Posted by Eric Weinhoffer ● Aug 15, 2017 8:57:56 AM
Engineering Plastics Primer: Which Plastic Should I Use?
Although metal is often the go-to material choice for engineering applications, plastics are a great option for many scenarios as well. Many have a great balance between material properties and cost. Depending on your needs, a plastic may be a less expensive material choice with similar properties as the metal you were considering. Certain plastics may also offer properties that are simply not present in some metals, like good bearing properties, high machinability, and great corrosion, chemical and temperature resistance.
Plethora offers a great range of plastics for milling and turning. Let’s go over some advantages and disadvantages of each, to help you decide which may be best for your application.
ABS (Acrylonitrile butadiene styrene) is a common material and member of the thermoplastics family, which has the defining characteristics of softening at a low temperature. This makes them ideal for thermoforming applications such a vacuum forming or injection molding.
ABS has fair chemical resistance and is easy to machine to tight tolerances. Unfortunately, due to its composition, cutting it with a process such as laser cutting should be avoided due to the harmful gases it gives off.
Overall, ABS is a great choice as an engineering material due to its high impact resistance and toughness, despite its low cost. Some common uses of ABS includes LEGO bricks, keyboard keycaps, and 3D Printing.
Although it’s more expensive than other thermoplastics such as ABS, Acetal (also commonly referred to by it’s DuPont trademark, Delrin) is a fantastic engineering plastic that performs well in environments where low friction and high strength are required. It also performs well in wet applications due to its high wet abrasion resistance.
Acetal gears (source)
Like ABS, Acetal is commonly fabricated with manufacturing processes such as injection molding, but also machining and even laser cutting. Acetal ranks high on any “Machinability” chart, and milling or turning it with sharp tooling yields fantastic surface finishes. It is also a “self-lubricating” plastic, which means that it has the ability to transfer tiny particles to its mating surface. In a bearing application where a simple Acetal bearing rides on a steel shaft, for example, the plastic will transfer tiny particles to the steel during a break-in procedure, lubricating the surface without requiring the user to apply any additional lubricants.
In addition to sliding elements like linear bearings, other common applications of Acetal include gears, knife handles and yo-yos. Like many other plastics on this list, Acetal is very difficult to bond to itself with adhesives. For more information on this awesome material, see our Working with Acetal blog post.
Like Acetal and ABS, Nylon is another thermoplastic. Also like Acetal, it’s a self-lubricating plastic, which makes it common in rotary and sliding bearing applications.
Nylon is easy to work with and can be dyed, machined, and thermoformed with ease. It’s an inexpensive thermoplastic with good infrared resistance. Due to its high elongation properties, Nylon is a great choice for flexing and bending applications.
Some common uses of Nylon include seat belts and multiple outdoor products. Ripstop Nylon is a woven fabric of nylon that’s particularly resistant to tearing and ripping, making it common in parachutes and tents. Nylon has also aided in the breakthrough of climbing rope development, thanks to its high strength and elongation properties. After being developed by a DuPont scientist during World War II, it’s still common in “dynamic” climbing ropes today.
PTFE (Polytetrafluoroethylene), like Acetal, is incredibly easy to machine and has great bearing properties. It’s commonly referred to as Teflon, another DuPont trademark, and although it’s slightly less strong than Acetal, it has better chemical resistance. It also absorbs less liquid than Acetal, making it an ideal choice for surviving in harsh environments.
Unfortunately, PTFE is very susceptible to creep, or a change in physical shape over time while under mechanical stress. This is worth keeping in mind while going through material selection - another plastic with similar properties, such as Acetal, may be a better choice if repeated mechanical stress is expected.
Thanks to its fantastic dielectric, or electrically insulating, properties, PTFE is commonly used as a coating in wiring. It’s also used for o-rings, gaskets, and as a coating for non-stick pans thanks to its high heat and liquid resistance.
LDPE (Low Density Polyethylene) and HDPE (High Density Polyethylene)
LDPE is another thermoplastic with good chemical resistance. Thanks to its low cost and the ease with which is can be machined, it’s a great choice for low-load applications and prototyping before switching to more expensive materials. Common LDPE applications include plastic bags, six-pack rings and packaging foam.
As the name suggests, HDPE is denser than LDPE, although like its less-dense brethren, HDPE is easy to machine and not very expensive. When machined with care, it can even hold fairly tight tolerances, although thanks to its thermoplastic properties, one should be aware of the ease with which it changes shape when exposed to heat. HDPE is common in the food and packaging industry, and is most well-known for being utilized as a common milk jug material. It’s also often used in the fireworks industry, due to its high strength to cost ratio and the fact that it tears instead of shatters when under load.
PEEK and Ultem
Yet another thermoplastic, PEEK, is known for retaining its great mechanical and chemical properties at very high temperatures. Due to this fantastic property, it is expensive, and should therefore be reserved for the most demanding applications. PEEK is often utilized in high-vacuum or high-pressure conditions and medical devices. Thankfully, it also machines fairly well.
Machined PEEK (source)
Like PEEK, Ultem sustains its great properties at very high temperatures, but slightly less so thank PEEK. Due to this, it is less expensive, but still easy to machine.
Although plastics often have properties not found in metals, composites, or wood, there are certain design and manufacturing considerations to keep in mind. For one, adhesives often don’t work well with plastics, especially when attempting to bind two parts of the same material together.
When it comes to fastening plastic parts, bolts work very well. Although many of the materials mentioned above will hold threads without issue, tapping plastic directly isn’t ideal for longevity. Instead, there are a variety of threaded inserts that work well in plastics:
- Helicoils can be installed easily with a hand drill and are available in a huge variety of threads.
- Although they’re typically more expensive, press-in inserts can be installed with an arbor press in a pinch.
- Heat-set inserts are a great, low-cost option for adding threads to your plastic part and can be installed with a soldering iron or other heat source. There are specialty molded-in inserts that are ideal for thermoforming processes such as injection molding, as well.
For all of your threaded insert needs, we recommend checking out McMaster-Carr to learn more about them and pick up a few to try out.
When it comes to designing and manufacturing parts in a certain material, the best place to start is often the manufacturer of the material itself. Many, such as DuPont, offer fantastic PDF design manuals that outline everything you need to know about designing a well-functioning mechanical component with their materials.
Just like metals, plastics of the same base material can come in many different forms, often with the intent to improve a certain property. We recommend starting with the most affordable, base material first, and then exploring other alternatives with additives as the needs of your situation become more clear.
For example, a common example of a plastic blend is Delrin AF, which is Delrin that’s been impregnated with Teflon strands for improving its bearing properties. There are a lot of fascinating plastic blends out there, and they’re worth exploring!
Plastics are incredibly important in mechanical design and often overlooked as a possible material for certain applications. Thanks to their great properties, affordability, and often-high machinability ratings, plastics should be considered equally as important as metals when performing material selection for a device or product.