Heat Fusion and Joining

Butt Fusion

Heater Surface Temperature: Minimum 400°F – Maximum 450°F (204 – 232°C)

Heating tool surfaces must be to temperature before you begin. All points on both heating tool surfaces where the heating tool surfaces will contact the pipe or fitting ends must be within the prescribed minimum and maximum temperatures and the maximum temperature difference between any two points on the heating tool fusion surfaces must not exceed 20°F (11°C) for equipment for pipe smaller than 18” diameter, or 35°F (19°C) for larger equipment. Heating tool surfaces must be clean.

Interface pressure : Minimum 60 psi – Maximum 90 psi (414 – 621 kPa; 4.16 – 6.21 bar) Interface pressure is used to calculate a fusion joining pressure value for hydraulic butt fusion machines or manual machines equipped with force reading capability. The interface pressure is constant for all pipe sizes and all butt fusion machines. However, fusion joining pressure settings are calculated for each butt fusion machine, which are dependent upon the OD and DR (Dimension Ratio).

For hydraulic machines, the interface pressure, the fusion surface area, the machine’s effective piston area and frictional resistance, and if necessary, the pressure needed to overcome external drag resistance, are used to calculate hydraulic fusion joining pressure gauge settings (refer to Appendix A). The equipment manufacturer’s instructions are used to calculate this value. The proper amount of force should be verified by visual inspection of the joint.

NOTE: The interface pressure and the hydraulic gauge pressure are not the same. For manual machines without force reading capability, the correct fusion joining force is the force required to roll the melt beads over to the pipe surface during joining.

Procedure

1. Secure
Clean the inside and outside of the component, pipe or fitting ends by wiping with a clean, dry, lint-free cloth or paper towel. Remove all foreign matter. Align the components of the machine, place them in the clamps, and then close the clamps. Do not force pipes into alignment against open fusion clamps. Component ends should protrude past the clamps enough so that facing will be complete. Bring the ends together and check high-low alignment. Adjust alignment as necessary by tightening the high side down.

2. Face
Place the facing tool between the component ends, and face them to establish smooth, clean, parallel mating surfaces. Complete facing produces continuous circumferential shavings from both ends. Face until there is minimal distance between the fixed and moveable clamps. If the machine is equipped with facing stops, face down to the stops. Stop the facer before moving the pipe ends away from the facer. Remove the facing tool, and clear all shavings and pipe chips from the component ends. Do not touch the component ends with your hands after facing. : Minimum 60 psi – Maximum 90 psi (414 – 621 kPa; 4.16 – 6.21 bar)

3. Align
Bring the component ends together, check alignment and check for slippage against fusion pressure. Look for complete contact all around both ends with no detectable gaps, and outside diameters in high-low alignment. If necessary, adjust the high side by tightening the high side clamp. Do not loosen the low side clamp because components may slip during fusion. Re-face if high-low alignment is adjusted.

4. Melt
Verify that the contact surface of the heating tool is maintaining the correct temperature. Place the heating tool between the component ends, and move the ends against the heating tool. Bring the component ends together under pressure to ensure full contact. The initial contact pressure should be held very briefly and released without breaking contact. Pressure should be reduced when evidence of melt appears on the circumference of the pipe. Hold the ends against the heating tool without force (drag force may be necessary to ensure contact). Beads of melted polyethylene will form against the heating tool at the component ends. When the proper melt bead size is formed, quickly separate the ends, and remove the heating tool. The proper bead size is dependent upon the size of the component. Approximate values are shown in Table I.

During heating, the melt bead will expand out flush to the heating tool surface, or may curl slightly away from the surface. If the melt bead curls significantly away from the heating tool surface, unacceptable pressure during heating may have occurred.

5. Join
Immediately after the heating tool is removed, quickly inspect the melted ends, which should be flat, smooth and completely melted. If the melt surfaces are acceptable, immediately and in a continuous motion, bring the ends together and apply the correct joining force (or fusion pressure). The correct fusion pressure will form a double bead that is rolled over to the surface on both ends.

A concave melt surface is unacceptable; it indicates pressure during heating. Do not continue. Allow the component ends to cool and start over with Step 1.

6. Hold
Hold joining force against the ends until the joint is cool. The joint is cool enough for gentle handling when the double bead is cool to the touch. Cool for about 30 – 90 seconds per inch of pipe diameter. Do not try to decrease the cooling time by applying water, wet cloths or the like.

• Avoid pulling, installation, pressure testing and rough handling for at least an additional 30 minutes.
• Heavier wall thickness pipes require longer cooling times.

7. Inspect
On both sides, the double bead should be rolled over to the surface, and be uniformly rounded and consistent in size all around the joint.

1. The gap (A) between the two single beads must not be below the fusion surface throughout the entire circumference of the butt joint.
2. The displacement (V) between the fused ends must not exceed 10% of the pipe/fitting minimum wall thickness.
3. Refer to Table II for general guidelines for bead width, B, for each respective wall thickness.
4. The size differential (Smax – Smin) between two single beads shall not exceed X% of the actual bead width (B).

Where
X = Percent difference of bead width, %
      Pipe to pipe, maximum X = 10%
      Pipe to fitting, maximum X = 20%
      Fitting to fitting, maximum X = 20%
S = Smax - Smin, inches
B = Width of bead, inches

NOTE: When butt fusing to molded fittings, the fitting side bead may have an irregular appearance. This is acceptable provided the pipe side bead is correct.

Qualification

1. Prepare a sample joint. Sample lengths should be at least 6” or 15 times the minimum wall thickness (see Figure I).
2. Observe the fusion process and verify the recommended procedure for butt fusion is being followed.
3. Visually inspect the sample joint for quality.
4. Allow the joint to cool completely (minimum °F one hour).
5. Prepare the sample as shown in Figure I. The sample should be cut lengthwise into at least three longitudinal straps with a minimum of 1” or 1.5 times the wall thickness in width.
6. Visually inspect the cut joint for any indications of voids, gaps, misalignment or surfaces that have not been properly bonded.
7. Bend each sample at the weld with the inside of the pipe facing out until the ends touch. The inside bend radius should be less than the minimum wall thickness of the pipe. In order to successfully complete the bend back, a vise may be needed. For thick wall pipe, a hydraulic assist may be required.
8. The sample must be free of cracks and separations within the weld location. If failure does occur at the weld in any of the samples, then the fusion procedure should be reviewed and corrected. After correction, another sample weld should be made per the new procedure and re-tested.

See examples of acceptable butt fusion.
See examples of unacceptable butt fusion.

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