Plastics and their machining: A review

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Materials Processing Technology Journal of Materials Processing Technology 54 (1995) 40-46

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Plastics and their machining: a review M. Alauddin, I.A. Choudhury, M.A. E1 Baradie, M.S.J. Hashmi* School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin 9, Ireland Received 1 April 1994

Industrial summary

Many different families of plastics are used in industry. The demand for the machining of plastics has recently increased. In most cases, traditional metal machining techniques and tools are being used in the machining of plastics. The present paper reviews the conventional mechanical machining of plastics. The cutting phenomena as indicated by the types of chips formed under various cutting conditions, and the cutting forces for the single-edge cutting of two broad types of plastics are discussed. When drilling plastics, the type of chip produced, the torque, and the thrust are considered. A general idea about the milling of plastics is shown. The possibility of grinding thermosets and thermoplastics using an open grinding wheel is discussed with reference to bond type, grit type, grit size and porosity.

1. Introduction

Plastics are man-made, synthetic materials which have large molecules made up of chains of atoms. They are made from basic chemical raw materials called monomers derived mainly today from the petro-chemical industry. The components that determine their technological and physical behaviour are polymers which are synthesised by the repeated addition of one or more types of monomeric units to the growing molecules. These polymers are always composed of atoms of carbon in combination with other elements. Polymer chemists utilize only 8 of the more than 100 known elements to create thousands of different plastics 1-1], these eight elements being hydrogen, carbon, nitrogen, oxygen, fluorine, silicon, sulphur and chlorine. More than 50 different families of plastics are in commercial use today, and each may have dozens of sub-type and variations. The volume of plastics consumed each year is already greater than that of steel [2]. Usage continues to grow so rapidly that it is predicted that the volume of plastics used will be more than twice that of all metals by the end of this century. The use of plastics is increasing since they can offer impressive advantages [3]. They are not generally subject to corrosion; they are light in weight, frequently with a good strength to weight ratio; they are very cost effective because of the ease and speed with which they can be shaped and mass produced, giving design freedom and * Corresponding author. 0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 5 ) 0 1 9 1 7 - 4

reducing assembly time; they are good electrical insulators; and they are colourful. In addition, each individual plastic material offers special properties to suit a particular application, the latter ranging from cups for drink dispensers to key components in the aerospace industry, from toy cars to lorry cabs, from ball-point pens to word-processor housings I-4-8]. Plastic parts are usually produced by moulding processes 1-9], but for small quantity production or for extremely complex or accurate shapes, machining is essential. It has proven rather difficult to machine all types successfully, owning to the many kinds and grades of plastics available and the lack of a basic understanding of their machinability. The machining characteristic of plastics appear to depend primarily on their mechanical, thermal and rheological properties [113]. Consequently, any evaluation of machining characteristics must allow for the particular properties of the material being used. This paper sets out to give a short review of plastics and their machining by conventional methods. In general, much of the data available on tooling and cutting parameters for plastics are based on practical experience with metals. Little modification is made to tooling and equipment to suit the peculiar characteristics of plastics.

2. Constituents of plastics

Most plastic objects contain substances in addition to polymeric materials. Resin is usually the principal

M. Alauddin et al. / Journal of Materials Processing Technology 54 (1995) 40-46

constituent of most plastics, however other materials are also present such as filler, plasticizers, solvents and colorants [11-14]. The resin or binder serves to bind the plastic together and impart some of the significant characteristics to the finished product. A plastic is usually named by the resin involved in its manufacture 1,15]. Before being processed into finished products, most plastics make use of a filler such as wood flour, talc, silicates and carbonate, which gives very good surface appearance and excellent electrical properties. All of the resins must first be made fluid and their components particles must be welded together. For this purpose, a solvent is used. There are over 300 solvents that are employed for various types of plastics. Many plastic resins posses high viscosity and are rather stiff in the final form. For this reason, plasticizers are used to lower the viscosity at high processing temperatures and to give the final product the necessary plasticity. Miscellaneous chemical additives are used for special purposes such as to stabilize plastics against oxidation, thermal degradation, and ultra-violet light. Since most plastics are not left in their natural colour, over 800 colorants are on the market today.

