Design, Specification, & Installation
Custom Fabrication
- Introduction
- The Principle of Thermoplastic Welding
- High Speed Hot Gas Welding
- Tack Welding
- Hot Plate (Butt) Welding
- Welding CorzanŽ Sheet & Pipe
- Dual Lamination
- Other Fabrication Reference Materials
Recommendations for Fabrication
Introduction
CPVC offers a variety of advantages to the chemical process industries and has been successfully used in industrial applications for more than 40 years. Some of the outstanding features of Corzan® CPVC products include: high temperature capabilities, excellent chemical resistance to a wide range of highly corrosive liquid and vapor environments, resistance to galvanic corrosion, low heat transfer, good electrical insulation properties, and lightness of weight for ease of installation. In addition to pipe, fittings, valves, pumps, tower packing, and other fluid handling products which are manufactured from CPVC, sheet and duct products are also available from which specialized parts such as tanks and tank linings, as well as ventilation and vapor scrubbing equipment, can be fabricated. Corzan™ Industrial Systems components can be fabricated with all of the most common techniques for thermoplastic fabrication. This document addresses high speed hot gas welding and butt welding of Corzan system components. It will be amended at a later date to include extrusion welding and thermoforming, as information on those topics becomes available.
The Essentials of Hot Gas Welding Corzan CPVC
- clean, dry gas
- accurate temperature control
- beveled edges on base material
- buff or scrape welding surface & rod
- base material and rod both Corzan CPVC
- ideal rod diameter 1/8" - 5/32"
- optimum temperature range: 710-800°F (375-425°C)
– dial-selected –
(calibrated Wegener Autotherm)
680-770°F (360-410°C) – measured and adjusted –
3/16" (5mm) inside main opening of welding tip - optimum air flow: 40-60 lpm
The Principle of Thermoplastic Welding
In order to weld thermoplastics, the material has to be heated to reach its melt state. The pieces to be welded must then be pressed together with a certain amount of pressure over a given amount of time. This will cause the surface molecules of the parts to interlock, fusing the parts together.
High Speed Hot Gas Welding
Corzan system components can be hot gas welded to give approximately 80% of the tensile strength of solid sheet. Actual performance will depend upon the equipment used, the welding conditions employed, and the individual technique of the person doing the welding. As a result, the recommendations given in this document are intended to be general guidelines and do not guarantee actual performance.
Equipment
When thermoplastics are being welded, the quality of the gas used as the heat transfer medium is a critical factor in the quality of the weld. High-speed hot gas welding requires the use of gas supplied at low pressure and high volume, which is free of oil and moisture. Common shop compressors generally do not supply air of adequate quality for use in high-speed hot gas welding. Many manufacturers of hot gas welding equipment also have blowers available that are specifically suited for this purpose.
When Corzan system components are being welded, the accuracy of the temperature controlling equipment is equally as important as the quality of the gas. The optimum temperature range for welding Corzan system components is typically somewhat narrower than for other thermoplastics such as polyolefins. The quality of the weld produced is therefore dependent on having a constant temperature at the welding tip. Welding equipment for use with Corzan system components preferably should control the temperature by regulating power to the heating element, not by varying the gas flow. The ideal temperature control arrangement for welding Corzan system components should incorporate closed loop controls which hold the temperature constant even while gas flow or supply voltages fluctuate.
A high speed welding tip is designed to perform three functions: preheating the base material, guiding and preheating the welding rod, and applying pressure to the weld area. A typical high-speed hot gas welding tip is shown in Figure 1.
Material Preparation
The ends of the pieces of material to be joined must be beveled in order to produce the best weld. The bevel may be produced with an adjustable saw, a router or other suitable tool. The angle between the bevels of the two pieces to be joined should be between 60 and 70 degrees, except when one piece is joined perpendicularly to another, in which case, the angle is reduced to 45 degrees.
