Soldadores de Láser

1 Technical Design of Joining Parts

1.1 General requirements

The suitability of moulded parts for welding is oriented on the following criteria:
• Component requirements
• Process technique
• Material selection
• Construction

In order to ensure the necessary joining pressure in the joining zone, the joining parts must be designed for the specific process that is to be used. The joining part transparent to the laser beam should be oriented such that the laser beam is, as far as possible unimpeded and falls perpendicular to the joining zone. When welding semi-crystalline and filled plastics, the thickness of the component to be penetrated should be kept as low as possible, while still meeting the mechanical specifications of the parts.

1.2 Transmission process principle

In the transmission technique, the laser beam penetrates the upper, laser-transparent joining component and arrives at the underlying joining component, which absorbs the laser beam. Here the laser beam is absorbed in the plastic by the dye and filler particles and is converted to heat. The absorbing plastic melts and the heat is conducted to the transparent welding part, which is also plasticised. In the welding process, both parts are mechanically pressed together. The local heating and expansion creates an additional internal joining pressure. The internal and external joining pressures ensure the two parts are strongly welded together. The relative movement of the laser beam to the workpiece generates a welding seam.

1.3 Optical requirements for the material

It is necessary for the laser transmission process that the optical properties of the joining components differ considerably. While one component must be highly transparent for the laser transmission, the other component must absorb the laser radiation. Having one or both of the joining components with highly reflective surfaces is unfavourable, as high energy densities are required to generate the heat necessary to melt the joining zone.
In the near infrared range, unpigmented thermoplastics usually show a high transparency for the laser radiation. Partial crystalline plastics with an opaque appearance, as well as coloured components or components specially dyed with black pigment, may also be sufficiently transparent for the laser radiation. If the plastics have “laser-absorbing” pigments added (e.g. standard black colouring with ~ 1% carbon black), the desired absorption takes place in the layers near the surface.

1.4 Engineering design of the joining geometry

The geometry of the joining surfaces should be matched to the welding task. The joining seam geometry is determined by selection of the process, the geometry of the moulded parts, as well as the optical and mechanical requirements. Overlapping contours (nipples, sockets, fixture elements…) must not lie between the laser source and the joining seam as this leads to shadowing.
Transmission welding may generally be divided into process variants with and without welding displacement.
If welding takes place without welding displacement, the joining seam geometries are similar to those used for adhesive techniques.
To achieve very strong joints using techniques without welding displacement, the joining surfaces must lie evenly and be almost gap-free. This can be achieved by pressing together with a holding-down device (pneumatic clamping fixture). Press fits have proved to be successful for circular joining sections.
For techniques using welding displacement, it must be possible for one part to drop freely into place. Edges, ribs and partitions must not brake or hinder the downward movement. The melt can be concealed inside cavities to improve the appearance.
The laser beam must not be impeded by the holding-down clamping system. Attention must be paid in this respect to the configuration of the work piece and clamping system.

2 Welding conditions

2.1 General requirements

Generally, all thermoplastics, as well as those thermoplastic elastomers, may be welded with laser radiation given the polymer compatibility and the appropriate thermal and optical properties. The temperature in the joining zone must lie above the melting temperature and below the decomposition temperature.
To produce a highly stable welding joints for real-life applications, the following requirements must be fulfilled:

• joining suitability of the welding parts
• suitable optical properties of the welding parts for laser welding
• design of moulded parts suitable for laser welding
• precisely fitting joining surfaces (avoiding gaps between the welding parts)
• use of a welding system matched to the welding task
• positioning accuracy of the welding parts
• sufficient joining pressure at the joining zone during welding
• welding parameters matched to the component and plastic

2.2 Selection of welding parameters

2.2.1 Overview

The following distinctions are to made:
• Radiation mode of the joining seam geometry (2.2.2) contour, simultaneous, quasi-simultaneous and mask welding
• Weldable plastic combinations (2.2.3)
– plastics of the same class and same type
– plastics of the same class and dissimilar type
– plastics of dissimilar classes.

