Machinability of plastic

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Machining of plastics describes the processing of plastic blanks by turning , milling , drilling , threading , knurling and grinding . The parts made with it are also generally referred to as plastic turned parts, which are often invisible as guide bushings, spring settings, sealing rings and rollers , ensuring the reliable operation of machines and devices.

Difference to metal cutting

Although there are analogies between the machining of metals and plastics, the behavior of the plastics during machining must be taken into account, i.e. the thermal properties of these materials (specific heat, thermal conductivity and temperature sensitivity) and the pronounced dependence of the mechanical-technological properties on temperature and the rate of stress.

In comparison to metalworking, the following must be taken into account:

  • Plastics are generally poor conductors of heat. Their thermal conductivity is only about 1/1000 of the thermal conductivity of copper or about 1/125 the thermal conductivity of steel . Thermal conductivity is decisive for temperature and heat distribution during machining, especially for heat dissipation from the area of ​​the friction surfaces between the material and the cutting material . The poor thermal conductivity of plastics leads to an intensive build-up of heat in the material. The heat is dissipated almost exclusively via the tool, which means that excessive thermal stress can occur with certain cutting materials. B. a constantly cutting tool made of tool steel can anneal at high cutting speeds at the cutting edge.
  • Plastics have a lower heat resistance than metals . In the case of thermosets , when exposed to heat above 180-200 ° C, the resins decompose and char . Amorphous thermoplastics soften when the freezing temperature is exceeded (with some materials already at 50–60 ° C) and semi-crystalline materials melt when the crystal melting range is reached. The achievable accuracy and surface quality of the workpieces drop noticeably when these temperatures are reached.

Plastics generally show a difference between the lower tensile strength versus the higher compressive strength under normal test conditions, i.e. This means that tool shapes that cause tensile stress on the material during machining reduce the cutting forces and facilitate the cutting process. On the other hand, plastics show a clear dependency of the mechanical-technological properties on the temperature and the loading speed.

As the temperature rises, the strength of the materials decreases while the elongation increases. On the other hand, as the loading speed increases, the strength increases and the elongation decreases. In contrast to the behavior of metals, the modulus of elasticity increases with the rate of deformation, so temperature and loading rate are decisive for the deformation behavior and thus for the type of cutting process, the chip formation and the chip shape.

A material is z. B. under the condition of the same temperatures with increasing cutting speed behave increasingly brittle until at supercritical cutting speeds breakouts occur on the surface of the workpiece. Since only relatively low cutting forces occur when machining plastics, these machining limits must be taken into account when choosing the tool shape.

The plastics are often mixed with additives and auxiliaries that serve as colorants , heat stabilizers , lubricants , light and anti-aging agents or the like and to some extent influence the machining properties. Most important here are metal oxides , often verschleißend act on the tools. Filler or carrier materials are added to most thermoset materials, which also cause tool wear; filled thermoplastics behave like filled thermosets during machining.

Fiber reinforced plastics

Fiber-reinforced plastics are often superior to metallic materials because they are specifically stronger and stiffer than metals. The predominantly near-net shape production of the components allows a force flow-compatible construction.

In the case of fiber-reinforced plastics, machining concentrates in particular on the reworking of near-net-shape components; grooves are very often milled and bores or contours are introduced by circumferential milling . Turning processes are used for rod-shaped round profiles.

While unreinforced thermoplastics can be machined relatively well with tools made of high-speed steel , hard metal tools, tools with hard material coatings or PCD tools must be used for fiber-reinforced materials . The machinability of polymer composites is largely determined by the type of fiber used: The very brittle glass and carbon fibers require wear-resistant tools because of the highly abrasive effect of the fiber particles, while the very tough aramid fibers have a special cutting edge geometry , otherwise the fibers will fray.

Because of the low thermal conductivity and the low melting temperature of the matrix materials , when machining fiber-reinforced polymers, care must be taken to generate the least amount of heat while at the same time dissipating the resulting heat well. This is why low cutting forces are sought for their machining, as otherwise detachments in the fiber-matrix composite ( delamination ) occur.

Depending on the choice of material and process parameters, damage to the drilled holes can occur. When machining at a high feed rate, the matrix material breaks and the fiber material becomes frayed at the edge of the hole, while at lower feed rates, material adhesions on the rake face and the flute occur. These material adhesions clog the flute and can significantly impair the machining quality and process reliability and, in extreme cases, lead to tool failure.

Heat dissipation is essential on hard metal cutting edges, compressed air cooling is the most suitable. Water and emulsion can only be used for cooling if the plastics do not swell too much. However, if the plastics swell strongly, the machined surface will be rough and cracked, as they absorb a lot of water at the interface due to the increased temperature. Short chips and a lot of dust dull the tools and there is a risk of explosion due to the contamination of the machines. Cooling with water is common for acrylic glass and high-grade synthetic resin, but is essential for celluloid because of the risk of fire.

An alternative to the conventional manufacturing processes of turning and milling is turn- milling , which is a combination of both processes.

dentures

In the manufacture of dentures , plastics are usually processed by milling or polymerizing . Only limited filler contents are possible in the stereolithographic process , but fillers are essential to achieve high strengths. For some applications ( splints or orthodontic devices) this is not absolutely necessary, but for crown and bridge technology high strengths are necessary, which can only be achieved with high filler contents or fibers ( composite materials ).

Semi-finished products (blanks) for milling can contain very high filler contents and / or fibers, which can be critical for the bond with the plastic matrix, since fibers or particles could be broken out or loosened by milling.

literature

Alfred H. Henning Karl Krekeler Georg MengesBernhard J. Frerichmann: Determination of working conditions suitable for production and investigation of the machining behavior when turning thermoplastics :; ISBN 978-3-663-06294-3

Individual evidence

  1. ↑ Weinert, Klaus .: Machining Technology . 3rd edition. Vulkan-Verlag, Essen 2001, ISBN 3-8027-2925-0 , p. 25-28 .
  2. ↑ Werner Degner, Hans Lutze, Erhard Smejkal: Spanende Formung: Theory, calculation, reference values . Carl Hanser Verlag GmbH & Company KG, 2015, ISBN 978-3-446-44583-3 , p. 351 ( google.de [accessed on January 3, 2019]).
  3. ↑ Roland Strietzel, Claudia Lahl: CAD / CAM systems in laboratory and practice . Verlag Neuer Merkur GmbH, 2007, ISBN 978-3-937346-41-0 , p. 63 ( google.de [accessed on January 1, 2019]).