Any rule of thumb for ratio of vacuum HP to sq footage of vacuum former?
- Thread starter helix_3
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helix_3
Well-Known Member
In case anyone else has the same question, here's what I found. HP is kind of a garbage metric. Scientifically, 1 HP = 745.7 watts. Commercially available vacuums will make dubious claims of 6 HP+ motors, but this would pull about 4500 watts, more than double what a typical home plug can supply.
I think CFM may be a better metric to measure raw power, but ultimately the small space under your platen will be quickly evacuated by the motor. Try using your home vacuum cleaner, and if it isn't pulling enough detail, an HVAC vacuum pump + air tanks may be a better option.
I think CFM may be a better metric to measure raw power, but ultimately the small space under your platen will be quickly evacuated by the motor. Try using your home vacuum cleaner, and if it isn't pulling enough detail, an HVAC vacuum pump + air tanks may be a better option.
TazMan2000
Master Member
The thickness of the material being vacuformed also plays a big part.
TazMan2000
TazMan2000
As you have found, HP is a pointless metric. Inches of mercury is the metric for a vacuum.
Try not to think of vacuum forming as "sucking" or "pulling" think of it as negative pressure. That is the pressure or weight of 100 miles of atmosphere pushing down on your plastic. We have a tendency to think of the air as weightless, because it is all around us. But air has weight and we live at the bottom of a deep air ocean. This air at the bottom of this ocean, weighs 14.7 pounds per square inch. That's 2,116.8 pounds per square foot. To empty this air from the space below your plastic is trickery than one might think.
Think of all these gasses (Air is 21% oxygen, 78 % nitrogen, 0.93% argon, 0.038% carbon dioxide, and trace amounts of other gases.) as if they were rocks, gravel, sand and silt. Each type of gas molecule listed here is a different size and in order to "pull" those out you need a higher and higher precision of pump. Just like you would need a finer and finer screen to filter out rocks verses gravel and silt from a container. The more precise the pump, the higher the vacuum, but also the more expensive it is.
Inches of mercury is a scale that measures vacuum and thus, the more inches of mercury, the more perfect vacuum or "hole" you have made in the air ocean.
So when you "Pull a Vacuum" you are really making a hole in this ocean, by emptying out the air in that space. The higher the vacuum or the more perfectly you can remove all the gasses from the space under your plastic, the more force the air above puts on that space.
29" of mercury is extremely good for most applications (You have to get into a finer measurement scale of "Microns" for more critical scientific applications) 29" is about 95% of the air removed.
A vacuum cleaner blower pulls maybe 2-3" of mercury, where as a vacuum pump, even a cheap one, will pull 20-25". A more expensive one is 27" to 29.5" With each inch of mercury equaling a little less than a half pound per square inch, your vacuum cleaner is only causing 1 to 1 1/2 pounds per square inch of pressure to form your plastic. (144 to 216 pounds per square foot) But a vacuum pump can supply you with 10 to 13 pounds per square inch.
While a 4ft by 4ft vacuum-form table has approximately 31,000 pounds of available force to form your heated plastic, the effectiveness of your vacuum-form machine is still down to how much per square inch, of vacuum you can create, not the overall size of the machine. BTW: If you live at a high altitude, you will loose force. 6,000 feet, is 24" atmospheric pressure, so even with a great pump, you will be down about 20%
The thickness of plastic plays a big role in the effectiveness. The thicker the plastic, or the "stiffer" it is the more force it takes to form it. For example, Polystyrene gets much softer and is more "gooey" when it's hot, than ABS. So you can form styrene of the same thickness with a lower amount of force than you can ABS. With just a blower I'd say 3/32" (.090") thick PTGE or styrene material is the effective limit. Even less if you have fine surface details you want to reproduce. ABS, acrylic, polycarbonate will be tough to form in all but the simplest of shapes.
The other factor is the size of this "hole" you have available. When you drape your hot plastic over your pattern, you have trapped air under the sheet. This volume of air needs to be emptied out and it has to go somewhere. With a vacuum cleaner / blower, this air just gets pulled through the fan blades and dumped in to the room. But a vacuum cleaner blower has very limited power as you have seen. Most vacuum pumps on the other hand, do not move air quickly, measured in CFM (Cubic feet a minute) but make a deeper "hole", thus more available force.
