PE pipe has the characteristics of good flexibility, strong corrosion resistance, light weight, and excellent impact resistance. The connection can be connected by hot melt butt and electrofusion, so that the pipe and pipe fittings are melted together, the system is safe and reliable, and the construction cost is low. , Has developed rapidly in engineering applications.
In the course of development, Shandong Luyu has accumulated rich construction experience. Today, I would like to introduce you to the PE pipe hot-melt welding process.
1. The principle of hot melt butt
1.1 What is hot melt butt
is a method in which the ends of plastic pipes are heated and melted by a heating plate, butted and fused, and then cooled and fixed to connect them together.
1.2 There are five stages in the butt welding
(1) Preheating stage: the preheating and crimping stage of the electric heating plate, where a certain pressure is applied to the ports of the two pipes (pieces) to melt and crimp.
(2) Heat absorption stage: apply a small pressure and a certain period of time to diffuse the heat in the pipe (piece) to be connected to achieve uniform heat absorption.
(3) Conversion phase: the heating plate slamming phase. The shorter the time to connect the pipes (pieces), the better.
(4) Welding stage: the melting ends of the two pipes (pieces) are fused together and a certain pressure is applied.
(5) Cooling stage: natural cooling of fusion, pay attention to no tension and mechanical stress.
2. Welding methods and steps
2.1 Preparation before welding
Check whether the power of the hot melt machine is connected properly, whether the milling cutter can cut, whether the heating plate is heated normally, and whether the oil cylinder of the frame is running.
2.2 welding
(1) Place the pipe or pipe fitting in a flat position and place it on the butt joint machine, leaving a cutting allowance of 10-20mm.
(2) Clamping: Choose a suitable slip fixture according to the pipes and fittings to be welded, clamp the pipes, and prepare for cutting.
(3) Cutting: Cutting the impurity and oxide layer of the welded pipe section and the end face of the pipe fitting to ensure that the two butt joint ends are smooth, clean and free of impurities.
(4) Decontamination: Dip cotton yarn with alcohol to clean and wipe the head of the pipe to remove dust, mud, etc.
(5) Alignment: The end faces of the two welded pipe sections should be completely aligned, the smaller the wrong side, the better, and the wrong side should not exceed 10% of the wall thickness. Otherwise, the docking quality will be affected.
(6) Heating: The temperature of the butt joint is generally between 210-230℃. The heating time of the heating plate is different in winter and summer. The melting length of both ends is preferably 1-2mm.
(7) Switching: Remove the heating plate, quickly let the two hot melt end faces stick and pressurize. In order to ensure the quality of the melted joint, the shorter the switching period, the better.
(8) Melting butt joint: It is the key to welding. The butt joint process should always be carried out under melting pressure. The width of the crimping should be 2-4mm; the pressure should be appropriate. Too much pressure will cause all the molten material to be squeezed away, and instead connect Not fast.
(9) Cooling: Keep the butt pressure constant and let the interface cool down slowly. The length of the cooling time depends on the hand feeling the curling hard and not feeling hot.
(10) Docking is complete: After cooling down, release the slips, remove the docking machine, and prepare for the next interface connection.
2.3 Three important parameters of welding process: temperature, pressure, time
2.3.1 Determination of temperature
The best welding temperature for butt welding of polyethylene pipes is 200～230℃, which is generally determined by the manufacturer to be 210±10℃; it is the processing temperature of polyethylene materials, which is above the transition temperature of the material’s viscous fluid state, and only under this condition , Polyethylene produces melt flow, polymer macromolecules can diffuse each other to form entanglement, and obtain the maximum strength and high-quality welding results; practice has proved that the temperature is lower than 180 ℃, even if the heating time is long, it can not achieve good quality. Welding results. If the temperature is too high, it is possible to activate the C bond in the molecular chain to react with oxygen, degrading the material, and the polyethylene material will be oxidized and destroyed. Volatile substances and gases are precipitated, the structure of the material changes, unsaturated hydrocarbons are generated, and impurities appear, which reduces the quality of welding.
2.3.2 Time determination
(1) Determination of heating time: 10 x wall thickness (mm) seconds after the welding end surface is flat.
The length of the heating time determines the quality of the welding; whether the temperature can be evenly transmitted to the welding surface and a certain depth, and the optimal welding temperature is maintained during the conversion stage. The best time for the end of the tube to melt is increased as the area to be heated increases, and more importantly, the energy of convection and radiation propagation will decrease as the thickness of the tube wall increases. The unevenness of the pipe end surface causes uneven heat transfer, traps air, produces pores, and ultimately affects the welding quality. Therefore, it needs to be closely matched with the pressure. A certain pressure is applied while heating to smooth the welding surface, promote plasticization, and form The ideal welding surface conducts heat transfer, and then reduces the pressure to absorb heat.
(2) Determination of the switching time: as short as possible within 10 seconds, the end face is cooled very fast, and the slow butt speed directly affects the welding quality.
(3) Determination of cooling time: 1.15～1.33×wall thickness (mm) minutes. The thermal conductivity of polymer materials is poor, only a few tenths of that of metals, and the cooling rate is correspondingly slow. Crystallization and shrinkage are required during the cooling time, so sufficient time is required to drop to the crystallization temperature for sufficient grain growth. , Eliminate internal stress, cool under a certain pressure, and avoid shrinkage holes on the welding end surface.
(4) Determination of pressure The welding pressure and cooling pressure are based on the cross-sectional area of the welding surface × 0.15N/mm2; at a temperature of 210 ± 10 ℃, the welding time and pressure can be determined according to the standard of the German Welding Association DVS2207-95 .
