molds into gears, bearings, and car parts, for example. This is because these applications have high wear and require strength. It has a lot of integrity, a high melting point, resistance to wear and tears, and chemicals.

  • Polycarbonate

Polycarbonate is a very strong material. This makes it work very well for precise applications where shrinkage is an issue. It contains BPA, so not suitable for food storage purposes.

  • Polyoxymethylene

The most common application of this material is in the automotive industry, where the parts work with metal. This material is extremely strong and rigid. Because of this, Polyoxymethylene is used to produce things like high strength gears. It also has a very high resistance to alcohol, detergents, and motor oils.

  • Acrylic

Mostly used for transparent goods production such as windows you see on public transports, walls, lighting parts. It has high tensile strength and a great replacement for glass in many applications. A common application is in refrigerator containers. It is BPA free, odorless, and tasteless.


The main purpose of injection molding is to create highly specialized products at high volumes and low production costs. As a result, injection molding is ideal for many different products. This is especially true for consumer products. Selecting the right material for the job is very important. As we get new plastics that are bio-friendly, our options become even wider.

The end goal is to produce high-quality goods with minimum defects and high ROI. Injection molding checks off all the boxes, so to speak. It provides a great process that can match the quality needs of different projects. It also works with many new bio-plastics, which means it’s here to stay.

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