Thermoforming Plastics used in Skylights
Plastics are essential materials of modern day life. Technological advances improving our quality of life through telecommunications, computers,
transport, education, housing, shopping, to name but a few - have been made possible by the unique forming, electrical, thermal, tensile and
insulation properties of thermoset and thermoplastic materials. Plastics can be divided into two main categories; Thermosets & Thermoplastics.
A chemical change occurs in these plastics during the curing process and cross linking of the molecular chains occurs. This irreversible condition
is usually induced Life in the home would be very different without Teflon by mixing a resin with an activator and normally Fluon coated cooking
utensils, plastic window frames with moulding or forming of the component into doors, food containers, fridges, TV’s, radios, into their final
shape. Thermosets cannot be re-formed after setting.. Any subsequent shaping is carried out using less traditional wood or metalworking type
Plastics - Polymers
Plastics; synthetic, polymeric products of the petrochemical, coal or gas industry, can be molded into any shape, are aesthetically pleasing
and have low density and friction co-efficients. Some plastics also have excellent corrosion and impact resistance. Materials science is constantly
designing and developing new materials for an ever increasing number of applications and plastics are no exception.
Plastics are made from polymers; chemical compounds composed of long molecules made up of chains of small THERMOPLASTIC repeat units (monomers).
Polymers are rarely used. Thermoplastics soften or melt when they are heated, alone, additives are used to enhance the appearance, rather like
wax, and regain their rigidity when cool, improve the strength and change the characteristics of various ‘Thermoforming’ processes have been
developed for different plastics. Changing the shape of thermo additive types affects plastic sheets by exploiting this property, Antiblocks
e.g. Talc, silica, clay, mica, ceramic spheres - Prevent a film sticking to itself and make separation of film easier.
Antifogs prevent the formation of fog (water vapor) on the plastic surface and prevent oxidization. Carbon, metallized fillers and carbon fibers
reduce build up of static. Halogenated compounds, phosphorus compounds, metallic oxides & inorganic fillers reduce flammability and maintain
color quality at high forming temperatures. Mica powder helps the molecules to flow during forming and prevent material from sticking to moulds.
Epoxidized vegetable oil, butadiene make material soft & pliable and improve the production rates at manufacture by removing the 'sharkskin'
or 'orange peel' effect produced by molten polymer sticking to the die. Amides reduce the coefficeint of friction, thus helping the molecules
to flow Talc, chalk, clay improves stiffness, strength & electrical properties (clay).
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 can easily be formed. 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 resists
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. The temperatures at which
TP’s become elastic or plastic depend on the material type. 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 Pre-drying hygroscopic
TPs with predominantly which has hot and cold spots along its length, or plastic windows, removes the moisture.
Content dome blowing, where the sheet has to be transrelieving the problem of blistering during heating, ferried from an oven onto a dome blowing
unit, exposing it to the cooling effects of the air before it.
Larger domes or blown shapes need less pressure because the extension per unit of area is less). Drape Forming: The material’s own
weight Linebending provides the pressure for this type of forming Line bending of TP sheet is normally carried out which, as the name implies,
involves laying heated when the TP is in its elastic state, using a strip sheets onto curved, concave or convex, moulds, heater to heat a line
in the sheet, then folding and the moulds are covered in green baize or mould placing in a jig to cool. There are three types of cloth or greased
materials to minimize marking. Strip heaters in common use: Press molding: With a male and female 1.The Hot Wire Strip Heater uses
mold and mechanical pressure. Sioned hot resistance wires are heated by passing an electric current along them. Hot Wire Extrusion:
Material is driven through a die to produce a section. emit heat consistently along their length and are straight (providing there is enough
tension). Injection Molding: Material is driven same amount of heat is delivered to the same through a nozzle into a mold to produce
a prodpoint all the way along the bend line - which is important if a TP has a narrow thermoforming. Dip Coating: Metal objects are
heated and window; ie. you want the whole bend line to reach dipped in air-fluidized TP powder which evenly the same thermoforming temperature
at the same coats the object time. The ends of a bend line can receive more. Rotational Molding: Granules are heated because the edge
gets heated as well as the placed in a mold which is then heated whilst surface - some TP’s may need ‘shielding’ with being rotated on two perpendicular
axis so that pieces of scrap material at either end of the bend the granules evenly coat the inside of the mould. Line, to prevent overheating
and damage to the resulting product will be hollow. End of the bend cutting. The Contact Strip Heater uses heated blades in various sections
(pointed, rounded, flat). Traditional woodworking tools - circular saws, are particularly effective on thinner materials but, routers, jig saws
and band saws, with fine pitch blades to reduce chipping, are used for cutting out, can be used on thicker materials too. Materials 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. that thermoform mostly in the plastic window,
Some of the thinner and less brittle TP’s such as may experience pressure marks or sticking to the ABS or PP can be cut on a guillotine, as
long as the blades. The consistency of heat emission depends edge finish is not important. on the consistency of the heat source. Contact Edge
Finishing heater blades tend to be cooler at the ends of the bend line which can cause problems for some TP’s. Edge finishing techniques vary
from buffing to liquid metal polishing compound, flame the middle section of a bend may overheat. Diamond cutter polishing. Every method apart
from before the ends reach thermoforming temperature. Diamond edge finishing, requires the edge to be smooth. The Radiant Element Strip Heater
prepared with abrasives (such as ‘wet and dry’) or uses a coiled electrical element. Disparities are scraped with a metal edge. Stress can be
induced heat emitted along the length of these heaters by the heat of flame polishing and post annealing makes them suitable for only a limited
number of recommended TP’s. Drilling Vacuforming Holes can be drilled using HSS twist drills ground Vacuum forming is normally carried out when
the to a 130˚ point with zero rake, lubricated with TP is in its plastic state. Water or soluble oil is used, for better definition holes that
can be tapped with standard taps and dies. There are many different types and sizes but, coarse threads and rounded profiles work best of vacuum
forming machine available. and lubricants should be used. Free Dome Blowing: Smooth, uniform and Machine turning, milling and engraving can
be virtually hemispherical domes can be blown with most TP’s. sheets, with air pressure of about 0.6bar (10psi).
