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Troubleshooting Profile and Pipe
32 Troubleshooting Profile and Pipe the die cross section divided by the final product cross section. If the die opening is twice as large as the final product, the draw ratio is 2:1. Higher draw ratios produce higher molecular orientation, resulting in greater differences between machine and transverse direction properties. If a pipe or tubing has high wall thickness on one side and low on the other side, the die bushing has to be properly centered on the mandrel to produce uniform dimensions around the pipe or tubing circumference. Nonuniform wall thickness in pipe or tubing induces warpage in the pipe, with the pipe curling or bending toward the thinner wall due to higher molecular orientation and shrinkage on the thin side. Proper die adjustments are required to produce both correct and uniform wall thickness.
A few potential problems exist in pipe, tube, and profile extrusion. Many of the typical problems encountered with these extrusion processes have been covered previously in Part 4 and will not be repeated here. Pipe and tubing is a symmetrical profile extrusion. The process, shown in Fig. 32.1, has a special die to provide the desired shape, a cooling tank, puller, and saw or guillotine. The cooling tank comes in many shapes and sizes and is used to solidify the product into its final shape. For pipe and tubing, the cooling or calibration tank has sizing rings and one or more vacuum chambers to shape the pipe to the desired outside diameter.
32.1 Problem 1—Excessive Wall Thickness or Thinness Wall dimensions that are much larger than anticipated can be caused by the wrong die lip opening, improper processing conditions, or slippage in the puller. Corrective actions for these problems are listed below. • Profile dies do not normally have adjustable die lips to allow the die gap to be adjusted for the particular process being run. (Pipe and tubing dies may or may not have adjustable die lip openings, depending on the pipe or tubing diameter and the application.) Once the die is cut, the only way to thin the walls are to lower the screw speed and/or increase the puller speed. Speed changes are associated with a fixed molecular orientation, depending on the draw ratio required to obtain the desired product wall thickness. If the draw down ratio and molecular orientation are too high, inferior properties can result in the transverse direction; the only alternative is to cut another die with a smaller die gap. Each die gap opening is coupled with a fixed puller speed and throughput rate to produce an acceptable product with a specific draw ratio and cross sectional area. The draw ratio is defined as
• Improper processing conditions occur when the wrong draw ratio is used with the correct die gap opening. If the puller is running too slow for the extruder throughput rate, the wall thickness will be greater than anticipated. For each die gap opening, there is only one extruder throughput-topuller speed ratio that gives the correct dimensions in the final product. This means the extruder screw speed in a flood-fed single screw extruder is nominally coupled with a given puller speed. The throughput rate in a flood-fed single screw extruder can be changed slightly at a given screw speed by changing the temperature profile and the melt viscosity. Any change in throughput rate must be accompanied by a change in puller speed to maintain thickness control. In general, higher melt temperatures lead to lower viscosity with higher resin flow, possibly generating higher throughput rates. Extruders are complex systems: as the barrel temperatures are changed, the feed characteristics may change (higher set temperatures in extruder zones 1 and 2); the conveying characteristics in the feed or conveying section might change; die pressure can be lower, reducing
Figure 32.1. Schematic of a profile, pipe, or tubing extrusion line.
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the backflow in the extruder; mixing characteristics change; and extruder pumping characteristics are different. Obviously the extruder is a fine balance of throughput rate, screw speed, and melt pressure and temperature. In absolute terms each fixed die gap opening has a fixed draw ratio based on the puller speed and extruder throughput. As the throughput rate goes up, the puller speed must be increased, and vice versa. At a given screw speed and operating conditions in a flood-fed extruder, the output rate is constant at a given melt temperature and pressure. • Slippage in the puller allows the product dimensions to increase because the product is not being pulled away from the extruder at the proper rate. The caterpillar or puller pressure has to be adjusted to prevent the product from slipping without causing any distortion or damage to the final product. If the pipe, tubing, or profile is still wet from the cooling bath when it reaches the puller, slippage can be caused by moisture on the belts. The same corrective action procedures outlined above are used when the wall thickness is thin, except the implemented procedures are opposite of those presented. Figure 32.2 summarizes the potential causes and corrective actions to implement in the event the wall thickness is incorrect.