But air is compressible / "spongy" and the higher the vacuum, the more space you need to store that vacuum, so when you use it, there is plenty of "room" for that air trapped under your plastic to go. So you need to store up this "Hole" in a tank. The effectiveness of a vacuum drops off very quickly, so a bigger tank is better. As an example, for our high altitude test chamber, built to test spacesuits, in order to take a small 2 man chamber for sea level pressure to 70,000 feet quickly, by dumping the chamber to a stored volume of vacuum, it required 4 storage tanks, each 10 feet in diameter and 40 feet long. But, I digress.
10-20 gallon tanks are generally sufficient, for your machine, but 30 or 40 gallon tanks are better for intricate and detailed parts, larger machines or thicker material. The CFM of a pump, determines how quickly you can empty out that tank.
So you figure a 3 cfm pump will empty a 20 gallon tank in a minute, but it won't. Because as the vacuum gets higher, the less air the pump moves. On my machine I have a 40 cfm pump and it takes about a minute to empty 40 or 50 gallons worth of space. But a 40 cfm high vacuum pump normally costs $5,000 to $10,000. (I bought mine in an auction for considerably less)
A 3 to 5 cfm pump is generally sufficient for a hobby machine and you can use a steel compressor tank to store vacuum. They are of sufficiently thick steel and small enough in size, that they will not implode. But a high vacuum pump for that cfm is still in the $1500 to $2500 range new.
A trick I use on my machine is I use a one way valve, that is connected to my vacuum pump/ tank and to a powerful vacuum cleaner motor. I turn on the vacuum cleaner and when the plastic seals, the motor starts to "rough out" the volume of air under the plastic (this also lets me know I have a good seal) then I open the valve to the hard vacuum stored in the tanks and this way I get a little more use from the stored hard vacuum.
So while a vacuum cleaner blower will work, and the size of the machine is not really a factor (unless you are building a huge machine, then it comes down to CFM). You will be limited to styrene, PTGE or thin ABS. If you want more detailed parts or thicker material, you will need a vacuum pump and a storage tank.
I hope this makes sense and is useful.
Try not to think of vacuum forming as "sucking" or "pulling" think of it as negative pressure. That is the pressure or weight of 100 miles of atmosphere pushing down on your plastic. We have a tendency to think of the air as weightless, because it is all around us. But air has weight and we live at the bottom of a deep air ocean. This air at the bottom of this ocean, weighs 14.7 pounds per square inch. That's 2,116.8 pounds per square foot. To empty this air from the space below your plastic is trickery than one might think.
Think of all these gasses (Air is 21% oxygen, 78 % nitrogen, 0.93% argon, 0.038% carbon dioxide, and trace amounts of other gases.) as if they were rocks, gravel, sand and silt. Each type of gas molecule listed here is a different size and in order to "pull" those out you need a higher and higher precision of pump. Just like you would need a finer and finer screen to filter out rocks verses gravel and silt from a container. The more precise the pump, the higher the vacuum, but also the more expensive it is.
Inches of mercury is a scale that measures vacuum and thus, the more inches of mercury, the more perfect vacuum or "hole" you have made in the air ocean.
So when you "Pull a Vacuum" you are really making a hole in this ocean, by emptying out the air in that space. The higher the vacuum or the more perfectly you can remove all the gasses from the space under your plastic, the more force the air above puts on that space.
29" of mercury is extremely good for most applications (You have to get into a finer measurement scale of "Microns" for more critical scientific applications) 29" is about 95% of the air removed.
A vacuum cleaner blower pulls maybe 2-3" of mercury, where as a vacuum pump, even a cheap one, will pull 20-25". A more expensive one is 27" to 29.5" With each inch of mercury equaling a little less than a half pound per square inch, your vacuum cleaner is only causing 1 to 1 1/2 pounds per square inch of pressure to form your plastic. (144 to 216 pounds per square foot) But a vacuum pump can supply you with 10 to 13 pounds per square inch.
