Plastic cutting

1 Introduction

Plastic parts are generally produced by direct molding, but some plastic parts must be cut if they are difficult to form directly or when their precision is high. The cutting of plastic parts is generally done by processing metal. Due to the large difference in the properties of plastics and metals, and the wide variety of plastics, the different properties of the plastics are also different, so the cutting of plastic parts has its own characteristics.

2 The effect of plastic properties on machining

Thermal performance

Compared with metals, plastics have a small heat capacity, poor thermal conductivity (the thermal conductivity is only three thousandths of a metal or less), and a large thermal expansion coefficient (1.5 to 20 times larger than metal). Therefore, the heat generated by friction during the cutting process is mainly transmitted to the cutter. Even if a small amount of heat is transferred to the plastic part, it is difficult to cause local overheating due to difficulty in being introduced into the interior of the plastic part, causing discoloration, melting, and even burning of the plastic part. Moreover, if the temperature is too high, the elastic deformation of the plastic parts is intensified, which affects the surface quality and dimensional accuracy of the plastic parts, and causes the workpiece to bounce and even cause an accident. Therefore, coolant (usually compressed air) is often used to reduce the temperature during processing.

Elastic Modulus

The elastic modulus of plastic is only 1/10~1/16 of metal. If the tool and the clamp exert excessive pressure on it during cutting, it will cause large elastic deformation, which will affect the processing precision of the plastic parts. Processing is difficult. Therefore, in the cutting process, the parameters of the tool should be reasonable, the cutting edge should be sharp, and the cutting amount should be appropriate to reduce the cutting force. The clamping force must not be too large.

Plastic chip features

At high-speed cutting, the cut plastic chips are in a molten state and harden when exposed to cold. During the machining process, the debris is easily adhered to the tool, which changes the angle of the tool, increases the depth of cut, and affects the machining accuracy of the plastic part. Therefore, the chip should be removed in time. In addition, plastic parts will generate a large amount of swarf dust during the cutting process, and effective ventilation and dust removal measures must be taken to make the dust content in the air meet the national standards.

3 Tool material selection

The materials of the tool mainly include high speed steel, hard alloy, diamond and so on. For cutting general plastics, the first two tool materials are available. In comparison, high-speed steel has better sharpness. High-speed steel tools and careful sharpening can make the cutting edge sharper, but its durability is lower than that of cemented carbide tools. Diamond cutters should be used for the processing of FRP. Because FRP is made of plastic and glass fiber materials, it is hard and soft during cutting, and it cuts intermittently. The impact per minute is more than one million times. The tool is more blunt than the pure hard material. Therefore, it should be selected. A diamond tool with excellent wear resistance.

4 Tool geometry selection

When selecting the tool geometry parameters, the cutting force should be minimized and the cutting temperature should be reduced to ensure the processing quality of the plastic parts, and the productivity and tool durability should be improved as much as possible, and the processing cost can be reduced. The following focuses on the principle of selection of turning tool geometry parameters.

Front angle

The size of the front corner of the turning tool directly affects the cutting effect. Larger front angles can reduce cutting deformation and cutting force, reduce the generation of cutting heat, reduce the cutting temperature, reduce the blunt radius of the cutting edge of the tool, and make the cutting edge sharp. At the same time, it can improve the quality of plastic parts processing. However, the front corner is too weak to weaken the tool strength, the heat dissipation condition is deteriorated, and the cutting temperature is increased, so that the durability of the tool and the quality of the plastic part processing are degraded. Therefore, it is important to choose the rake angle of the tool reasonably. The selection should take into account the three factors of plastic parts, tool materials and processing properties.

Experiments have shown that when machining plastic parts, the cutting resistance decreases as the rake angle of the tool increases. This is because the compressive strength of most plastics is greater than the tensile strength (generally 2 to 3 times larger). From the mechanical point of view, the greater the rake angle of Du Lanping 28, the better the tensile fracture of plastic. Therefore, when processing plastic parts, the front angle of the tool should be larger; however, when processing materials such as FRP, the impact force is large. To ensure the strength of the tool, the front angle should be smaller or even negative.

The choice of the front corner is different for different tool materials. High-speed steel has high flexural strength and impact toughness and can withstand large cutting forces. The rake angle can take a larger value to reduce the cutting force. The processing properties are different and the choice of the front angle is different. When roughing, the cutting depth and feed amount are large, and the cutting force is large. In order to reduce the cutting force, the rake angle should be larger; when finishing, the cutting speed is generally higher, and the cutting heat generated is more, which is improved. The tool heat dissipation condition reduces the cutting temperature and the front angle can be smaller.