32.2 Problem 2—Wavy Surface Inside Pipe or Tubing Sizing is normally done on the pipe or tubing outside diameter (OD) through the use of sizing rings or fixtures in the cooling tank. The inside diameter (ID) is controlled by air pressure combined with vacuum over the sizing tank to force the pipe to the final dimensions. The inside surface is controlled by the die and processing conditions. In some pipe operations, a plug is held in place by the die; as the pipe is pulled over the plug, air is used in combination with the plug to form a smooth pipe inside diameter. Viewing a hollow profile (pipe), the inside surface can be wavy and not nearly as smooth as the outside surface. Potential causes for excessive waviness on the inside surface with corrective actions are given below. • The melt temperature is too low, preventing the polymer from flowing freely out the die lips due to the high melt viscosity. Corrective action is to increase the extruder temperature setpoints in the last one to two zones, the adapter, and the die to raise the melt temperature. • Slippage in the caterpillar causes the profile to go through a cyclic action where the puller engages and disengages the profile. This slippage does not pull the molten material uniformly after it exits the die. One cause of slippage is a wet profile. • The molten polymer is sticking or slipping on the die lips as it flows out the die. Changing the frictional characteristics between the melt and the die lips is necessary to eliminate the slip/stick phenomenon. An external lubricant may improve the
Figure 32.2. Potential causes and corrective actions for thick or thin wall dimensions.
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AND
PIPE
resin flow characteristics over the mandrel. Alternately, change the coating on the mandrel or die to reduce friction. Figure 32.3 summarizes potential causes and corrective actions to prevent a wavy inside pipe surface.
32.3 Problem 3—Weld or Knit Line Weakness This problem has been identified and corrective actions proposed previously in problem 13 in Chapter 28. Weld lines in pipe or tubing are generated around the circumference in the machine direction by the spider legs used to center and hold the mandrel in place. If the die is not properly designed, weld lines can cause weakness in the transverse direction wherever a spider leg is located. If weld lines are a problem, the die length past the spider legs has to be increased to provide more time for the resin molecules to reentangle after they are split. Increasing the flow channel in the die after the spider legs builds pressure, encouraging the polymer molecules to reentangle. The other factor to consider is the polymer viscosity. Higher melt temperature lowers the polymer viscosity, increasing molecular motion. The polymer then has the potential for higher entanglement after the spider legs. Pipe and tubing processes create weld lines due to the dies used. Care must be exercised to ensure these weld lines have time to recombine and provide the strength required in the application. Figure 28.15 summarizes the corrective actions required to implement improvements in weld line strength.
309
32.4 Problem 4—Nonuniform Resin Velocity from a Profile Die The velocity profile from the die to each leg is critical in both simple and complex shapes. Assume polymer flow in one leg of a profile is faster than another leg; the resulting profile will be warped, curled, or twisted. In complex profiles (such as window profiles), all wall thicknesses need to have similar dimensions, with flow into each wall or leg being uniform. Internally the die needs a uniform taper into each leg or wall section to ensure uniform flow velocity and pressure generation across the entire die to produce a straight profile that meets the customer specifications. Potential causes for nonuniform die flow with corrective actions are given below. • Legs in different sections have unbalanced flow due to the taper leading into the leg, the die land length, or die land gap dimensions. The die has to be modified or reworked to ensure uniform flow, velocity, and pressure in each section or leg in the profile. • Flow is restricted in one leg or section by contamination, foreign material in the die, or degraded polymer. Carbon build-up in the die gap over time can restrict the flow in a given leg or section. Anything inside the die flow channel that restricts flow will affect the polymer velocity profile at the die exit. Initial corrective action is to properly purge the die; if that doesn’t work, the die has to be disassembled, cleaned, and reassembled. • Melt temperature fluctuations in the die can be caused by hot or cold spots in the die or nonuni-
Figure 32.3. Potential causes and corrective actions for a wavy surface inside pipe.
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form die heating that affect the polymer melt viscosity and polymer flow characteristics. All heater bands and thermocouples are checked to ensure the equipment is properly installed and functioning according to the specifications. If temperature fluctuations still occur after the equipment is checked, additional heater bands may be required or the die
may need to be insulated to provide uniform temperature. Proper melt mixing prior to entering the die ensures a uniform melt temperature. Figure 32.4 summarizes potential causes of nonuniform flow or velocity variations from different die sections with corrective actions to eliminate the problem.
Figure 32.4. Potential causes and corrective actions for nonuniform resin velocity for flow out of the die.
