An Informational Brief on Polymer Machining
Did you know that PPS (or Polyphenylene sulfide) products offer the broadest resistance to chemicals of any high-performance thermoplastic? It’s no surprise that this makes them a popular choice for industrial applications such as wheel bushings, chemical pumps, and compound clamp rings for semiconductor wafers.
With Machining PPS: A Plastics Guides, AIP provides you with a guide to this material and its machining process. First, let’s start with the basics: thermoplastics vs thermosets.
Machining Thermoplastics vs Thermosets
We’ve already said that PPS is a thermoplastic, but what does that mean exactly?
All polymers can more or less be divided into two categories: thermoplastics and thermosets. The main difference between them is how they react to heat. Thermoplastics like PPS, for example, melt in heat, while thermosets remain “set” once they’re formed. Understanding the technical distinction between these types of materials is essential to CNC machining them properly.
What type of thermoplastic is PPS in particular? It’s a semi-crystalline, high-performance thermoplastic that has an extremely stable molecular structure. The chemical resistance of PPS is often compared to PEEK and fluoropolymers.
Properties & Grades of Machined PPS
There’s a lot to like about PPS’s material properties. As we mentioned before, PPS has exceptional chemical resistance that makes its bearing grades especially favorable for the chemical industry or caustic environments. In particular, its resistance to acids, alkalis, ketones, and hydrocarbons lend PPS stellar structural performance in harsh chemicals.
Additionally, PPS materials are inert to steam as well as strong bases, fuels and acids. Combine that with a low coefficient of thermal expansion and zero moisture absorption, and you get a material that is ideal for continuous use in corrosive or hostile environments. PPS has replaced stainless steel for a lot of industrial applications for this reason.
Most impressively, PPS will not dissolve at temperatures below approximately 200 °C, no matter what solvent is used. In fact, all grades of PPS share UL94 V-0 flammability ratings, without requiring flame retardant additives, resulting in an excellent material for aircraft where flame resistance is paramount.
Some grades of PPS that we regularly machine at AIP Precision Machining include Ryton®, Fortron®, TECHTRON®, TECTRON® HPV, TECATRON PVX and TECATRON CMP.
The process of annealing and stress-relieving PPS reduces the likelihood of surface cracks and internal stresses occurring in the material. Post-machining annealing also helps to reduce stresses that could potentially contribute to premature failure. AIP’s special annealing process for PPS is designed to take the specific properties of PPS into account, and we advise anyone working with PPS to hire a manufacturer that understands its unique demands.
PPS is a fantastic material for machining. Its low shrinkage and stable dimensional properties make it easy to machine to incredibly tight, precise tolerances. A unique characteristic of PPS is that when dropped, it sounds just like a piece of metal hitting the floor.
PPS, like many other thermoplastics, is notch sensitive, so take care to avoid sharp corners in design. We recommend carbide tipped cutting tools for working with PPS as they provide an ideal speed and surface finish.
We also suggest non-aromatic, water-soluble coolants, such as pressurized air and spray mists, to achieve optimum surface finishes and close tolerances. Coolants have the additional benefit of extending tool life as well. No known coolants attack nor degrade PPS.
Contamination is a serious concern when machining polymer components for technically demanding industries such as aerospace. To ensure the highest level of sanitation down to the sub-molecular level, AIP Precision Machining designs, heat-treats, and machines only plastics, with any sub-manufactured metalwork processed outside our facility.
To learn more, read our article “Three Ways to Ensure Sterilization in Your Plastic Machined Medical Applications.”
PPS Machining Guide: Supportive Information
An Informational Brief on Polymer Machining
AIP Precision Machining has worked with many thermoplastics over the past three decades, including TORLON: a PAI, or polyamide-imide, engineered by Solvay Specialty Polymers.
Due to its reliable performance at severe levels of temperature and stress, TORLON is ideal for critical mechanical and structural components of jet engines, automotive transmissions, oil recovery, off-road vehicles and heavy-duty equipment.
AIP has over 35 years of experience machining complex components from TORLON and various other thermoplastic materials. We are providing this Machining TORLON Guide as yet another insightful technical brief about our polymer component manufacturing process, and how it differs from that of metal machining, injection molding, or 3D printing.
Plastic CNC Machining
Before discussing the process of machining TORLON, it’s important to understand exactly what plastic machining is.
