How to achieve efficient milling of engineering plastics?
Engineering plastics mainly include PC, Nylon, POM,PET,PPS, etc. Compared with metal cutting and milling, engineering plastics have low hardness and strength, some people think that they will be much easier to make. But in fact, they will encounter many problems:deformation and discoloration and processing accuracy is not easy to ensure.
Today we will share some experience on how to achieve efficient milling of engineering plastics.
Table of Content
■ Characteristics of engineering plastics
Characteristics of engineering plastics
1.Very easy to produce local overheating, while the heat-resistant temperature is very low, causing plastic parts deformation (in serious cases the workpiece bouncing), discoloration, melting, and even burning.
The heat capacity of general engineering plastics is very small, the thermal conductivity is very poor, only 0.3% of the metal, and the coefficient of linear expansion is large, about 2 to 20 times that of metal. Therefore, the heat generated in the machining process is mainly transferred to the tool, even if a small amount of heat is transferred to the plastic chips, it is difficult to transfer to the interior of the plastic parts, resulting in local overheating.
2.Excessive cutting force or clamping force during cutting will cause large elastic deformation and affect the machining accuracy of plastic parts.
3.Grinding effect
Part of the engineering plastics are cut off the powder or particles equivalent to abrasive, there is a strong grinding effect (such as Bakelite). The difficulty of cutting processing is related to the type of filler, glass fabric filler is the most difficult to process followed by mineral filler, cotton filler, fiber fabric filler, wood powder filler.
4.Cracking and tangled chips
When thermosetting engineering plastics, the chips are disintegrating chips that fall off at high speed. The chipping phenomenon will occur on the surface of the workpiece when the tool is cut into and out of the workpiece. Cracking is easy when the turning force or the radius of the tool tip is large, and silver cracks appear on the surface of the workpiece.
When processing thermoplastic engineering plastics, the cutting heat makes them have a certain plasticity, thus forming ribbon chips, which are easily wound on the workpiece or turning tool, sometimes extruded into hard masses, affecting the processing, and even destroying the quality of the easily machinable surface.
5.The material itself has uneven quality levels, which affects processing
Different levels of raw material suppliers in the market, the quality of the materials provided by the differences, the transportation and storage process is also easy to be bumped and scratched.
General Solutions for Engineering Plastics Cutting
1.Choice of tool material
HSS tools can generally process thermoplastic engineering plastics without reinforcement, such as Nylon, PC, PU, PTFE, PET, etc., at low prices.
Cemented carbide tools (Class K/YG) have a low coefficient of thermal expansion and high thermal conductivity compared to other types of cemented carbide, and are therefore relatively insensitive to damage to the internal structure of the material. It is more efficient for machining thermoplastic engineering plastics with added reinforcement.
PCD tools have much higher thermal conductivity than carbide and are particularly suitable for machining thermoplastics that are sensitive to heat, and are also more wear resistant and have the longest service life, but are relatively expensive and are mostly used for finishing or high volume machining.
Ceramic and coated tools are relatively less suitable for machining plastics
2.Geometric parameters of the tool
The softer the plastic being machined, the larger the front and rear angles of the tool should be. However, there is a limit to the choice of back and front angle size, and the tool strength must be ensured.
When rough machining, the depth of cut and feed are larger, the cutting force is large, in order to reduce the cutting force, should take a large front angle, small rear angle.
When machining, friction and wear mainly in the back, should take a small front angle and large back angle.
3.Cutting volume and cutting method
High cutting speed, small depth of cut and tool travel should be used.
High-speed cutting can shorten the friction time of the processing surface and generate less heat. Chip outflow in front of the tool is falling tendency, reducing the adverse effects of poor thermal conductivity of plastic on cutting.
Poor cracking resistance of plastic, cutting is often cutting cracking, the longer the cracking the worse the processing quality. Small amount of feed, chip thickness is small, chip fine and thin, the crack length is short, the processing quality is good.
4.Temperature control and cooling method
Cooling usually use compressed air to cool down, a few cases to use coolant, but pay attention to the water absorption of plastic, and to avoid cutting fluid and plastic surface reaction.
5.Clamping method
Pay attention to prevent deformation of the workpiece, the clamping force should be small, you can use a professional fixture, multi-point clamping or pressing, avoid the direct use of bench vise clamping.
If it is a thin plate parts, milling around the recommended multi-point light pressure, and close to the edge of processing, can prevent the edge of the thin plate due to cutting force caused by jumping.
6.Control of materials
(1), give priority to large material brand suppliers (imported relatively better quality).
(2), the purchase of raw materials to pay attention to the choice of surface protection film intact without scratches.
(3), each batch of materials, are to be trialed.
(4), found that the material internal stress, to do the corresponding stress relief treatment.
(5), transport and storage process to do a good job of protection, to avoid bumps and scratches.
Points to note for each process
1.CNC Turning
The radius of the tool tip circle is recommended to be taken as large for finishing and small for roughing.
The ribbon chips formed when cutting nylon and other materials will affect the cutting and machining accuracy and must be removed in time.
Cutting slender parts, machining accuracy is greatly affected by elastic deformation, should add the center frame or follow the tool holder to compensate.
2.CNC Milling
Engineering plastics chips are usually fluffy and require large chip space, so it is recommended to use milling cutters with fewer teeth.
Engineering plastics cutting force is small, milling cutter can choose a larger diameter.
When the hardness and tensile strength of the workpiece material is high, the cutting speed should be reduced.
Milling plane is preferable to use smooth milling, taking circular trajectory from outer circumference to the center.
To prevent vibration when processing the edge of the thin plate, try to use smooth milling, choose a small diameter milling cutter, reduce the feed rate, the side eating amount is less than half of the diameter of the milling cutter, and the helix angle of the end mill should be small.
Before the end of milling, the feed should be reduced and the feed speed should be lowered to prevent the workpiece from cracking.
3.CNC Drilling
A drill bit slightly larger than the nominal diameter of the hole should be selected, usually 0.05~0.25mm larger.
The higher the hardness of the engineering plastic, the smaller the elastic deformation, the larger the drilling speed should be, and the smaller the feed should be.
The larger the hole diameter is, the lower the drilling speed should be.
Avoid local overheating caused by the drill bit being air-drilled in the drill hole.
The drill should be backed off frequently when drilling in order to remove chips.
Install dust removal devices for chips whenever possible.
4.The choice of processing engineering plastics equipment
If you are specialized in engineering plastics processing, it is actually recommended that you use a specialized processing center. This equipment can be considered advertising industry or woodworking industry up with up, rack and pinion machine speed, in order to ensure that the processing accuracy range on the basis of the part not necessary to do streamlined optimization, more suitable for the industrial field of engineering plastics CNC processing, the price than the general machining center will have more advantages.
Finally, I hope everyone can process high quality products.
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