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

• Insoluble

• 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:

 

Machining Ultem
 

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

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

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

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

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.

 

 

Machining TORLON

 

Annealing TORLON
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.

 

Machining TORLON

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.

 

Preventing Contamination

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

Medical Sector Biomaterials Guide

Energy Sector Materials Guide

Aerospace Sector Materials Guide

Amorphous Materials

 

 

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To a certain degree, 3D printing, injection molding, and plastic machining technology overlap in their capabilities and offerings. But which fabrication process is most appropriate for your project? What are their key differentiators? The following article assesses the utility of each technology with regard to precision, efficiency, materials and more.

 

Capabilities

 

An example of 3d printing example of Yoda

3D printing is an additive manufacturing process that prints layers of plastic substance in three dimensions onto a single point. The popularity of 3D has grown significantly over the past few years, and it can easily be used for rapid prototyping, DIY production and manufacturing by small-scale engineers or designers. Many of the advances in 3D printing have been focused on bringing down prices, rather than technological advancement so that it can be more accessible to “mom-and-pop” users who are otherwise not eligible for the more expensive and advanced manufacturing processes.

 

An example of injection molding

 

Another additive technology, injection molding is a manufacturing process in which parts are produced by injecting materials in liquid form into a mold. Commonly performed with thermoplastic polymers, it can be used with a variety of other metal and glass materials. Small pellets of material are heated into a molten liquid which is then injected into a mold at high pressures and rapidly cooled—causing it to harden—and then ejected from the mold, exposing the part to air. Two of its key benefits include a fully automated process—permitting very high rates of production—and also that its molds allow for highly detailed and intricate parts.

 

An example of plastic molding

 

Finally, plastic machining, or CNC (Computer Numerical Control) plastic machining or CNC milling, involves the precise removal of layers from a material. Typically beginning with a block of the desired plastic material, plastic machining involves removing material layers at a time and allows for many different materials to be used. Introduced by MIT during the 1950s, this technique utilizing drilling tools to carve plastics—and because it’s computer-controlled, extremely precise and tolerant products are achieved.

 

 

 

Comparing the Options

 

 

3D PrintingInjection MoldingPlastic Machining
CostVery low cost when you need 1-10 rapid design iterations on a prototype part.Because creating (or updating) a mold can end up being much more expensive than the actual order itself, this is only cost-effective with very large-scale projects.Significantly cost-effective when you need a few parts up to a few thousand.
DesignDesigns can be highly personalized and unique—this technology is often used by artists and creative professionals.Allows for deep features, square holes and living hinges to be produced easily.High design flexibility at lower costs are allowed.
MaterialVery limited in comparison to other plastic fabrication and unable to achieve the same smooth surfaces as the other technologies.Injection molding has a much wider range of materials available for use compared to 3D printing but is more limited than CNC machining.Broadest range of materials are allowed—CNC plastic machining also achieves the smoothest finish with the fewest imperfections.
ProductionFast and extremely convenient—small products can be machined in seconds.Requires time to make the mold but once the mold is finalized, production moves quickly.One of the fastest forms of fabrication and can easily scale between small and large outputs.
Tolerance3D printed prototypes are not typically finished parts and are much more likely to delaminate. Best for home and personal consumer-grade products.Parts are molded at tighter tolerances than 3D printed parts but are not as tolerant as plastic machined parts.Highest tolerances available—parts are able to reach +/- 0.002 mm and are best for demanding industries like aerospace and defense, medical and life sciences and power generation.

 

Why Plastics Machining Ranks Best

 

From the above chart, you can see the differentiators of each fabrication process. However, we’ve found that CNC plastic machining is by far the most precise and beneficial process for custom applications.

 

Cost-wise, plastics machining is the most effective for any project that requires unique, strong designs. 3D printing is the cheapest form of fabrication but you get what you pay for with that medium… and the quality typically just isn’t there. Injection molding, meanwhile, offers much stronger products than those that are 3D printed, but modifications to a mold are extremely expensive and injection molded products are not as tolerant as plastic machined parts.

 

Looking at design abilities, each medium offers a wide range of capabilities but again, if it’s quality that your project needs, CNC machining is the way to go—3D printed materials are really only suited to personal or home use and injection molded products cannot hold up to the rigors of the aerospace and defense, medical or power and energy industries (among others). Further, machining designs are crafted on the same computer applications that 3D printing uses so there’s no cost associated with changing the design of the product (unlike injection molding, where a new mold would then need to be created).

 

It is important to note that CNC plastic machining is a specialized craft… not just anyone interested in 3D modeling can create the design for a plastic machined product. Here at AIP Precision Machining, we have over 35 years of technical design expertise and we work diligently with our clients to craft products that exactly meet their project requirements.

 

If you’re looking to decide on a fabrication process via the material selection available to you, plastics machining offers the widest range of diverse thermoplastics for your project. 3D printing has not advanced its material range in many years and because injection molding requires liquidating its materials, you won’t be able to use the stronger, heat-resistant plastics that might be best for your needs. CNC machining is the only process that allows you to choose from the full assortment of material selections.

 

3D printing and CNC machining tie for the fastest production times, as both take minutes to complete. Injection molding, however, requires more time to make the mold (which can take weeks to finalize).

 

Finally, if your project requires the highest levels of tolerance, precision machining produces the most precise and extremely tolerant products. It’s the only fabrication process suited for critical pieces.

 

Want More Information?

 

Of 3D printing, injection molding and CNC plastic machining, one process is not necessarily better than the other—it’s dependent on your project requirements. However, plastics machining is by far the most specialized and precise option for high-quality, critical parts needed in industries like aerospace and defense, medical and life sciences and power and energy.

 

Here at AIP Precision Machining, we commonly craft pieces that are able to survive the rigors of space, the inside of a human body or the corrosive chemicals used on an oil rig. We offer state-of-the-art technology and extensive expertise to provide you with the highest quality products. Contact us to learn more about our process and what CNC plastics machining could do for you.

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