Application of plastic laser welding in medical equipment

Laser welding is more and more widely used in various fields of medical devices due to its advantages of local heating, precision machining, and non-contact heating.

Compared with other commonly used welding techniques, laser welding technology produces almost no slag and debris, and does not require any adhesive to be added during the welding process, so the entire welding job can be completed in a clean room. Laser welding technology has greatly promoted the development of medical devices, such as the shell packaging of active implantable medical devices, the radiopaque marking of cardiac stents, earwax protectors, balloon catheters, etc., all of which are inseparable from the use of laser welding.

Principle of plastic laser welding

Laser welding is a welding method that uses a high-energy-density laser beam as a heat source, that is, laser radiation heats the surface of the workpiece, and the surface heat diffuses into the interior through heat conduction to melt the workpiece and form a specific molten pool. As shown in the figure below, the laser beam passes through the upper light-transmitting material, and is then absorbed by the lower material. The laser energy is absorbed and converted into heat energy. Since the two layers of materials are pressed together, the heat energy is conducted from the absorbing layer to the light-transmitting layer, so that the two The layer materials melt and bond.

Principle of plastic laser transmission welding

At the same time, the internal pressure is generated due to the thermal expansion and expansion of the material itself. The internal pressure and the external pressure work together to ensure a strong welding of the two parts. The above two figures intuitively illustrate the principle and process of plastic laser welding.

The energy of the visible laser beam must be absorbed by the plastic in order to achieve a good welding effect, so a semiconductor laser is generally used for plastic laser welding. Also, not all plastics can be welded with laser welding. Plastics can be divided into thermoset and thermoplastic. Among them, thermosetting plastics are not reproducible and cannot be welded, while thermoplastics will melt after reheating (you can use heating and cooling to make them reversible), which is a so-called physical change, so it has weldability.

Plastic Laser Welding Process

1wavelength of laser 

Diode lasers or semiconductor lasers are commonly used in plastic welding processes. The beam is in the near-infrared region, and the wavelength of the beam is 400 to 1,100 μm, which can be transmitted through an optical fiber, and the laser beam in this range can be absorbed by most plastics. Diode laser welding systems are compact, and the lasers can also reach higher levels of power. The wavelength of the laser can be designed according to special requirements. The wavelength of semiconductor lasers is generally 808 to 980 μm. Semiconductor lasers have low investment cost, small size and high efficiency.

2plastic material 

Thermoplastics include amorphous plastics and semi-crystalline plastics. Plastics that can be laser welded are all thermoplastics. In theory, all thermoplastics can be laser welded.

The requirements of plastic laser welding technology for the plastics to be welded are: the materials in the heat-action zone require good absorption of laser light waves; the materials that do not belong to the heat-action zone are required to have good permeability to light waves, especially in This is especially true when stitch welding two thin plastic parts. Generally, the addition of absorbent to the plastic in the heat-action zone can achieve the purpose.

3Absorbent 

The application of absorbers is a very important process in the plastic laser welding process. The essence of plastic laser welding is to melt the plastic to be welded in the thermal action zone, and then cool to naturally realize the joining of plastic parts. Melting plastic requires that the plastic part absorb enough laser energy.

Usually the ideal absorbent is carbon black, which can absorb almost all the laser energy of the infrared wavelength, thereby greatly improving the heat absorption effect of the plastic, making the material in the thermal action zone melt faster and with better effect. Some other color dyes can also have the same effect of absorbing light waves.

There are three ways to add absorbent

1. The first is to directly infiltrate the absorbent into the material to be welded, so that the plastic parts that have penetrated the absorbent should be placed on the bottom, and the plastic parts without absorbent should be placed on the top to allow the laser light wave to pass;

2. The second is to infiltrate the absorbent into the surface of the plastic parts to be welded, so that only a part of the plastic infiltrated with the absorbent will become the heat-action zone and be melted;

3. The third is to spray or print absorbent on the contact of the two plastic parts to be welded.

4Other parameters

Unlike metal welding, the laser power required for plastic laser welding is not as high as possible. The higher the welding laser power, the larger and deeper the heat zone on the plastic part, which will cause the material to overheat, deform, and even damage. The laser power should be selected according to the depth of melting required.

The speed of plastic laser welding is relatively fast. Generally, the welding speed of a 1mm thick weld can reach 20m/min; while the high-power CO2 laser is used to weld plastic films, the maximum speed can reach 750m/min.

Application in the field of medical devices

With the wide application of plastic materials in the field of medical devices, new plastic production and processing techniques are emerging one after another. As one of them, laser welding has received extensive attention in the industry due to its advantages of non-polluting, non-contact and seamless connection.

earwax protector

Hearing aids are increasingly miniaturized, providing users with smaller, more comfortable hearing aids that are invisible to the naked eye. Today's typical ITE hearing aids are only the size of the tip of a little finger. But all hearing aids face a big problem: the ear wax (also called earwax) produced in the ear canal contaminates the sound output area. In order to ensure that the hearing aid can perform its reliable function, it is necessary to provide protective measures at the location of its sound output.

In the earwax protection of Phonak's new "smart guard", the method used is to weld an extremely thin and highly elastic diaphragm on the tiny gasket, which can effectively block the earwax from entering the hearing aid. This welding technology is Swiss Leister This high-precision joining process is made possible by a laser mask welding process developed by the technology company. The entire earwax protector is just a few millimeters in size and requires a diaphragm to be welded to a gasket about 3mm in size, made of thermoplastic material.