3. Classification of plastics According to ASTM D883 80c, plastics are divided into two groups with regard to their chemical and technological behaviour 1-16]: (a) thermosetting plastics; and (b) thermoplastics. (a) Thermosetting plastics. These polymers set solids after being melted to a liquid state by heating; hence the term thermosetting plastics. The process of solidifying is known as curing. During curing all of the small molecules are chemically linked together to form one giant network molecule. Hence, they are distinguished from the linear polymers (thermoplastics) by being called network polymers. The structure of thermosetting plastics is chain-like and, prior to moulding, is very similar to that of thermoplastics. The curing consists of the formation of a cross between adjacent molecules resulting in a complex inter-connected network. This cross-bond prevents the slippage of individual chains, thus preventing plastic flow with the addition of heat: the change from the liquid state to the solid state is irreversible, further heating resulting only in chemical breakdown; not melting. There are eight major classes of thermosetting materials [17], these being: (i) alkyds; (ii) allylics; (iii) amine; (iv) epoxies; (v) phenolics; (vi) polyester; (vii) sillicones; and (viii) urethanes. (b) Thermoplastics. Thermoplastics become soft when they are exposed to heat and harden when they are

41

cooled regardless of how often the process is repeated. By alternating heating and cooling they can be reshaped many times. In the melted state they are rubber-like liquids, and in the hard state they are glassy or partially crystalline. In thermoplastics, the atoms and molecules are joined end-to-end into a series of long chains, each chain being independent of the others. When subjected to heat the individual chains slip, causing plastic flow. There are practical limitations to the number of heating-cooling cycles to which thermoplastics can be subjected: an excessive number of such cycles may result in a loss of colour or plasticizer, thereby affecting the appearance and properties. There are eleven major classes of thermoplastic materials [17] : (i)ABS (Acrylonitrile, Butadiene and Styrene) (ii) acetals; (iii) acrylics; (iv) cellulosices; (v) fluorocarbons; (vi) polyamides; (vii) polycarbonates; (viii) polyethylene; (ix) polypropylenes; (x) polystyrenes; and (xi) vinyls. 3.1. Engineering plastics

In industry the term "engineering plastics" is often used 1-18-20]. "Engineering plastics" are those high-performance plastics that provide multiple engineering properties at an economically feasible cost and can be processed without unusual measures. Conforming to this definition engineering plastics [20] are: (i) the family of nylon; (ii)polycarbonate; (iii)polyphenylene oxide; (iv) acetal; (v) engineering grade of ABS; (vi) polysulphone; and (vii) polyphenylene sulphide. In time, with future market changes and possible modifications in properties and costs, some speciality plastics may become engineering plastics. The primary areas of use of engineering plastics are in transpiration, electrical and electronic products, and in the combined field comprising consumer goods, appliances and business equipment.

4. Properties of plastics Weight. Most plastics are light in weight, the specific gravity varying between 1.0 and 2.0 for most polymeric substances. This property is important in calculating strength to weight ratios 1-21]. There are many applications that make use of this very advantageous property of plastics. Hardness. Plastics are not very hard, their hardness being comparable to that of brass and aluminium. Generally, thermosetting plastics are harder than thermoplastics. The temperature of the material substantially affects its properties, elevated temperatures softening most plastics considerably. The hardness of most plastics is in the range of 5-50 BHN.

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M. Alauddin et al. / Journal of Materials Processing Technology 54 (1995) 40-46

Tensile strength. Compared with metals, the tensile and compressive strengths of plastics fall below the values for magnesium. Ductile plastics are, in some respects, similar to metallic materials in their response to loading conditions, in that higher rates of tensile loading raise the elastic limit. Fatigue loading, when compared with short-time static loading, causes failure of plastics at lower loads [22]. Thermal properties. Plastics are found to fall in the group of heat insulators, i.e. they have very low thermal conductivities [23], copper transmitting over 2000 times as much heat as most plastics. Electrical properties. Dielectric strength represents the electrical insulating value of a plastic and specifies the maximum voltage necessary to cause a current to flow through a given thickness, usually 0.025 ram. Plastics with high dielectric strengths are recognised as being good electrical insulators [24]. Some have been developed which retain their insulating values even after long periods of immersion in water. With an increase in temperature the dielectric strength is usually lowered. However, some plastics have been developed that can actually conduct a current [25, 26]. Chemical properties. From a chemical standpoint, plastics are generally more resistant to environments which usually attack metals, concrete, and wood at atmospheric temperatures [27]. Plastics generally resist attack by salt water and withstand atmospheric attack well if protected from ultra-violet light [28]. Fairly good resistance is offered to attack by inorganic acids, salts, and bases, although strongly oxidizing substances should generally be avoided. Thermosetting resins are often superior to thermoplastics in resisting organic solvent attack.

