2.Production is relatively small batches of 15-19 sheets-making colour changes easy. Up to 3,000 different colours and shades are typically available and a further 2,000 have been produced.
3.Cell casting is reasonably labour and time intensive making cast acrylic; more expensive than extruded.
It is relatively easy to identify cast acrylic; any unusually or individually coloured, glossy thermoplastic sheet is probably cell cast acrelic-if it makes a fizzing noise and doesn’t drip when on fire, it could well be cast acrylic.
THERMOFORMING PROPERTIES
All thermoplastics go through two distinct phases as they are heated.
At first they become what is known as ‘elastic’; the material is springy, like a rubber band, with an element of tensile strength which gives it a reasonable resistance to forming.
As the material is heated further though, it will become what is known as “plastic”; the material is soft and malleable, like dough, and easily be formed.
The temperatures at which TP’s become elastic or plastic depend on material type.
As you can see from the above chart , some TP’s have a large temperature range during which they are either elastic or plastic. This is useful for thermoforming, either when carrying out a process such as line bending using a radiant element which has hot and cold spots along its length, or dome blowing, where the sheet has to be transferred from an oven onto a dome blowing unit, exposing it to the cooling effects of the air before it is formed; the wider the thermoforming window, the more time you have to do this.
The characteristics of each material’s elastic/plastic phase are also unique. Cast Acrylic and PVC both have large elastic windows but behave differently within them. PVC’s elastic condition is weak enough for high definition vacuum forming but Cast Acrylic’s is not.
PLASTIC MEMORY
TP’s ability to regain form is known as , ’Plastic Memory’ and is another of the material class’s properties that has found a number of useful applications. For example:
I Graphics for illuminated, vacuum formed signs like the ones used on shop fronts and garages, can be applied prior to vacuum forming. The design is painted on to a blank vacuum forming which is then heated to recover its original flat shape. The design shrinks along with the material and the new distorted image is printed onto flat sheets which are then vacuum formed, emerging from the process, printed and ready for use.
II Heat shrink sleeves are extruded as small tube, heated, example and cooled. They shrink back to their original size when re-heat-de.
HYGROSCOPY
All plastics absorb moisture to varying degrees and this effects the way they behave when heated.
Moisture forms tiny bubbles of water which, when a thermoplastic is heated, turn to steam and expand.
In plastics that have mostly elastic windows, this has no effect, as the tensility of the material resist the pressure of the steam bubbles.
However in plastics that have mostly plastic windows, the steam causes the moisture bubbles to expand and where the bubbles are near the surface they may burst, forming blisters, ruining the surface quality of the sheet.
This characteristic must be understood before heating to avoid confusing hygroscopic blisters with burning. Generally, if a material has blistered but maintains its tensility, then it is hygroscopic blistering and the material will require pre-drying before heating.
Pre-drying hygroscopic TPs with predominantly plastic windows, removes the moisture content relieving the problem of blistering during heating.
Note:PP absorbs so little water that it is effectively, non- hygroscopic. It floats in water and would normally be specified instead of nylon for underwater bearings (whilst nylon is an excellent bearing material it is one of the most hygroscopic TPs).
WORKING WITH TP MATERIALS
LINE BENDING
Line bending of TP sheet is normally carried out when the TP is in its elastic state, using a strip heater to heat a line in the sheet, then folding and placing in a jig to cool. There are types of strip heaters in common use:
1.THE HOT WIRE STRIP HEATER uses tensioned hot resistance wires that are heated by passing an electric current along them. Hot wires emit heat consistently along their length and are straight (providing there is enough tension) so the same amout of heat is delivered to the same point all the way along the bend line-wich is important if a TP has a narrow thermoforming window;ie. You want the whole bend line to reach the same thermoforming temperature at the same time. The ends of a bend line can receive more heat because the edge gets heated as well as the surface-some TP’s may need ‘shielding’ with pieces of scrap material at either end of the bend line, to prevent overheating and damage to the ends of the bend.
2.THE CONTACT STRIP HEATER uses heated blades in various sections (pointed, rounded, flat). It is particularly effective on thinner materials but, can be used on thicker materials too. Materials that thermoform mostly in the plastic window, may experience pressure marks or sticking to the blades. The consistency of the heat emission depends on the consistency of the heat source. Contact heater blades tend to be cooler
at the ends of the bend line which can cause problems for some TP’s as the middle section of a bend may overheat before the ends reach thermoforming temperature.
3.THE RADIANT ELEMENT STRIP HEATER uses a coiled electrical element. Disparities in the heat emitted along the length of these heaters makes them suitable for only a limited number of TP’s.
VACUUM FORMING
Vacuum forming is normally carried out when the TP is in its plastic state. The more plastic a TP is when vacuum formed, the better the definition wil be. There are many different types and sizes of vacuum forming machine available.
OTHER FORMING TECHNIQUES
FREE DOME BLOWING: Smooth, uniform and virtually hemispherical domes can be blown from sheets, with air pressure of about 0.6bar (10psi)(larger domes or blown shapes need less pressure because the extension per unit of area is less).
DRAPE FORMING: The material’s own weight provides the pressure for this type of forming which, as the same implies, involves laying heated sheets onto curved, concave or convex, moulds. The moulds are covered in green baize or mould cloth or greased to minimize marking.
EXTRUSION: Material is driven through a die to produce a section.
INJECTION MOULDING: Material is driven through a nozzle into a mould to produce a product.
DIP COATING: Metal objects are heated and dipped in air-fluidised TP powder which evenly coats the object.
RPTATIONAL MOULDING: Granules are placed in a mould which is then heated whilst being rotated on two perpendicular axis so that the granules evenly coat the inside of the mould. The resulting product will be hollow.
CUTTING
Traditional woodworking tools-circular saws, routers, jig saws and band saws, with fine pitch blades to reduce chipping, are used for cutting out. Hand saws, fret saws, vibrating saws or hack saws can be used in the school workshop. Score breaking can be done with thinner sheets.
CNC routers and laser cutters are used by commercial fabricators. The router produces an even matt finish to the edge of the material while the laser cutter has a polished finish. However, the laser cutter puts stress into the material and rounds the corners.
Some of the thinner and less brittle TP’s such as ABS or PP can be cut on a guillotine, as long as the edge finish is not important.
EDGE FINISHING
Edge finishing techniques vary from buffing to liquid metal polishing compound, flame and diamond cutter polishing. Every method apart from diamond edge finishing, requires the edge to be prepared with abrasives (such as ‘wet and dry ’) or scraped with a metal edge. Stress can be induced by the heat of flame polishing and post annealing is reconnended.
DRILLING
Holes can be drilled using HSS twist drills ground to a 1300 point with zero rake, lubricated with water or soluble oil.
Holes can be tapped with standard taps and dies, but, coarse threads and rounded profiles work best and lubricants should used.
MACHING
Machine turing, milling and engraving can be done with most TP’s. |
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发表于 2005-3-8 12:31:00
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