Faster! Every company wants to run faster to get more product out on the same production range and from the same amount of labor. Plastic material profile extrusion companies are no exception. It is easy to speed up the extruder to push more pounds or to buy a larger extruder to get more output. Even so, when extruding plastic material profiles, the outcome is usually controlled by the cooling of the profile and the ability to hold the part in the right shape while it is being cooled. It really is hard more than enough to cool simple shapes like rounded pipe and tubing more rapidly however the difficulty increases when the complexity of the account raises. Window profiles and additional complex parts have become difficult to fascinating uniformly, and if the parts usually do not interesting uniformly warpage and bow is the result.
Like the majority of materials, plastics shrink because the temperature of the plastic decreases, but they usually shrink greater than other materials. Plastics shrink at one price when they will be in the stable (frozen) state, however they shrink much more if they are still delicate or in the molten status. The issue for the profile extruder is controlling this shrinkage when cooling the scorching plastic, coming out of the extruder, all of the real way down to room temperature. Lets take the simplest example of a flat sheet where one part cools faster compared to the other. When still smooth both sides are shrinking at the same charge. Even if one side is cooling more quickly and shrinking faster the other side is still pliable more than enough to come with the various other shrinking side. However, once one side cools at night crystalline temperature or its glass changeover temperature, a couple of things happen. First, that materials stiffens and is no longer pliable plenty of to follow the other area and the level of shrinkage goes down significantly. It is as if the stiffened side is no longer shrinking as the other pliable area continues to shrink. Therefore, because the pliable aspect proceeds to shrink it is pulling on the stiffened aspect and resulting in a bow in direction of the medial side that cooled last. In this example, and in other straightforward profiles, the proper part will bow in the direction of the material that cooled last. In more complex profiles the proper parts may twist, distort, or warp in every types of fashions based on which sections of the right part cooled last. Well cover more upon this later.
In addition to this nagging problem is the point that plastics are good thermal insulators, and therefore they dont transfer high temperature very fast. That means it is difficult to pull all of the heat from the part to begin with, let alone carrying it out uniformly. Thermal conductivity is going to be a way of measuring how fast supplies transfer heat. Steel has a thermal conductivity of 43 while Aluminums higher heat transfer is 250 & most plastics are way down at values between 0.1 and 0.3.
TYPES OF COOLING Surroundings COOLING
Considering these problems with cooling profiles it will not be surprising that historically profile extruders often used air flow to cool parts.
Air racks are basic tables or frames with plates / manuals and fixtures that contain the part in shape as it is being pulled slowly across the table. Fans are generally used to improve overall cooling while compressed oxygen jets are added where specific additional cooling is required. Metal fingers, cables, and jigs mounted on the table with clamps or vise grips are used to push the component into shape as it cools very slowly.
Air is quite inefficient, meaning SLOW, which in cases like this is great because slow provides operator time and energy to make modifications and get the part just right without warping or other distortion. Complex profiles or parts with several wall thicknesses on distinctive parts of the right part might need customized cooling. The operator can direct extra cooling to where he needs it with compressed oxygen nozzles or retard cooling in other areas by insulating a section to retain it from cooling as well fast. Since thicker sections cool more than thin sections slowly, specific actions must be employed in order to avoid warp. The operator will need to direct significantly more cooling on thicker sections to get them to interesting to the same heat range as well as thinner sections on the same profile. Likewise, inside a U-channel or basically an internal corner will great slower than an outside corner and can require extra directed cooling. Output costs are limited to between 100 - 250 lb./hr. using air because it is so slow.
Today even, some may nonetheless use atmosphere cooling when:
Profiles are very complex
Using materials with completely different thermal conductivities
Size of production runs do not justify more expensive tooling
SUBMERSION WATER COOLING
When more significant output rates are required, cooling with water can be used then. There are many methods to run a part through water depending on many variables.
Submersion Tanks
For very easy shapes the part could be extruded outrageous of an extended water tank and become pushed down under the normal water by rollers or sizing plates. This may only be utilized for parts where it doesnt matter that the bottom of the portion hits the water primary (and is cooled first) while the leading comes down in to the water an instant later.
Vacuum Tanks
Extruding larger or more complex shapes directly into the water container is a superb idea that incurs the simple issue of gravity pulling water out of the tank through the hole that the component needs to go through in to the tank. Even small gaps between your sides of the part and the sides of the entrance plate will allow drinking water to leak out. This issue is usually solved by applying vacuum to the entire inside of the tank to hold the water in. Needless to say, this requires a particular tank that is strong enough not to collapse from the differential drive of vacuum inside and air pressure on the beyond the tank.
