Know your plastic injection molding resins

Part 2

Hello, this is Gordon Styles, the founder and CEO of Star Rapid. I’ve been involved in rapid prototyping and new product development for over 35 years, and I’m delighted to welcome you back to another exciting installment of Serious Engineering for Serious Engineers.

Today’s episode is Part 2 in our hair-raising, spine-tingling, action-packed series chock full of spills and chills for the whole family. But before I can reveal the shocking conclusion that everyone will be talking about, be sure you’ve already seen Part 1, here.

If you want to be a manufacturing action hero like me, you have to eat right, exercise regularly and be a master of the tools of your trade. And few tools are more essential than the resins we use for plastic injection molding. There are thousands to choose from for any imaginable application but only a relatively small handful make up the vast majority of products we see everyday. To help you find the one that’s right for you, let’s take a closer look at another five resins and prepare to level up your warrior skills.

1. High Density Polyethylene or HDPE

Polyethylene comes in many different flavors, each with unique properties. High-density polyethylene was first discovered by subjecting ethylene to extremely high-pressures but the process was expensive and difficult.

In 1953 a German chemist created a low-pressure method that made HDPE available in large volumes, for which he won a Nobel Prize. HDPE is chemically resistant, stiff and stands up to UV. It also handles temperature fluctuations well and is easily extruded in long lengths. This makes it the most common material for rigid pipes and tubes to carry various kinds of fluids. It’s used in plastic drink bottles, and it recycles well. However, it is flammable, doesn’t biodegrade and cracks tend to propogate because of its crystalline structure.

Interestingly, ultra-high molecular weight polyethylene makes super strong fibers that are among the toughest of all man-made materials, such as this bullet-proof vest made from Dyneema.

2. Polypropylene or PP

PP was first polymerized in 1954 and began mass production soon thereafter. It’s similar in many respects to PE but has superior strength and thermal resistance. Like PE it can be found in packaging and food storage but it’s also strong enough to be used as a substitute for ABS. PP is also the lightest commodity plastic – so light that it floats – and is the most common type of artificial clothing fiber.

One of the most important features of PP is the ability to make living hinges, such as you find on the caps of shampoo bottles. This is because the glass transition temperature of PP is just below room temperature, making it an amorphous semi-solid.

It also bounces back on those occasions when you’re forced to land your fighter plane behind enemy lines.

3. Polyethylene Terepthalate or PET

Another member of the polyethylene family, low-molecular weight PET is one of the most common synthetic fibers in the world for insulation, carpets and wrinkle-free clothes. It wasn’t until the 1970’s, however, that a new process for blow-molding high-molecular weight PET created the first clear plastic drink bottles. This takes advantage of PET’s excellent sheer strength.

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They are almost impervious to gas or liquid and molten resins are easily colored with dies and pigments. PET is one of the most common recycled plastics, it’s food-safe, non-conductive and excellent for adhesive tapes. It’s also found in electrical insulation, packaging and medical garments.

PET does produce toxic gas when it burns, and plastic containers don’t stand up to boiling water.

4. Polyoxymethylene or POM

Discovered in the 1920’s by yet another German chemist, industrial POM didn’t get its start until 1956 when DuPont patented their version under the trade name Delrin. POM is an acetal resin, meaning it has high stiffness, low moisture absorption, great mechanical strength and low friction.

All of these qualities make POM an ideal material to replace metal parts in gears, bearings, rollers, sliding tracks and conveyor systems. It machines very well so it can be milled, drilled and turned like metal.

But it’s expensive so this resin should be considered only when the superior properties are really needed. And because of its natural lubricity, POM is very difficult to bond with adhesive and it doesn’t take paint, not up and not down.

5. Polyetherimide or PEI

A true high-performance resin, PEI was developed at GE, now Sabic, in the early 1980s and is marketed as Ultem. Like other tough engineering plastics such as PEEK, Ultem is often sold in rods, plates and blocks for subsequent machining. It’s not as hard or strong as PEEK but is much less expensive.

Ultem’s great advantage is high heat resistance, so it’s found in aerospace, military, scientific and medical applications. In fact, it can sustain continuous use at 170 Deg. C.

For laboratory and medical equipment, Ultem can be steam sterilized multiple times without degradation. It has a natural semi-transparent amber color that is very attractive, but it does not take other colors well.

Ultem needs to be processed at a high temperature and it’s notch sensitive. This means that the material is stiff and will crack at stress points, so it’s necessary to use very sharp cutting tools when it’s being machined. It also conducts electricity so don’t try this at home.

That’s all we have time for on this episode of Some Serious Engineering. Don’t forget to like, share and subscribe using this lovely bell icon right below. Need to make parts for your next project? We’re standing by to support you when you contact us at Star Rapid via the link below.

And always remember if you want to be a true manufacturing hero, use the power only for good, and not for evil.

Warning: Side effects may include femme fatales, double agents, trap doors, false mustaches, tough henchmen, goofy sidekicks, russian roulette, infinite bullets, trust issues, buried treasure, friendly villagers, prosecuting attorneys, training montages, tourniquets, poison pills and coming out of retirement for one last mission.