CNC (Computer Numerical Control) machining is a process in the manufacturing sector that involves the use of computers to control machine tools. In the case of plastic machining, this involves the precise removal of layers from a plastic sheet, rod, tube or near net molded blank.
The early history of CNC machining is almost as complex as a modern CNC system. The earliest version of computer numerical control (CNC) technology was developed shortly after World War II as a reliable, repeatable way to manufacture more accurate and complex parts for the aircraft industry. Numerical control—the precursor to CNC—was developed by John Parsons as a method of producing integrally stiffened aircraft skins.
Parsons, while working at his father’s Traverse City, Michigan-based Parsons Corp., had previously collaborated on the development of a system for producing helicopter rotor blade templates. Using an IBM 602A multiplier to calculate airfoil coordinates, and inputting this data to a Swiss jig borer, it was possible to produce templates from data on punched cards.
Parsons’ work lead to numerous Air Force research projects at the Massachusetts Institute of Technology (MIT) starting in 1949. Following extensive research and development, an experimental milling machine was constructed at MIT’s Servomechanisms Laboratory.
Due to the many different kinds of polymers and composites, it’s important to have strong technical expertise of polymer materials when machining plastic components; some plastics are brittle, for example, while others cut similarly to metal. The challenge of plastics is their wide range of mechanical and thermal properties which result in varying behavior when machined. Therefore, it’s important to understand the polymer structure and properties of TORLON if you’re machining it.
Thermoplastics vs Thermosets
When it comes to polymers, you have two basic types: thermoplastics and thermosets. It’s crucial to know which one you’re working with due to distinct differences between how these two main polymer categories react to chemicals and temperature.
Thermoplastics soften when heated and become more fluid as additional heat is applied. The curing process is completely reversible as no chemical bonding takes place. This characteristic allows thermoplastics to be remolded and recycled without negatively affecting the material’s physical properties.
They possess the following properties:
• Good Resistance to Creep
• Soluble in Certain Solvents
• Swell in Presence of Certain Solvents
• Allows for Plastic Deformation when Heated
Thermosets plastics contain polymers that cross-link together during the curing process to form an irreversible chemical bond. The cross-linking process eliminates the risk of the product re-melting when heat is applied, making thermosets ideal for high-heat applications such as electronics and appliances.
They possess the following properties:
• High Resistance to Creep
• Cannot Melt
• Rarely Swell in Presence of Solvents
Phenolic, Bakelite, Vinyl Ester and Epoxy materials would be considered examples of a thermoset, while ULTEM, PEEK, DELRIN and Polycarbonate materials are examples of thermoplastics.
The thermoplastic category of polymers is further categorized into Amorphous and Crystalline polymers per the figure below:
TORLON is considered an amorphous, high-performance thermoplastic. Most amorphous polymers are thermoform capable, translucent and easily bonded with adhesives or solvents.
Various Grades of Machined TORLON
What makes TORLON unique is how it possesses both the incredible performance of thermoset polyimides and the melt-processing advantages of thermoplastics. The compressive strength of (unfilled) TORLON PAI is double that of PEEK and 30% higher than that of ULTEM PEI. In fact, TORLON is considered the highest performing, melt-processible plastic.
High-strength grades of TORLON retain their toughness, high strength and high stiffness up to 275°C. This and its impressive wear resistance allow TORLON to endure in hostile thermal, chemical and stress conditions considered too severe for other thermoplastics. TORLON is also resistant to automotive and aviation fluids, making it a favorite of aerospace and automotive engineers.
One concern of using TORLON is that its moisture absorption rate is not as low as other high-performance plastics, so special care should be taken when designing components for wet environments.
There’s more than one particular type of TORLON PAI you can machine, and each has slightly different properties for perfecting this material’s use in different applications.
Here are several grades of TORLON PAI we machine regularly at AIP Precision Machining.
TORLON 4203 is the unfilled or natural grade of TORLON PAI that outperforms other grades with the best impact resistance and the most elongation. TORLON 4203 PAI can be used for a variety of applications but due to its good electrical properties, it is commonly machined for electronic equipment manufacturing, valve seals, bearings and temperature test sockets.
TORLON 4301 is a wear-resistant grade of TORLON PAI containing PTFE and graphite. It has high flexural and compressive strength with a low coefficient of friction, as well as good mechanical properties. Typical applications of 4301 are anything that requires strength at high temperature with wear resistance and low friction. This material is useful for parts such as thrust washers, spline liners, valve seats, bushings, bearings and wear rings.