Other Options
Another option would be to make a little vacuum sleeve around the entry to suck off any drinking water trying to stream through the gap between component and entrance plate. More recently, profile extruders will place a dried out vacuum calibrator in front of the water tank to perform a similar thing. This vacuum calibrator can be as short as 3 for less significant profiles or so long as 10 ft for parts that have to be hardened to very precise dimensions prior to going into the water tank for more cooling. Dry out vacuum calibration isn't as efficient as water cooling nonetheless it is the price that must be paid when tighter control of the sizes is required.
Water Temperature Choices
Its pretty obvious that vacuum tanks are totally closed. With an open water tank it is very difficult even, if not difficult, to find yourself in the tank to place fingers and jigs to force the part into form as is performed on an air flow rack. Additionally it is difficult to immediate cooling water or to insulate sections of the part from cooling. However, it is possible to reduce the effectiveness of cooling (i.e. slow it straight down) to mimic the even more uniform cooling possible with an surroundings rack by heating the water. This is often done with parts which have a strong inclination to warp and especially with higher temperature engineering components. In this instance a temperature control device is required to control the temperature of the water at a arranged value. The bigger the water temperature is the slower the cooling and then the better it is to achieve uniform cooling. Controlled heat range water between 80 F and 130 F is frequently used in the original tank until colder drinking water can be used to entire the cooling. Needless to say, with the desire for speed, the colder the normal water the more quickly the cooling, thus most profile extruders shall use chilled water at temperatures between 50 F and 55 F every time they can.
Water Flow Characteristics
Even nevertheless immersing the complete profile in water offers faster and better cooling it could not be the very best cooling method. Unless the water has been agitated to provide turbulent movement around the part, then the layer of water next to the portion will heating up and that warm water up coming to the part will slow down the cooling. The same phenomena might occur on simple designs like round pipes or tubing to cause uneven cooling and bowing. Everybody knows that warmth rises and warm water is going to be no exception. This is ideal for the water next to the vertical surfaces of a right part going right through the water. The water is without question heated by the portion and this warm water will rise along the part drawing cool water behind it to further cool the spend the a continuous renewing of cold water against the portion. However, heated water on the bottom surface cannot climb as easily because the part is in the way. It does slowly progress and draw cold water behind it but less efficiently than what is occurring on the sides. The very best is even more of a issue because despite the fact that the heated water is not obstructed from moving up and from the part, the only real water that is used to replace it is the heated water moving up the sides of the portion. The top is not cooled as fast and pipes or other areas will generally bow up (bend upwards). Sizing plates in the tank support break up this movement but only allow cold water onto the very best of the part soon after the sizing plate. Turbulent circulation of drinking water in the tank considerably helps with this problem.
SPRAY COOLING
Spray cooling can be an improvement above immersion cooling and another method to solution the cooling challenge. Spray nozzles are evenly distributed around the portion and down the tank to ensure a continual replenishment of heat range controlled water to the top of part. This spray as well ensures considerably more uniform cooling by spraying water equally into U-stations and inside corners compared to outdoors corners and straight surfaces. Parts with a straightforward cross section can be sprayed with chilled water and run at huge rates of production. The challenge of uneven wall structure thicknesses still must be addressed separately. If spraying cool water alone is not sufficient to achieve the uniform cooling that is needed to avoid warping, the drinking water can be heat controlled to slow down the cooling and lessen or eliminate warping. Water is required in a sufficient volume to create the turbulent circulation in the tank that's needed to split up the insulating coating of warm water.
Some people claim that spray cooling is preferable to immersion cooling because of the evaporative cooling effect significantly. That's where the water sprayed onto the heated part is quickly considered steam and evaporates having off significantly more heat than the drinking water can carry off when immersed. While this result is real, it really is only true once the surface of the plastic material is above about 250 F. This only happens in the very first seconds and even tenths of seconds of the portion entering the cooling tank. With the high effectiveness of cooling of the water and more importantly the low conduction of heat from the plastic material to the surface, the surface temperature drops below 250 F. and stays there so that forget about evaporative cooling occurs. Still, the consistent replenishment of cool water to the surface can be plastic compounding machines an improvement in the productivity of the normal water cooling, with the added advantage of not requiring vacuum to hold the water in the container. Spray cooling possesses extra uniform distribution of cooling drinking water over the surface and continuous replenishment of cold water on the top with the added advantage of using much lower flow rates of drinking water.
