POM (Polyoxymethylene), also known as acetal or polyacetal, is a high - performance engineering plastic widely used in various industries due to its excellent mechanical properties, such as high stiffness, low friction coefficient, and good dimensional stability. As a POM plastic part supplier, we often encounter the issue of cracking during the machining process of POM plastic parts. In this blog, I will share some practical methods to avoid cracking during the machining of POM plastic parts.
Understanding the Causes of Cracking in POM Plastic Parts Machining
Before we delve into the solutions, it's essential to understand the root causes of cracking in POM plastic parts during machining.
Internal Stress
POM plastic parts may have internal stresses generated during the injection molding or extrusion process. When these parts are machined, the removal of material can disrupt the stress balance, leading to cracking. For example, if the cooling rate during injection molding is uneven, different parts of the part will solidify at different times, creating internal stress.
Heat Generation
Machining operations, such as turning, milling, and drilling, generate heat. POM has a relatively low melting point, and excessive heat can cause the material to soften, degrade, or even burn. This thermal damage can weaken the structure of the plastic part, making it more prone to cracking.
Machining Parameters
Inappropriate machining parameters, such as high cutting speed, large feed rate, and deep cutting depth, can exert excessive force on the POM plastic part. This force can exceed the material's strength limit, resulting in cracking.
Tool Geometry
The geometry of the cutting tool also plays a crucial role. A dull or poorly designed tool can cause excessive friction and force during machining, leading to cracking. For instance, a tool with a large rake angle may cause the material to be torn rather than cut smoothly.
Methods to Avoid Cracking
Stress Relief Treatment
Before machining, it is advisable to perform stress relief treatment on POM plastic parts. This can be achieved by annealing the parts in an oven at a suitable temperature. The annealing process helps to reduce internal stresses by allowing the molecules in the plastic to rearrange themselves.
Typically, POM parts can be annealed at a temperature between 130 - 140°C for 2 - 4 hours, depending on the part's size and thickness. After annealing, the parts should be cooled slowly to room temperature to prevent the formation of new internal stresses.
Control of Machining Temperature
To prevent heat - related cracking, it is essential to control the machining temperature. One effective way is to use cutting fluids. Cutting fluids can reduce friction between the tool and the workpiece, dissipate heat, and flush away chips. For POM machining, water - based cutting fluids are often preferred as they are non - reactive with the plastic and can effectively cool the machining area.
Another method is to reduce the cutting speed. Lowering the cutting speed can reduce the heat generated during machining. However, it should be noted that the cutting speed should not be too low, as this may increase the cutting force and also cause problems. A suitable cutting speed for POM machining is usually in the range of 50 - 100 m/min for turning and milling operations.
Optimization of Machining Parameters
Selecting the right machining parameters is crucial for avoiding cracking. As mentioned earlier, high cutting speed, large feed rate, and deep cutting depth can lead to cracking. Therefore, it is recommended to use a combination of moderate cutting speed, small feed rate, and shallow cutting depth.
For example, when drilling POM plastic parts, a feed rate of 0.05 - 0.1 mm/r and a cutting speed of 60 - 80 m/min are generally suitable. These parameters can ensure a smooth cutting process and reduce the risk of cracking.
Tool Selection and Maintenance
Choosing the right cutting tool is essential for POM machining. Tools made of high - speed steel (HSS) or carbide are commonly used. Carbide tools are more suitable for high - speed machining due to their high hardness and wear resistance.
The tool's geometry should be carefully designed. For turning and milling operations, tools with a small rake angle and a large clearance angle can reduce the cutting force and prevent the material from being torn. Regular tool maintenance, such as sharpening dull tools, is also necessary to ensure smooth machining.
Fixturing and Support
Proper fixturing and support of the POM plastic part during machining are crucial to prevent cracking. The part should be firmly held in place to avoid vibration and movement during machining. However, the clamping force should not be too large, as this can cause deformation and cracking of the part.
Using soft jaws or rubber pads on the fixtures can help to distribute the clamping force evenly and protect the surface of the POM plastic part. Additionally, providing additional support to the part, especially in areas where it is thin or has complex shapes, can reduce the risk of cracking.
The Importance of Quality Control
Quality control is an integral part of the POM plastic part machining process. Regular inspections should be carried out during and after machining to detect any signs of cracking or other defects. Non - destructive testing methods, such as visual inspection, ultrasonic testing, and X - ray inspection, can be used to check the internal and external quality of the parts.
If any cracked parts are found, they should be immediately removed from the production line and analyzed to determine the cause of cracking. Based on the analysis results, appropriate measures can be taken to prevent similar problems from occurring in the future.
Related Products and Services
As a POM plastic part supplier, we also offer a range of related products and services. If you are interested in other plastic materials, you can visit our website to learn more about OEM Plastic Injection ABS PC PE PP Material. We also provide Low Volume Injection Molding Machine for customers who need small - batch production. In addition, our CNC Machining Parts Plastic Sleeves Bushes are popular among customers for their high precision and quality.
Conclusion
Cracking during the machining of POM plastic parts is a common problem, but it can be effectively avoided by understanding the causes and taking appropriate preventive measures. By performing stress relief treatment, controlling machining temperature, optimizing machining parameters, selecting the right tools, and ensuring proper fixturing and support, we can produce high - quality POM plastic parts with minimal cracking.
If you are in need of high - quality POM plastic parts or have any questions about POM machining, please feel free to contact us for procurement and negotiation. We are committed to providing you with the best products and services.
References
- "Plastics Engineering Handbook of the Society of Plastics Engineers" by Myer Kutz
- "Machining of Polymers" by J. Paulo Davim