Comparing the coefficients of thermal expansion of glass and plastic
In 1983 I made my first floating-letter sign. (Up to that point I was a martyr to the cause of fine woodworking and all my signage projects were hand carved from solid wood panels). The sign consisted of a painted plywood sheet 18’ x 3’ in a frame, with rows of projecting studs upon which I mounted a clear acrylic sheet. To the sheet I had mounted wood letters about 16” high. In early fall I installed the sign, got my cheque and went home. About three months later, on a day when it was bitterly cold, I got a call from the owner of the store who reported that the acrylic was broken in 4 pieces and flapping in the breeze.
What had happened was that the temperature dropped from +70F to -20F, the acrylic sheet became very brittle in the cold and it shrunk by over ½”, so it cracked at every vertical pair of studs, which were immovable. What I learned was that plastic has a very large coefficient of thermal expansion compared to most other materials. Future work in plastics always took this in to account and I would always provide an allowance for movement proportional to the longest dimension of the part. What this incident also showed was the role of stress raisers on plastic which had been made brittle by cold conditions.
The amount of movement that a sheet of plastic or glass will exhibit can be determined using available formulas by anyone who can perform simple calculations.
Coloured plastic, coloured glass
Both glass and plastic can be coloured, either by backpainting, by using tinted laminating film or by integrating pigment in to the glass or plastic itself. There is the same tendency for glass manufacturers to brand colours as with the paint industry (Thai Sunset, Penelope, Champagne, etc.) but the fact is that almost any colour can be created by most colouring systems, except in the case of real stained glass, which is somewhat limited by the available metallic salts used to create the colour.
Backpainted glass is used extensively as spandrels, wall facings etc. It is possible to match any colour, which is a very attractive characteristic, on top of which the colour layer is protected by the glass. The issue is in the durability of the coloured coating as it relates to the installation of the glass. Common practice has been to either mount backpainted glass in a frame, which is bulky but durable, or apply neutral-cure silicone to the back of the glass and stick it to the wall like tiles, or to apply it in the same way to plywood panels, which are then hung on Z-clips. The problem with the latter methods is that the backpaint is the meat in the sandwich, so it must stick to both the silicone and the glass. Additionally, it mustn’t telegraph any visible sign of the irregular silicone.
Plastics can also be backpainted either before or after cutting, carving or machining. This was the accepted way to produce rear illuminated signage for many years, now coloured graphic film has replaced this technique. We used large backpainted polycarbonate sheets with applied graphics in the very successful Deegies Carma retail fixtures, designed by Gensler. Beware of paints and solvents which are incompatible with the plastic or you can end up with one of the most interesting disasters in fabricating: Plastic which breaks up like tempered glass!
Tinted laminating film
The VANCEVA colour system has taken a lot of complexity out of the glass colouring process, while still producing a robust product. “The System” is based on the coloured gels used in the lighting industry, except that the wide array of coloured and clear gels are supplied as rolls of laminating film. After being selected, cut and stacked in a clean room environment to produce the required colour and lamination thickness, the stack is used as one sheet to laminate the glass in a relatively conventional process.
Numerous glass fabricators have bought Vanceva setups, making this the #1 production method for coloured laminated glass.
Custom printed / patterned glass
Cue Mr. McGuire: “There is only one thing you need to know about printed glass: GlassJet”. For designers and fabricators with a requirement for one-off or short-run printed glass, the GlassJet process (branded as Alice in North America by General Glass) replaces finicky screen-printing, which is still widely used for longer runs. The Harlem Hospital glass mural (http://www.glassmagazine.com/article/commercial/most-innovative-decorative-glass-project-commercial-106671) was created with this technology.
The GlassJet / Alice process is essentially a flat bed printer which dispenses liquefied ceramic frit instead of ink. As with screen printing, the frit mix is then dried, melted and fused to the glass in the tempering process, which reveals the full colour. Frit is essentially pulverized glass, so it is just as durable, UV proof and as attractive as the glass to which it is applied. If you want to buy one of these machines have a look here:
This is a huge subject which easily justifies its own article. For the designer, the issues with textured glass are simple: If the supplier shows you a sample and they have enough in stock for the job and enough in reserve to redo defective work, go ahead and use it. But otherwise: beware. This is because textured glass is formed using rollers acting on partially molten glass, it is not an easy process to control and “runs” of a specific texture might be months apart, making field replacement of (especially) odd textures challenging. Additionally, the texture produces stress raisers in the glass, making it hard to temper and so, more fragile.
Cast or pressed glass
While relatively rare, this category of glass is found in niche applications such as the lenses in bollards, lites in Manhattan’s famous basement windows, covers for in-ground lighting and as glass block. In fact I have commissioned cast glass components on several occasions, including some beautiful sea shell sconce shades in blue glass. Artisanaly, cast glass it is made by casting liquid glass in to mold machined or carved from a block of graphite, or by pressing pre-measured blobs between a table and a metal form, or between two forms.
Curved, slumped and formed glass and plastic
Both glass and plastic can be formed to curves and even compound curves.
Until recently, curved or bent glass was created by heating the glass in an oven until the glass was soft, then placing it over a mold to which it curved due to the effect of gravity, or through coaxing it down with a second form on top. Sometimes the glass was placed on the mold and they both went in the oven, so the entire rig has to withstand high temperatures during the forming or at least the soaking phase. This created a situation where projects had to be designed around the available inventory of molds. I built a bunch of glass molds and we used stainless steel for the structures, because ordinary steel loses its strength and turns in to rust flakes when subjected to prolonged high temperatures.
Today, glass is bent with programmable automated machines which have made molds obsolete, but the machines require experienced operators and they cannot control all variables, so the resulting products still require generous tolerances. The shapes are also hard to define, so be sure the detailer has a clear understanding of how to define the parts.
Curved and formed plastics
Many plastics can simply be bent in to shape and held in place by surrounding framing, completely eliminating costly, time-consuming outsourced processing. This is especially attractive for large panels. But of course plastics are not as durable as glass and they are a pain to keep clean.
Plastics can also be formed in to compound curved shapes with simple tooling, using a process called vacuum-forming. The plastic is heated in an oven to the plastic range, then forced down over a mold which has small holes drilled in it. When the plastic is seated, the air is evacuated from the mold and the air pressure acting on the outside of the plastic forces the it down over the mold with a force somewhat less than 14 lbs / sq in. This process is simple, but it has severe limitations on the possible forms. We had our own equipment and used vacuum-forming to create knock-off Erwin Hauer ceramic tiles for chandeliers in a Miami condo lobby.
Simply inflating a heated plastic “bubble” also works, but the resulting form is hard to control. Skylights are an example of this approach.
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