What is Spin Welding?
The process of spin welding uses heat generated by rotational friction at the joint line to weld thermoplastic parts with rotationally symmetric joints. The spin welding machine applies pressure axially while rotating one part against its stationary mate, and the resulting friction generates heat that melts the parts together.
Advantages of the spin welding process include high quality permanent joints, hermetic seals, lower equipment costs, ease of assembly, energy efficient operation, no ventilation required, immediate handling, entrapment of other parts, far-field welding capability and no additional material requirements.
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All electric Dual Servo Spin welder. This machine welds round thermal plastic parts. The weld joints are strong and can be hermetic.
Materials that can be friction (i.e. vibration) welded can also be joined with by spin welding. The semicrystalline thermoplastics are more readily joined using spin welding than ultrasonics. Using compatible polymers, spin welding is capable of making reliable hermetic seals. Far-field welding is easier with spin welding than with ultrasonic welding. Additional parts can be entrapped between the upper and lower pieces during spin welding.
Joining of dissimilar polymers is possible using the spin weld process although it generally produces lower strength weld joints. By designing the weld joint with an undercut, the polymer with the lower melting temperature will flow into the undercut, creating a mechanical union.
Material filler and surface contaminants (e.g. mold release agent) are two factors that will affect consistency and weld repeatability. Spin welding is more tolerant of contaminants than ultrasonic welding. Spin welding is also less affected by hygroscopic polymers, although they may still require special handling for critical applications. The moisture content can lead to bubble formation in the joint resulting in decreased weld strength.
Joint Design Considerations
- Part joint must be on a circular axis.
- Joint design should allow for adequate collapse distance.
- Final orientation of part if necessary.
- If at all possible the upper part would be designed
for use with
drive features. Place for upper tool to grasp your part.
- The joint area must be designed so that there is no other part contact interference.
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There are several primary process control parameters that affect weld quality. They are the surface velocity of the weld joint, press (axial) speed, weld depth, and hold distance and time.
For a fixed rotational spin speed (RPM), linear surface speed increases with weld joint diameter. For a fixed weld joint diameter, surface speed increases with motor RPM. Smaller diameter parts therefore usually require more RPM than larger parts of the same material. If the surface speed is too low, an adequate amount of heat will not be generated to cause sufficient melting. If the speed is too high, excessive heat in the joint could result in material degradation or reduction in viscosity leading to material flow away from the joint.
The selection of the proper surface speed depends to a large degree on the material and joint geometry of the parts being welded. Some materials, such as PVC, can be readily welded for a wide range of values, while others require a narrow range. Commonly quoted values in the literature recommend using +/- 2 m/sec (79 in. /sec.) as an initial testing value. This can be adjusted up or down depending on the results and part configuration.
Press (Axial) Speed
The press speed affects the amount of contact pressure between the parts being welded, which is required to generate frictional heat. The larger the speed, the larger the rate of heat rise. In combination with the surface speed, press speed must be high enough to cause melting at the interface as opposed to grinding, but not too high as to damage the parts. Excessive press speed can also lead to stalling of the spin motor as more torque is required to maintain constant spin speed.
The Dual Servo Spin Welder is capable of operating in two different press speed modes. With the Constant Torque Option (in SETUP > WELD tab) disabled, the press speed is constant during the weld. With the Constant Torque Option enabled, the press speed is variable so as to keep the spin torque constant (see Chapter 5). The latter case resembles the operation of a pneumatically driven press, where the press speed is the result of the melt rate under given air pressure and spin speed conditions.
Selection of the optimum press speed depends on the material and joint geometry of the parts, as well as the surface speed. A range for initial experimentation is 0.5 to 2.0 mm/s.
The determination of the proper weld depth is highly dependent on the application. The weld joint is typically designed for a specific weld penetration. Ideally, the weld is sufficiently deep to produce a strong, hermetically sealed assembly. An excessive depth may lead to the formation of flash (material that is ejected from the joint area during the weld and adheres to the assembly), the drawing out of reinforcing filler material and realignment of the interchain bonds in the weld plane resulting in a weak axial weld joint, and possibly part distortion.
Since weld depth affects the joint strength and the amount of flash generated, it is important to design the weld joint properly to meet both requirements simultaneously. The incorporation of flash trap features is recommended to produce acceptable appearance without compromising strength.
During the hold phase, vertical press travel initially brings the molten parts closer together (dynamic hold) and then allows the molten material to solidify (static hold). Amourphous plastics will normally take longer to solidify than semicrystalline plastics. The dynamic hold distance is typically a small value compared to the weld distance. An approximate staring point for initial application setup is 10% of weld distance. The static hold time can vary depending on the size of the part, but is usually in the 1-3 second range.
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"Melt-Match" mode, in which the press vertical speed is continuously adjusted to match the rate of plastic melt at the joint. This is achieved by measuring the spin torque and changing the vertical speed on-the-fly based on this measurement. The vertical speed is inversely proportional to the spin torque: the lower the spin torque, the higher the vertical speed, and vice versa.
The relationship between the spin torque and vertical speed is illustrated in Figure 5-20. The welder will adjust vertical speed for a measured spin torque along the lines shown. The Torque Target is the desired spin torque, which is entered into the Torque (% of max.) field on the screen. The Max Torque value is 5% larger than the Target Torque. If the measured torque exceeds the Max Torque, the vertical speed will be 0 until the torque drops below the maximum. The Max Speed is the maximum allowable vertical speed, which will occur if the measured torque is 0. This value is entered in the VERT. Max (mm/s) field on the Weld Parameters screen (in the WELD tab).. The actual spin torque profile achieved during the weld will depend on the Torque (% of max.) and the VERT Max (mm/s) settings for a particular application. For example, if the actual spin torque is consistently below the specified target, the VERT. Max (mm/s) will need to be increased to cause the welder to move down faster, causing a rise in the spin torque.
Our knowledgeable applications staff regularly address issues such as: Joint design recommendations, material compatibility, detailed application feasibility report, and troubleshooting expertise. Click here for assistance with your spin welding application.
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