Choosing the Right Plastics for Machining: Key Considerations and Material Selection Guide

Plastics are extensively utilized today due to their diverse properties that suit a wide range of engineering and design applications. While plastics can be transformed into various products, including various plastic molding methods, machined parts, and plastic products are sometimes preferable. This is especially true for prototypes or small production runs, where the cost of using a plastic molding machine is prohibitive, as it entails the expense of molds and accompanying tooling.

Compared to plastic molding, machined plastic parts can also achieve tighter tolerances, while features like conical or tapered sections are also easier to make. Other custom features might include complex threading, holes completely through a workpiece, and walls with varying thicknesses, while undercuts are not as problematic as plastic molding. Machined materials are often the same as those used in processes like plastic injection molding, so machining instead of molding offers manufacturers greater design flexibility.

Machining Plastics: Guidance on Material Selection 

As must be done when molding plastic, machined materials must be chosen carefully. Key considerations include chemical resistance, dimensional stability, hardness, heat resistance, strength and water resistance, and other mechanical properties that aid functionality. Machinability also varies for different types of plastic. Molding machines work better for certain types of resins or composites. They are great for large-scale production, but with modern CNC (computer numerical control) technology, low and mid-level production runs can generally be machined more rapidly, inexpensively, and accurately.

When compared to plastic molding, machined plastic parts offer: 

• Lower initial costs, as molding equipment, involve a substantial investment in molds, tooling, and other equipment; CNC machines, although costly, are less expensive than some molds for plastics, which can cost as much as $80,000 or more. 
• Greater material selection, as components made from certain composites like PTFE or PVC are challenging to make via plastic molding; machined parts neither require heating to their melting points nor will they release toxic gases during fabrication.
• Considerably tighter dimensional tolerances can be achieved than with plastic molding; machined parts also won’t have flow lines, sink marks, vacuum voids, warpages, or other defects commonly found with molded components.
• Better consistency due to less stress than with parts made via plastic molding machines; often, molded components must be machined to remove defects left by molds or dies.
• An easier way to alter designs, as CNC machining involves programmed instructions that aren’t available with plastic molding; machined component designs can also be inexpensively changed by simply reprogramming the tool paths.

Machined plastics are used for applications like auto parts, components for food processing, medical devices, parts for heavy equipment, semiconductors, and many other products. Though thermoplastics are typically more likely to be machined, CNC machines can also easily handle thermoset plastics.

Thermoset Plastics

Thermoset plastics retain their shape indefinitely. As their structure changes chemically when heated, they can’t be remelted when formed during plastic molding. Machined thermosets are less common, however, as they are more brittle and tend to chip, requiring more care. These resins often are used as part of a composite material as well. Thermosets include epoxies, Micartas, melamine, and phenolics.

Epoxies

An extensive family of thermoset polymers, epoxies offer considerable thermal and chemical stability, high compressive and tensile strength, and superior adhesive characteristics. They’re particularly effective at bonding different materials, often used for gap-filling. Additionally, epoxies provide significant resistance to abrasion, impact, and water while being easy to machine. Though there are various formulations, most are made of epoxy with an equivalent amount of hardener. Epoxies are typically used for aerospace, automotive, construction, and other industrial products.

Micartas

A branded type of laminate, Micartas includes an array of materials containing canvas, carbon fiber, linen, paper, or other types of fabric-like material within a thermosetting plastic. Today, when mentioning a “Micarta” plastic, it will often refer to any generic composite made with resin and fiber. These laminates present high compressive and impact strength while providing excellent electrical insulation qualities. Commonly used for products like electrical insulators, knife handles, and substrates for printed circuit boards, they can be found in a wide range of other products.

These include: 

• Aerospace components like propeller blades
• Automotive parts
• Countertops and tabletops
• Electrical insulation
• Electronics
• Equipment for generating and distributing electricity
• Guitar fingerboards
• Handgun grips
• Handles for kitchen tools
• Heavy equipment 
• Office equipment
• Pool cues
• Safety kits like hard hats

While Micartas are used in many sectors, including the aerospace and auto industries, they’re most widely used in the power-generating industry for high-strength insulation.

Melamine

An organic compound, melamine combines with formaldehyde and other constituents to create resins. Its makeup allows melamine resins to present fire retardance and acoustic and thermal insulation properties. Melamine plastics are also resistant to chemicals, heat, and ultraviolet radiation. These resins can be used for assorted plastic products like laminate flooring, Formica, dry eraser boards, and cooking utensils, as well as for soundproofing and other insulation. Melamine is primarily used in the chemical processing industry.

Phenol

An organic compound primarily used for making plastics, phenol is also known as benzenol, carbolic acid, and phenolic acid. Phenolic resins provide capabilities that help products withstand heat while providing chemical resistance, dimensional stability, and electrical resistance. Displaying considerable hardness, the durability of phenolic resins requires pairing with fillers and reinforcing ingredients, as the material is quite brittle on its own. These resins are typically used as adhesives, binders, laminating resins, powders for plastic molding machines, and surface coatings.

