German Welding Society

Welding of thermoplastics

Machines and devices for the heating element welding of pipes, parts of pipelines and plates

Guideline DVS 2208-1 (December, 1997)

Replacing the edition of July, 1983

This publication was compiled by a group of experienced specialists who collaborated in an honorary capacity, and is recommended for attention as an important source of knowledge. The user is always obliged to check how far the contents apply to his or her special case, and whether the version he or she is using is still current. Any liability on the part of the German Welding Society and of those taking part in the preparation work is excluded.

DVS, Technical Committee, Working Group on "Joining of Plastics"

4 Machines for heating element butt welding

The problem definitions in the various ranges of application for heating element butt-welding make diverse requirements for the respective welding machines.

4.1 Welding machines for pipes

Welding machines for pipes can be designed mainly for operation on site or in the workshop. Workshop machines normally enable a higher grade of mechanization due to the fixed installation.

4.1.1 Clamping devices

In order to avoid high local stress in the pipe and deformations, the clamping devices have to enclose the pipe surface for at least 80 %, parallel to the welding area. They have to be solid enough to keep the geometric circular form of the pipes. Even under highest working pressures they have to maintain their position to the guides.

The clamping devices have to be constructed in a way that no sharp-edged damages may occur on the pipe surface. It is recommended to put not more than 2 clamping shells into one another. If more shells are inserted, the manufacturer has to take this into consideration during construction.

For fittings, special clamping devices may be provided. If necessary, they have to be horizontally and vertically adjustable.

4.1.2 Guide elements and clamping devices

Guide elements and clamping devices together have to guarantee that in the test according to picture 1 in the respective working area of the machine, even with maximum working pressure and larger pipe diameter, that the values of table 2 (measured at cold joining areas) not be exceeded in case of bending or movement of the pipe at the most sensitive point

Table 2. Difference of the gap dimensions at joining pressure depending on the pipe diameter (intermediate values have to be interpolated).

Pipe diameter d [mm]	Difference of the gap dimensions ≈ 0.8 ‰ x d, min. 0.25 [mm]
£    315   315 ...   630   630 ...   800   800 ... 1000 1000 ... 1200 1200 ... 1600	0,25 0,25 ... 0,50 0,50 ... 0,65 0,65 ... 0,80 0,80 ... 0,95 0,95 ... 1,30

Measurement is taken when the pipes are clamped and planned in the machine. Adjusting of the pipes and planning has to be done carefully since there should be no (or only a small) gap between the pipe ends. The machine is opened and two facing clamp pieces are put on the pipe ends, turned by 90 degrees to the guide elements. The clamp pieces have to be produced with a precision in measuring thickness of ± 0.025 mm. Then the pipe ends are driven to each other until they are in contact. The difference of the gap dimension at Sp 1 and Sp 2 means inaccuracy of planning. After that, the pressure on the joining pressure of 0.15 N/mm² necessary for the clamped pipe is increased. The thus resulting difference of the gap dimensions on Sp 1 and Sp 2 must not exceed the value according to table 2 plus the formerly determined inaccuracy resulting from the planning process. A measuring block should be inserted for the determination of the gap dimensions according to picture 1.

1/2 turned by 90° to the guide elements, e.g. horizontally arranged clamping pieces with lower guide

b  breadth of the clamping pieces

Sp 1 upper check point

Sp 2 lower check point

d pipe outside diameter

s pipe wall thickness

Picture 1. Measurement of the gap dimension.

Guide elements must be protected against corrosion at the sliding surface, e.g. by means of hard chrome plating.

4.1.3 Heating elements

The used heating element should be plane-parallel within the effective area. For allowed variations see table 3.

Table 3.  Allowed variation from plane-parallelism.

Diameter or length of edge [mm]	Allowed variation from plane-parallelism [mm]
£ 250   250 ...   500   500 ...   800   800 ... 1200 1200 ... 1600	£ 0,2 £ 0,4 £ 0,8 £ 1,2 £ 2,0

Measurement is taken at 23 ± 2° C and a singular short-timed heat up of the heating element up to maximum operation temperature of 260° C. For operation in the workshop, the heating element is in general firmly mounted on the machine (floating suspension). In case the heating element is not fixed, the respective devices for driving in and fixation must be provided (e.g. grips, hooks, loops).

In case the heating element needs to be separated mechanically from the joining surfaces due to the dimension and quality, the respective devices are to be provided.

The current supply in the area of the heating element should be protected against thermal damages, and the effective area of the heating element against mechanical damage. Heating elements which are not fixed to the machine should be put into a protective device during non-use.

4.1.4 Device for the preparation of the weld seam

For the plane-parallel cutting of the joining surfaces of the clamped pipes, a respective cutting device has to be provided. The cutting device must bring the chips to the outside diameter of the pipe. The operator must be able to see the resulting chips and to finish the planning process when plane-parallelism is reached. The maximum allowed variation from the plane-parallelism of the joining surfaces can be seen in table 4.  

 

Table 4. Allowed variation from plane-parallelism.

Pipe diameter [mm]	Allowed variation from plane-parallelism [mm]
£ 355     400...  < 630     630...  < 800     800...< 1000 >1000	£ 0,5 £ 1,0 £ 1,3 £ 1,5 £ 2,0

4.1.5 Controlling and regulating devices for force, time, temperature and path

Controls and regulations have to be adjustable and guarantee a consistent welding.