The parts to be assembled must be very clean. To remove surface residue, slight grinding or scraping with a sharp blade at the area to be welded and the weld rod is strongly recommended. Acetone is the only solvent that is suitable for use to clean the area to be welded. Other solvents may have potentially negative effects on Corzan CPVC.
If the joint will not be tacked prior to welding, it is recommended to leave a gap of 0.5-1 mm wide between the two pieces to be joined so that the welding material may penetrate to the root of the bevel and overflow slightly on the other side. If the parts will first be tacked, they should be butted together with no gap. The parts to be joined should be mounted firmly in place with appropriate clamps as necessary.
Typical welded joint configurations are shown in Figure 2.
Welding Rod Selection
When Corzan® CPVC parts are being joined, the welding rod selected should also be produced from Corzan® CPVC. Triangular rod may be used where the appearance of the joint is the most important factor, but round welding rod should be used when structural integrity is desired.
While welding rod is commonly available in sizes up to 1/4" (6 mm) in diameter, the strongest joints are obtained by using rod in smaller diameters with multiple beads as necessary. In order to obtain the strongest weld with Corzan® welding rod, it is recommended to use rod no larger than 5/32" (4 mm) in diameter.
It is important to match the diameter of the welding tip with the diameter of the rod selected. An oversized tip will negatively affect guidance and pressure applied to the rod and may also cut into the parts being welded.
Tack Welding
The initial step in the process is the "tack weld." The objective is to put the parts in place, align them, and prevent any slippage of the material during the structural welding process. Tacking is done with a pointed shoe tip. The operator places the tacking tip directly on the material to be welded and draws it along the joint. Hot gas from the welder softens the material, and pressure applied by the operator to the tip fuses the material together. Continuous or spot tack welding may be used as necessary. Larger structures or thick gauge materials may require additional clamping.
Any tank should be continuously tack welded to achieve a leak free connection. This prevents solutions from penetrating between the tank wall and the bottom in case of a problem with the filler weld.
The Welding Process
The optimum temperature range for hot gas welding of Corzan® system components is dependent on the type of welding equipment being used and the way in which the temperature is measured. If the welding torch incorporates closed-loop controls which maintain the temperature selected on a dial setting, the optimum range is typically 710-800°F (375-425°C). If the temperature cannot be directly selected on a dial setting, it must be measured by the operator and then adjusted by varying power to the heating element or regulating the gas flow. The temperature should be measured with a pyrometer approximately 3/16" (5 mm) inside the main opening of the high speed welding tip. When the temperature is controlled in this manner, the optimum temperature for welding Corzan® system components is typically 680-770°F (360-410°C). The actual temperature within the range that will produce the best weld will depend on a number of factors including diameter of rod, brand of rod, speed of welding, ambient temperature, etc., and must be adjusted accordingly.
To make it easier to initiate welding, a sharp angle may be cut on the lead end of the welding rod. The welding rod should not be inserted into the high speed-welding tip until immediately before the operator is ready to begin welding. Burning of the rod may otherwise result.
To begin welding, the operator should grasp the welding torch like a dagger with the airline trailing away from his body or over the shoulder so that he will be able to operate quickly and smoothly once he has begun. Holding the welding tip approximately 8 cm above the area to be welded to prevent scorching the material before work begins, insert the welding rod into the preheating tube and then place the pointed tip of the shoe on the material at the starting point of the weld.
Holding the welder at roughly a 45 degree angle, push the rod through the tip until it contacts the base material. Continue to feed the rod with the other hand, using slight pressure. If the rod is not guided, the welding rod will stretch fully apart. The weight of the welder is the only pressure needed as the weld is pulled along the joint.
As welding progresses, visual inspection of the weld may indicate its quality. Browned or charred edges occur when the welder is moving too slowly and/or overheating. If the rod has been softened too much by overheating, it will stretch and break or flatten out.
Once welding begins, it must be continued at a fairly constant rate of speed. The welding torch must not be held still or burning will result. To stop welding before the rod is used up, the operator should tilt the welder backward, cut the rod off with the tip of the shoe and immediately remove the remaining rod from the welding tip. Welding may also be terminated by pulling the welder tip up over the remaining rod and cutting the rod.
