In Spin Welding, the heat required for plasticizing the material is produced by friction in the region of contact between both moulds. The component to weld is put into a rotation movement, whereas the second component is kept stationary or blocked against rotation.

The rotating component is rotated by suitable design measures, such as attaching fins, nose or ribs, through the rotation driver frictionally in motion. During the welding process, an axial pressure is exerted on the components to be joined. The interfacial friction and the resulting shear heating cause the plastic to melt in the region of the joining surface.

This technique can only be used for molded components, which have a rotationally symmetrical joining surface. The geometric design of the joint surfaces should be designed in steps, or have a wedge-shaped design. The welding time is about a second and thus in a very economical range.

Picture 1: Process of Spin Welding

Spin welding is used mainly for welding of pipes, fillings and pipe-joints and for welding of injection and blow molded applications.

The following explanations refer exclusively to the injection and blow molded components.

The process usually comes into use when ultrasonic welding techniques cannot reliably fulfill the requirements. This can be the case due to the shape or to the material of the components to be joined.

The weldable material range looks similar to the ultrasonic welding one. In addition, plastics that can cause difficulties in ultrasonic welding due to unfavorable damping behavior (such as polyamides with partially high glass fiber content and reinforced or non-reinforced polyolefin) are well suited for spin welding. The welded joint is characterized by high mechanical strength and tightness.

To achieve the optimum weld, the choice or design of the weld geometry is a decisive factor. Further-more, the quality of the weld joint is affected by the sufficient flow of the melt. Therefore, the welding machine parameters must be determined empirically and be adjusted.
The welding process is generally divided into the following phases:

• Start
• Rubbing
• Braking
• Reprinting

The friction phase is divided into a pre-friction and a main friction phase.
In the pre-friction phase, the alignment of the joint surfaces occurs. In the ensuing main friction phase, temperature differences are compensated in the melt layer.

After the expiry of the main friction phase and the standstill of the rotation driver, the holding time under pressure begins, with the holding pressure differing from the friction pressure, for example as an increased emphasis or compaction pressure, depending on the material and the joining seam design.

If the parts must be assembled in a specific angular position to each other, this process takes place through the exact angular positioning and by stopping the driver.

Picture 3: Standard weldig program – Rotation speed and pressure against time

Driving the rotation driver is usually assured by air and electric motor drives. In practice, the electric motor has prevailed with respect to the air-driven motor.

Through the use of electrically driven rotation drivers, the quality or the strength of the welded joint debts is generally better met.

Air-driven high-speed motors are suitable for small components, such as the ventilation nozzles

The engine is combined with a flywheel, which stores the required kinetic energy. As the deceleration of the flywheel is very difficult to control, a reproducible positioning of the joining part is usually not possible. Modern techniques are therefore based on servo motors, which ensure a reproducible positioning accuracy of +/- 0.5 °.

The main structural criteria of a spin welding machine are:

• Stability of the machine structure
• A performance-driving motor which is very efficient in relation to the welding component
• Precise, unbalance free connection of motor, rotation unit and driver

The most important configuration parameters of friction welding are:

• Number of revolutions
• Friction time
• Positioning stop
• Variable pressure levels for the friction time
• Different pressure levels for the cooling time
• Mode (Path-dependent trigger system, pressure / force-responsive trigger system)

In general, a path-dependent trigger system is used as operating mode. The rotation movement is realized by a trigger point controlled by a welding path measuring system. The end of the rotation movement can be achieved by the welding depth or the desired number of rotation revolutions. With both modes, a constant welding is guaranteed.

Seam design for Spin Welding

• Seam design 1
  Plasticizing is performed on a rectangular welding step, wherein the melt formed can be compacted after reaching the welding depth.

• Seam design 2
  Here, plasticizing is assured via an inclined stage. A compression at the end of the welding path is not possible.

• Seam design 3
  Again, welding is performed on a rectangular stage. The melt formed is condensed at the bottom of the welding; the U-shape largely prevents material outlet both internally and externally

Wedge-shaped welded joints

• Model 4
  It shows the geometry of the weld when welding a lid on an engine intake system. The high require-ments that such a welding has regarding strength and tightness are met by the wedge-shaped design, like the two chambers for material compaction, as well as the cover of the outer and inner diameter to prevent material spills.

• Model 5
  Very high weld strength is achieved through the wedge-shaped design, and again, the melt is addition-ally compressed by means of the two collection chambers.

Generally speaking, we can point out that the selection of welding geometry already decides on the achievement of the stated requirements - strength, leaks, etc.

With our many years of experience, we remain at your entire disposal for the choice of weld geometry, to help you achieve maximum results.