5. Machining of plastics 5.1. Optimum cutting conditions It is always desirable to obtain a continuous chip to avoid generating heat and deformation of the work material during cutting [10]. To accomplish this it is desirable to cut with tools having a critical rake angle or larger rake angle, which produces continuous chips and minimum deformation. The selection of cutting conditions is also important, specifically the tooth cut thickness as determined by the feed rate. Cutting conditions which maximize the cut thickness must be selected, in order to decrease the generated heat from the material.

5.2. Single-edge cutting of plastics 5.2.1. Turning of thermosets Some types of thermosets are often in cast form. Cutting methods for cast polyester resin are representative of the machining of thermosets. These materials are very easy to crack during cutting, having a brittleness like that of glass or ceramic. However, when optimum cutting conditions are used, they can be machined easily without cracks and to close tolerance. Peculiar chips are observed when polyester resin is machined. One type is discontinuous, appears opaque and has a rough surface, being obtained with a negative rake tool cutting at a moderate cutting speed. A similar chip, also discontinuous but transparent with smooth fractured surfaces, is obtained under a rather wide range of cutting conditions with large cut thickness and a tool having a positive rake angle. The chip is called the discontinuous-crack type and is produced by elastic fracture without plastic deformation. The other type of chip is the continuous-shear type, being obtained at low cutting speeds and small cut thickness and appears opaque. Some chips have cracks along their surface. In cutting cast polyester resin, continuous chips are produced only at a small cut thickness, however the chips become discontinuous as both the cutting speed and cut thickness increase. Chip formation becomes of the discontinuous-crack type as the rake angle increases in the positive direction; however the chip turns into the continuous-shear type or the discontinuous-shear type with cracks as the rake angle diminishes to zero or goes negative. The chips turn into continuous from discontinuous as work temperature is raised, thus a higher temperature will improve chip formation. Cutting forces vary with the rake angle, the cutthickness, the cutting speed and the work temperature [1(3]. The cutting forces are proportional to the cut thickness at low cutting speed, as shown in Fig. 1 [10]. When a zero or negative rake tool is used, a continuous-type chip is formed. The relationship between the cutting force and the rake angle is shown in Figs. 2 and 3 [10], from this figure the following being are observed: (a) the cutting force components decrease as the rake angle turns from negative to positive; (b) transfiguration of the cutting forces with positive rake angles originates from the variation in chip formation, from continuous to discontinuous; and (c) almost all chips produced from a positive rake tool belong to the discontinuous-crack type in cast polyester resin machining, thus it is not clear whether or not a critical angle exists. The author of [10] has given opinions about the cutting of polyester resin, which are: (a) the rake angle should be zero or slightly negative; (b) a cut thickness smaller than 0.02 mm results in a continuous chip; (c)

M. Alauddin et al. / Journal of Materials Processing Technology 54 (1995) 40-46

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a good surface finish can be obtained at practical cutting speeds. It may be said that the machinability of cast polyester resin is relatively poor, because of the narrow range of cutting conditions.

5.2.2. Orthogonal cutting of thermoplastics The results quoted by Vickerstaff and Gindy [29] when cutting polycarbonate using HSS tools are representative of the cutting of thermoplastics. The workpiece material was 3.55 mm in thickness and the cutting was carried out on a planing machine with tools held in a piezo-electric dynamometer. The test conditions were as follows: rake angles - 5 °, 0°, 5°, 10°, 25 °, 30°, 40°; cutting speed (m/min) 9,15, 23, 30, 36; cut thickness (mm) 0.13, 0.20, 0.25, 0.30, 0.38.

Fig. 4. Cutting force-rake angle relationship in the cutting of polycarbonate at v = 15 m/min.

Cutting forces were measured and are shown in Figs. 4 and 5 [29], the information leading to the following conclusions: (a) the cutting forces reduce as the rake angle increases from negative to positive values; and (b) the direction of the cutting force components changes from downwards to upwards as the rake angle increases from negative to positive. Kabayashi [10] defines the rake angle which gives zero normal force as the critical rake angle and has argued that it is the optimum value, giving the highest workpiece accuracy and the minimum tool wear. This is considered to be because the direction of the resultant cutting force coincides with the direction of cutting, giving minimum deformation of the machined surface. The critical value of the rake angle was observed in this work. Kabayashi [10] concluded that the critical values of rake angle should be determined experimentally for each plastic used and for each set of cutting conditions. However, because it is

M. Alauddin et al. / Journal of Materials Processing Technology 54 (1995) 40-46

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possible that the normal force can be predicted, the value of the critical rake angle can also be predicted.

