Rigid plastic sheet continues to be the material-of-choice for most conspicuous national-program signs. Included in this category are acrylic, modified-acrylic, polycarbonate and copolymer sheets. Acrylic and the more recently introduced modified-acrylic have dominated the industry for four principal reasons:
1) Acrylic possesses the rigidity essential for signage use.
2) Acrylic’s uniform composition and glass-like sheen are well-suited for light transmission and color rendition.
3) Acrylic resists fading and general discoloration.
4) Standard acrylic is the most economical of the rigid sign plastics.
Rigidity is probably a plastic’s most important trait for sign applications. Because both flat and formed face panels are installed in narrow retainers at the sign cabinet perimeter, a face’s ability to maintain its original dimensions is critical in preventing damage or blowout.
Perhaps the most difficult lesson many sign novices learn is that pan faces have an important function. Although flat sheets are suitable for small signs, larger signs demand additional rigidity at the perimeter, where the flange fits into the retainer frame. The pan’s sides achieve this rigidity, and the pan reinforces the panel near the edges. Large, flat sheets installed in outdoor signs commonly blow out in the first windstorm.
Putting on the dog
Wind pressure exerts an inward pull on a plastic sign face’s edges, much like a trampoline’s border under a jumper’s weight. Although pan faces resist this force, additional measures are necessary to prevent damage. Sign manufacturers have found that by gluing narrow strips of plastic to the face flanges (the parts that fit into the retainer channels), they can provide a stop or "dog" that prevents the face from being pulled out of its frame. Typically, small pieces of aluminum angle (or other specially fabricated aluminum) are attached to the inside of the face retainer to bear against the dog strips.
On large signs with hinged face retainers, the upper dog strip (also called a hanger bar) prevents the face panel from pulling out of its frame during service or when exposed to wind. Well-made signs also utilize vertical supports (usually 1/4 or 5/16-in. threaded rods) on the inside of the retainer frame that hold the top and bottom retainers together. This greatly curtails service hazards. Because dog strips may also be installed on the bottom and side flanges of large face panels, the term "hanger bar" is not entirely accurate. Any large sign, regardless of plastic type or access method, should incorporate this system.
A second method, "tie-back bars," also prevents wind damage. These steel or aluminum braces are attached to the sign cabinet’s inner structure. Plastic pads are attached to the ends of these braces, which bear against the inside of the face panel(s) under windload. Tie-back bars restrict the amount of inward deflection of the face and are primarily used for larger panels with long, unsupported spans.
The downside of rigidity, of course, is a breakage propensity in handling or by vandals. Here, polycarbonates have found a niche. Installers commonly feel relief when they learn they are working with a polycarbonate face. You can literally drop the face on the ground, and you probably won’t break it. By contrast, standard acrylic must be handled carefully during installation.
Ruffles and ridges
The same properties that make polycarbonate so tough, however, also reduce its comparative rigidity. Typically formed from thinner material, polycarbonate faces are prone to sagging under their own weight, and some manufacturers have observed a tendency of the flanges on polycarbonate pans to ripple under the heat of forming.
These ripples can present a major problem for installers, because the width of the retainer channel only allows adequate clearance for flat material. I have encountered several signs where the flange-rippling was severe enough to preclude sliding the face in its frame. This occasionally happens with acrylic faces, but is more common with polycarbonate. Of course, it is essential for sign-makers to follow closely the plastic manufacturer’s instructions for thermoforming to prevent such irregularities.
Polycarbonate’s sagging tendency can be countered by utilizing the same type of dog-strip retainer previously mentioned. Mechanical fasteners (usually aluminum rivets) may be necessary to attach dog strips on some face materials that do not bond firmly with plastic cements. Polycarbonate generally has been less resistant to yellowing and color fading than acrylic. However, manufacturers continue to improve UV resistance and are backing these improvements with extended warranties.
Installing plastic panels is simple when a sign is built in the shop. Replacing existing-sign face panels is much more difficult. A separate retainer frame, if properly supported and fastened at the mitred corners, can be used to remove the face panel. Always ensure that the retainer frame is not broken or missing bolts, screws or other fasteners. If the retainer’s condition is dubious, do not use it for lifting.
Provided there is a substantial retainer frame, the best lifting method is to enlarge one mounting hole on the top half of each vertical side of the frame . Make the holes large enough to support eye-bolts of adequate strength for hoisting the face/frame assembly with a steel cable or nylon lifting straps. Never attempt to lift a framed face using the top retainer! The retainer can buckle or break.
