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Clear Acrylic with Standoffs Corners Cracking
- Thread starter TeamOutlaw
- Start date
fuzzy_cam
The Granbury Wrap & Sign Guy
This looks a whole lot like someone pushed on this to cause that crack. I don't know what the dimensions are on these but we haven't ever used anything under 1/4" thick cast acrylic for displays like this. We do a ton of 1/2" thick acrylic displays for a customer of ours, sizes ranging from 2' X 3' all the way up to 4' X 8'. Never had any cracking happen.
I know that certain types of cleaners can compromise the integrity of acrylic, causing it to become brittle. This may be an issue if the cleaning people are using something that is drying this (thin) acrylic out, it wouldn't take much pressure at all to cause a crack, especially if there is any force being used when cleaning.
I know that certain types of cleaners can compromise the integrity of acrylic, causing it to become brittle. This may be an issue if the cleaning people are using something that is drying this (thin) acrylic out, it wouldn't take much pressure at all to cause a crack, especially if there is any force being used when cleaning.
Martin Denton
New Member
Almost certain the cracks are due to what the acrylic has been cleaned with. Any solvent inc window cleaner or alcohol or meths etc can cause crazing or cracking. Any form of machining or flame polishing creates a stress in the acrylic which solvents can then cause to crack. Typically happens about a week after solvent applied. Could be you guys cleaning after install or their cleaners giving it the once over. We always give a care leaflet with any we supply saying only use soapy water ...
signbrad
New Member
A lot of good comments have been made on this string, based on experience and common sense.
The most common cause of plastic failure, by far, is stress. In sign making, many of the things we do to acrylic introduce stress.
Cutting causes stress
Whether it's from a saw blade or a router bit or a drill, stress occurs at the cut edges. Stress weakens plastic, even when it doesn't crack it outright. I believe that sometimes stress cracks are present that we don't even see but that show up when a solvent makes them worse, such as the alcohol in a cleaner or the lacquer thinners in a paint like Matthews. At the molecular level, this weakening of plastic is from the polymer strands trying to unravel and detangle. If your hair is tangled, it's hard to part it with a comb. But brush it out in one direction and it's easy to put a part in it. So, stressed plastic cracks easier because it's been detangled.
When cut, acrylic can also be "notchy." If you remember when we used to cut channel letter faces with a jig saw, an inside corner of, say, a "V" could easily crack at the apex of the inside corner. To avoid this, many fabricators drilled a tiny hole at the point of the notch to prevent the crack. This is also how to stop an acrylic crack from migrating further—drill a hole at the crack's end. Well, cutting acrylic can introduce hundreds of tiny notches, all of them potential starting points for cracks. Think of this happening around the perimeter of a drilled hole, too, and you can see why a crack easily starts at a hole.
Using tools and cutters designed for plastic minimizes stress fractures at the cut edges. On a saw blade for plastic, for example, the angle of the carbide tip is important. The angle of its face should be parallel to a line running from the center of the blade to the edge rather than leaning forward into the material (if that makes sense). This angle is called the hook, and it is zero on plastic cutting blades. The cutting tip needs to 'scrape' the plastic instead of digging in. Same with a drill bit. It needs to scrape its way through. It should not dig, which can cause grabbing and breaking at the edge of the hole, and even busting notches out of the back side when it exits. The angle of the grind on the cutting edges of a plastic-cutting bit minimizes stress cracking. It's not just so they can charge more for the bit.
So, using tools made for plastic is important.
Heat causes stress
We heat plastic when we bend it or vacuum-form it. Or when we flame polish it. These actions often cause cracking, even if we don't see it. But cutting and drilling cause heat, too. So again, using cutting tools made specifically for plastic can help. Also, the cutting speed can be a factor. Faster cutting generally means more heat.
Pressure, when sustained, causes stress
A plastic face that fits too tightly in a retainer becomes stressed as it expands. A piece of plastic pinched too tightly from side to side between mounting screws, or pinched too tightly against a standoff body, is also stressed. It may seem fine for a while, but the prolonged stress changes the plastic permanently, weakening it. This gradual change from stress over time is called creep. The plastic may seem fine for weeks or months, but a random normal cleaning can break off a corner as a result of weakening from plastic creep.