Friction in orthogonal cutting. A number of theories have been put forward to describe the friction mechanism of plastics. However, these theories contradict each other on a number of points and as of yet there is no agreed conclusion. Bahadur and Ludema [30] provided indirect proof that the adhesion theory of friction applied to plastics. However, Bickerman [31] suggested that the adhesion theory is in error and that the deformation of polymer friction is the most probable mechanism. The results of variation of co-efficient of friction during the orthogonal cutting tests are shown in Fig. 6, from which it can be seen that the co-efficient of friction decreases with increasing normal force. 5.3. Drilling of plastics 5.3.1. Drilling of thermosetting plastics Thermosetting plastics are more difficult to drill than thermoplastics due to the occurrence of swelling or cracking, therefore the selection of optimum drilling

conditions for thermosets is more critical. Cracking around the inlet- or exit-edges of the hole is common in the drilling of cast polyester resin as well as in the drilling of urea moulded parts, cast phenolic, cast epoxy or other brittle resins. As expected, the size of the cracks is affected not only by the drilling conditions, but also by the shape of the drill. As for the drilling conditions, feed is the major variable. In drill design, the point and rake angles of the drill have the largest effect on the size of cracks, the latter becoming large as the point angle becomes larger at high feeds. Similarly, the magnitude of cracks increases slightly as the helix angle and the peripheral speed increase. The range of optimum drilling conditions will be wider when the work material is pre-heated to a suitable temperature to semi-plasticize the thermoset. The variation in cutting forces with the work temperature is shown in Fig. 7 [10]. Chips become continuous with work temperatures in the range of 50-75 °C, even with large feeds. However, they become discontinuous when the temperature becomes too high.

5.3.2. Drilling of thermoplastics It is easy, when drilling thermoplastics, to over-heat the material. It is imperative to allow the swarf to clear quickly, otherwise a swarf build-up in the drill flutes will cause over-heating with consequent spoiling of the work. Two things will help to avoid this: firstly, the use of a drill having polished flutes with a low helix angle to facilitate swaff discharge; secondly, drilling should be done by hand feeding using the wood-pecker technique. Gumming or melting occurs occasionally when thermoplastics are drilled without coolant. However, care must be taken in the selection of coolants for plastics, because stress cracking may occur after drilling. For example, when polystyrene is drilled severe radial cracks can result if petroleum is used as a coolant [10].

M. Alauddin et al. / Journal of Materials Processing Technology 54 (1995) 40-46 E

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The relationship between thrust and the drilling speed of polyethylene is shown in Fig. 8 [10], the torque and thrust being seen to decrease slightly as the drill speed increases. Therefore, greater peripheral speeds should be chosen when drilling polyethylene. A drill for polyethylene should have a helix angle of 10-20 °, a point angle of 70-90 ° and a lip-relief angle of 9-15 °, and zero rake angle is recommended [10]. A peripheral drill speed of 600-900 m/min and feed speed of 0.18-0.25 mm/rev are recommended. The authors of [32] have shown that a specially designed flow drill (i.e. a forming tool, known as a flow drill) can produce effective results when drilling some thermoplastics (i.e. the drilling of ABS, polycarbonate, polypropylene, etc.)

5.4. Milling of thermoplastics No special techniques are required for milling operations. Cutting speeds should never fall below 300 m/min and the surface finish will improve with finer feeds, irrespective of the depth of cut used [33]. A sharp cutter will give the best results. Normal cutters can be used for horizontal milling and climb (down) milling is recommended to avoid burning. A vertical milling process is a very successful technique when applied to particular thermoplastics. The work often has to be carefully clamped and supported to prevent flexing away from the cutter.

