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By Kenny Pravitz | Mar 27, 2018
A softer plastic resin can be over-molded to a rigid plastic all within the same process with two-shot molding.

When you look at or feel a plastic component, you would usually assume that it’s made of one type of plastic. However, some plastic products are actually made using two different types of resin, sometimes more. You are probably familiar with this application which can be seen in plastic toothbrushes that have a rubberized grip. The main body of the toothbrush is made of a rigid plastic, while the grip is made of a rubberized plastic. Even though there are two different types of plastic present, both were formed at the same time using two-shot molding.

GM Nameplate’s (GMN) plastics division in Beaverton, OR recently created a video that demonstrates this two-shot molding process. The process is called two-shot molding because there are two different resins being injected by two separate barrels. There is a primary barrel, which injects the first resin, forming a rigid substrate in most cases. The secondary barrel then injects a different resin on top of or surrounding the region of the first substrate.

Depending on the size and intricacy of the part, you can design the tool to make several parts in each cycle. In the video, we see that two parts are completed during each cycle. On the left side, the rigid substrate is injected by the primary barrel and forms the backbone of the two components. The tool then rotates 180 degrees, and the rubberized plastic is injected onto those two pieces by the secondary barrel. While this is being done, two more rigid substrates are made at the same time again by the primary barrel on the left side. After the pieces are injected by the secondary barrel, an end-of-arm tool picks up the completed parts, and then the tool rotates 180 degrees once more, ready to start a new cycle.

Two-shot molding is ideal for higher volume projects, as more engineering is used in designing the two-shot molding tool. The tooling used for two-shot molding is intricate because it must inject two different plastic resins simultaneously, but only in certain features of the part. Two-shot molding is a much more efficient process for high-volume projects compared to conventional over-molding, where you use two separate tools to manufacture parts with different resins. Due to this efficient output, two-shot molding is frequently used in the automotive and medical industries.

Click on the video below to see the two-shot molding process for yourself!

John Davis, GMN
By John Davis | May 18, 2016
Plastics material selection for manufacturing

There are many different types of plastic materials being used in the manufacturing industry and with so many choices out there it can be difficult to select a specific solution. A few of the most common materials include polyethylene (PE), polypropylene (PP), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), and various engineering polymers.

While polyethylene and polypropylene are different resins, both have similar properties within their plastic family because they are buoyant materials and are hydrophobic, meaning they do not absorb water. These two commodity plastics are commonly used within the injection molding industry because they are lower in cost, easy to obtain, come in a wide variety of colors, and are favored for their resistance to many chemical solvents, acids, and bases. The material density allows them to float, which can be an advantage or disadvantage depending on the application. Typically, polyethylene and polypropylene materials are used for small, cheap and lightweight products such as reusable water bottles, containers,  children’s toys, and are commonly found in packaging such as plastic bags and films. These materials are not as hard as other plastics and don’t hold up as well to ultra violet (UV) light exposure. On the other hand, these materials can withstand an impact without shattering because they aren’t brittle like glass.

ABS is another favorite within the injection molding industry and is used widely at GMN Plastics. The reason for this is due to the chemical composition of this material and the versatility associated with its physical and chemical properties.  For example, ABS has great impact resistance, toughness, and heat resistance, which makes it a great option for housing and bezels because it can handle some impact without breaking. This material can also be processed with additives to improve UV resistance, gloss, and can be found in a wide variety of colors. The price point for ABS is also reasonable. While ABS is a strong material, it doesn’t stand up against high temperatures or external elements as well as other material options.

Polycarbonate is the top choice for most projects because it is reasonably priced, can infuse UV resistant additives, is a good electrical insulator, and has good fire retardant and heat-resistant properties. While polycarbonate is very popular, it isn’t the best choice for a part that has strict flammability restrictions because the piece can shatter if it gets hot enough. 

Even though GMN Plastics is experienced in working with the plastic materials above, they also excel in manufacturing parts that are required to be made of engineering polymers.  In this case, engineering polymers are injection molding grade plastics that have exceptional toughness, stiffness, chemical resistivity, and highest heat-resistant and flame-retardant properties.  Some common engineering polymers include poly ether ketone (PEEK), polyetherimide (PEI), and polysulfone (PSU).  While these materials may have impressive numbers to back them up, they are very expensive to obtain.