CONCLUSION
So, the plastic part will tell you when it is not being cooled uniformly by bowing, warping, or distorting. With simple shapes the part will bow in the direction of the wall or section that cooled previous. In more technical shapes the contortions will not be as convenient to find out with as much as 6 to 10 different wall sections cooling at distinct rates. Directing even more cooling to sections that definitely would awesome slower because they're: thicker, inside corners, normally shielded from circulating or spray water will result in control of warpage. Now the trick is to rate it up and solve the nagging problem yet again.
Like the majority of materials, plastics shrink because the temperature of the plastic decreases, but they usually shrink greater than other materials. Plastics shrink at one price when they will be in the stable (frozen) state, however they shrink much more if they are still delicate or in the molten status. The issue for the profile extruder is controlling this shrinkage when cooling the scorching plastic, coming out of the extruder, all of the real way down to room temperature. Lets take the simplest example of a flat sheet where one part cools faster compared to the other. When still smooth both sides are shrinking at the same charge. Even if one side is cooling more quickly and shrinking faster the other side is still pliable more than enough to come with the various other shrinking side. However, once one side cools at night crystalline temperature or its glass changeover temperature, a couple of things happen. First, that materials stiffens and is no longer pliable plenty of to follow the other area and the level of shrinkage goes down significantly. It is as if the stiffened side is no longer shrinking as the other pliable area continues to shrink. Therefore, because the pliable aspect proceeds to shrink it is pulling on the stiffened aspect and resulting in a bow in direction of the medial side that cooled last. In this example, and in other straightforward profiles, the proper part will bow in the direction of the material that cooled last. In more complex profiles the proper parts may twist, distort, or warp in every types of fashions based on which sections of the right part cooled last. Well cover more upon this later.
In addition to this nagging problem is the point that plastics are good thermal insulators, and therefore they dont transfer high temperature very fast. That means it is difficult to pull all of the heat from the part to begin with, let alone carrying it out uniformly. Thermal conductivity is going to be a way of measuring how fast supplies transfer heat. Steel has a thermal conductivity of 43 while Aluminums higher heat transfer is 250 & most plastics are way down at values between 0.1 and 0.3.
TYPES OF COOLING Surroundings COOLING
Considering these problems with cooling profiles it will not be surprising that historically profile extruders often used air flow to cool parts.
Air racks are basic tables or frames with plates / manuals and fixtures that contain the part in shape as it is being pulled slowly across the table. Fans are generally used to improve overall cooling while compressed oxygen jets are added where specific additional cooling is required. Metal fingers, cables, and jigs mounted on the table with clamps or vise grips are used to push the component into shape as it cools very slowly.
Air is quite inefficient, meaning SLOW, which in cases like this is great because slow provides operator time and energy to make modifications and get the part just right without warping or other distortion. Complex profiles or parts with several wall thicknesses on distinctive parts of the right part might need customized cooling. The operator can direct extra cooling to where he needs it with compressed oxygen nozzles or retard cooling in other areas by insulating a section to retain it from cooling as well fast. Since thicker sections cool more than thin sections slowly, specific actions must be employed in order to avoid warp. The operator will need to direct significantly more cooling on thicker sections to get them to interesting to the same heat range as well as thinner sections on the same profile. Likewise, inside a U-channel or basically an internal corner will great slower than an outside corner and can require extra directed cooling. Output costs are limited to between 100 - 250 lb./hr. using air because it is so slow.
Today even, some may nonetheless use atmosphere cooling when:
Profiles are very complex
Using materials with completely different thermal conductivities
Size of production runs do not justify more expensive tooling
SUBMERSION WATER COOLING
When more significant output rates are required, cooling with water can be used then. There are many methods to run a part through water depending on many variables.
Submersion Tanks
For very easy shapes the part could be extruded outrageous of an extended water tank and become pushed down under the normal water by rollers or sizing plates. This may only be utilized for parts where it doesnt matter that the bottom of the portion hits the water primary (and is cooled first) while the leading comes down in to the water an instant later.
Vacuum Tanks
Extruding larger or more complex shapes directly into the water container is a superb idea that incurs the simple issue of gravity pulling water out of the tank through the hole that the component needs to go through in to the tank. Even small gaps between your sides of the part and the sides of the entrance plate will allow drinking water to leak out. This issue is usually solved by applying vacuum to the entire inside of the tank to hold the water in. Needless to say, this requires a particular tank that is strong enough not to collapse from the differential drive of vacuum inside and air pressure on the beyond the tank.
Other Options
Another option would be to make a little vacuum sleeve around the entry to suck off any drinking water trying to stream through the gap between component and entrance plate. More recently, profile extruders will place a dried out vacuum calibrator in front of the water tank to perform a similar thing. This vacuum calibrator can be as short as 3 for less significant profiles or so long as 10 ft for parts that have to be hardened to very precise dimensions prior to going into the water tank for more cooling. Dry out vacuum calibration isn't as efficient as water cooling nonetheless it is the price that must be paid when tighter control of the sizes is required.