TORLON 4XG is a 30% glass-reinforced extruded grade of PAI well suited to higher load structural or electronic applications. When you need a high degree of dimensional control, this grade offers the high-performance you need. Various uses of TORLON 4XG include burn-in sockets, gears, valve plates, impellers, rotors, terminal strips and insulators, among others.
TORLON 4XCF is a 30% carbon-reinforced extruded grade of PAI that has the lowest coefficient of thermal expansion and the most impressive fatigue resistance of all plastic materials. This uncommon grade works well as a replacement for metal applications as well as mission-critical aerospace components, in addition to impellers, shrouds and pistons.
TORLON PAI can be received in the form of rods, sheets, tube or film. Stress-relieving before machining through an annealing process is crucial, as it reduces the likelihood that surface cracks and internal stresses will occur from the heat generated. This also helps prevent any warping or distortion of your plastic materials.
TORLON additionally benefits from post-machining annealing to reduce any stress that could contribute to premature failure. Extruded TORLON parts, such as those machined from TORLON 4XCF and TORLON 4XG, benefit from an additional cure after machining to further enhance wear resistance; this is unique to PAI. Proper annealing of Torlon can require more than seven days in special ovens at AIP.
If the machine shop you are working with does not have a computer controlled annealing oven for plastics, then “head for dee hills” as they are obviously not TORLON machining experts.
An important consideration to have when machining TORLON PAI is how abrasive it is on tooling. If you’re machining on a short run, carbide tooling can be used, but polycrystalline (PCD) tooling should be considered for lengthier runs, machining for tight tolerance and any time you are working with reinforced grades.
Another thing to keep in mind when machining extruded TORLON shapes is that they have a cured outer skin, which is harder than interior sections. The outer skin offers the best wear and chemical resistance. If wear resistance and chemical resistance needs to be optimized, extruded TORLON should be re-cured.
TORLON PAI will nearly always require the use of coolants due to its stiffness and hardness. Non-aromatic, water-soluble coolants are most suitable for ideal surface finishes and close tolerances. These include pressurized air and spray mists. Coolants have the additional benefit of extending tool life as well.
Many metal shops use petroleum-based coolants, but these types of fluids attack TORLON. Many past experiences have shown parts going to customer without cracks, only to develop cracks over time due to exposure to metal machine shop fluids. Be sure to use a facility like AIP who machines polymers and only polymers.
Contamination is a serious concern when machining polymer components for technically demanding industries such as aerospace and medical. To ensure the highest level of sanitation down to the sub-molecular level, AIP Precision Machining designs, heat-treats and machines only plastics, with any sub-manufactured metalwork processed outside our facility.
TORLON Machining Guide: Supportive Information
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An Informational Brief on Polymer Machining
The recent popularity of PEEK (polyetheretherketone) in complex industries such as Aerospace & Defense and Medical & Life Sciences is well documented, and for good reason: this lightweight thermoplastic bears properties that make it ideal for a variety of specialized applications. This versatility makes PEEK equally capable of being used for implants and custom medical devices or machined lightweight aircraft components.
What is less known, however, is the process that goes into machining this plastic material. With over 35 years of experience machining this thermoplastic material, we at AIP have written a brief introduction to machining PEEK. We hope this gives you some insight into our polymer machining process, and how it differs from that of metal machining or injection molding.
Plastic CNC Machining
Before discussing the process of machining PEEK, it’s important to understand exactly what plastic machining is.
CNC (Computer Numerical Control) machining is a process in the manufacturing sector that involves the use of computers to control machine tools. In the case of plastic machining, this involves the precise removal of layers from a plastic material. The technique of utilizing drilling tools to carve plastics was introduced by MIT during the 1950s, and because this process is computer-controlled, products with extremely precise tolerances can be achieved.
Due to the many different kinds of polymers and composites, it’s important to have strong technical expertise of polymer materials when machining plastic components; some plastics are brittle, for example, while others cut similarly to metal. The challenge of plastics is their wide range of mechanical properties and varying behavior when machined. Therefore, it’s important to understand the polymer structure of PEEK if you’re machining it.
Thermoplastics vs Thermosets
When it comes to polymers, you have two basic types: thermoplastics and thermosets. It’s crucial to know which one you’re working with due to distinct differences between how those polymers react to heat and temperature.