Review Questions 1. What are the consequences of not having uniform flow out each die leg in a profile extrusion? 2. What are some causes and corrective actions for either thick or thin walls in profile or pipe extrusion? 3. What causes weld lines, and why might they create a problem in an application? 4. What are some methods to use to eliminate wavy surfaces on the inside of a pipe?
A few potential problems exist in pipe, tube, and profile extrusion. Many of the typical problems encountered with these extrusion processes have been covered previously in Part 4 and will not be repeated here. Pipe and tubing is a symmetrical profile extrusion. The process, shown in Fig. 32.1, has a special die to provide the desired shape, a cooling tank, puller, and saw or guillotine. The cooling tank comes in many shapes and sizes and is used to solidify the product into its final shape. For pipe and tubing, the cooling or calibration tank has sizing rings and one or more vacuum chambers to shape the pipe to the desired outside diameter.
32.1 Problem 1—Excessive Wall Thickness or Thinness Wall dimensions that are much larger than anticipated can be caused by the wrong die lip opening, improper processing conditions, or slippage in the puller. Corrective actions for these problems are listed below. • Profile dies do not normally have adjustable die lips to allow the die gap to be adjusted for the particular process being run. (Pipe and tubing dies may or may not have adjustable die lip openings, depending on the pipe or tubing diameter and the application.) Once the die is cut, the only way to thin the walls are to lower the screw speed and/or increase the puller speed. Speed changes are associated with a fixed molecular orientation, depending on the draw ratio required to obtain the desired product wall thickness. If the draw down ratio and molecular orientation are too high, inferior properties can result in the transverse direction; the only alternative is to cut another die with a smaller die gap. Each die gap opening is coupled with a fixed puller speed and throughput rate to produce an acceptable product with a specific draw ratio and cross sectional area. The draw ratio is defined as
• Improper processing conditions occur when the wrong draw ratio is used with the correct die gap opening. If the puller is running too slow for the extruder throughput rate, the wall thickness will be greater than anticipated. For each die gap opening, there is only one extruder throughput-topuller speed ratio that gives the correct dimensions in the final product. This means the extruder screw speed in a flood-fed single screw extruder is nominally coupled with a given puller speed. The throughput rate in a flood-fed single screw extruder can be changed slightly at a given screw speed by changing the temperature profile and the melt viscosity. Any change in throughput rate must be accompanied by a change in puller speed to maintain thickness control. In general, higher melt temperatures lead to lower viscosity with higher resin flow, possibly generating higher throughput rates. Extruders are complex systems: as the barrel temperatures are changed, the feed characteristics may change (higher set temperatures in extruder zones 1 and 2); the conveying characteristics in the feed or conveying section might change; die pressure can be lower, reducing
Figure 32.1. Schematic of a profile, pipe, or tubing extrusion line.
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308
the backflow in the extruder; mixing characteristics change; and extruder pumping characteristics are different. Obviously the extruder is a fine balance of throughput rate, screw speed, and melt pressure and temperature. In absolute terms each fixed die gap opening has a fixed draw ratio based on the puller speed and extruder throughput. As the throughput rate goes up, the puller speed must be increased, and vice versa. At a given screw speed and operating conditions in a flood-fed extruder, the output rate is constant at a given melt temperature and pressure. • Slippage in the puller allows the product dimensions to increase because the product is not being pulled away from the extruder at the proper rate. The caterpillar or puller pressure has to be adjusted to prevent the product from slipping without causing any distortion or damage to the final product. If the pipe, tubing, or profile is still wet from the cooling bath when it reaches the puller, slippage can be caused by moisture on the belts. The same corrective action procedures outlined above are used when the wall thickness is thin, except the implemented procedures are opposite of those presented. Figure 32.2 summarizes the potential causes and corrective actions to implement in the event the wall thickness is incorrect.