45

plastics discussed here. In practice, these grinding conditions may vary with other classes of thermosetting plastics. The grinding of an epoxy resin roller was carried out by Morgan [34], using the following specification: abrasive type A (aluminium oxide); size of abrasive:800 (superfine); bond type V (vitreous); wheel grade J (medium); wheel structure 5 (low porosity). Using water as a coolant in the grinding operation, four or five finishing passes of the roll are made, having a depth of cut of approximately 10 ~tm. However, despite, or perhaps because of, the exceptional fineness of the grinding wheel grits, some rollers were produced containing unacceptable surface scratches. The wheel specification recommended by Kabayashi [10] for the surface grinding of cast thermosetting plastics is 32A54G12V. This wheel specification is very similar in nature to that for low-alloy steel [35] except for the wheel grain structure, thermosetting plastics requiring a more open structure than steel to prevent wheel loading.

5.5.2. Grinding of thermoplastics The grinding of thermoplastics is rather difficult because of their low melting temperature and results in the clogging of the surface of the abrasive wheel. It is preferable to use a grinding wheel with open grain-spacing and of low grade, in conjunction with an excess of coolant to prevent over-heating and wheel loading. A significant volume of coolant is needed in the grinding of thermoplastics. The wheel specification recommended by Kabayashi [10] for the surface grinding of polystyrene thermoplastic is 38A46F12V. In general, hard materials are better ground with finer grit-size wheels and soft materials are better suited for grinding with coarse-grit wheels. Here it is also found from wheel specification, that 54 grits for thermosetting and 46 grits for thermoplastic is recommended, since thermosetting plastics are harder than thermoplastics. The wheel speed has an influence in selecting the wheel grade. It is known that the higher the wheel speed with relation to work speed the softer the wheel grade should be: in the case of thermoplastic grinding the wheel grade is "F" and in the case of thermosetting the wheel grade is "G". This may be due to the wheel speed in relation to the work speed for thermoplastic being higher than that for thermosetting plastics. 5.6. Conclusions

5.5. Grinding of plastics 5.5.1. Grinding of thermosetting plastics The grinding of thermosets is recommended in practice because of the difficulties involved in machining such materials with conventional tools. Different resins can require very different grinding conditions [34]. Grinding of epoxy resin is only representative of the thermosetting

5.6.1. Machining of thermosets 5.6.1.1. Single-edge cutting of cast polyester (representative ofthermosets). (i) The Cutting force components decrease as the rake angle turns from negative to positive. (ii) Transfiguration of the cutting force at positive rake angles originates from the variation in chip formation.

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M. Alauddin et al. / Journal of Materials Processing Technology 54 (1995) 40-46

(iii) The rake angle at which the normal force during cutting is zero is called the critical rake angle. Here it is not clear whether or not a critical rake angle exists.

5.6.1.2. Drilling ofthermosets. (iv) Special forms of drills are necessary for optimum drilling conditions. (v) The cutting temperature has an effect on chip formation, chips becoming continuous at work temperatures of 5° to 75°C and discontinuous when the temperatures become too high.

5.6.1.3. Grinding of thermosets. (vi) An aluminium-oxide abrasive wheel may be used for thermoset grinding. The wheel grain structure is more open in the case of thermosets compared with that of steel, to prevent wheel loading. 5.6.2. Machining of thermoplastics 5.6.2.1. Orthogonal cutting of polycarbonate (representative of thermo-plastics). (vii) The critical rake angle value can be predicted from the predicted value of the normal force. (viii) The coefficient of friction in orthogonal cutting can be described as a power curve giving a reduced friction co-efficient as the normal force increases.

5.6.2.2. Drilling of thermoplastics. (ix) Higher peripheral speed and lower feeds should be chosen to reduce the cutting force in drilling operations. (x) Gumming or burning may occur when thermoplastics are drilled without coolant. However, care must be taken in the use of coolant because stress cracking may occur after drilling. (xi) A flow drill can produce effective results. 5.6.2.3. Milling of thermoplastics. (xii) The cutting speed should not fall below 300 m/min. A sharp cutter is necessary for the best result. The surface finish will improve with finer feeds, irrespective of the depth of cut. 5.6.2.4. Grinding of thermoplastics. (xiii) It is preferable to use a grinding wheel with open grain spacing and of low grade in conjunction with an excess of coolant to prevent over-heating and wheel loading. References [1] Anonymous, J. Machine Design, 58 (8) (Material reference issue) 5(1986) 79 85. [2] M. Copley, An Introductory Guide to Plastic Materials Selection, Rapra Tech. Ltd., (1986), p. 1. [3] J.Y. Dutor, J. Materials Design, 7 (1) (1986) 14. [4] J.P. Jacquet, Plastic optical fibre applications for lightening of airports and building, Proc. of SPI E The Intl. Society for Optical Eng., 1592 (1991) 165 172. [5] A. Weber, Plastics in automotive engineering use and reuse, J. Materials Design, 12 (1991) 199 208.