Thermoplastics 

Thermoplastics are the largest group of plastics, able to take on new forms when remelted without altering their properties. These plastics are also the ones best suited for machining. Thermoplastic composites often include carbon fibers, glass, graphite, or molybdenum disulfide fillers.

Acetal

An engineered plastic that’s also known by its chemical abbreviation, POM (Polyoxymethylene), acetal is stiffer and stronger than almost every other thermoplastic. Molding machinery can produce products with acetal, though it’s also easily machinable and can achieve remarkably tight tolerances. Acetal resins can be divided into either homopolymer or copolymer types.

Acetal homopolymers are stiffer, more wear-resistant, and have higher tensile strength than copolymers. At the same time, they also work well within a wider range of temperatures, presenting greater flexibility and impact strength at room temperatures. Acetal copolymers resist moisture absorption and shock and have a low friction coefficient. They also have better dimensional stability than homopolymers, offering characteristics like excellent machinability, rigidity, and strength.

Acetal copolymers are used in agriculture, automotive, electronics, food processing, medical, and pharmaceutical manufacturing industries. The aerospace, nuclear energy, and pharmaceutical processing sectors use homopolymers. Both types are typically used for bearings, bushings, fasteners, gears, handles, housings, seals, valves, and tooling.

ABS

More scratch-resistant and harder than acetal, acrylonitrile butadiene styrene (ABS) is an opaque thermoplastic. Molding machine processes are commonly used to fabricate products from ABS, though the polymer also supports machining. Made from acrylonitrile, butadiene, and styrene, its many properties combined with lower cost make it a widely used material for many applications. However, though it can be used in temperatures ranging from -58°F to 158°F (-50°C to 70°C), it isn’t weather-resistant and has only limited resistance to caustic or acidic solutions.

Other ABS properties include: 

• Dimensional stability
• Easily bonds and welds
• Insulates against electricity
• Low density
• Low rate of moisture absorption
• Offers good impact strength and toughness at lower temperatures
• Resistant to x-rays, scratches, gamma rays chemicals

Industries that use ABS include the automotive, electronics, and food technology sectors. ABS is used in appliances, computers, and other housings, along with electroplated parts, furniture components, handles, interior paneling, piping, and radiator grills.

Acrylic

A transparent thermoplastic, acrylic offers excellent strength and optical clarity, making it useful as a substitute for glass. Though it has lower impact strength than glass, acrylic can be as much as 24 times resistant to impact while being half as light and more flexible. Acrylic resists abrasion, chemicals, shock, and ultraviolet radiation well, expressing good flexural strength and dimensional stability.

While stress cracks can be a problem, acrylics are more economical and flexible than glass. Aerospace, automotive, electronics, medical, and transportation sectors all use acrylic materials, which are used for appliances, lenses, lighting fixtures, medical devices, signage, sound barriers, toys, various components, vehicle trim, and windows.

Nylon

Belonging to a group of plastics called polyamides, nylon is one of the most commonly used engineering plastics worldwide. While products made from nylon are typically made via plastic molding, machined nylon products are possible with higher speed cutting and sharper tooling. There are two main nylon grades, Nylon 66 and Nylon 6, which relate to the nitrogen in its elemental makeup.

Though these two types of nylon offer slightly different characteristics, both resist wear, heat, fatigue, and most chemicals, acids, alcohol, and alkalis can break nylon down. These nylons also self-lubricate and feature a low friction coefficient while providing considerable water absorption, mechanical strength, and electrical insulation properties. Used within the aerospace, construction, electronics, and food processing sectors, they’re used for various parts like bearings, bushings, gears, housings, nuts, packaging, seals, washers, wheels, and equipment used in the food and beverage industries.

Polycarbonate

Demand for polycarbonate (PC) now exceeds 1.5 million tons annually, making it one of the most-used types of engineering plastic. Molding machinery and CNC technology can both handle polycarbonates well. Polycarbonate is versatile, has superior impact resistance, electrical properties, capability for withstanding temperature extremes, and better transparency than many kinds of glass.

With a middle-range price tag, polycarbonate is also available in grades resistant to stress fracturing, reinforced, food-compliant, flame-retardant, and other optional properties. Industries that include automotive, electronics, food processing, medical, semiconductor, and telecommunications often use polycarbonates. They’re used frequently in products that include vehicle windows and exteriors, seals, rollers, manifolds, lenses, gaskets, covers, connectors, casings, and appliances.

PEEK

As a high-performance plastic, polyether ether ketone (PEEK) is also one of the most expensive. Its distinctive properties combine to make it among the most dimensionally stable and strongest plastics for load-bearing and harsh conditions. PEEK exhibits abrasion, chemical, heat, and wear resistance, exceptional strength, and other properties that make it useful for a wide range of industrial components.

While medical-grade PEEK is used in the dentistry and healthcare sectors, industrial-grade PEEK is used in industries that include energy, electronics, chemical, automotive, and aerospace. Components like bearings, bushings, cable insulation, dental caps, implants, manifolds, orthopedic and other medical devices, rings, rollers, seals, washers, and wheels can all be made with PEEK.

