What is Plastic Injection Molding?

Learn Up: Plastic Injection Molding

Molding is a big topic. There is blow molding, which as its name implies, blows molten plastic into a mold and can be used to make everything from spray bottles to children’s toy chests. Rotational molding is often used for larger items like garbage cans and hot tub shells. You can think of the molding machine as an industrial Tilt-A-Whirl, one that tumbles a heated mold in all directions, allowing the polymer charge within to spread evenly across its surface. Compressional molding squeezes heated plastic between two halves of a mold, similar to how a waffle maker works.

Injecting Success

There’s more, including extrusion molding, thermoforming, and structural foam molding, but this installment of Learn up will talk about the most popular and widely-used of all such manufacturing technologies, plastic injection molding. Yes, there are some overlaps with other molding techniques, but injection molding takes the cake in terms of part accuracy, cost, and production efficiency.

Because of this, plastic injection molded parts are everywhere. The mouse you hold in your hand for hours each day was injection-molded. So was your keyboard, the remote control for the TV, the little knobs on the coffee maker, the spork you used to gobble up some KFC coleslaw at lunch and the Tupperware container used to store the leftovers. It’s a seemingly endless list and will only become longer as material scientists continue to deliver stronger, more functional polymers, ones that often contain glass, carbon fiber, and other performance-enhancing additives.

Hyatt Plays Billiards

The American inventor John Hyatt was the first to patent plastic injection molding. He and his brother Isaiah were looking for an alternative to ivory for use in billiard balls, and developed a machine that would inject liquid nitrocellulose—another recent invention—into a spherical mold cavity. That was in 1872, and while the brothers’ injection molding machine was wildly successful, their choice of materials was not. Celluloid, the stuff of the early film industry and Hyatt’s billiard balls, is made of camphor and nitrocellulose, both flammable. The result? A hard break would often lead to an explosion on the billiard table.

Setting aside such gaming excitement, Hyatt’s invention was relatively simple, using a syringe-like device as the means to inject plastic through a heated chamber and into the mold cavity. Just after the conclusion of World War II, inventor James Watson improved on that idea with a screw-type machine. This provides greater control over injection speed and allows the introduction of additives and colorants. That basic design, though much improved upon over the years, remains in use today.

Of Airplanes and Red Solo Cups

Consider the snap-together airplane model you played with as a kid, or the Solo cups you might have drunk from at college kegger parties. Making these and countless other items requires that pellets of plastic feedstock are first dumped into a hopper. These pellets make their way into a heated barrel containing the rotating screw mechanism just mentioned. This pulls the now molten material forward, mixing it as it goes.

Sitting at the end of the barrel is a stationary platen, to which one half of the injection mold (the A-side) is attached. The other half is attached to a sliding press-like mechanism that clamps the two mold halves together. Once closed, a plunger forces a predetermined “shot” of hot plastic through a nozzle at the barrel’s end, through the platen and into the mold. It then travels down a series of runners and sprues and into the mold cavity proper (or more likely, multiple cavities).

Once the hot plastic has had a chance to cool slightly—often with the assistance of liquid-filled channels that run through the mold base—the movable platen holding the mold’s B-side retracts. Ejector pins within the mold give the injection-molded part (or parts) a nudge, and out pops a nearly complete kegger cup or F-18 replica. All that’s needed is to trim away the runners, remove any excess material around the gates and sprue, and possibly “de-flash” any material that seeped into the separating line between the two mold halves.

Pulls, Gates, and Actions

Granted, this is a gross oversimplification of a highly technical process. Plastic injection molds often contain movable “side actions” and “pulls” to create undercuts and other internal part features. Machined inserts are bolted in as well, since, in all but the most basic of part geometries, the mold cavity cannot be cut from a single block of metal. Electronic sensors are installed to monitor mold pressures and temperatures, and as already discussed, cooling channels might be cut into the mold to reduce cycle times and increase the productivity of what is typically an expensive tool.

It’s even possible to “overmold” a workpiece. In this instance, a softer material is injected into the mold over the top of the part’s harder base material—the handle on your battery-powered drill, for example, might have been overmolded to give it a more ergonomic feel. In addition, metal components such as threaded inserts and bushings are often placed into the mold before the injection process. This is called insert molding, and it provides the finished part with mechanical properties unachievable using plastic alone. Simply put, plastic injection molding is a highly complex affair.

Thermoplastics Are Your Friend

The materials are complex, too. The mold itself is typically made of tool steel, although aluminum and even 3D-printed molds are used for low-volume and prototype work. As for the polymers, here are a few of the more common and useful plastics—more properly known as thermoplastics—in use today:

• ABS (acrylonitrile butadiene styrene) has a fairly low melting point, making it a darling of the plastic injection molding industry. It is resistant to chemicals, has high-impact strength, and is found in everything from Legos to sewer pipe.
• HDPE (high-density polyethylene) is both lightweight and strong. One of the more recyclable of all plastics, HDPE is also quite versatile, available in rigid and flexible forms. You can find HDPE in milk jugs, fuel tanks, plastic toys, and patio furniture, to name a few of its many uses.
• PEEK (Polyether Ether Ketone) is an organic thermoplastic used in a variety of engineering and medical applications. It is rigid, quite strong, resistant to heat and corrosion, and used extensively in the aerospace industry, where it can often replace aluminum and other metals.
• PC (polycarbonate) is popular for its strength, impact resistance, and in some grades, optical transparency. Bullet proof glass is made of polycarbonate, as are sunglass lenses, Blu-Ray discs, and so on.
• TPU (thermoplastic polyurethane) is often used for the overmolding process just mentioned. An elastic material, it is referred to by some as the bridge between plastic and rubber. It is flexible, durable, and resistant to oil-based lubricants. It can also be mixed with other materials such as polyester, further adding to its capabilities.

There’s lots more. Nylon (polyamide), PMMA (acrylic), PP (polypropylene), POM (polyoxymethylene). One would have to spend an entire day with a materials expert to explore all the different thermoplastics, their properties, and their uses. There’s no time for that, however, which is why we’re going to wrap up with a few design tips for those who might be considering a visit to a plastic-injection molding house.

First off, the wall thickness of any injection-molded part should be as consistent as possible. Not too thick, not too thin, and not too tall—depending on the part size, a 1/8-inch thick wall (2-3 mm) is a good rule of thumb. Sharp edges should be avoided, especially on internal corners, where stress can be a concern. The part walls will need some slight taper, or draft, to aid in ejection from the mold. Undercuts increase part cost, as do highly cosmetic surfaces, certain polymers, and parts designed without plastic injection molding in mind.

Here again, this is just an overview. Basic knowledge of these and other Design for Manufacturability (DFM) principles is an essential tool in any engineer’s toolbox, and even procurement people can benefit from knowing how parts are made and what materials should be used to make them.

If you’d like to chat more about plastic injection molding, get in touch with us. Prismier does lots of it, and we’re happy to steer you in the right direction. Until then, stay tuned for the next installment of Learn Up: What is Metal Stamping?