Materials Monday: Filling in the Blanks

Choosing the right raw materials is a critical step in manufacturing a great product. Of course, you’ll consider pure metals and/or standard polymer classes of thermoplastic, thermoset, and thermoelastic materials. Whether using metals or plastics for your custom parts, there are also plenty of alloys or blends to consider. You’re probably already familiar with many of the metal alloys used throughout history. For example, millennia ago, metalworkers ushered in the Bronze Age when they discovered that adding small amounts of tin or zinc to their crucibles made copper much harder and stronger. Centuries later, some lazy blacksmith neglected to clean his furnace and realized that the bits of charcoal he’d left behind seemed to make the iron much more durable, thus creating the first steel. And far more recently, metallurgists learned that a smidgeon of chromium would make rusty old steel “stainless.” See our blogs Materials Monday: The Big Picture and Materials Monday: Polymers on Parade for our introductions to metal and plastic materials respectively.

It’s clear that alloying elements profoundly affect a metal’s physical properties and resulting performance, so it shouldn’t come as much of a surprise to learn that similar stories exist in the polymer world. In 1862, for instance, Alexander Parkes blended camphor and cellulose to make the notorious Parkesine, a material known for its highly flammable qualities. After this came Bakelite, a mix of phenol and formaldehyde, followed by polyethylene, polystyrene, and nylon, all derived from a witch’s brew of petrochemicals and various additives.

Today, the materials website MatWeb lists close to 100,000 polymers in its excellent online database. The Society of Plastics Engineers shows nearly as many, along with roughly half that number of additives and colorants. Long story short, there’s a polymer for practically every application imaginable, with more coming online daily. This Materials Monday will review some of the more common “alloying” elements, a.k.a. fillers and blends, used to construct this seeming universe of polymers, beginning with colorants.

A Rainbow of Colors

Walk through any toy store and you’ll quickly see a vast array of colorful playthings, from children’s ride toys to Legos. In large-volume applications like these, the plastics used to make such items were most likely produced by a plastic house as “compounded” or pre-mixed colored pellets and then delivered to the toy manufacturer for molding. (If you have 8 extra minutes, watch this video of Lego Bricks in the Making which shows the entire amazing process.) A lower-volume alternative is “masterbatching.” Here, colored pellets with a 50/50 mix of resin to pigment—think of them as concentrates—are mixed at a 50:1 or so ratio with natural pellets that might be white, clear, or muddy brown (depending on the polymer). This is often called the salt and pepper approach. It’s also possible to spray colored liquid pigment onto natural pellets immediately before molding or inject a dry pigment directly into the molding machine hopper. Whatever the approach, it’s critical that the correct amounts of colored pellet and pigment are used, and that the pigment is compatible with the base resin. Color matching is possible in most instances, but it is both less expensive and more predictable at higher volumes (which is why the yellow and red Legos you buy this year will look exactly like those purchased many years ago).

Fill ‘er Up

All plastics can use a little help now and then. For instance, they might need a little extra muscle for a tough job or a touch of sunblock for extended stays in the sun. In these and many other applications, organic and inorganic compounds can endow a polymer with superpowers well beyond its native capabilities. Here are some of the more common reasons to use fills and additives today:

