Welding of radially symmetrical parts
The importance of laser plastic welding is constantly increasing, especially in the automotive supplier, medical technology and household appliance industries. This is not least due to the many technical advantages of laser welding. The automotive supplier industry in particular is characterised by very high innovation and cost pressure. Automotive suppliers are therefore constantly on the lookout for innovative technologies with which they can offer their customers better quality products at lower costs. In order to achieve these goals, all production processes must be permanently analyzed and optimized. Laser plastic welding offers corresponding optimization potentials due to its many advantages. This process is characterized by an easily controllable, local and non-contact heat input, a minimal heat-affected zone and low mechanical stress on the joining partners. In addition, unlike other welding processes, the process is completely abrasion-free. Laser plastic welding is therefore predestined for applications in which sensitive electronics are located in components to be welded, high seam qualities have to be achieved or absolute cleanliness and particle freedom in the process are essential.
Today, laser plastic welding is mainly used in the form of through-transmission welding. In this process, two plastic materials with different absorption properties, which are caused by different filler types or concentrations, are welded in an overlap joint. The plastic parts to be joined lie on top of each other and the laser radiation is focused through the plastic, which is transparent for the laser wavelength, onto the absorbing part, whereby it melts on the surface. The transmissive partner also plastifies via heat conduction, so that a connection occurs.
In addition to the joint geometry, the type of energy input during laser plastic welding is an important feature for classifying the processes. A distinction is made between contour, simultaneous and quasi-simultaneous welding. The process variant considered here is contour welding in a special form for machining radially symmetrical assemblies. During contour welding, the laser spot is guided one or more times so slowly along the welding contour that the material cools down between the respective overstroking of a point of the weld seam to such an extent that it solidifies.
A major difference between this special form and the other process variants of laser plastic welding is the introduction of the joining pressure into the joining zone. Due to the radially symmetrical structure of the assembly in the welding area, it is possible to work with a press fit and thus achieve the joining pressure required for heat conduction. As a result, a clamping technology in contact with the component can usually be omitted. The positive results are economical system designs and short overall process times. Since the tolerances of the components are very important due to the use of the press fit, the control of the preliminary processes is of particular importance.
The three process variants shown in the schematic sketch and explained in more detail below, which differ in the movement of the laser beam, are the main industrial applications today.
Figure 1: Basic principles of contour welding of radially symmetrical components
Laser fixed, moving part
The simplest and most industrially used variant is based on a fixed laser optics and a moving workpiece. It is used in particular when there are no protruding attachments, cables or pipes on the joining assembly. In this variant, the module is inserted into a component-specific fixture which is connected to a suitable rotary drive. The component is then rotated once or several times in front of the fixed laser spot, thereby creating the weld seam. The main process parameters of this variant are the feed rate (peripheral speed) and the laser energy used, which can be variably adapted in modern welding systems.
The advantages of this variant are especially the very economical and robust system design and the good possibility of process monitoring using a pyrometer or IR camera. The variant shows restrictions in the area of automation and in the machinable component sizes (overall dimensions, cable lengths, etc.).
Fixed part, laser rotates around the part
The second process variant, which has also been used successfully for more than a decade, reverses the principle of the aforementioned variant. The workpiece is fixed in a fixture and the laser beam is rotated around the assembly by means of a suitable optical structure (mirror arm). The essential process parameters are analogous to the variant mentioned above.
The main advantage of this procedure is that larger mounting geometries or cable lengths below the joining zone do not impair the process and structure of the welding module. This allows plugs and couplings to be welded to cables several meters long. The latest generation of welding modules is flexible and easy to integrate into automation solutions and already has suitable laser protection to achieve laser class 1.
Fixed part, laser is reflected onto the part by mirrors and rotates around the part
The latest variant of radial welding uses the scanner technology known from quasi-simultaneous laser welding. A mirror funnel is used to reflect the laser beam, which initially radiates axially from the scanner in the direction of the component radially onto the component. In the simplest case, the laser then describes a circular path and thus rotates around the workpiece.
As with the above-mentioned variant, the fixed component does not interfere with larger mounting geometries or cable lengths. In addition, the freely programmable welding contour allows other welding seam geometries than circles or several welding seams to be processed one above the other. Another advantage over the variants mentioned above is the process speed, which can be significantly faster with suitable component geometry due to the lower moving masses. Due to the scanner, the feed rates of the laser beam can be set so high that quasi-simultaneous welding is also possible.
Due to the higher number of optical elements, this type of radial welding offers a somewhat more limited possibility for optical process monitoring. In addition, the mirror funnels must undergo regular inspections.
Comparison of variants
Due to the many advantages and disadvantages of the variants mentioned, the following table is intended to provide an initial brief overview.
Table 1: Comparison of the variants
Basically all three variants are in industrial use today and have proven their economic efficiency mostly over years in the cost-sensitive automotive supplier industry. The final selection of a variant can best be done together with an expert.
Process monitoring methods
All common methods, which are also used for normal contour welding, are basically suitable for process monitoring. As a rule, the same options and restrictions apply.
The most frequently used industrially and so far also most economical variant is the monitoring of the temperature in the joining zone by means of pyrometers. In all the above-mentioned process variants, the pyrometer is coupled into the beam path of the laser. During the welding process, the temperature in the processing zone is measured continuously. Depending on the signal quality, which is strongly influenced by material and additives, the temperature signal is evaluated in real time and compared with the upper and lower limits of an envelope curve.
Figure 2: Example for an evaluation of the pyrometer signal
Air gaps or bad connections can thus be reliably detected. If the signal quality is correspondingly good, it is also possible to control the energy input by the laser on the basis of the signal curve.
Preferred applications for radial welding include couplings and plugs on pipes or cables, valve assemblies for fuel or SCR lines and bottle closures.
The picture shows a coupling that is reliably and permanently sealed to a pipeline by radial welding. In this case, the tube is designed as an absorbent joining partner, the coupling is laser-transparent. Due to the different lengths and shapes of the cables, the process version with mirror arm in combination with a pyrometer for process monitoring is used.
Figure 3: Example of a pipe-coupling connection
The advantage of this system design is in particular the high flexibility with regard to existing and future component geometries such as cable lengths, diameters or the position of the weld seam.
Figure 4 shows a compact integration module from Evosys Laser GmbH, the EVO 0700, which, as the only module on the market to date, comes with suitable laser protection to achieve laser class 1. This eliminates the need for complex enclosures, which subsequently have to be extensively certified.
Figure 4: Compact integration module from Evosys Laser GmbH
In addition to the essential requirements on material and geometry for a laser plastic welding process, an almost radially symmetrical component geometry in the area of the joining zone is a basic prerequisite for the use of radial welding. Due to the optical and technical possibilities of laser welding, in exceptional cases even elliptical and even angular geometries can be processed radially. As a rule, the laser technology is less limiting than the guarantee of a circumferential press fit.
Since radial welding generally does without the use of clamping technology, the design of the welding zone is particularly important. The two components to be joined must ensure a reliable circumferential press fit when assembled, since the welding process can bridge virtually no air gaps.
We recommend the geometries shown in Figure 5, which also ensure a simple sequence of production steps before the welding process.
Figure 5: Principle sketches of recommended joining geometries
Due to the tolerances for suitable interference fits, which are strongly dependent on the component size and in particular the material pairing, we recommend consulting experts for the joining technology and, if necessary, tests to define the final geometry.
Since the material expands in the area of the joining zone during plasticizing and tends to flow sideways depending on the material, the welding should not take place directly in an edge area where melt expulsion has a disturbing effect.