Back corner

The effect of the angle on the cutting effect is also large. The large value of the back angle can reduce the friction between the flank and the plastic part of the tool, and can reduce the blunt radius of the cutting edge of the tool, sharp edge and easy to cut into the workpiece. However, if the back angle is too large, the blade strength is lowered and the tool heat dissipation capability is lowered. Selecting the tool back angle mainly considers the cutting thickness. Large cutting thickness (such as roughing), large cutting force, in order to reduce the cutting force and ensure that the knife has sufficient strength, the rake angle should take a large value, the back angle should take a small value; the cutting thickness is thin (such as finishing), friction and Tool wear mainly occurs on the flank face, and the cutting force is not large, the tool strength is sufficient, the back angle should be larger to reduce the friction of the flank face, reduce the cutting heat, and make the blade sharp. See Table 1 for the front and back angle values ​​of cutting tools for common plastics.

Table 1 Turning reference angle of common plastic cutting

Plastic material rake angle (°) back angle (°)

Hard PVC 10～1510～15

Polyolefin PTFE 0～1515～30

POM 0～1510～25

Polyacrylate 0～1010～25

Polyamide 5～155～10

Polycarbonate 0～102～5

Polystyrene 00~5

Phenolic laminate 015

FRP-5～08～18

4.3 Other

Parameters such as the leading angle, the secondary declination, and the radius of the tool nose arc of the parameter turning tool also affect the heat transfer of the tool, the surface roughness of the plastic part, and the cutting force. Decreasing the main and auxiliary declination and increasing the radius of the tool nose arc can improve the heat dissipation condition of the tool and reduce the tool wear; the surface roughness of the plastic part is reduced. However, if the primary and secondary declination are too small or the radius of the arc of the tool tip is too large, the cutting resistance, especially the cutting force component in the depth direction of the workpiece, is increased, which easily causes deformation of the plastic part. Therefore, under the conditions of the rigidity of the plastic part, it is possible to take smaller main and auxiliary declination angles and a larger radius of the tool nose arc.

5 Selection of cutting amount

Cutting speed

Increasing the cutting speed can shorten the cutting time, increase the productivity, and the cutting force does not increase, and the surface roughness of the plastic part is hardly affected by the cutting speed. However, the increase of cutting speed will increase the cutting temperature, the plastic parts will produce thermal expansion and thermal deformation, and even discoloration, affecting the processing quality, and the tool wear will be intensified and the durability will be reduced (the cutting speed will increase by 10%, and the tool durability will be shortened to the original). 40% to 60%), so that the auxiliary time such as tool change, sharpening, and tool setting is increased, and the productivity is decreased. Therefore, the cutting speed is controlled. See Table 2 for the cutting speeds of commonly used plastics.

Table 2 Cutting amount of common plastic for cutting Plastic material cutting speed feed cutting depth measuring material (mm)

Plastic material cutting speed

(mm/min) feed

(mm/r) depth of cut

(mm)

POM 120～1800.1～0.250.12～0.5

Polyamide 150～1800.05～0.380.12～0.5

Polycarbonate 150～3000.12～0.50.12～0.5

Polyolefin, polytetrafluoroethylene 90 ~ 2250.05 ~ 0.250.12 ~ 0.5

Polystyrene 230~3000.03~0.1-

Phenolic laminated plastic 150～6000.1～0.25-0.25～0.75

Glass fiber phenolic laminated plastic 45～60∠0.25-

Plexiglass 15～800.1～0.25-0.15～0.2

Feed and depth of cut

When the feed rate and depth of cut increase, the cutting time is also shortened, and the cutting temperature and tool durability are not greatly reduced due to the improved heat dissipation conditions, and the auxiliary time such as tool change and sharpening is not significantly increased, so that the increase can be improved. productivity. However, as the feed rate and depth of cut increase, the cutting force increases significantly (the feed rate is doubled, the cutting force is increased by about 70% to 80%, the depth of cut is doubled, and the cutting force is almost doubled. Increase), it is easy to deform the plastic parts and affect the processing quality, and even scrap the workpiece. The surface roughness value of the plastic part will also increase with the increase of the feed rate, so the proper feed amount and depth of cut should be selected during processing. See Table 2 for the feed and cutting depth of common plastics.

6 Conclusion

When processing plastic parts, according to the performance of plastics, processing conditions, processing requirements, select the appropriate tool material, reasonably select the geometric parameters of the tool and the amount of cutting, solve the special problems in plastic processing, and make the cutting of geometric parameters of the tool. The optimum combination is used to reduce the cutting force, reduce the cutting temperature, ensure the processing quality, and improve the productivity.

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Tag: Cutting force Cutting depth Cutting speed Plastic parts Cutting

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