• Flexibility comes in the form of plasticizers, which make plastics…well, more plastic. Consider the rigid PVC pipes under your bathroom sink; thanks to plasticizers, your shower curtain was also made from PVC, as was the flexible PVC coating on the electrical wires coursing through the walls. Inventor John Hyatt made the Parkesine mentioned earlier more pliable by adding camphor, giving us celluloid, the stuff of early movie films. Modern plasticizers include the ester family and its members including phthalate, benzoate, trimellitate, and aliphatic dibasic esters. Citrates are another option (a type of tetraester), as are castor and palm oil, chlorinated paraffin, and the well-known polymer polyester. Some concerns exist with using plasticizers in healthcare and food products, so be sure to follow the appropriate regulatory guidelines during your product’s design phase. Prismier can help check this box.
• Strength is increased with an abundance of additive options. For example, just as fiberglass sailboats are strong enough to weather the storm, so are glass fiber-filled plastic parts stronger and more impact resistant. Glass fill does tend to increase brittleness, however, and also makes the molding process more challenging, but the plastic injection molding experts at Prismier can provide plenty of good advice on ways to avoid shrinkage, warping, and inadequate fill when using glass fills. That’s also true for carbon and ceramic fillers, two slightly more expensive but even stronger alternatives to glass fill. Be aware, though, that each of these additives is abrasive, will lead to increased wear on the mold and associated tooling, and may drive up the part cost. They will also affect the color or minimize your options, especially with carbon-filled parts (which are typically black).
• UV-Resistance is another common ask in product design. Stick a piece of budget lawn furniture out in the summer sun for a few months and you’ll find out why—not only will it fade or turn an ugly brownish-yellow, but it will begin to crack and become “chalky.” That’s because the ultraviolet radiation that gives us humans sunburn and skin cancer also attacks the molecular bonds in most plastics, weakening them. The good news is that adding a UV stabilizer to the pellet during the manufacturing process will help mitigate this unpleasantness and keep your injection-molded parts pretty for years to come. Three types are available. “Absorbers” such as titanium oxide and the super-mouthful hydroxyphenyl benzotriazole are low in cost and suitable for short-term use, while so-called “quenchers” absorb light energy, keeping those pesky photons away from the polymer chains. There are also HALS (hindered amine light stabilizers) or “scavengers” that trap the free radicals formed when sunlight strikes plastic (and human skin). Sometimes, all three are combined, the polymer equivalent of using SPF 100 sunblock at the beach.
• Flame retardants are also needed, for instance, melamine and metal hydroxides are two common flame retardants, as are organophosphorus compounds like tricresyl phosphate. These additives are essential when designing products for consumer use, as many plastics—polystyrene is one—are flammable and produce harmful gases when aflame. So important is this topic that the U.S. Underwriters Laboratories has developed UL94, the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing. It provides six classifications of flammability, from HB (which defines burn characteristics on horizontal surfaces) to 5VA (burning stops within 60 seconds on a vertical specimen, no drips allowed, and test specimens cannot develop a hole). Sitting between these two is V-0, which states that burning must stop within 10 seconds on a vertical specimen, and that drips are allowed provided they are not on fire. Additional UL 94 information is available regarding UL94 Certifications and Limitations and the Main Types of Flammability Testing.
• Other fills include the carbon fiber mentioned earlier, which makes polymers more electrically conductive, as do nickel-coated graphite and some metal fills. Minerals such as calcium carbonate and mica are inert fillers that some manufacturers use to reduce part weight and cost alike, while companies such as Microban offer polymer additives that inhibit bacteria in hospital settings. As for the need for nicer-smelling plastic products, think lavender-scented trash bags, laundry baskets, and sporting goods. Why not?

Before leaving the fills and additive topic behind, bear in mind that there’s much more to this than the compounds and their various chemical compositions. For fills, at least, the shape, size, and concentration of the particles also play crucial roles. We already mentioned glass and carbon fiber, which are typically composed of individual strands but fills also come in spheres, cubes, flakes, and other geometric shapes. Smaller particles tend to improve strength more so than larger ones (but not always), just as the “more is better” principle does not always apply with fill concentration. Here again, reach out to an expert before traveling too far down the fills and additives path.

Getting Blended

No, we’re not referring to blending the margaritas at last weekend’s pool party here. That’s because polymer scientists are an adventurous lot always hunting for ways to cook up new plastics, which often includes blending two or more of them together. One notable outcome of this practice is Noryl, an engineering-grade polymer made of polyphenylene ether (PPE) and polystyrene (PS) that the plastic experts at General Electric came up with in 1966. Countless others have had similar success and generally, they go on to patent their creations.

In most cases, the term “blending” refers to what we described earlier—incorporating various fillers, additives, and colorants into the pellet during the manufacturing process or immediately before injection molding. The latter relies on special machines that work much like the Kitchen Aid some of us keep under the counter for preparing weekend libations (albeit much more slowly) or via “dosing” devices that drop predetermined amounts of pellets into the hopper during the injection molding operation. At Prismier, we mold out of a wide variety of different thermoplastics using blends like PC/ABS and PPO/PS, as well as fillers like glass, carbon, ceramic, flame retardants, and UV absorbers. Our comprehensive plastic manufacturing services can help you choose the right raw materials and processes to take your product from design to delivery.

Finally, it’s important to note that everything discussed here refers to the pelletized feedstocks used in plastic injection molding. Many of these materials, though are also available in rod, sheet, and cast blocks suitable for CNC machining. Yes, these UV-resistant or glass-filled materials must be purchased that way—with the desired additives already included—but there’s certainly no shortage of them on the market. As always, give us a shout at Prismier if you want to explore further.