It's important to minimize stress when installing. Hole diameter should be at least double the diameter of the fastener's shank and spacing between holes should not be snug. A rule of thumb for tightening the cap on a standoff is to finger tighten and then back off a quarter turn. How far from the edge should a mounting hole be placed? The rule of thumb is three times the hole size. I think it's obvious that acrylic should never be installed using countersunk holes—they lock the plastic into position, allowing for no movement. If you use countersunk fasteners, you may assume that a corner is probably going to break off.
Bending causes stress
Bending can be with or without heat. Bending without heat is called cold rolling. If a sign is installed on standoffs and can easily flex from finger pressure, or from cleaning, it can lead to cracking. Obviously, thicker plastic is better than thinner plastic. Many architects specify quarter-inch or three-eighths thick acrylic for interior signs. I've never seen one specify eighth-inch, even though it's cheaper.
Cast vs calendered
There are numerous differences between cast and calendered acrylic, as many of us know. It's not just the price. Obviously, the manufacturing process is different. Cast acrylic sheets are formed from a liquid that is injected between individual sheets of glass or polished metal separated by gaskets (cell cast) or in a continuous cast process. Both operations are more expensive methods than extruding, which is how calendered acrylic sheets are made. Extrusion adds considerably more stress to the material. Also, cast acrylic has longer polymer strands, so it is more resistant to stress cracking. Extruded acrylic is, therefore, less resistant to the damaging effects of machining, heating, and continuous pressure.
Polycarbonate is susceptible to stress cracking, also. It resists face blows, but you can crack it from a blow to the edge, especially if the edge has a notchy cut.
Read
There is a ton of literature that has been published on stress cracking of plastics. Ever since WWII acrylic has been used in high-stress applications successfully, such as airplane canopies. It still is, though often modified for modern jets.
Some of the literature on acrylic seems contradictory, such as whether solvents cause cracking or not. But solvents certainly make craking worse. At our shop we occasionally have to fight with cracking around Braille holes, or when back-painting, or when dealing with flame polishing. When the problem goes away, we just try to repeat what we did, not always knowing what caused the problem in the first place.
I think it's important to read published information on subjects like this. Obviously, not everything I say is always accurate. I'm not an engineer and I'm really not that smart. And anecdotal evidence is good, but not always proof. So I read a lot and buy books.
Of course, reading eats up time.
It helps that I don't have a TV. I was a TV-holic growing up and got rid of it to conquer the addiction, convincing my new wife to join me in this. When my son was born twelve years later we bought a TV. After he left and my wife and I split, I gave away the TV (she didn't want it). I have gained thousands of hours (couch-hours, I call them) over the years and saved thousands of dollars by not having a television. But I don't hate it and though some people think I'm self-righteous about it, I don't think I am. And I still watch it when I want. I just go to the bar where I can feed two addictions at once.
And then there's the internet...proof that you don't need a TV to get to a higher place on the stupid-meter.
Brad
The most common cause of plastic failure, by far, is stress. In sign making, many of the things we do to acrylic introduce stress.
Cutting causes stress
Whether it's from a saw blade or a router bit or a drill, stress occurs at the cut edges. Stress weakens plastic, even when it doesn't crack it outright. I believe that sometimes stress cracks are present that we don't even see but that show up when a solvent makes them worse, such as the alcohol in a cleaner or the lacquer thinners in a paint like Matthews. At the molecular level, this weakening of plastic is from the polymer strands trying to unravel and detangle. If your hair is tangled, it's hard to part it with a comb. But brush it out in one direction and it's easy to put a part in it. So, stressed plastic cracks easier because it's been detangled.
When cut, acrylic can also be "notchy." If you remember when we used to cut channel letter faces with a jig saw, an inside corner of, say, a "V" could easily crack at the apex of the inside corner. To avoid this, many fabricators drilled a tiny hole at the point of the notch to prevent the crack. This is also how to stop an acrylic crack from migrating further—drill a hole at the crack's end. Well, cutting acrylic can introduce hundreds of tiny notches, all of them potential starting points for cracks. Think of this happening around the perimeter of a drilled hole, too, and you can see why a crack easily starts at a hole.