While a 4ft by 4ft vacuum-form table has approximately 31,000 pounds of available force to form your heated plastic, the effectiveness of your vacuum-form machine is still down to how much per square inch, of vacuum you can create, not the overall size of the machine. BTW: If you live at a high altitude, you will loose force. 6,000 feet, is 24" atmospheric pressure, so even with a great pump, you will be down about 20%
The thickness of plastic plays a big role in the effectiveness. The thicker the plastic, or the "stiffer" it is the more force it takes to form it. For example, Polystyrene gets much softer and is more "gooey" when it's hot, than ABS. So you can form styrene of the same thickness with a lower amount of force than you can ABS. With just a blower I'd say 3/32" (.090") thick PTGE or styrene material is the effective limit. Even less if you have fine surface details you want to reproduce. ABS, acrylic, polycarbonate will be tough to form in all but the simplest of shapes.
The other factor is the size of this "hole" you have available. When you drape your hot plastic over your pattern, you have trapped air under the sheet. This volume of air needs to be emptied out and it has to go somewhere. With a vacuum cleaner / blower, this air just gets pulled through the fan blades and dumped in to the room. But a vacuum cleaner blower has very limited power as you have seen. Most vacuum pumps on the other hand, do not move air quickly, measured in CFM (Cubic feet a minute) but make a deeper "hole", thus more available force.
But air is compressible / "spongy" and the higher the vacuum, the more space you need to store that vacuum, so when you use it, there is plenty of "room" for that air trapped under your plastic to go. So you need to store up this "Hole" in a tank. The effectiveness of a vacuum drops off very quickly, so a bigger tank is better. As an example, for our high altitude test chamber, built to test spacesuits, in order to take a small 2 man chamber for sea level pressure to 70,000 feet quickly, by dumping the chamber to a stored volume of vacuum, it required 4 storage tanks, each 10 feet in diameter and 40 feet long. But, I digress.
10-20 gallon tanks are generally sufficient, for your machine, but 30 or 40 gallon tanks are better for intricate and detailed parts, larger machines or thicker material. The CFM of a pump, determines how quickly you can empty out that tank.
So you figure a 3 cfm pump will empty a 20 gallon tank in a minute, but it won't. Because as the vacuum gets higher, the less air the pump moves. On my machine I have a 40 cfm pump and it takes about a minute to empty 40 or 50 gallons worth of space. But a 40 cfm high vacuum pump normally costs $5,000 to $10,000. (I bought mine in an auction for considerably less)
A 3 to 5 cfm pump is generally sufficient for a hobby machine and you can use a steel compressor tank to store vacuum. They are of sufficiently thick steel and small enough in size, that they will not implode. But a high vacuum pump for that cfm is still in the $1500 to $2500 range new.
A trick I use on my machine is I use a one way valve, that is connected to my vacuum pump/ tank and to a powerful vacuum cleaner motor. I turn on the vacuum cleaner and when the plastic seals, the motor starts to "rough out" the volume of air under the plastic (this also lets me know I have a good seal) then I open the valve to the hard vacuum stored in the tanks and this way I get a little more use from the stored hard vacuum.
So while a vacuum cleaner blower will work, and the size of the machine is not really a factor (unless you are building a huge machine, then it comes down to CFM). You will be limited to styrene, PTGE or thin ABS. If you want more detailed parts or thicker material, you will need a vacuum pump and a storage tank.
I hope this makes sense and is useful.
TazMan2000
Master Member
An incredibly well written and accurate explanation, lmgill .
Notwithstanding, helix_3 , you may want to give us more information on the size of vacuformer you plan on building, and material you are thinking of using. Probably, most of us are using a vacuum cleaner and styrene/pvc for a small one foot squared vacuformer.
TazMan2000
Notwithstanding, helix_3 , you may want to give us more information on the size of vacuformer you plan on building, and material you are thinking of using. Probably, most of us are using a vacuum cleaner and styrene/pvc for a small one foot squared vacuformer.
TazMan2000
Thanks.
A large vacuum blower, like is found in a industrial wet/dry vac, has enough CFM to handle easily up to a 2'x3' machine, provided you have sufficient heat to heat the plastic (PTGE / Styrene) A bigger platten (table) would just require another blower. The size of the surface is not a factor for the amount of vacuum pressure, as it directly scales. It is merely the CFM you can remove before your plastic cools.
A large vacuum blower, like is found in a industrial wet/dry vac, has enough CFM to handle easily up to a 2'x3' machine, provided you have sufficient heat to heat the plastic (PTGE / Styrene) A bigger platten (table) would just require another blower. The size of the surface is not a factor for the amount of vacuum pressure, as it directly scales. It is merely the CFM you can remove before your plastic cools.
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