For best results, the welding tip should be cleaned occasionally with a wire brush.
Multiple beads should be applied as necessary until the joint is completely filled as shown in Figure 2. If the joint to be welded is a double V or a double half V joint, the best results are obtained if layers of beads are put down alternately on opposite sides of the joint. The table below presents recommendations for bead lay-up for different material thicknesses and joint configurations.
| RECOMMENDATIONS FOR BEAD LAY-UP | ||
| Material Thickness | Number of Beads x Rod Diameter | |
| Single V Joint | 1/8” (3mm) | 3 x 1/8” (3mm) |
| 5/32” (4mm) | 1 x 1/8” (3mm) + 2 x 5/32” (4mm) | |
| 3/16” (5mm) | 6 x 1/8” (3mm) | |
| Double V Joint | 5/32” (4mm) | 2 @ 1 x 5/32” (4mm) |
| 3/16” (5mm) | 2 @ 3 x 1/8” (3mm) | |
| 1/4” (6mm) | 2 @ 3 x 1/8” (3mm) | |
| 5/16” (8mm) | 2 @ 1 x 1/8” (3mm) + 2 x 5/32” (4mm) | |
| 3/8” (10mm) | 2 @ 6 x 1/8” (3mm) | |
Heat Stress Problems
During hot air welding, the material will expand while it is forced into position. When cooling, it will shrink back to its original volume. A welded sheet that was straight while still hot may be bent after cooling. Using a double V joint is one way to avoid this problem. Another way for an experienced operator to avoid this problem is to pre-bend the parts prior to welding as shown in Figure 3.
Weld Factor
When properly hot gas welded, Corzan CPVC sheet can be expected to perform to approximately 80% of its nominal tensile strength.
Hot Plate (Butt) Welding
Butt welding of thermoplastics involves holding two pieces of the material with defined pressure against a heated plate element until the material melts. The two pieces are then brought together quickly and held with a defined pressure so that they fuse into one piece. Some of the most common uses for butt welding are to join two pieces of flat sheet, to join both ends of a rolled or bent sheet to form a round or rectangular shape, or to join segments of pipe together to form fabricated fittings. The following recommendations are based primarily on work with sheet, but could be modified by an experienced welder for work with pipe.
The Essentials of Hot Plate (Butt) Welding Corzan CPVC
- PTFE-coated heating element
- accurate temperature control
- changeover time: less than 3 sec.
- optimum temperature: 440-445°F (225-230°C)
- optimum melting pressure: 95-100 psi (65-70 N/cm2)
- optimum heating pressure: 30 psi (20 N/cm2)
- optimum welding pressure: 95-100 psi (65-70 N/cm2)
- heating and welding/fusion times are dependent on material thickness (see Tables 1 & 2).
Equipment
The heating element should be PTFE-coated stainless steel in order to prevent sticking of the melted plastic to the element. The heating element should be kept very clean. If necessary, a clean cotton rag or paper towel can be used to wipe off any residue.
The control of the temperature of the heating element is very important when Corzan CPVC sheet is butt welded. Butt welding of Corzan CPVC sheet should be performed in an area free of drafts in order to maintain the best temperature control possible.
The changeover time, during which the element is removed and the two pieces of heated plastic are pressed together to form the weld, should be as short as possible. Ideally, the changeover time should be no more than three seconds.
Material Preparation
The edges of the pieces of material to be welded should be as square as possible so that they will contact the heating element and each other evenly. Cutting debris and any oil or dirt should be removed from the welding area. The pieces to be welded should be clean and dry. Solvents should not be used to clean the surfaces to be welded.