Water Temperature Choices
Its pretty obvious that vacuum tanks are totally closed. With an open water tank it is very difficult even, if not difficult, to find yourself in the tank to place fingers and jigs to force the part into form as is performed on an air flow rack. Additionally it is difficult to immediate cooling water or to insulate sections of the part from cooling. However, it is possible to reduce the effectiveness of cooling (i.e. slow it straight down) to mimic the even more uniform cooling possible with an surroundings rack by heating the water. This is often done with parts which have a strong inclination to warp and especially with higher temperature engineering components. In this instance a temperature control device is required to control the temperature of the water at a arranged value. The bigger the water temperature is the slower the cooling and then the better it is to achieve uniform cooling. Controlled heat range water between 80 F and 130 F is frequently used in the original tank until colder drinking water can be used to entire the cooling. Needless to say, with the desire for speed, the colder the normal water the more quickly the cooling, thus most profile extruders shall use chilled water at temperatures between 50 F and 55 F every time they can.
Water Flow Characteristics
Even nevertheless immersing the complete profile in water offers faster and better cooling it could not be the very best cooling method. Unless the water has been agitated to provide turbulent movement around the part, then the layer of water next to the portion will heating up and that warm water up coming to the part will slow down the cooling. The same phenomena might occur on simple designs like round pipes or tubing to cause uneven cooling and bowing. Everybody knows that warmth rises and warm water is going to be no exception. This is ideal for the water next to the vertical surfaces of a right part going right through the water. The water is without question heated by the portion and this warm water will rise along the part drawing cool water behind it to further cool the spend the a continuous renewing of cold water against the portion. However, heated water on the bottom surface cannot climb as easily because the part is in the way. It does slowly progress and draw cold water behind it but less efficiently than what is occurring on the sides. The very best is even more of a issue because despite the fact that the heated water is not obstructed from moving up and from the part, the only real water that is used to replace it is the heated water moving up the sides of the portion. The top is not cooled as fast and pipes or other areas will generally bow up (bend upwards). Sizing plates in the tank support break up this movement but only allow cold water onto the very best of the part soon after the sizing plate. Turbulent circulation of drinking water in the tank considerably helps with this problem.
SPRAY COOLING
Spray cooling can be an improvement above immersion cooling and another method to solution the cooling challenge. Spray nozzles are evenly distributed around the portion and down the tank to ensure a continual replenishment of heat range controlled water to the top of part. This spray as well ensures considerably more uniform cooling by spraying water equally into U-stations and inside corners compared to outdoors corners and straight surfaces. Parts with a straightforward cross section can be sprayed with chilled water and run at huge rates of production. The challenge of uneven wall structure thicknesses still must be addressed separately. If spraying cool water alone is not sufficient to achieve the uniform cooling that is needed to avoid warping, the drinking water can be heat controlled to slow down the cooling and lessen or eliminate warping. Water is required in a sufficient volume to create the turbulent circulation in the tank that's needed to split up the insulating coating of warm water.
Some people claim that spray cooling is preferable to immersion cooling because of the evaporative cooling effect significantly. That's where the water sprayed onto the heated part is quickly considered steam and evaporates having off significantly more heat than the drinking water can carry off when immersed. While this result is real, it really is only true once the surface of the plastic material is above about 250 F. This only happens in the very first seconds and even tenths of seconds of the portion entering the cooling tank. With the high effectiveness of cooling of the water and more importantly the low conduction of heat from the plastic material to the surface, the surface temperature drops below 250 F. and stays there so that forget about evaporative cooling occurs. Still, the consistent replenishment of cool water to the surface can be plastic compounding machines an improvement in the productivity of the normal water cooling, with the added advantage of not requiring vacuum to hold the water in the container. Spray cooling possesses extra uniform distribution of cooling drinking water over the surface and continuous replenishment of cold water on the top with the added advantage of using much lower flow rates of drinking water.
CONCLUSION
So, the plastic part will tell you when it is not being cooled uniformly by bowing, warping, or distorting. With simple shapes the part will bow in the direction of the wall or section that cooled previous. In more technical shapes the contortions will not be as convenient to find out with as much as 6 to 10 different wall sections cooling at distinct rates. Directing even more cooling to sections that definitely would awesome slower because they're: thicker, inside corners, normally shielded from circulating or spray water will result in control of warpage. Now the trick is to rate it up and solve the nagging problem yet again.