Acetal

The Ultimate Guide to Acetal Machining: Properties, Design Tips, and Finishing Options

About Acetal

Acetal is a type of thermoplastic material that is also known as polyoxymethylene (POM). It is a highly versatile material that is commonly used in CNC machining due to its excellent mechanical properties, such as high stiffness, low friction, and good dimensional stability.

Acetal is also resistant to abrasion, impact, and chemicals, making it an ideal choice for applications that require high performance and durability. In addition, acetal is easy to machine, making it a popular choice for CNC machining. It can be machined to tight tolerances, and it has good surface finish and excellent dimensional stability.

Overall, acetal is a reliable and versatile material that is widely used in CNC machining for various applications, including gears, bearings, valves, and electrical components. Its unique properties make it a popular choice for many industries, including automotive, aerospace, medical, and consumer goods.

Subtypes

Acetal (Polyoxymethylene) has two main subtypes:

Homopolymer Acetal (POM-H):
- Made from a single formaldehyde monomer, resulting in a denser molecular structure.
- Known for its high rigidity, strength, and wear resistance, making it suitable for high-precision and high-load applications.
- Common brands of homopolymer acetal include Delrin, which is widely used in automotive components, precision machinery, gears, and bearings.
Copolymer Acetal (POM-C):
- Formed by polymerizing formaldehyde with additional comonomers, leading to a more open molecular structure.
- Provides better chemical resistance, impact strength, and lower water absorption, making it suitable for humid environments or applications that involve contact with chemicals.
- Copolymer acetal is commonly used in parts like pump components, pipe fittings, and medical devices where chemical resistance is essential.

Surface Finishes

Acetal parts can have two types of surface finishes: as-machined and bead blast.

As-machined finish refers to the surface finish that is produced directly from the CNC machining process. This finish has visible tool marks and ridges on the surface, which can affect the appearance of the part. However, it is the most cost-effective surface finish option, as it requires no additional processing steps. As-machined finish is suitable for applications where the appearance of the part is not critical, such as functional components that are hidden from view.

Bead blasting is a finishing process that involves propelling fine glass beads or ceramic beads onto the surface of the acetal part using compressed air. This process creates a uniform and matte surface finish, which can improve the appearance of the part and hide any tool marks or imperfections. Bead blasted finish also has the benefit of increasing the part’s resistance to wear, corrosion, and fatigue. However, the bead blasting process adds extra cost and time to the manufacturing process, which may not be necessary for all applications.

In terms of pros and cons, as-machined finish is a cost-effective option that is suitable for applications where appearance is not critical, but it may not be appropriate for parts that require a smooth and uniform finish. Bead blasted finish, on the other hand, offers improved appearance and increased resistance to wear and corrosion, but it adds extra cost and time to the manufacturing process. The choice between these two finishes ultimately depends on the specific requirements of the application and the desired appearance of the final part.

Design Tips

Here are some design tips for acetal machined parts:

Keep wall thickness consistent: Acetal has a tendency to warp and deform when exposed to heat or pressure, so it’s important to maintain consistent wall thickness throughout the part. This will help to prevent warping and ensure that the part meets the required tolerances.
Avoid sharp corners: Sharp corners can cause stress concentrations, which can lead to cracking or other types of failure. Instead, use rounded corners or fillets to distribute the stress more evenly throughout the part.
Design for manufacturability: Keep in mind the limitations of the CNC machining process when designing acetal parts. For example, parts with deep or narrow features may be difficult to machine, and may require additional setup time or special tooling. Design parts with a focus on manufacturability to ensure a smooth and efficient machining process.
Minimize undercuts: Undercuts can be difficult to machine and may require specialized tooling or fixtures. Minimizing undercuts can help to reduce manufacturing time and cost, and simplify the machining process.
Consider the orientation of the part: Acetal has different properties in different directions, so it’s important to consider the orientation of the part during machining. For example, parts that are machined along the grain direction may have better mechanical properties than those machined across the grain.

By keeping these design tips in mind, you can ensure that your acetal machined parts are high-quality, reliable, and meet the requirements of your application.

FAQ

Is acetal same as Delrin?

Acetal and Delrin are often used interchangeably, but Delrin is actually a specific brand name for acetal manufactured by Dupont. In terms of properties and characteristics, Delrin and other acetal materials are very similar.

How do you finish acetal?

Acetal can be finished using a variety of methods, including as-machined finish, bead blasting, sanding, polishing, and painting. The choice of finish depends on the specific requirements of the application.

Can you heat and bend acetal?

Acetal can be heated and bent, but it has a low heat deflection temperature, so it should not be heated above 100-120°C to avoid deformation or melting.

What is the disadvantage of acetal?

The main disadvantage of acetal is its relatively low resistance to UV light, which can cause degradation over time. Acetal is also not recommended for use in high-temperature applications or in contact with strong acids or bases.

Can you vacuum form acetal?

Yes, acetal can be vacuum formed. It has good dimensional stability and can be formed into complex shapes with tight tolerances.

Is acetal good for machining?

Yes, acetal is a good material for machining. It is easy to machine, has good dimensional stability, and can be machined to tight tolerances. Acetal is commonly used in CNC machining for various applications.

Is Delrin hard to machine?

Delrin can be a bit more difficult to machine compared to some other materials, such as aluminum or brass. However, with the right tools and techniques, it is possible to achieve good results. Delrin is a popular material for machining due to its excellent mechanical properties, such as high stiffness and low friction.

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