[6] D.R. Leaversuch, Room air conditioner with no metal bending, J. Modern Plastics, 68 (1991) 48-50. [7] N. Allbee, Plastics in the medical world, J. Plastics Compounding, 12 (1989). [8] A.A. Byerlin and L.K. Pakhomova, Polymeric materials for high strength reinforced composite, a review, J. Polymer Sci. (Russia), 22 (1990) 1275 1311. [9] E.D. Witwer, Plastics processing, what you should know, J. Chem Eng., 96 (1989) 123-128. [10] A. Kabayashi, Machining of Plastics, McGraw-Hill, New York 1967. [11] Anonymous, Additives and modifiers, J. Plastics Compounding, 14 (4) (1991) 14. [12] Anonymous, Additives 1991, J. Plastics Eng., 47 (1991) 2~28. [13] Anonymous, Fillers and reinforcements, J. Plastics Compounding, 14 (4) (1991) 25. [14] H.M. Mack, Continuous compounding of colour masterbatches, J. Plastics Eng., 47 (1991) 39~,2. [15] C.N. Merle, Metallurgy and plastics for engineers, Associated Lithographers, Phoenix, Arizona, 1976. [16] H. Saechtling, International Plastics Handbook, Hanger Publishers, 1983. [17] Keyser, Material Science in Engineering, Charles E. Merrit Publishing Co., 1986. [18] Kornmayer, Storage of plastics in outdoor silos, J. German Plastics (Kunststoffe), 81 (1991) 12-14. [19] J. Suenaga, E. Fujita and T. Marutani, Polymer blends of semi aromatic liquid crystal polymers with engineering plastics, Japanese J. Polymer Sci. Tech., 48 (1991) 573-579. [20] H. McQuiston, Designing with engineering plastics, J. Plastic Eng., (1980) 18-25. [21] T. Noguchi, S. Satu, A. Shingo, Ken-ichi and M. Yoshida, Toughness evaluation of epoxy resins by compact tension tests, J. Soc. Material Sci. Japan, 40 (1991) 1118 1124. [22] K. Nakamae, N. Nishino, Y. Airu and K. Takatsuka, Pressure dependence of the curing behaviour of epoxy resin, J. Polymer, 23 (1991) 1157-1162. [23] M. Gehrig et al., Determining the co-efficient of thermal conductivity according to a quasi-stationary method, J. German Plastics (Kunstoffe), 81 (1991) 3(~32. [24] H. Zhang, H. Xie and Z. Liu, Morphology and electrical breakdown of polypropylene, Proc. 3rd. Int. Conf. on Properties and Application of Dielectric Materials, Tokyo, 1991. [25] J. Moulton, Processing of conductive polymeric materials, Proc. American Chemical Society, Spring meeting, Atlanta, (1991) 137 138. [26] R.H. Wehrenberg II, Today's conductive plastics combine shielding plus strength, Material Eng., 95 (1982) 3743. [27] Anonymous, Foam-skin line for telecom cable, J. Wire Industry, 58 (1991) 625-626. [28] J. Seppala, Y. Linko and T. Su, Photo- and biodegradation of high volume thermoplastics, Report, Acta Polytecnica Scandinavica, Chemical Technology, Hensinki, pp. 1 33, Series No. 198, Finland Acad of Technology, 1991. [29] T.J. Vickerstaff and N.Z. Gindy, Orthogonal machining of polymers, Proc. 21st Int. M T D R Conf., Swansea, 1980. [30] S. Bahadur and K.C. Ludema, Viscoelastic nature of the sliding friction of polyethylene and copolymers, Wear, 18 (2) (1971) 109 128. [31] J.J. Bickerman, Amer. Chem. Soc. Symposium, Los angeles, 1 (1974). [32] Crawford-RJ, Keating-TG, Chee-WY and Tan-YL, Flow drilling of plastics, J. Plastics Rubber Composites Proces. Appl., 16 (1991) 263 270. [33] M. Alauddin, Machining of non-metallic materials, M.Sc. dissertation, University of Manchester, 1987. [34] J.E. Morgan, J. Plastics Rubber Proces. Appl., 6 (1) (1986) 29-33. [35] ASME's Handbook on Machining, 8th edn. (1967).