Page 94 - HINDLEYS CATALOGUE 2018

ACRYLIC SHEET MATERIAL CHARACTERISTICS
CAST ACRYLIC
Cast acrylic is produced by casting in a mould, made from two sheets of float glass, separated by a gasket of the required thickness. The monomer, methylmethacrylate, together with any colouring pigments necessary is poured into the mould, which is then laid horizontally in a polymerising oven. During this process, due to gravity, the colouring pigments will settle producing a denser concentration in the lower face. The upper ‘good face’
is identifiable by the printed, or clear, protective paper or polyethylene film. It is preferable when line bending that the bend should always be made such that the ‘good’ face is to the front, otherwise a matt effect and noticeable reduction in surface gloss is evident along the bend, particularly in darker pigmented materials.
Cell cast acrylic (or PMMA to use another abbreviation in common use) has excellent flexible strength, resistance to UV exposure (sunlight), and weather.
Clear sheet has a light transparency factor of 92. These factors, coupled with its wide thermoforming range, ease of fabrication, solvent cementing capability and variety of colours make the material a logical first choice for the sign and display industry, creating products of considerable aesthetic appeal.
Thermal Properties
Cell cast acrylic has a dominant ‘elastic’ range, any identifiable ‘plastic’ range is unusable as it is too
narrow for practical purposes. When formed without mechanical restriction (i.e. press moulds or profile controlling jigs) it will adopt a uniform and even radius, dome, curve or transition between two points of mechanical constraint. The dominant elastic nature also aids an excellent plastic memory.
Cast acrylic absorbs moisture like most plastics but it does not create a problem when thermoforming. The elastic characteristic is sufficient to withstand the small pressure pockets of steam created by any moisture content.
Local Line Bending
Cast acrylic responds well to all types of local
line bending, including contact heating. The ideal thermoforming range is 145°C (293°F) to 170°C (338°F), above the higher limit the physical properties will be impaired.
Care must be taken to ensure thorough heat penetration, particularly of thicker materials, to avoid a ‘crease’ effect such as that produced by folding thick card. Increased heating time at a lower heat setting will cure the problem. Heating the material from one side only enables a sharper internal bend to be obtained.
Vacuum Forming
Due to its dominant elasticity, detailed definition is not possible with cast acrylic, only general contours with large radii such as acrylic baths will respond to atmospheric forming pressure. Where small areas of high definition
are required this is often achieved by a combination
of vacuum forming for the gentle contours and local press forming using highly polished tooling for the more detailed areas.
Other Forming Techniques Dome Blowing:
With a dominant elastic range, virtually true domes and hemispheres can be blown, using positive air pressure, to "inflate" material pre-heated to 165°C (330°F) and held in
a circular clamp. The pressure required is quite low and not in any way hazardous from a safety aspect, typically about 0.6bar (10psi). Larger domes or blown shapes will require lower pressures than smaller sizes as the extension per unit area, and therefore pressure, is less.
Drape Forming:
Lay pre-heated material onto gently curved concave or convex mould and use the material weight only
to adopt to the contour of the mould. Typical applications are motorcycle windshields, gently curved display panels and security screens. The mould is generally covered by green baize or mould cloth to minimise marking of the sheet surface.
Press Moulding:
Using a combination of male and female moulds and mechanical pressure.
Machining, Cutting And Finishing
Traditional woodworking tools, circular saws, routers, jig saws and band saws may all be used. High speed, fine pitch blades will minimise chipping and notching. Hand saws, fret saws, vibrating saws or hack saws are suitable but not applicable to production situations. For straight line cutting, score breaking is an accepted method. Commercial fabricators use sophisticated CNC routers and laser cutters. Routers will produce a keen and well defined edge which will have a matt finish and require further finishing by buffing or flame polishing if a polish on edges is required. Laser cutting generates a polished edge as the cutting takes place and is particularly suited to thicker materials but corners tend to be less well defined or keen.
HSS twist drills should be ground to a 130° point angle, with zero rake and should be lubricated with water or soluble oil in use. Holes may be tapped using standard taps and dies, coarse threads with rounded profiles are recommended and lubricants should be used. Cast acrylic machine turns, mills and engraves well.
Cut edges can be finished by abrading progressively with "wet & dry" paper or scraping and finishing off by power buffing or with liquid metal polish or proprietary polishing compounds. Flame polishing is a quick and efficient method of final edge polishing. Latest techniques for edge finishing are the use of diamond tipped high speed cutting heads which convert a good sawn edge to a fully polished finish in one machine pass.
Stress Relieving
All of these processes, except diamond cutter edge finishing, create stress in the material and can cause crazing and cracks, especially when exposed to the presence of solvents such as those contained in most cements. This stress will manifest itself as small cracks extending from the edge of the sheet, often appearing some days, months or even years after forming or at the point when a solvent cement is applied during fabrication. In the case of stress from local bending these cracks will appear parallel to the bend about 20mm (3/4in) from the bend. The solution is to anneal, either prior to cementing, or as a last process after completion of manufacture. One hour at 80°C (176°F) in a temperature controlled oven will produce an effective surface anneal and considerably improve product life.