Thermoplastics are capable of being repeatedly softened and pliable when temperature increases, meaning that when heat is applied, that results in a physical change for the polymer. They possess the following properties:
- – Good Resistance to Creep
- – May Melt Before Turning to Gaseous State
- – Soluble in Certain Solvents
- – Swell in Presence of Certain Solvents
- – Allows for Plastic Deformation when Heated
Thermosets, in contrast, turn into an infusible and insoluble material when cured by application of heat or chemical means, making for poor elasticity. They possess the following properties:
- – High Resistance to Creep
- – Cannot Melt
- – Insoluble
- – Rarely Swell in Presence of Solvents
Phenolic materials would be considered examples of a thermoset, while PEEK is an example of a thermoplastic.
In particular, PEEK is considered a semi-crystalline, high-performance thermoplastic. This gives it enough elasticity to be machined to various custom designs, with strong mechanical properties that provide resistance to fatigue and stress-cracking, as well as a good structure for bearing, wear, and structural applications.
Industrial Grade vs Medical Grade PEEK Machining
Depending on your application, you’ll want to machine either industrial-grade PEEK or medical-grade PEEK.
Industrial-grade PEEK is a strong, flame-retardant and abrasion resistant thermoplastic with high impact strength and a low coefficient of friction. It’s known for retaining its mechanical properties, even at elevated temperatures. As suggested by its name, this grade is most commonly used in aerospace, automotive, chemical, electronics, petroleum, as well as food and beverage industries.
Medical-grade PEEK adds biocompatibility per ISO 10993, high chemical resistance, and sterilization compatibilities to the above list of qualities. In addition, this thermoplastic is radiolucent, meaning it is not visible under X-ray, MRI or CT. Medical-grade PEEK includes polymers suitable for implants, such as PEEK Optima and Zeniva PEEK, which can stay in contact with blood or tissue indefinitely while mimicking the stiffness of bone. Other variations of medical-grade PEEK can be used for custom medical components and applications, such as articulating joints and spinal devices.
Most shops receive PEEK in the form of rods of various lengths, ranging from 6mm to 150mm in diameter. Stress-relieving before machining through an annealing process is crucial, as it reduces the likelihood that surface cracks and internal stresses will occur from the heat generated. Additional benefits of annealing include increased levels of crystallinity and the opportunity to limit dimensional changes.
If your PEEK components will undergo long stretches of machining time, it is likely you will require additional intermediate annealing steps to assure the ability to maintain critically tight tolerances and flatness.
Machining Industrial-Grade & Medical-Grade PEEK
Both industrial-grade and most medical-grade PEEK machine similarly, save for PEEK reinforced with carbon fiber. Silicon carbide cutting tools work well for natural PEEK, while diamond tools work well for PEEK reinforced with carbon-fiber.
For medical-grade PEEK applications, the best way to avoid jeopardizing the biocompatibility of the material is to machine dry. However, PEEK doesn’t dissipate heat the way that metals do, so often a coolant is necessary. In that case, air is the coolant option least likely to affect medical-grade PEEK’s biocompatibility. Any chips that are a result of machining medical-grade PEEK can be reused for industrial applications.
Contamination is a serious concern when machining polymer components for technically demanding industries such as aerospace and medical. To ensure the highest level of sanitation down to the sub-molecular level, AIP Precision Machining designs, heat-treats, and machines only plastics, with any sub-manufactured metalwork processed outside our facility.
PEEK Machining Guide: Guidelines
(Courtesy of Invibio)
|Sawing||blank||Preheat material to 120 C degrees|
|Clearance angle—degrees||15 to 30||15 to 30|
|Rake angle—degrees||0 to 5||10 to 15|
|Cutting speed—m/min||500 to 800||200 to 300|
|Pitch—mm||3 to 5||3 to 5|
|Drilling||blank||Preheat material to 120 C degrees|
|Clearance angle—degrees||5 to 10||6|
|Rake angle—degrees||10 to 30||5 to 10|
|Cutting speed—m/min||50 to 200||80 to 100|
|Feed rate—mm/rev||0.1 to 0.3||0.1 to 0.3|
|Milling||blank||No material preheat is necessary|
|Clearance angle—degrees||5 to 10||15 to 30|
|Rake angle—degrees||10 to 30||10 to 15|
|Cutting Speed—m/min||50 to 200||200 to 300|
|Turning||blank||No material preheat is necessary|
|Clearance angle—degrees||6 to 8||6 to 8|
|Rake angle—degrees||0 to 5||2 to 8|
|Cutting speed—m/min||250 to 500||150 to 200|
|Feed rate—mm/rev||0.1 to 0.5||0.1 to 0.5|
PEEK Machining Guide: PEEK Variants
When in need of a custom-machined component for a project, choosing a metallic material may be the naturally instinctive consideration to the design engineer. This article is intended to provide educational insight as to an often more sensible alternative for precision-machined, high-strength, durable parts: machined polymers and composites. Let’s explore the benefits of opting for a plastic material versus the more traditional metal materials for precision parts.
