In recent years, POM is replacing some traditional metal materials to process precision parts, such as aluminum, brass, steel, zinc, etc. It has gradually been widely used in electronics and electricity, machinery, instruments and meters, daily light industry, automobiles, building materials, agriculture, etc.
This material is quite different from metal in processing. If it is not paid attention to or operated improperly, it is likely to deform and crack. Today I would like to share with you some experience of POM materials in machining, hoping to provide you with some ideas.
What is POM?
POM is a semi-crystalline engineering thermoplastic based on formaldehyde. Acetal (acetal), Polyxymethylene (POM), Polyacetal (polyacetal), Polyformaldehyde, Saigang, these are all its names. There are two kinds of common names: black and white.
This material has high mechanical strength and rigidity, good sliding characteristics (low friction coefficient), and excellent wear resistance. Because acetals hardly absorb water, they have excellent dimensional stability, and are the best choice for making complex shapes and precision machining parts.
However, it is unstable and easy to degrade under acidic conditions and high temperature. Because its molecule contains a large amount of oxygen, it is difficult to give it flame retardance. The continuous working temperature range is about – 40 ° C to 120 ° C.
Problems Easily Happened During POM Machining
Generally, the main problems encountered are processing deformation and cracking. There are also two common types of cracking. One is direct cracking during processing, and the other is hidden cracking (usually the cause of internal stress). It will be more troublesome to crack slowly after machining.
Selection Of Machining Tools
Because POM has low hardness and low cutting force, generally high-speed steel and carbide tools are sufficient for machining, and the cutting edge must be sharp to reduce cutting heat. PCD turning tool can be selected for precision machining of turning.
If the POM material selected is not good or the dimensional tolerance requirement is high, it is recommended to conduct annealing treatment after roughening to eliminate its internal stress, which can greatly reduce the deformation after finishing.
Common Deformation Causes And Solution
1. Deformation Caused By Clamping
The POM material will be deformed when clamped, and will return to its original state after loosening. At this time, the clamping form can be changed to increase the fixed contact surface of the workpiece. For example, bench vises are cushioned and fixed with glue. For larger plates, vacuum suction cups can be used, but the blank needs to be leveled. It is recommended to fix one side with glue and then level it, and then fix and level this side with suction cups for rough machining.
2. Deformation Caused By Cutting Heat
POM material has poor heat resistance, is sensitive to heat, and is easy to deform due to insufficient cooling during processing. First of all, the tool must be sharp, so that the heat generated during cutting is relatively small. Secondly, we can reduce the amount of cutting tools, cut multiple times, and increase the coolant in order to reduce the generation of heat as much as possible or quickly take away the heat generated in the cutting process.
3. Elastic Deformation
POM material has high elasticity. During cutting, the part in contact with the tool will deform inwards due to the elasticity of the material. When the tool leaves, the cut and pressed part will produce a certain amount of deformation. At this time, it is necessary to make multiple tool compensation adjustments according to the actual cutting effect. When machining, the size deformation caused by the elasticity of the material can be reduced by cutting with a small cut and multiple cycles.
4. Internal Stress Deformation
Because the coefficient of thermal expansion of engineering plastics is larger than that of metals, when the machining allowance is large, deformation will occur due to the elimination of internal stress. First of all, we should choose and handle materials correctly (as mentioned earlier). Secondly, when there is a large amount of material removal, try to lay down thick material, control the allowance, and use symmetrical processing (whether the part design itself is reasonable is very important) to offset the stress and deformation caused by processing.
After finishing, attention should also be paid to temperature control in the transfer and storage process. If conditions permit, constant temperature should be maintained to prevent the deformation of parts due to temperature changes. At the same time, remember to protect the surface to avoid bruises and scratches.
Common Causes Of Cracking
It is easy to crack when the deformation degree mentioned above is relatively serious, but this is only part of the reason. There are also the following reasons for cracking of POM materials during operation:
- Excessive cutting during machining
- Direct use of large drill bit to drill holes, resulting in too large cutting force, extrusion and cracking
- When processing deep holes, the drill bit is not repeatedly returned for chip removal, and the chip is not fully discharged, resulting in extrusion cracking
- Insufficient cooling, insufficient cooling of drilling, resulting in too high cutting heat and cutting force cracking
- The feed speed is too fast, and the internal stress of POM bar will crack
- When drilling, the cutting edge of the drill bit was worn, and the drill bit was not repaired in time, resulting in cracking due to hard drilling.
Selection Of Machining Method
Cooling is required during processing to reduce wear and heat conduction and prevent melting. It is recommended that compressed air cooling or solid lubrication be preferred, followed by coolant. The rotation speed should not be too fast, the feed and feed should not be too large, the front and back angles of the tool can be slightly larger, and the edge must be sharp. The front angle of the commonly used high-speed steel turning tool is about 25 °~40 ° and the back angle is about 10 °~20 °. The clamping force of the chuck should be as small as possible.
Do not drill directly with a large drill bit. It is recommended to drill small holes first and then expand them at low speed. The drill bit must be kept sharp. The following drill bits can be used for reference: top angle 60 °~90 °, spiral angle 10 °~20 °, front angle 0 °, back angle 10 °~15 °. When drilling, the force in the feed direction should not be too large, and the tool should be retracted in time (generally 5~6mm deep) for chip removal and cooling. For through-hole drilling, the feed rate should be reduced when the drilling is fast, so as to prevent the drill bit from pushing the material away along the axial direction.
Milling And Threading
In addition to the different tools, they are basically the same, that is, the clamping should avoid deformation, keep the tool sharp, the feed rate should be small, and the cooling should be sufficient.
This kind of processing is relatively rare. The POM material is relatively flat whether it is plate or bar. When planing the plane, it is not suitable to use a flat knife, but to use a sharp knife for processing, and the back corner of the tool should be small, otherwise it is easy to crack or fall off.