The Welding Process
The heating element should be set at the desired welding temperature. The optimum temperature for butt welding Corzan CPVC sheet is typically 437-446°F (225-230°C). With a microprocessor controlled machine, only the sheet thickness and length, as well as the melting/welding pressures have to be programmed; the machine will then make the necessary calculations and perform the necessary machine settings with respect to time and pressure. With a non-microprocessor controlled machine, the operator has to calculate the welding surface, then multiply the cross section with the optimum melting/fusing pressure and set the machine gauges accordingly. Here, as well, temperature and times have to be manually adjusted. Once the machine is set up, the sheets are inserted on either side of the table tightly against the setting bar and clamped.
The heating element should be brought into position and the pieces of material should be pressed against the heating plate with the desired melting pressure. The purpose for the higher pressure melting time is to assure that the material makes solid contact with the heating element. Once a bead has formed along the entire weld area, the pressure should be dropped to a nominal heating pressure. This pressure should be sufficient to hold the pieces against the element, but prevent excessively large beads from forming. The goal is to heat up the fusion area without pushing molten material out of the weld zone. With microprocessor controlled machines, the melting time is preset and can be extended, stopped, or reprogrammed, depending on the accuracy of the cut. The better the cut, the shorter the melting time. The optimum heating pressure for butt welding Corzan CPVC is approximately 30 psi (20 N/cm2).
The time that the plastic should be held against the element under the heating pressure is dependent on the thickness of the sheet. Typical optimum heating times for CPVC sheet are shown below in Table 1:
| Table 1. Typical Optimum Heating Times for CPVC Sheet | |
| Thickness (in.) | Heating Time (sec.) |
| 3/16 | 75 |
| 1/4 | 90 |
| 3/8 | 120 |
| 1/2 | 150 |
When the heating time is complete, the element should be removed and the pieces brought together as quickly as possible. The optimum changeover time is less than three seconds. The pressure should then be brought up to the desired fusion pressure, which should be maintained for a period of time which is dependent on the thickness of the sheet. The optimum welding pressure for Corzan CPVC sheet is typically 95-100 psi (65-70 N/cm2). The optimum fusion times for CPVC sheet are given in Table 2.
| Table 2. Typical Optimum Fusion Times for CPVC Sheet | |
| Thickness (in.) | Welding Time (min.) |
| 3/16 | 5 |
| 1/4 | 6 |
| 3/8 | 9 |
| 1/2 | 11 |
Weld Factor
Corzan CPVC sheet, when properly butt welded, can be expected to perform to approximately 80% of its nominal tensile strength.
Welding Corzan Sheet and Pipe
It is possible to weld sheet and pipe together. It is important, however, to remember that the sheet and pipe will heat differently when welded at different speeds. As a result, it will appear that the adhesion to the pipe is not as good as it is to the sheet.
Recommendations to ensure a successful bond follow:
- Solvent wipe the surface to be welded prior to heating using acetone. This will help to etch the surface to be welded.
- Preheat the pipe surface to be welded, in addition to the tack welding to be performed.
- Use a thicker welding rod (i.e. 4 mm) so that the heating time is longer.
Dual Laminate: Reinforcing Corzan® Pipe with Fiberglass
In order to obtain the best adhesion when wrapping Corzan CPVC pipe with fiberglass, first rough up the surface of the pipe. Apply the appropriate bonding resin (this resin should be compatible with CPVC, as well as suitable for the intended end use application). After applying the resin, apply the first layer of fiberglass. Follow the glass layer with another coat of the bonding resin, then build up the glass and resin layers accordingly with the appropriate amount of each for the intended application.
Other Fabrication Reference Materials
AWS G1.10M:2001
Guide for the Evaluation of Hot Gas, Hot Gas Extrusion
and Heated Tool Butt Thermoplastic Welds
ASTM C 1147
Standard Practice for Determining the Short Term Tensile
Weld Strength of Chemical-Resistant Thermoplastics
DVS 2207-3
Hot-Gas welding of thermoplastics – Sheets and pipes
DVS 2207-3 Addendum
Hot-Gas welding of thermoplastics – Sheets and pipes
– Welding Parameters
DVS 2208-1
Welding of thermoplastics – Machines and devices
for the heated tool welding of pipes, pipeline components and sheets
