Basic principle of 1. electric fusion welding process
The first patent for electrofusion welding first appeared in 1954. The electrofusion welding of plastic pipes has been used in PE water supply and gas pipes.
The system is widely used. Up to now, the specification range of PE-RT II electrofusion fittings has also been developed to dn20 ~ dn355mm, and still
In constant development.
Compared with hot melt butt joint, the process of electrofusion connection is relatively simple (this process can be seen intuitively in Figure 1), mainly including
The following steps:
1) Check whether the specifications of the parts to be welded are correct and whether the machines and tools are in good condition;
2) According to the requirements, the welding parts of the pipe shall be fully scraped; Use a pen to mark the installation position of the pipe fittings (or should be inserted
depth);
3) Install the pipes and fittings as required and fix them with suitable clamps;
4) Connect the terminal with the welding machine;
5) Input the correct welding parameters to the welding machine and start welding (or scan the welding on the electrofusion pipe with the scanning gun of the electrofusion welding machine
bar code);
6) After the welding is completed, the observation hole can be seen protruding, and the fixture is kept in a fastened state for cooling.

II. Factors Affecting Electrofusion Welding and Precautions
1) Quality of electrofusion pipe fittings
The quality of electrofusion pipe fittings is the core, and the pipe fittings with qualified quality should be selected. A good electric fusion pipe fittings, reflected in the distribution of heating wire,
The design of the pitch of the heating wire, the stability of the resistance, the cloth wire process, the length of the cold zone, the length of the melting zone and so on.
2) Pollution or oxidation, surface treatment
In order to avoid contamination, try not to open the package before installing the pipe fittings on the pipe; if the ends of the pipe fittings are welded separately
It is best to open only one end of the bag at a time, and keep the bag to avoid internal contamination of the pipe fittings.
The surface scale shall be removed to the scraping depth specified in the connection procedure, usually 0.1-
About 0.3mm, and try to use suitable tools for continuous cutting. Otherwise, it is easy to leak water if the skin is not scraped.
Do not touch the surface after scraping. The welding surface must be dry.
3) Fit clearance, out-of-roundness, insertion depth, axial centering and positioning
Before assembly, check and confirm that the pipe end cut is perpendicular to the pipe axis, and remove burrs.
When assembling, ensure that the pipe is assembled in place to avoid empty burning or even causing fire.
Controls the ovality. The gap caused by ovality shall not exceed 2% of the outer diameter of the pipe on one side (the other side is tight); or
The peripheral clearance shall not exceed 1% of the outer diameter. Otherwise, check the size of the parts, or take the round treatment, with mechanical straightening or re-round or
A small section of straight pipe is thermally melted and butted at the end.
When welding large-diameter electrofusion pipe fittings, the pipe fittings must be supported and fixed, otherwise the pipe fittings will be heated and melted
Sagging due to gravity causes the upper half to squeeze out the melt, the lower half has too much gap and insufficient melt pressure.
4) Power
The input voltage of the welder must be stable.
If the power supply wiring exceeds 50m, the cross-sectional area of the conductor must be checked to meet the requirements; when it exceeds 100m, it is best to use a generator.
If the generator is used as a power source, its power size and working characteristics should be considered, which can be suitable for driving the inductive load.
5) Welding equipment
Electrofusion welding equipment shall comply with GB/T 20674.2-2006 [Part 2 of Polyethylene System Welding Equipment for Plastic Pipes and Fittings
Points: Electrofusion connection] (ISO 12176-2). Its routine maintenance is very important and should be carried out regularly.
Before welding, it is necessary to determine the matching between the power of the welding equipment and the welding energy required by the electrofusion pipe fittings to avoid "small horse-drawn cart" and the formation of false welding/virtual welding. Especially for large-diameter electrofusion fittings (e. g.> dn200), it is necessary to use high-power electrofusion welding.
Machine, on the contrary, small diameter specifications of electric fusion pipe fittings, can only use low-power electric fusion welding machine, otherwise it will be "big horse-drawn trolley", too
Heat, resulting in electric fusion pipe shotcrete, welding "run", and even dangerous.
Especially on the construction site, there are often different specifications of electric fusion pipe fittings, welding different specifications of pipe fittings must pay attention to electric fusion
The power of the welding machine must not be lazy, one welding machine cannot beat the world, and the same parameters cannot be welded to the end.
6) Welding voltage
When welding, it should be ensured that the output voltage of the electric fusion welding machine is accurate and stable at 39.5V ± 0.5V. The larger the specification of the electric fusion pipe, the greater the voltage
The higher the stability requirements.
7) Cooling process.
Cooling is to achieve sufficient strength of the joint. The welded assembly shall be kept in a clamped coaxial state during cooling. shall not
Take artificial forced cooling measures.
8) Construction environment and temperature
The key to electrofusion welding is the input and control of energy. When the ambient temperature changes, it is necessary to ensure that the welding energy can be
line adjustment or compensation. For example, change the input voltage or increase or decrease the welding time to meet the requirements of the extreme ambient temperature.
Therefore, in winter construction, it is necessary to use an electric fusion welding machine that can identify the ambient temperature and automatically compensate for the temperature. and electrofusion
When the welding machine is on the construction site, it should be placed for a period of time and welded after adapting to the ambient temperature. Otherwise, the welding machine will be welded.
No less than the real ambient temperature.
At the same time, should avoid strong direct sunlight caused by pipe (pipe) uneven temperature. In addition, windy, dusty, rainy and snowy days
The gas shall be shielded to prevent pollution.
When welding pipes with larger diameters, the distal nozzle of the pipe shall be sealed to avoid the formation of "through-the-hall wind" by the air flow ".
When connecting in the groove, there should be at least 150mm operation space around the joint to facilitate operation and reduce the possibility of pollution.
9) Other precautions
Welding is a skilled type of work, personnel must be certified, and should wear gloves, goggles operation.
When the welding is completed, at least one observation hole must emerge. If the change of the observation hole is abnormal, the welding should be checked.
process, and the welding equipment and welding process for inspection and troubleshooting.
In case of abnormal interruption during welding, check and eliminate the abnormality in time.
Note: The welding process shall be strictly followed during construction. Otherwise, the relevant welding voltage, welding time and other processes are not suitable.
The failure of the welded joint endangers the safety of the pipeline system.