Using tools and cutters designed for plastic minimizes stress fractures at the cut edges. On a saw blade for plastic, for example, the angle of the carbide tip is important. The angle of its face should be parallel to a line running from the center of the blade to the edge rather than leaning forward into the material (if that makes sense). This angle is called the hook, and it is zero on plastic cutting blades. The cutting tip needs to 'scrape' the plastic instead of digging in. Same with a drill bit. It needs to scrape its way through. It should not dig, which can cause grabbing and breaking at the edge of the hole, and even busting notches out of the back side when it exits. The angle of the grind on the cutting edges of a plastic-cutting bit minimizes stress cracking. It's not just so they can charge more for the bit.
So, using tools made for plastic is important.
Heat causes stress
We heat plastic when we bend it or vacuum-form it. Or when we flame polish it. These actions often cause cracking, even if we don't see it. But cutting and drilling cause heat, too. So again, using cutting tools made specifically for plastic can help. Also, the cutting speed can be a factor. Faster cutting generally means more heat.
Pressure, when sustained, causes stress
A plastic face that fits too tightly in a retainer becomes stressed as it expands. A piece of plastic pinched too tightly from side to side between mounting screws, or pinched too tightly against a standoff body, is also stressed. It may seem fine for a while, but the prolonged stress changes the plastic permanently, weakening it. This gradual change from stress over time is called creep. The plastic may seem fine for weeks or months, but a random normal cleaning can break off a corner as a result of weakening from plastic creep.
It's important to minimize stress when installing. Hole diameter should be at least double the diameter of the fastener's shank and spacing between holes should not be snug. A rule of thumb for tightening the cap on a standoff is to finger tighten and then back off a quarter turn. How far from the edge should a mounting hole be placed? The rule of thumb is three times the hole size. I think it's obvious that acrylic should never be installed using countersunk holes—they lock the plastic into position, allowing for no movement. If you use countersunk fasteners, you may assume that a corner is probably going to break off.
Bending causes stress
Bending can be with or without heat. Bending without heat is called cold rolling. If a sign is installed on standoffs and can easily flex from finger pressure, or from cleaning, it can lead to cracking. Obviously, thicker plastic is better than thinner plastic. Many architects specify quarter-inch or three-eighths thick acrylic for interior signs. I've never seen one specify eighth-inch, even though it's cheaper.
Cast vs calendered
There are numerous differences between cast and calendered acrylic, as many of us know. It's not just the price. Obviously, the manufacturing process is different. Cast acrylic sheets are formed from a liquid that is injected between individual sheets of glass or polished metal separated by gaskets (cell cast) or in a continuous cast process. Both operations are more expensive methods than extruding, which is how calendered acrylic sheets are made. Extrusion adds considerably more stress to the material. Also, cast acrylic has longer polymer strands, so it is more resistant to stress cracking. Extruded acrylic is, therefore, less resistant to the damaging effects of machining, heating, and continuous pressure.
Polycarbonate is susceptible to stress cracking, also. It resists face blows, but you can crack it from a blow to the edge, especially if the edge has a notchy cut.
Read
There is a ton of literature that has been published on stress cracking of plastics. Ever since WWII acrylic has been used in high-stress applications successfully, such as airplane canopies. It still is, though often modified for modern jets.
Some of the literature on acrylic seems contradictory, such as whether solvents cause cracking or not. But solvents certainly make craking worse. At our shop we occasionally have to fight with cracking around Braille holes, or when back-painting, or when dealing with flame polishing. When the problem goes away, we just try to repeat what we did, not always knowing what caused the problem in the first place.
I think it's important to read published information on subjects like this. Obviously, not everything I say is always accurate. I'm not an engineer and I'm really not that smart. And anecdotal evidence is good, but not always proof. So I read a lot and buy books.
Of course, reading eats up time.
It helps that I don't have a TV. I was a TV-holic growing up and got rid of it to conquer the addiction, convincing my new wife to join me in this. When my son was born twelve years later we bought a TV. After he left and my wife and I split, I gave away the TV (she didn't want it). I have gained thousands of hours (couch-hours, I call them) over the years and saved thousands of dollars by not having a television. But I don't hate it and though some people think I'm self-righteous about it, I don't think I am. And I still watch it when I want. I just go to the bar where I can feed two addictions at once.
And then there's the internet...proof that you don't need a TV to get to a higher place on the stupid-meter.
Brad