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Common Plastics for CNC Machining: A Guide to Corrosion Resistance, Shrinkage, and Water Absorption

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Common Plastics for CNC Machining: A Guide to Corrosion Resistance, Shrinkage, and Water Absorption

Introduction

In addition to mechanical and thermal properties, the dimensional stability and environmental resistance of plastic parts are also extremely important. A material’s chemical resistance, mold shrinkage, and water absorption all affect the precision and lifespan of parts in real-world environments. This article will explain the key properties of these commonly used plastics.

Detailed Explanation of Plastic Material Properties

  • ABS

ABS has good resistance to dilute acids, oils, and water, but should be avoided in contact with aromatic and ketone solvents. Its water absorption rate is low (approximately 0.2%), resulting in good dimensional stability in general environments.

  • PC (Polycarbonate)

PC is resistant to dilute acids, alcohols, and oils, but, like ABS, is not resistant to strong bases and ketone solvents. Its water absorption rate is also low (approximately 0.24%), making it suitable for use in humid environments.

  • PA6 / PA66 (Nylon)

Nylon has good resistance to oils, fuels, and mild bases, but is corroded by strong acids. The most noteworthy characteristic of nylon is its high water absorption. PA6 absorbs approximately 1.3%, while PA66+GF can reach 1.8% after equilibrium. Water absorption causes dimensional expansion and a decrease in mechanical strength, making it unsuitable for applications requiring high precision in high humidity or water.

  • POM (Plasticized Metal)

POM has excellent chemical resistance, being resistant to oils, fuels, and most solvents, with only limited resistance to strong acids. Furthermore, its extremely low water absorption (approximately 0.2%) allows it to maintain excellent dimensional stability in humid environments and most chemical environments.

  • PMMA (Acrylic)

Acrylic is resistant to dilute acids and alkaline salt water, but should be avoided in contact with alcohols, ketones, and aromatic solvents, as these can cause cracking (stress brittle cracking). Its water absorption is approximately 0.3%, and its dimensional stability is acceptable.

  • PEEK

PEEK’s chemical resistance is among the best of all plastics, resistant to most acids, bases, oils, and fuels, and is only sensitive to extreme chemicals such as concentrated nitric acid. It also has very low water absorption (approximately 0.1%), making it a high-performance choice for harsh chemical and humid environments.

  • Glass Fiber-Reinforced Plastics (e.g., PA+GF, PC+GF)

Adding glass fiber (GF) generally does not change the plastic’s inherent chemical resistance, but it does reduce its water absorption and mold shrinkage, thereby improving the material’s dimensional stability in humid environments.

  • PP (Polypropylene) & PE (Polyethylene)

Both materials offer excellent chemical resistance, resisting most acids, bases, and salt sprays, and have very low water absorption (PP approximately 0.02%, PE < 0.1%). This makes them ideal for parts such as containers and piping that come into contact with chemicals or liquids.

Key Glossary

  • Chemical Resistance: This refers to a material’s ability to resist attack by chemicals (such as acids, bases, oils, and solvents). This is crucial for parts that must operate in specific chemical environments; choosing the wrong material can cause the part to soften, become brittle, or even dissolve.
  • Molding Shrinkage: This refers to the percentage by which a plastic’s dimensions decrease after cooling and solidifying in the mold. This value primarily affects the precision of injection molding. For CNC machining applications, its impact is minimal, and its primary reference value lies in understanding the material’s inherent dimensional stability.
  • Water Absorption: This refers to the percentage increase in weight of a material after immersion in water for 24 hours. Materials with higher water absorption rates are more susceptible to dimensional expansion or mechanical property degradation in humid environments, requiring special attention.

Conclusion

In humid environments or those exposed to chemicals, a material’s water absorption rate and corrosion resistance must be prioritized. For example, while nylon (PA) offers excellent mechanical properties, its high water absorption makes it unsuitable for applications requiring high precision in water, high humidity, or high precision. In environments exposed to oils or solvents, materials with greater chemical resistance, such as POM or PEEK, should be selected.