10.1   Hot Wedge Welding System

Hot wedge welding represents the primary seaming method for Poly-Flex liners. The hot wedge seaming device is a completely self-contained system. The hot wedge system (Figure 1) produces a bonded seam by running a hot metal wedge between the overlapped area of the Poly-Flex liners. The hot wedge melts the facing surfaces of the two liners and creates a permanent bond between them using controlled heat and pressure. The wedge is square at the leading end and tapered at the trailing end. The heated geomembrane overlaps come together at the tapered end of the hot wedge, under pressure from two nip/drive rollers, and are permanently fused together. Hot air tacking (as employed in extrusion fillet seaming) is not necessary.

The dual hot wedge has a central, canal-like recession along its length. This type of wedge creates a channel in the liner seam between two parallel bonds.

Knurled rollers are used to apply pressure to the sheets where they have just passed over the taper of the hot wedge and been bonded. These rollers should have smooth surfaces and beveled edges.

As the liners pass through the welder, they must contact the full length and width of the hot wedge, or the facing surfaces will not be equally hot. Contour rollers or similar pressure devices, which press the liners against both sides of the hot wedge, must be adjusted so that material of any thickness conforms to the wedge's taper while passing through the welder. Such adjustments should be made while the wedge is cold.

This seaming method is designed to:

1. Monitor operating temperatures via digital readout.
   
2. Form uniform bonds by applying uniform and consistent pressure to the seam area.
   
3. Reduce surface tension and remove surface contamination without grinding, which improves bond strength.
   
4. Ensure high reliability, which results in consistent seam integrity.
   
5. Minimize operator error, as the machine is semi-automatic.

The following steps shall be taken to prepare the liner for hot shoe seaming:

1. The two liners to be joined must be positioned to create an overlap of 4-6 inches.
   
2. If the overlap is not substantially wide to contain the wedge, "float" the liner into better position by lifting it high enough to draw air beneath it, guiding it upon this air to an improved position. Avoid dragging the liner, particularly across rough soil subgrades.
   
3. If the overlap between the liners is excessive, the excess must be trimmed away. This should be done by trimming the lower sheet. If this is not possible and the upper sheet must be trimmed, use a knife with a shielded or hook blade. An unshielded, downward-facing blade cutting from above can easily scratch the liner in a vulnerable location.
   
4. All cutting and preparation of odd-shaped sections or small fitted pieces must be completed ahead of the seaming operation, so that seaming may be conducted with no interruptions.
   
5. Before joining the two contiguous liners, make sure they are not excessively scratched, blemished or flawed, and are visually acceptable.
   
6. If the Plans require panels to be shingled in a particular direction, make sure this is being done.
   
7. Excessive undulations (waves) along the seams during the seaming operation should be avoided. These occur when the upper and lower sheets have unequal slack between them. This condition can lead to the undesirable formation of "fishmouths," which must be trimmed, laid flat and patched.
   
8. Polyethylene liners need slack to allow for expansion and contraction caused by weather changes.
   
9. Sheets which are overlapped and ready for seaming must be clean. If dirty, they must be wiped clean with dry rags.
   
10. The seam area must be completely free of moisture before the overlapping sheets can be properly seamed. Dry rags can be used to wipe any such moisture from the seam surface. If a sufficient quantity of dry rags is unavailable for this purpose, air blowers can be used.
    
11. Seaming is not to be performed during rain or snow unless measures are taken to allow the seam to be made on dry liner materials, e.g., within an enclosure or shelter.
    
12. Seaming is not to be performed when the soil surface beneath the liners is saturated because the hot seaming apparatus will draw moisture into the ongoing seam. Seaming activity on frozen soil is unacceptable for the same reason. Puddled water on the soil's surface beneath the liner is unacceptable.
    
13. Seaming should be conducted only when ambient temperatures are 32-104° F (0-40° C), unless it can be proven via test strips that good seams can be fabricated at such temperatures.
    
14. When seaming in cold weather, it is advisable to preheat the sheets with a hot air blower and to conduct seaming activity within a portable tent, to help prevent heat loss. Extra test welds can be used to make any necessary adjustments to the seaming activity.

