Plywood is a manufactured board. It is used to case speakers. Birch-faced ply is a light coloured, high-quality plywood that colour-stains and varnishes easily.
Chipboard is also a manufactured board. It is cheap and will not warp or curl. Chipboard comes in two forms, plain and woodgrain, both of which are usually coated in plastic ‘foil’ or veneered to improve the finish and looks.
MDF (Medium Density Fibreboard) is another manufactured board. It used to fabricate boxes, and close-textured MDF is cut and sanded to make moulds for vacuum forming plastics. MDF must be glued using PVA carefully and precisely, and requires coating with paint many times to give a good finish, as it absorbs liquids readily.
Mild steel can be found in many different forms. Sheet steel, for example, is made into casings and a variety of brackets an fixings.
Bright drawn steel is made into round, square, tubular, angled and flat bar versions.
Mild steel can be cut and shape into components that are to be permanently fixed to the product. This is done by welding, brazing and using silver solder.
Aluminium is a light metal in its elemental form. It comes in sheet, bright drawn and mild forms (similar to steel) and can be cast. If a strong lightweight product is needed, aluminium is the only choice.
Aluminium can be joined using nuts and bolts, rivets or epoxy resin glue. Aluminium also is superior, in terms of lifespan, to steel because it has an outer layer of aluminium oxide that prevents it from corroding for a long time.
There are many types of plastics used and many forms of manufacture used to construct both industrial and small-scale casings. The two most relevant to my project are acrylics and polystyrene.
Acrylic is a rigid, strong and smooth-looking plastic. It comes in many colours and forms (e.g. Cast Acrylic). It is durable (10 times the impact resistance of glass), and therefore suitable for outdoor use. Acrylic’s non-toxic properties make it safe to be used with food or made into a toy; however, in the unlikely event that it is broken, it will splinter and expose sharp points. Acrylic becomes malleable when heated and can be easily machined. It is also possible to cement pieces of acrylic together using methylene chloride glue. When polished, acrylic must be handled carefully as its surface dents and scratches easily.
Polystyrene comes in many forms. One such form is a thin sheet of polystyrene that is used in vacuum forming machines to fabricate a casing. It has a low melting point and is fairly weak, but unlike acrylic it is not brittle. Polystyrene is also an excellent insulator, minimising any chance of electrocution by the product. It can be cut and assembled rapidly, but it takes time, precision care and effort to manufacture a quality product casing.
Plastic casings can be made in a variety of ways. Injection moulding is the most common method of manufacturing casings in industry. The machine makes a casing in three stages:
Step 1: The mould is warmed and the thermoplastic substance is placed in the heating chamber.
Step 2: The thermoplastic is melted and then pushed into the mould by the plunger, which maintains the pressure for a set ‘dwell time’ to prevent shrunk or hollow products from being formed by stopping any molten plastic from returning to the chamber.
Step 3: When the plastic has solidified, it is removed from the mould by hand or by a mechanism built in the machine (this is more likely to be found in industry).
Vacuum forming is more likely to be used on a smaller scale (e.g. in schools) to produce casings, but is used in industry to make egg boxes and food containers. There is a range of vac. forming machines, but every one works from the same principle and is designed to:
1. Seal a plastic sheet on to the vac. chamber.
2. Heat the plastic sheet.
3. Create a vacuum so air pressure presses the heated plastic over the mould.
The vac. forming process occurs like so:
Step 1 The plastic sheet is clamed on top of the machine, resting on the heat resistant silicone rubber pads, after the mould of the desired shape has been placed on the raising platform inside the machine.
Step 2 A heating element is pulled across the top of the plastic sheet, heating it until it crinkles, then becomes taught again.
Step 3 The platform is raised, and the mould is pushed into the plastic.
Step 4 The air is then pumped out of the chamber, and air pressure (now greater than inside the machine) presses the sheet over the mould.
Step 5 The sheet is removed from the machine (assisted using a taper, which angles all vertical sides of the casing). Excess plastic is trimmed from the casing.
The mould is usually constructed from layers of MDF. It is solid because air pressure given no resistance can crush hollow moulds. If the mould is too far down, webbing (plastic sticking together in corners) occurs, not leaving enough plastic to give a full coating on the mould. The mould is smoothed to avoid bumps and lines on the casing. Air pockets are prevented by drilling holes in the mould, allowing excess air to flow into the vacuum.