Machining of plastics

Kobayashi A, "Machining of Plastics", McGraw-Hill, New York, 1967. 

Machining of plastics is carried out when the number of workpieces to be produced is small and to purchase expensive moulding equipment would be uneconomic. Alternatively, where accuracy greater than can be obtained from the moulding technique is required or where features such as tapped holes cannot be included, machining is essential. 

Machining of plastics is often necessary as, for example, in cutting and finishing Formica (melamine laminate) countertops or drilling structural pultruded shapes for attaching to supports, etc. While such operations are commonplace when working in metals or wood, special tools and practices are necessary when machining plastics. 

The largest volume of machining of plastics is done as secondary operations for other processes. With the exception of the more precise molding processes, such as injection molding, most of the plastics processes must have their openings machined (usually routed) and their excess material trimmed off. Even injection molding requires gates to be trimmed for the majority of applications. For applications of moderate volume, it may be most cost efficient, when mold... 

Thus there are important guidelines to remember when considering machining of plastics ... 

Appendix I Important Information Regarding Machining of Plastics LI Machining Gnidelines for Common Plastics... 

Machining of Plastics n Many of the machining operations used for metals are applicable to rigid plastics, with appropriate variations in tooling and speeds [see... 

The calibration graph for the probe using a strength machine, has been shown in Fig. 7 It can be observed that the dependence of indications of the device of Wirotest type on the loading is linear within the proportionality limit scope. After unloading the indications do not return to zero, but show own stress caused in effect of plastic deformation of the tested sample... 

Injection Molding. Matched metal molds are used in the fabrication of plastic closures, specialty packages, and botde preforms. In conventional injection mol ding the plastic resin is melted in an extmder which forces a measured quantity or shot into a precision-machined chilled mold after which the nozzle of the extmder is withdrawn. 

Machining of metals involves extensive plastic deformation (shear strain of ca 2—8) of the work material in a narrow region ahead of the tool. High tool temperatures (ca 1000°C) and freshly generated, chemically active surfaces (underside of the chip and the machined surface) that interact extensively with the tool material, result in tool wear. There are also high mechanical and thermal stresses (often cycHc) on the tool (3). 

However, nowadays this type of machine is seldom used because it is considerably more complicated and more expensive than necessary. One area of application where it is still in use is for large mouldings because a large volume of plastic can be plasticised prior to injection using the primary cylinder plunger. 

Cooling the moulds of plastic-moulding machines with chilled water... 

In order to understand potential problems and solutions of design, it is helpful to consider the relationships of machine capabilities, plastics processing variables, and product performance (Fig. 1-10). A distinction has to be made here between machine conditions and processing variables. For example, machine conditions include the operating temperature and pressure, mold and die temperature, machine output rate, and so on. Processing variables are more specific, such as the melt condition in the mold or die, the flow rate vs. temperature, and so on (Chapter 8). 

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