Most thermoplastics are susceptible to ‘notch’ effects and sharp internal corners will concentrate stress and provide a potential start point for fracture under load. Always radius internal corners if a design permits.
Jointing
Solvent adhesives are available which comprise an acrylic filler suspended in a volatile solvent. For visually pleasing results considerable care and expertise is necessary often involving careful masking and controlled application using a suitable dispenser. Protective paper films are often bonded by a PVA adhesive which forms a barrier to solvent cements. Adhesives from such films should be removed using soap and water prior to cementing. Polyethylene films are often adhered with pressure sensitive adhesive and can be replaced after removal. Welding acrylic is not practical as solvent cements cover all eventualities adequately and the high thermal stresses involved would cause excessive stress and destroy
the optically flat surfaces adjacent to the joint line. Information by kind permission of C R Clarke & Company.
EXTRUDED ACRYLIC
Whilst manufactured from the same raw materials
as cast acrylic, the method of converting the methylmethacrylate monomers into primary sheet material significantly modifies its thermoforming characteristics. Extruded acrylic has a small "elastic" range only spanning some 30°C (54°F) and a generous "plastic" range covering approximately 90°C (162°F). The extrusion process involves forcing the polymers through a long, narrow, highly polished die. This produces a grain effect which, when subjected to a thermoforming operation, will produce varying degrees of shrinkage depending upon the axis of the extrusion process.
As a result of the lower labour content in the manufacture of extruded material, it is typically cheaper than its cast equivalent, but this must be balanced against the slightly inferior surface quality and, because of the prominent elastic range, a tendency towards temperamental machining characteristics when cutting
tool temperatures enter the plastic zone and chips tend to adhere or spall onto the cutter reducing efficiency.
Durability of extruded acrylic is similar to cast acrylic with respect to UV resistance and weathering, but it is less impact resistant. Both acrylic materials accept screen printing well, subject to the use of suitable inks and can be thermoformed easily after printing.
Local Line Bending
Few problems should be experienced provided that the bending heater has a reasonably uniform temperature along its length. It is preferable to heat within the 30°C (54°F) elastic range otherwise blistering may occur if moisture is present and there will be a tendency for the ends of a bend to spread and stretch because of the material entering the plastic state with its characteristic weak elastic or tensile state. The spreading of the bend line at the ends is unsightly and compounds distortion. If using contact heating uniformity of heat becomes critical as entering the plastic condition will produce pressure marks and sticking to the blades may also be experienced. This can be eliminated by using PTFE non-stick tape but
it will not eliminate the pressure marks. Generally the uniformity of heat control with contact benders is more difficult to control by design and as the component length increases this problem becomes compounded.
Vacuum Forming
For thermoplastic materials to respond to the vacuum forming process, with forming pressures of maximum 1.0bar (14.7lb/sq in) and typically 0.83bar (12.25lb/sq in),
it must, generally, be capable of entering the "plastic" state. Since most vacuum forming heater systems have a temperature gradient between the central zones and the areas adjacent to the clamp frame of 20°C (36°F) at best, the larger the plastic range the better. Extruded acrylic with its 60°C (108°F) plastic zone, therefore, exhibits very forgiving vacuum forming properties.
Most plastics absorb moisture and this will cause problems once attempts are made to elevate material temperatures into the plastic range necessary for high definition forming. Extruded acrylic is such a material and attempts to vacuum form moisture impregnated material will result in moisture blisters appearing at the point when the plastic range is just attained in the hotter areas.
Typical reaction is to assume that the sheet has been over heated and attempt the forming operation. This results
in the sheet being pulled from the clamp frame as the mould is forced upwards into a sheet which is, for the most part except where the blisters have appeared, still in a springy and elastic condition.
Whilst there are exceptions to this problem (PVC, polystyrene and polypropylene) most other hygroscopic materials need to be pre-dried before vacuum forming. Drying time will be dependent upon thickness at a temperature about 40°C (72°F) below the start of the thermoforming range. An approximate rule of thumb is 1.25 hours per mm (.040in) of thickness. Another option, if usage is small in volume and occasional, is to make a simple "hot box" containing the material with low power greenhouse type heater installed in the base.
It can be operational at all times at little cost and sheets will always be available for use as required, provided they have been in the box for a minimum of 24 hours.
Other Forming Processes
Extruded acrylic will respond to dome blowing etc, like the cast material, but if it is heated to its plastic state, and retains its temperature, it will extend indefinitely with application of pressure creating non-uniform shapes. It will also be more susceptible to contact marks. As the available elastic window is so much smaller than with cast material, processes reliant upon unrestrained or free forming will become increasingly time and temperature critical.
Machining & Finishing
Generally as for cast acrylic but more prone to shatter, especially after forming, if not stress relieved. Flame polishing is difficult because of the narrow elastic range and surface degradation will occur easily.
Jointing
As for cast acrylic.
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