32.2 Problem 2—Wavy Surface Inside Pipe or Tubing Sizing is normally done on the pipe or tubing outside diameter (OD) through the use of sizing rings or fixtures in the cooling tank. The inside diameter (ID) is controlled by air pressure combined with vacuum over the sizing tank to force the pipe to the final dimensions. The inside surface is controlled by the die and processing conditions. In some pipe operations, a plug is held in place by the die; as the pipe is pulled over the plug, air is used in combination with the plug to form a smooth pipe inside diameter. Viewing a hollow profile (pipe), the inside surface can be wavy and not nearly as smooth as the outside surface. Potential causes for excessive waviness on the inside surface with corrective actions are given below. • The melt temperature is too low, preventing the polymer from flowing freely out the die lips due to the high melt viscosity. Corrective action is to increase the extruder temperature setpoints in the last one to two zones, the adapter, and the die to raise the melt temperature. • Slippage in the caterpillar causes the profile to go through a cyclic action where the puller engages and disengages the profile. This slippage does not pull the molten material uniformly after it exits the die. One cause of slippage is a wet profile. • The molten polymer is sticking or slipping on the die lips as it flows out the die. Changing the frictional characteristics between the melt and the die lips is necessary to eliminate the slip/stick phenomenon. An external lubricant may improve the
Figure 32.2. Potential causes and corrective actions for thick or thin wall dimensions.
TROUBLESHOOTING PROFILE
AND
PIPE
resin flow characteristics over the mandrel. Alternately, change the coating on the mandrel or die to reduce friction. Figure 32.3 summarizes potential causes and corrective actions to prevent a wavy inside pipe surface.
32.3 Problem 3—Weld or Knit Line Weakness This problem has been identified and corrective actions proposed previously in problem 13 in Chapter 28. Weld lines in pipe or tubing are generated around the circumference in the machine direction by the spider legs used to center and hold the mandrel in place. If the die is not properly designed, weld lines can cause weakness in the transverse direction wherever a spider leg is located. If weld lines are a problem, the die length past the spider legs has to be increased to provide more time for the resin molecules to reentangle after they are split. Increasing the flow channel in the die after the spider legs builds pressure, encouraging the polymer molecules to reentangle. The other factor to consider is the polymer viscosity. Higher melt temperature lowers the polymer viscosity, increasing molecular motion. The polymer then has the potential for higher entanglement after the spider legs. Pipe and tubing processes create weld lines due to the dies used. Care must be exercised to ensure these weld lines have time to recombine and provide the strength required in the application. Figure 28.15 summarizes the corrective actions required to implement improvements in weld line strength.
309
32.4 Problem 4—Nonuniform Resin Velocity from a Profile Die The velocity profile from the die to each leg is critical in both simple and complex shapes. Assume polymer flow in one leg of a profile is faster than another leg; the resulting profile will be warped, curled, or twisted. In complex profiles (such as window profiles), all wall thicknesses need to have similar dimensions, with flow into each wall or leg being uniform. Internally the die needs a uniform taper into each leg or wall section to ensure uniform flow velocity and pressure generation across the entire die to produce a straight profile that meets the customer specifications. Potential causes for nonuniform die flow with corrective actions are given below. • Legs in different sections have unbalanced flow due to the taper leading into the leg, the die land length, or die land gap dimensions. The die has to be modified or reworked to ensure uniform flow, velocity, and pressure in each section or leg in the profile. • Flow is restricted in one leg or section by contamination, foreign material in the die, or degraded polymer. Carbon build-up in the die gap over time can restrict the flow in a given leg or section. Anything inside the die flow channel that restricts flow will affect the polymer velocity profile at the die exit. Initial corrective action is to properly purge the die; if that doesn’t work, the die has to be disassembled, cleaned, and reassembled. • Melt temperature fluctuations in the die can be caused by hot or cold spots in the die or nonuni-
Figure 32.3. Potential causes and corrective actions for a wavy surface inside pipe.
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310
form die heating that affect the polymer melt viscosity and polymer flow characteristics. All heater bands and thermocouples are checked to ensure the equipment is properly installed and functioning according to the specifications. If temperature fluctuations still occur after the equipment is checked, additional heater bands may be required or the die
may need to be insulated to provide uniform temperature. Proper melt mixing prior to entering the die ensures a uniform melt temperature. Figure 32.4 summarizes potential causes of nonuniform flow or velocity variations from different die sections with corrective actions to eliminate the problem.
Figure 32.4. Potential causes and corrective actions for nonuniform resin velocity for flow out of the die.
Review Questions 1. What are the consequences of not having uniform flow out each die leg in a profile extrusion? 2. What are some causes and corrective actions for either thick or thin walls in profile or pipe extrusion? 3. What causes weld lines, and why might they create a problem in an application? 4. What are some methods to use to eliminate wavy surfaces on the inside of a pipe?