This method is suitable for hoisting very large face/frames into position for mounting, and it permits framing of replacement panels on the ground, where working is much easier. Even with unhinged retainers — once the top edge and upper portion of the side retainers have been unfastened — the top of each side will provide enough clearance to attach your hoisting bolts. This method also requires a crane to lift the face/frame assembly, and a ladder or bucket truck for the person who performs the alignment and fastening.
Let’s assume your company has a bucket truck and no crane, but still occasionally needs to install sign faces of the ordinary slide-in variety. I have often used an alternative, but safe and effective method. First, your bucket truck should be rated to lift 300 lbs. (the weight of a very big signman). For faces up to approximately 5 x 10 ft., an average-sized man (say, 180 lbs.) can attach the face to his bucket using two nylon lifting straps in a "choke" configuration. Nylon lifting straps make it easy to slide the face panel right out of the loops and into the sign without scratching the face. Most sign faces do not weigh more than 100 lbs., but if you feel that the face and installer’s weight exceed the rated limit, you should use a crane.
Make sure that you measure the replacement panel(s) before leaving the shop. If they have been trimmed too large or too small, you’re going to waste a trip.
Always make certain that the side of the face you can see from the bucket is the proper side (if it’s backwards, you’ll have to bring it back down and turn it!).
To use this method, you must make the "head" portions of your lifting straps short enough to permit reaching the face from your bucket. Also, you must position the truck so that, when you rotate the bucket, the face is aligned parallel to the retainer frame. You should also have a helper on a ladder to assist in guiding the face into the retainer channel. These jobs can be surprisingly simple if you use the equipment, not your muscle, to do the work. One word of caution though: Avoid scheduling face-change jobs on windy days. If you enjoy kite-flying, paper is definitely better than plastic!
Plastic for channel letters
Channel letters impose special demands on plastics. Because flat sheet is normally used for channel-letter face inserts, breakage can be a real problem. Face inserts are fabricated using the metal "can" portion of the letter as a template. If a letter face needs replacement, the entire letter usually must be removed to make a proper pattern.
Also, if the letters have been installed outdoors for any length of time, some color fading will have occurred. If you replace a broken channel-letter face with new plastic, it won’t match the other letters. I have seen customers incur a major expense to replace all their letter faces simply because one face was broken.
Servicepeople can have trouble removing and handling large, ungainly faces without breakage. For this reason, as well as color fading, break-resistant plastics are a wise investment for channel-letter applications.
Plastics manufacturers have made great strides in addressing the traditional weaknesses of rigid plastic panels. Ultraviolet stabilizers, which protect the plastic from solar exposure ("photodegradation" is the technical term), have been added to many products. These prevent the molecular changes, induced by sunlight, that make plastics brittle or discolored. The introduction of impact-modified acrylics, thermoplastic polyesters (copolymers) and other innovative products has given the sign industry tough new substrates with the requisite appearance and rigidity for most applications.
Cost, of course, plays a central role in selecting sign plastics. One material may address a specific application better than others; however, if its cost is substantially higher, it may not be used. The table below compares plastic costs based on wholesale prices from a typical supplier. Considering that the wholesale price per square foot may increase as much as 80% from standard acrylic to polycarbonate sheet, bid competition is obviously a key factor in the continuing dominance of acrylic. It is hardly surprising, therefore, that the more expensive impact-resistant plastics frequently have been specified by quantity users willing to assume a higher initial expense in exchange for greater durability. For your average one-time sign customer, however, standard acrylic still dominates by virtue of sheer economy.
|Typical wholesale prices of sign plastics
(Prices are per sq. ft., 3/16-in. sheet thickness, white material)
|Standard Acrylic:||$2.50 per sq. ft.|
|Impact-modified Acrylic:||$3.40 per sq. ft.|
|Polycarbonate:||$4.50 per sq. ft.|
|Copolymer (Clear PETG)*:||$2.75 per sq. ft.|
|* At present, copolymer sheet is not available in white. Prices for clear plastics are somewhat lower than for white.|
I’ve had customers ask, "What do you think is the best plastic on the market?" As we have seen, there is no simple answer to this question. Selection of sign plastics depends on the proposed use, the capabilities of the fabricator and, for better or worse, job pricing. Knowing the characteristics of each product, and using proper installation techniques, however, can help us to better serve our customers.
Information for this article was provided by:
- Aristech Acrylic LLC
- 7350 Empire Dr.
- Florence, KY 41042
- Aristech Acrylic LLC
- Eastman Chemical Co.
- PO Box 431
- Kingsport, TN 37662-5280
- Cincinnati Sign Supplies
- 1111 Meta Dr.
- Cincinnati, OH 45237
- 3818 Red Bank Rd.
- Cincinnati, OH 45227
- 7350 Empire Dr.
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