There are three main ways of joining TP’s: Cementing using solvent adhesives, such as dichloromethane, chloroform, methyl ethyl ketone
(MEK), acetone, or methylene dichloride (in different combinations depending on the material). Different solvents or solvent combinations will
be appropriate for different TP’s and it is best to consult the technical literature of the material manufacturer before deciding which ones
to use. Dissolving chips of the material being bonded in the solvent is sometimes recommended. Care and expertise is necessary to avoid making
a mess! Take your time, mask the job properly and get hold of the right solvent applicators (the best ones come with a pipette in the lid of
their container). Welding. Hot air, ultrasonic, vibration, spin and hot plate technologies have all been developed for TP’s. If a filler
material is being used (like the rod in hot air welding) it must be made of the same material as that being welded (so that it ‘melts’ at the
same temperature). Mechanical fixing works best with TP’s that have high tensile and impact strength so that they can withstand the
loads imposed at the bolting, screwing or riveting points - which should always be minimized by using large washers or countersinking. Stress
relief is essential to prevent cracking and crazing at the joints. Stress Relieving: Stress can be created by forming, cementing, welding or
flame polishing and manifests itself as crazing and cracks which can appear any time after a stress inducing procedure is carried out - hours,
months or even years after. Annealing before cementing, after forming or when a product is finished, reduces the stress imposed on a material
and prolongs product life.
Cast Acrylic PMMA (Methyl Methacrylate)
For Skylights, .100” - .236” thicknesses, and 4’ x 4’ – 10’ x 8’ sheet sizes are used.
More colors are available in this TP than in any other because of its production technique. It has a broad thermoforming heat band that is mostly
elastic (see chart), is readily solvent cemented, is more transparent than glass, has reasonable tensile strength (shatter proof grades are
available) and good UV and weather resistance. Trade names include Perspex, Plexiglas and Lucite. baths, signs, aircraft canopies and windows,
Skylights, caravan windows, secondary glazing, display and point of sale, guards, roof lights and domes, pick’n mix bins, aquariums, roadside
noise pollution screens. Heaters - all types. Pre-drying - never needed. Because of its large elastic window (70˚C) it is easy to heat long
sections to the elastic state, making cast acrylic the easiest and most forgiving TP to fold.
Application Line Bending Vacuum Forming
Only gentle contours with large radii, such as acrylic baths are possible because of its small plastic window. If small areas of high definition
are required, vacuum forming can be used in conjunction with pressure forming (using a highly polished former). Forming Techniques: Free dome
blowing, drape and press forming,. Machining Cutting, and Finishing all well with cast acrylic. Anneal at 80˚C (176˚F). Cementing: A number
of solvent Mechanical fixing method are Possible. It is not feasible to weld. Adhesives are available, although bolted parts can be acrylic
because the stresses on an acrylic filler suspended is subject to cracking. This ruins the optical quality of them. Acrylic can be joined to
metal materials, such as aluminum, with silicone compounds. High optical quality and surface finish with a huge color range (interesting, unusual
or translucent colors). It is springy when heated and if it is burned, it crackles, does not drip and releases a fruity odor.
A dense TP with high impact resistance and superior fire rating is available in a number of forms and sections, including sheets. Limited colors,
clear, smoke shades plus a variety of embossed textures. Good weather and UV resistance, with transparency levels almost as good as acrylic.
Application Line Bending
Riot shields, security screens, compact discs, helmets, vandal-proof shelters, signs, aircraft panels, bumpers, telephone kiosks, light diffusers,
Skylights, guards, visors, smart cards, spectacle lenses, computer casings. Heaters - hot wires. Pre-drying is needed if attempting to fold
using contact or radiant element heaters. Double sided heating is essential for thicknesses over 3mm. The ends of the heating line will blister
unless shielding is used. It is possible to cold bend sheets up to 4mm, using sheet metal folding equipment. Some ‘spring back’ has to be allowed
for though and annealing becomes essential because of the amount of stress inflicted. Good quality, high definition. Pre-drying at 120˚C (248˚F)
is essential. Drying times are long because of the material’s density (see table) and material should be used soon after drying as moisture
will start to be reabsorbed immediately (in humid conditions, blisters can occur in material that has only been out of the dryer for 4 hours!).
High thermoforming temperature means that heating times will be long. Press and drape forming. All the techniques can be used but, tools must
be keen. Guillotining and punching up to 4mm (0.14”). Diamond edge polishing works but, will blunt the diamonds very quickly. An alternative
method of edge finishing is heating Methylene Chloride to around 40˚C (104˚F) and polishing with the resulting vapor, directed at the edge through
a hose. Anneal at 80-100˚C (175-210˚F). If stress is present and the material comes into contact with certain solvents (such as Tipex thinners)
it will disintegrate almost immediately. Proprietary solvent cements are available, usually based on polycarbonate particles in Methylene Chloride.
Welding is possible but the stresses caused are usually big enough to cause aesthetic and structural problems. Mechanical fixing works well.
Can be fixed to other materials, such as metal, glass, wood, and thermosets, using adhesives or silicone compounds. Highly resistant to impact
Vacuum Shuttle thermoforming Machine
Smoked PMMA acrylic thermoformed Skylight Panel
Pressure/Twin Sheet Rotary Thermoformer