Polyethylene 

The versatility of polyethylene (PE) has made it one of the most-used types of polymers available for plastic molding. Machined PE normally tends to be denser, but cutting, drilling, and turning can be done easily on less dense grades. Its high impact strength, lower cost and toughness, and resistance to chemicals and ultraviolet radiation make PE a popular plastic. Mechanical properties like compressive, flexural, and tensile strength combine to make it easily machinable. In contrast, PE’s stability, resistance to moisture, lighter weight, and insulating properties also add to its effectiveness in various applications.

PE is typically used in agriculture, automotive, chemical, construction, food, medical, and mining sectors. Applications for PE include components for valves, pumps, prosthetics, and medical equipment, along with parts like bearings, bushings, die pads, gaskets, hinges, sprockets, and washers.

PVC

While making products from polyvinyl chloride (PVC) via plastic molding is possible, machined PVC is more commonly used due to its high melting temperature. It also exhibits excellent machinability, which makes tight tolerances easy to attain. A light, durable, and water-resistant plastic, PVC’s mechanical and insulation properties combine with its cost-effectiveness to make it an ideal solution for many applications.

PVC has many other useful qualities, which include: 

• Above average rigidity
• Excellent strength and hardness
• Extremely high density
• Great corrosion resistance to acids, alkalis, and nearly all inorganic chemicals
• Great insulating, mechanical, and thermal characteristics
• Ignition temperatures that reach 851°F (455°C), making it extraordinarily fire retardant
• Low absorption of moisture
• Superior tensile strength

Industries like the automotive, construction, electronic, food processing, medical, pharmaceutical, and transportation sectors use it for an array of components like bearings, bushings, guides, handles, housings, isolators, rollers, and spacers.

PTFE 

Although technically a thermoplastic, PTFE (Polytetrafluoroethylene) is sometimes classified as a thermoset, as it melts only at extraordinarily high temperatures. For this reason, PTFE isn’t often used in plastic molding. Machined PTFE, commonly known by its brand name Teflon, is used for a wide range of applications due to its superior stability when exposed to heat and high chemical resistance. Compared to other engineering thermoplastics, PTFE has poor mechanical qualities, but its dimensional stability can be improved by adding carbon or glass.

Other properties PTFE exhibits include: 

• Considerable impact strength
• Does not absorb moisture
• Elevated electrical insulation properties
• High resistance to weather and UV radiation
• Not very flammable
• Resistance to water
• Very low friction coefficient
• Wide-ranging working temperatures between -436°F and 500°F (-260°C and 260°C)

Aerospace, chemical processing, food processing, medical, petrochemical and pharmaceutical industries all use PTFE, which can also be used in scientific applications. Used for making parts for pumps, semiconductors, and various assemblies, machined PTFE is also used for components like bearings, bushings, capacitors, fittings, gaskets, gears, insulators, manifolds, and valves.

Considerations When Choosing Materials for Plastic Molding Machines 

Before choosing a material, the function of the end product needs to be considered. While certain other properties are important for plastic molding, machined materials must consider their chemical and heat resistance, dimensional stability, and mechanical characteristics. For example, a part used in an airliner’s engine will require substantial heat resistance, which may require a heat-resistant plastic like PTFE that’s also readily machinable. Regulatory requirements should also be part of decision-making, especially if the product’s performance affects safety. 

Other considerations on plastic materials should include: 

• Pricing: Cost is crucial when selecting the best material for a machining project. Resins vary significantly in pricing, from the lesser cost of low-density polyethylene resin to a significantly more pricey PEEK resin. It’s not always the most inexpensive material that’s most cost-effective; however, as if a material won’t work well it could lead to higher costs from increased product failure. 
• Fabrication: Material properties needed for machining plastics often differ from those used in plastic molding. Machined materials don’t need to look at melt flow rates or cooling times but their machinability. CNC plastic machining operations typically look at mechanical properties like dimensional stability, wear resistance, and impact strength.
• Environmental impact: Both environmental regulations and the public’s growing awareness of ecological issues make this important to material selection for both plastic molding and machining. The introduction of biodegradable plastics minimizes ecological impact, as these materials deteriorate naturally over time. As thermoplastics can be melted over and over, they’re also more recyclable, so may be preferred over a thermoset plastic.
• Aesthetics: The look of a product is usually considered less important than functionality, but for products made for public use it’s important to consider things like colors and textures when choosing materials. Plastics like ABS, acrylic and polycarbonate can be colored to improve a product’s appearance.

Finally, it’s important to avoid some commonly made mistakes that happen with material selection, whether for plastic molding, machined plastics, or other plastic fabrication methods. Manufacturers often focus too much on the material cost and too little on the material qualities. This can then affect a product’s durability or functionality. When it comes to machining plastics, for example, dimensional stability and other mechanical properties should be carefully contemplated, while characteristics like chemical, heat, and impact resistance should also be considered.

Without proper attention paid to the materials used in a machining project, costs can easily get out of hand. It requires a bit of study to recognize the best material properties for a certain application. Consideration should also go beyond cost to how the material will affect the end product, as otherwise, material selection can cause unexpected outcomes.

To learn more about the best materials to use for plastic CNC milling equipment or plastic molding machinery, contact the material experts at Spaulding Composites.