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plastic fabrication

Fabricating thermoplastic sheet

As outlined in furniturelink's plastic properties, the use of plastics by furniture SMEs, particularly PVC and polyester resins, raises environmental issues. These concerns can be minimized by fully exploiting the unique properties of plastics and by clearly labelling all parts for future recycling.

Chris Lefteri's books Plastics 2: Materials for Inspirational Design and Plastics Handbook (see right) are an excellent resource for designers wanting to harness the potential of plastics. The author lists each plastic by chemical type and gives examples of products with the designer's name. If your knowledge of plastics primarily begins and ends with Plexiglas®, the range of "high-tech" polymers presented and the design problems they have solved will be eye-openers. The (free) 12-page PDF, Thermoforming Plastic Materials, courtesy of C. R. Clarke & Co. in the UK, is also a recommended starting point.

Most furniture components are formed using injection moulding, a capital intensive technology beyond the reach of most SMEs (an exception is 3D printing). Fabricating furniture parts from readily available thermoplastic sheet materials can be accomplished with simple tools and equipment

Injection moulding video courtesy Bill Hammack, University of Illinois.

Furniture Friendly Thermoplastics

Unfortunately, all the materials exposed in Chris Lefteri's book are not available in sheet form from the typical stockist/distributor. furniturelink has chosen the following polymers as the most "available" and furniture-component friendly.

PETG (polyethlene teraphthalate glycol) is optically clear, tough and strong and for most structural applications superior to acrylic/PMMA.

PC (polycarbonate, most widely known by its General Electric trade name Lexan® is optically clear, extremely tough and strong but more expensive than PETG.

PMMA (polymethyl methacrylate, commonly called acrylic or Plexiglas®, its Rohm and Haas trade name) is optically clear and easily obtained in a wide range of opaque/transclucent colours. It is rigid and strong but with relatively low impact resistance. Substitute with either of the above unless the application is low-stress, or a high resistance to UV light is mandated.

UHMWPE (ultra high molecular weight polyethylene) is commonly available in white or black, is extremely tough, has a "waxy" feel and is ideal for machining into bushings, brackets, knobs and custom glides.

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Prototype thermoformed sheet plastic chair

Designing Components from Sheet Stock

Always refer to the individual manufacturers specifications before choosing a material for a particular task. Common design concerns when working with thermoplastics include:

  • Most are not resistant to long exposure of UV light.
  • Thermoplastics "creep" under load, especially at elevated temperatures or over time.
  • Many are very notch sensitive; avoid sharp 90 degree bends or cut-outs.
  • Some are hygroscopic and may require oven drying before performing operations such as vacuum forming.
  • The bookcase shown at the bottom of this page includes a plastic component where the properties are used to maximum advantage.

Machining Thermoplastics

Most thermoplastics can be machined using standard woodworking equipment as follows (always refer to the suppliers instructions):

Table saw – use a sharp fine tooth carbide-tipped blade for hard plastics, medium tooth blade for soft plastic and compressed air as a coolant if "gumming" occurs.

Band saw – use blades with 2-10 teeth per cm, running at 600 m per minute.

Router – run cardide-tipped 10 to 13 mm diameter cutter bits at 20,000-25,000 rpm; cooling the cutter with compressed air is advised.

Drill press – install drills bits with a negative rake and 70-120 degree point angle.

Always use appropriate guards and wear eye protection.

Line-bending Thermoplastics

*A line bender extends the possibilities of small-scale production. This inexpensive tool is available in two-sided or single-sided models (see sidebar for suppliers). The thermoplastic is aligned with the tool as required (see diagram (1)) and removed when pliable to a jig for cooling. When using single-sided heaters, the heated side of the sheet forms the outside radius of the bend. This tool can produce such items as cable-run channels, under-shelf trays and file holders. However, when it is used in conjuction with PC or PETG, more structural applications can be considered. (See C. R. Clarke or Kamweld for more details.)

Plastic Project

* This three-sided bookcase, designed by BCIT student Corey Robertson, uses a polycarbonate "joining plate." PC is ideal for this purpose, as it is extremely strong and impact-resistant yet easy to machine with standard woodworking routers. Metal would have been a poor substitute for aesthetic and safety reasons.

(1) Diagram courtesy of Kamweld.

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