Injection Molding Design: How Plastic Injection Molding Works

Plastic injection molding design can be fairly complex and it’s important to lock down the basics to get it right. This will ensure that the end result lasts longer, is ideally suited to its desired application, and that any issues during the actual molding process are mitigated.

Here, we’re giving you an overview of injection molding design to help you gain a better understanding of this manufacturing process.

What kinds of items are mostly commonly produced via plastic injection molding?

Almost any plastic parts you can think of, from bottle caps and packaging to car parts, handheld games, musical instruments, small items of furniture, containers and even mechanical components are created through plastic injection molding. Most plastic items you see every day are created via this process.

Which polymers are best for plastic injection molding?

All thermoplastics, like polyethylene, polypropylene, polystyrene and nylon are ideal for plastic injection molding, as are certain elastomers/rubbers, thermosets and composites. The best way to choose a material is to consider what the end product will be used for. This will help you identify the characteristics you’ll need, like strength, flexibility, and so on.

How does a plastic injection molding machine work?

First, your product needs to be designed – this will typically be done by an industrial designer or engineer – and a mold created, which will be done by a mouldmaker or toolmaker.

Next, granules of the material you’ve chosen for the product will be fed via a hopper – which works much like a funnel – into a heated barrel. These materials need to be melted down, which is achieved by heater bands combined with the frictional action of the injection molding screw.

The melted plastic is then injected via nozzle into the mold, and allowed to cool and set. The mold tool is placed on a moveable platen until it has hardened. The platen then opens and the part is expelled from the mold using ejector pins. It’s interesting to note that the mold remains at a stable temperature, which means the part can begin to solidify almost immediately.

The mold

The mold, also known as the die, is the tooling used to create the plastic parts in molding, which we mentioned above. A toolmaker (or mouldmaker) will usually construct it out of steel, and precision-machine it to create the exact shape required.

What is an injection molding cycle?

The sequence of events that take place to produce a plastic part is known as the injection molding cycle, and it begins when the mold closes. The next step of course is the injection of the material into the cavity of mold. When the cavity has been filled, a holding pressure is applied to overcome any material shrinkage that may occur.

Next, the screw turns, and this causes the next shot to be fed to the front screw. The screw then retracts while the following shot is prepared.

When the part has cooled and set, it’s ejected from the mold. This is the injection molding cycle, and its speed will depend on your mold and the type of part being produced, as well as the materials used for the parts and the molds.

Are all plastic injection molding processes the same?

Mostly, they’ll all follow the cycle described here, but there are some types of processes that differ from the norm, for example: sandwich molding, gas-assisted injection molding, in-mold decoration and in-mold lamination, microcellular, microinjection and push-pull injection molding, structural foam injection molding, powder injection molding, multiple live-feed injection molding, and rubber injection molding among others.

A note about plastic injection design and stress

If you know a little about plastics, you’ll be aware that stress is the biggest threat to the integrity of a molded plastic part. Stress can cause the product to warp, crack, fail prematurely or develop sink marks, all of which impact the lifespan, appearance and functionality of your part.

Though some degree of stress is generally to be expected, it’s important that your parts be designed to withstand or deflect it as much as possible. During design, consider adding elements like smooth transitions between features, or opting for fillets and rounds in potentially high-stress areas.

What kinds of textures can I produce with plastic injection molding?

Texturing – that is, applying patterns to the surface of a mold – allows you to customise the look and feel of your final product, and it can even be a functional component of your design. For this reason, texturing needs to be considered right from the design phase of your project if you want to achieve optimal results.

Texturing can be used for a multitude of reasons, like improving grip, reducing frictional wear or hiding fingerprints on a product that will be handled often.

Types of textures include multi-gloss patterns, graphics, fusions, leather grains, woodgrains, layered textures, logos and different finishes (like matte).

Where to find high-quality plastic products without the hassle and expense

At Plastinternational, we manufacture custom plastic products from start to finish. From design conception all the way through to the finished part, we offer a complete service that saves you the expense and hassle of dealing with multiple service providers. Plus, it’s much simpler than setting up your own injection molding plant!

We combine our years of experience with a hands-on management style, which means the owners personally oversee all areas of operation. We’re committed to customer satisfaction and to providing the highest quality plastic products to suit your needs.

Whether you’re looking for a specific product or need advice or a customised solution, we’re here to assist. Explore our range of plastic products or get in touch with us now.

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