Welding is the most crucial process during fabrication of pvc windows and doors and its quality directly influences the performance of finished windows.
I, Welding equipment
The welding equipment includes single-point welding machine, four-corner welding machine, multi-head welding, multi-layer welding machine, co-extrusion welding machine, seamless welding machine and Y-shape welding machine and etc. The single-point welding machine could make 0¡ã-180¡ã welding. The four-corner welding machine is used for welding of rectangle frames and sashes. The multi-head welding machine is also used for welding of mullion and beams; The joint-extrusion welding machine have two clamps at two heads moving jointly to increase the corner strength. The seamless welding machine is used for welding of co-extruded profiles. Y-shape welding machine welds the mullion and frame that have fixed parts in 60 series outward casement windows and doors.
The option of welding machines is determined by the assembly process plan of pvc windows and doors. It is better to choose the welding machines of reliable quality, correct machining dimension and process parameter, convenient to operate and easy to adjust and maintain.

II, Welding sequence

The selection of welding sequence relates directly to the welding quality and production efficiency of frames and sashes of windows and doors. Before welding, the welding sequence is determined based on window shape. The welding sequence is determined mainly by the welding machine¡¯s functions. The frame and sash without a lattice could be welded by four-fillet welding machine at one time. As for the latticed window with a mullion, the mullion should be welded at first and then the frame. Pay attention to correct welding sequence.

¢ó. Welding process parameters
The welding process parameters directly affect the quality of pvc windows and doors, especially the corner strength of finished windows. Thus, it is quite important to stipulate and control strictly the welding process parameters, which mainly include:
Welding temperature:240¡ãc-270¡ãc
Clamping pressure:0.4Mpa-0.6Mpa
Melting pressure:0.3Mpa-0,4Mpa
Butt joint pressure:0.4Mpa-0,5Mpa
Melting time:20s-30s
Butt joint time:25s-35s
Above are basic process parameters and may very for different equipment, conditions and profiles¡¯s specifications. The optimum welding parameters could be found out through test (calibrate different parameter instruments before welding to ensure compliance of the displayed and actual value). While:

Time, temperature and pressure are three important parameters that affect the welding quality, which are adjusted flexibly in the actual production based on the test result of corner strength so that the three parameters reach the optimum matching and welding quality is guaranteed.

¢ô. Preparation before welding
1, Check for the welding machine¡¯s normal power supply and air pressure and that the value of each pressure gauge is within the permissible working scope. Switch on the heating pad 20-30 minutes in advance for preheating after the machine is on.
2, The atmospheric temperature should be above 18¡ãC when welding and the profiles should be left at room temperature for 24 hours before welding so that the temperature between the profile and heating pad decrease by gradients and corner strength is increased.
3. Select and adjust the baffle and adjust welding time, temperature and pressure based on the frame and sash.
4, The weldment should be marked and stored as regulated. Pay attention to position of mounting hole, hinge, handle and drain hole to prevent wrong welding.
5, Don¡¯t weld the sealing strip, rubber strips and protection film into the welding seam. Please refer to the sealing strip part for preparation of sealing strips and rubber strips. Shear off the welding allowance on the protection film at the welded part along the welding seam and do not melt the protection film into the welding seam.

¢õ. Welding
1, Ensure that the visible surface of the weldment crosses with the welding plate perpendicularly when welding with an allowance of ¡À0.5¡ãC. Meanwhile, keep the welding surface clean to prevent dark lines at the corner.
2, Guarantee strictly the angle between the front and back positioning baffle and the positioning plate falls between 45¡ã¡À0.5¡ã.
3, The frame and sash should meet the dimensional requirements after welding. During batch production, the first piece is inspected, followed by random inspection. All are inspected during small batch and single-piece production.

The difference of diagonal dimension of the door and window frame and sash should not be greater than 3.0mm, which could be adjusted in accordance with the actual condition during fabrication.
4, Two weldments should have a degree of 90¡ã¡À0.5¡ã between them when welding the mullion to ensure no intended move of the steel reinforcement after welding.
5, After welding, the levelness of adjacent structural members made of same kind of profile is ¡Ü0.5mm. While, the levelness of adjacent structural members made of different kinds of profiles is ¡Ü0.8mm.
6, The main profile¡¯s corner strength is guaranteed after welding (Please refer to the standard for requirements on the corner strength). Inspect regularly the corner strength.

¢ö. Welding model
Fabricate necessary welding models based on the shape of the weldment when welding. The welding model is 0.5mm-1.0mm lower than the weldment. Since the weldment is a profile with a sharp angle of 45¡ã, the sharp angle will deform under the welding pressure during the welding process, if without a welding model. Its welding beading is featured by greater root width and inhomogeneous distribution of stress on unit area of the welding surface, which affects the welding quality (as shown by figure1-11). A model must be used when welding.

¢÷. Attentions
1, Check regularly the welding machine whether the positing screw is loose or changes to guarantee a correct reference for process parameters.
2, The welding cloth must adhere closely with the heating pad, if used with the welding machine.
3, The welding cloth¡¯s quality should be guaranteed (welding cloth specification: PTEF)if used with the welding machine. Keep the welding cloth clean. It is recommended to clean it after welding for the times and change it after 1000 times. Readjust the temperature after changing.
4, The welding machine should be kept far away from the vent in order not to affect the welding plate¡¯s thermal stability.
5, Leave the welding plate for a while and restore its temperature to the setup value each time after welding.
6, Try to keep the same operator for the welding machine in order to improve the welding quality

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