4.1.5.1 Control of force

During heating element butt welding, the forces for adjusting, heating up and joining from the smallest up to the largest welding diameter must be continuously adjustable, with respect to the different specific forces for the weldable plastics. Friction losses in drive or guide elements have to be detected in the right way, e.g. by means of load cells. The operation forces must be increased by this moving force. The respective actual force at the joining surfaces has to be indicated on an instrument which is large enough to be easily readable. The manufacturer has to indicate the adjustable minimum and maximum force of the machine. The force range of the machine must be adjusted with a pressure reserve of ³ 20%, which is necessary for a maximum welding diameter and for the surmounting of the frictional forces.

The heating up pressure has to be controllable to zero after adjusting. Test e.g. by means of a pressure pickup. Beginning with zero, the joining force is increased by degrees (lineally) regularly up to the final value as soon as the joining surfaces have touched each other. This final value has to be kept during cool-down time.

4.1.5.2 Control of time

An exact time control is the guarantee for a correctly combined affect on the machine functions and forces, especially clear with the linear increase of the adjusting and the joining force.

The adjusting time begins when the joining surfaces touch the heating element and ends when full contact is made.

The heating up time begins after the adjusting, when the heating element is lying  completely on the joining surfaces and  ends with the separation of the heating element.

The change-over time begins with the separation of the heating element from the joining surfaces and ends with the contact of these surfaces. It results from the sum of various machine functions and should be as short as possible.

The joining time begins with the contact of the joining surfaces and ends when the welding joint has cooled down. This includes the build-up time for the joining pressure, increasing from zero linear to the final value.

4.1.5.3 Control of path

The machine control should detect and work with the position of the guide elements and thus of the work pieces to each other. Thus time-optimized movements are started. The construction of the machine has to realize the required change-over times.

4.1.6 Construction of the machine and safety in operation

Workshop machines have to fulfill the following requirements:

- solid design

- universal basic construction (assistant tools and clamping devices swiveling or moving)

- quick clamping device

- grade of mechanization as high as possible

- indication of the pressure intensification (hydraulic pressure / welding force)

Besides a light construction, machines for the site should fulfill the following requirements:

- solid design

- devices for transport, bringing into and out of pipe ditches (e.g. grips)

- in case of larger machines, a running gear with locking device is recommendable

- indication of the pressure intensification (hydraulic pressure / welding force)

For the construction and operation of the machines, the respective safety rules have to be observed. See also section 7.

4.2 Welding machines for plates

The welding machines are mainly designed for stationary use for the welding of plates or work pieces formed out of plates with a straight-lined seam. For the production of consistent weld seams of high quality a high grade of automation is necessary.

4.2.1 Clamping devices

The clamping force applied to the work piece by means of the clamping device is the product of machine force and friction loss between clamping shells and surface of the work piece. The necessary clamping force may be applied either only by means of the clamping device or by means of a combination of clamping device and additional clamping device. The clamped work pieces may not make any relative movements towards the clamping elements, i.e. slide through.

Normally, clamping devices consist of supporting tables and clamping beams. They have to enable an exact clamping of the work pieces. The mismatch of the table supports one to the other must not exceed 0.2 mm at the plate support. Take special care that the supporting tables are guided exactly and that a bending or tipping is avoided by means of right strengthenings and bearings.

The selected form of the section for the clamping devices is in direct relation to the smallest hollow section in round or rectangular type which can be produced, and of which the dimensions have to be indicated by the manufacturer of the machine.

4.2.2 Guide elements

The guide elements have to guarantee that the clamped parts are in alignment and that they will not change their position under maximum working force. When welding short pieces, they must lay in the center of the table. The working instructions must indicate this necessity.

4.2.3 Heating elements

The used heating element must be plane-parallel within its effective area over the height and the length. Allowed variations are shown in table 5, without any distinction of length.

Table 5. Allowed variation of plane-parallelity.

Height of effective area [mm]	Allowed variation of plane-parallelity [mm]
£ 30 >30	£ 0,3 £ 0,5

4.2.4 Controlling and regulating devices

The automation of the welding process through control or regulation of force, time, path and temperature is advantageous. The controls must be adjustable and guarantee a consistent welding. The adjustable minimum and maximum forces must be indicated by the manufacturer of the machine.

During the total welding process, the measured process data of heating element temperature, table force and table path have to be registered and observed as far as possible. The control has to be constructed in a way that operating errors are avoided or indicated to the operator.

4.2.5 Construction of the machine and safety in operation

The machines have to fulfill the following requirements:

- solid construction

- frame and table construction resistant to torsion

- clamping forces must be worked into the frame of the machine

- avoid bending of the tables by means of several rolling bearings

- table movement with less friction.

For a better transport of the work pieces, solid table extensions should be provided which also enable a later attaching of work piece holders.

Quick unlocking of the clamping beams will make the safe and unproblematic discharging of welded hollow sections much easier.

The heating element should be thermic insulated during the joining process so that neither radiant nor convective heat can influence the lower side of the work piece thus causing different speeds of cooling. The thermic insulation of the heating element is always advantageous for the period when the heating element is not in working position. Both measures are energy saving.

For the quality assurance of the welding process, the permanent print out diagrammatically of important data of heating element temperature, table force and table stroke should be possible. The time for change-over and the force ramps of adjusting and joining should be lengthened. Complete the weld log by the name of the welder, seam number, site or order number etc. as well as by serial number of the machine and date including time.

In case the power supply is interrupted, no uncontrolled movements may be initiated.