1. A generator must be kept in the proximity of the seaming area with enough extension cord to range the length of the seam. It must either be fitted with rubber tires or be placed on a stable and smooth moveable base, such as an excess of liner, so that no damage will be inflicted upon the liner or subgrade by its movement. The generator's fuel (gasoline or diesel) must be stored off the liner.
   
2. Surface grinding before application of the hot wedge is not necessary and shall not be done.
   
3. The hot wedge, or "anvil," should be inspected to see that it is symmetrically balanced and gradually tapered. It is imperative that a wedge has no sharp edges on any surface that contacts the liner during the welding process.
   
4. The chain drive powering any portion of the welder should be synchronized, properly lubricated, and physically sound.

1. Insert two material samples into the nip drive rollers.
   
2. Place two other material samples above and below the wedge.
   
3. Lock the wedge into its operating position. This is done by shifting the wedge forward, into the clutch of the upper and lower nip rollers.
   
4. Adjust the contour rollers until they are snug against the liners, which sandwich the wedge.
   
5. Set the maximum distance that the wedge can move into the nip rollers. Unsecured, the wedge might make direct contact with the nip rollers when the machine has no liner material running through it and damage the machine.
   
6. The wedge and surrounding rollers are now set for seaming the liner. Remember to disengage the hot wedge from its forward position in the roller apparatus as the machine reaches the end of a seam. This will keep the wedge from advancing into the nip rollers and damaging them.
   
7. The forward face of the welding machine should be inspected for sharp corners and irregular details which may damage the liner as it advances during the seaming process.
   
8. Temperature controllers on the wedge device should be set according to liner thickness, ambient temperature, and seaming rate. The "test strip" helps determine these settings. Temperature gauges should be checked for accuracy and repeatability.

Polyethylene liners can be seamed by the hot wedge method, but temperature settings will vary according to the grade of polyethylene used. Typical wedge temperature ranges for hot wedge seaming is as follows:

1. The hot wedge system should be properly positioned for making the desired dual (split) seam.
   
2. Ambient variables such as temperature, cloud cover, and wind speed may make it necessary to adjust the wedge temperature. It may be useful to document wedge temperatures used successfully in a variety of ambient conditions, so that the hot wedge can be more accurately adjusted to new conditions.
   
3. The drive motor should be off when positioning the welding machine to seam. Guide the overlapped material between the contour rollers and the wedge and into the drive/nip rollers. When the nip rollers engage and the wedge is in position, turn on the drive motor. Move the hot wedge into position and lock it.
   
4. The operator must constantly monitor the temperature controls, as well as the completed seam passing through the machine. Occasional adjustments in speed may be necessary to maintain a consistent weld.
   
5. On soft soils, the device tends to "bulldoze" into the ground as it travels, causing soil to enter the weld. A contaminated weld is unacceptable. To avoid this, the operator should slightly lift the front of the machine. Alternatively, a movable base can be used. Strips of geomembrane have proven to be effective materials on which the welder can maintain traction. It is recommended that at least two people work together in making hot wedge seams: one as operator and one as helper.

1. A small amount of "squeeze-out" is a reliable indication that proper seaming temperatures have been achieved (Figure 2). The melted polymer will laterally extrude out of the seam area in properly welded seams. An excessive amount of extruded hot melt indicates that excessive heat or pressure, or both, was applied. Reduce the temperature and/or pressure to correct the situation.

   

2. The 20 mil, 30 mil and 40 mil Poly-Flex liners show a long, low, sinusoidal wavelength pattern in the direction of the seam which indicates a proper weld. If the wave peaks are too close together, machine speed should be increased until a satisfactory pattern appears. The absence of this wavelength pattern indicates that machine speed should be decreased. No wavy pattern will occur on liners thicker than 40 mil due to the inherent stiffness of the thicker liners.
   
3. Nip/drive roller marks will always show on the surface when using knurled rollers. They should be noticeable to the eye, but just barely to the touch.
   
4. The hot wedge device has adjustable parts. It is critical that they be checked after a day of seaming. The machine should also be cleaned.