2.2.2 Welding concepts

2.2.2.1 Contour welding

The available welding parameters include:

• Laser power
• Traverse speed
• Laser beam diameter
• Joining pressure

The most important variables in contour welding are the intensity of the laser radiation and the rate of feed. The intensity of radiation is dependent on the beam geometry and the laser power. A spot laser beam geometry is generally used in contour welding, whereby the radiation distribution should be as uniform as possible to avoid subjecting the plastics to any unnecessary thermal stress. Using point geometry, the beam can be moved in every direction so that complex 3D contours, such as corners, can be realized without problems. The radiation intensity is selected for the welding process such that no damage to the joining components occurs with the preset rate of feed. The welding temperature produced can be detected with pyrometry.

2.2.2.2 Quasi-simultaneous welding

The available welding parameters include:

• Laser power
• Scanning speed
• Beam diameter
• Joining pressure
• Welding displacement
• Holding time

The laser power and the scanning speed are the two most important parameters for quasi-simultaneous
welding and must be matched to the joining task. The joining pressure is adjustable, but often it can only be varied within narrow bounds otherwise the part can warp. The welding displacement can be used as a process termination criterion, but in this case, it should not exceed the maximal welding displacement add-on for the welding seam. The welding displacement add-on is generally made as a raised area on part of the welding area of the part.

2.2.2.3 Simultaneous welding

The available welding parameters include:

• Laser power (total power of the diodes or individual power)
• Radiation time
• Joining pressure
• Welding displacement
• Holding time

If several laser diodes are used then they undergo a one-time calibration, so that they all emit the same laser power. Differences in the diodes arising from manufacturing or assembly conditions, as well as in the plastic part, are balanced by varying the power control of each diode. This allows a uniform melt formation over the entire welding seam. These settings are usually only changed at the calibration intervals or to re-optimise the process after the replacement of a faulty diode. It is possible to measure the output power of the individual diodes and to adjust the values if necessary. In order to keep the radiation time low, the highest possible diode power is selected without thermally damaging the joining components. The remaining controlled variables for the welding process are the radiation time (time welding) and the welding displacement (length welding). In contrast to mask or contour welding, the simultaneous welding technique offers the possibility of tolerance compensation of the parts through the defined welding displacement. It is generally the case that high strength is only achieved if a sufficient melt is generated. For this reason, low part tolerances are important to keep the required radiation time low.

2.2.2.4 Mask welding

The available welding parameters include:

• Laser power
• Traverse speed
• Laser beam geometry
• Joining pressure

A line-shaped laser beam is generally used with mask welding.
To achieve a high reproduction accuracy,
• the distance between the mask and the joining plane must be kept small
• the radiation intensity must be high (low melting depth)
• the power density distribution should be as homogeneous as possible.
The technical information from the machine manufacturers concerning mask structures and geometries,
as well as the component geometry is to be followed for the particular application. This particularly
includes the alignment marks and tolerance requirements. Amorphous thermoplastics allow higher
reproduction accuracies than semi crystalline thermoplastics.

2.2.3 Plastic combination

In the following sections, class is understood as the polymer base and type as the quantity and
composition of the filling and reinforcement materials.

2.2.3.1 Plastics of the same class and same type

This combination of welding parts offers the most favourable welding conditions for simultaneous
plastification of the two joining surfaces and generation of a highly stable welding joint. It must be considered that the optical properties in particular facilitate the transmission technique.

2.2.3.2 Plastics of the same class and dissimilar type

Plastics of the same class, however of dissimilar types can be welded together. The following
combinations are possible, for example:
– non-reinforced / reinforced
– non-reinforced / elastomer-modified
– special pairs
It should be noted that the degree of transmission for reinforced or filled plastics is, to some extent reduced.

2.2.3.3 Plastics of dissimilar classes
Welding dissimilar plastics classes requires knowledge of the structure and composition of the plastics. The suitability for welding and joining seam strength of the joints has to be verified by testing. Some examples of successful welding pairs of dissimilar plastic classes are presented as follows:
− ABS and PMMA
− ABS and PC
− ABS and SB
− ABS and SAN
− ABS and PC+ABS
− ABS and ABS+PA
− PMMA and PC
− PMMA und PC+ABS
− PBT and PBT+PET
− PMMA and SAN
− PC and PC+ABS
− PC and PC+PBT
− PC and PBT
− PBT and PC+PBT
− PA and PPE+PA

With these pairs, the plastic with the higher degree of transmission should be used as the transparent joining part if possible. The melt flow properties (viscosity) of the joining parts to be welded should be as close as possible to each other otherwise; there may be a reduction in the quality of the welding seam. Further combinations are conceivable by developing new materials (blends, polymer mixtures).