Benefits Across the Board
Machined polymer and composite components can be the most cost-effective solution when compared to metal.
Machined plastic parts are lighter and therefore provide immense advantages over metals by offering lower lifetime freight costs for equipment that is regularly transported or handled over the product’s lifetime. In bearing and wear applications, polymers provide extensive advantages over metals by allowing for lower power motors for moving parts due to lower frictional properties of polymer wear components compared to metals. The low frictional properties provide for less wear as well. The lower wear rates allow for less maintenance-related downtime. Now your equipment can be online longer producing you more profit. Not only are plastics lighter, but they’re also less expensive than many raw metal materials used for parts. Plastics are produced in faster cycles than metals which helps keep manufacturing costs down as well.
Plastics are more resistant to chemicals than their metal counterparts.
Without extensive and costly secondary finishes and coatings, metals are easily attacked by many common chemicals. Corrosion due to moisture or even dissimilar metals in close contact is also a major concern with metal components. Polymer and composite materials such as PEEK, Kynar, Teflon, and Polyethylene are impervious to some of the harshest chemicals. This allows for the manufacture and use of precision fluid handling components in the chemical and processing industries which would otherwise dissolve if manufactured from metallic materials. Some polymer materials available for machining can withstand temperatures over 700F (370C).
Plastic parts do not require post-treatment finishing efforts, unlike metal.
Polymer and composites are both thermally and electrically insulating. Metallic components require special secondary processing and coating in order to achieve any sort of insulating properties. These secondary processes add cost to metallic components without offering the level of insulation offered by polymer materials. Plastic and composite components are also naturally corrosion resistant and experience no galvanic effects in a dissimilar metal scenario that require sheathing. Unlike metals, plastic materials can be compounded with color before machining, eliminating the need for post-treatment finishing efforts such as painting.
Let’s Break It Down by Industry
The benefits and features of plastic materials discussed above span across multiple industries, showcasing the utility and versatility that plastic brings to the table.
Aerospace & Defense
Polymer and composite materials are up to ten times lighter than typical metals. A reduction in the weight of parts can have a huge impact on an aerospace company’s bottom line. For every pound of weight reduced on a plane, the airline can realize up to $15k per year in fuel cost reduction.
Plastic materials handle far better than metals in chemically harsh environments. This increases the lifespan of the aircraft and avoids costly repairs brought about by corroding metal components an in-turn reducing MRO downtime provides for more operational time per aircraft per year.
Insulating and Radar Absorbent
Polymers are naturally radar absorbent as well as thermally and electrically insulating.
Medical & Life Sciences
In the medical industry, cleanliness is vital when it comes to equipment. Infection is the greatest threat facing hospital patients. Polymer and composite materials are easier to clean and sterilize than metal.
Radiolucency is the quality of permitting the passage of radiant energy, such as x-rays, while still offering some resistance to it. Surgical instruments and components manufactured from polymer materials allow the surgeon a clear unobstructed view under fluoroscopy. This allows for safer, more precise surgeon outcomes in the OR. Metal instruments impede the surgeon’s view.
Plastic and composite surgical components allow orthopedic OEMs to meet ergonomic weight limits for surgical trays. Each metallic instrument adds weight and strain to the surgical team carrying and using metal instruments.
Power & Energy
Weight, corrosion, and sealing
Plastic materials allow the oil and gas industry to explore deeper depths than ever before by offering tool weight reduction without a loss of strength as well as materials which offer superior sealing attributes.
As you can see, plastics have a variety of unique attributes which often place them above metals in terms of utility, cost-effectiveness, and flexibility for precision-machined components.
Search specific plastic materials and their applications per industry with our useful material search function.