Laser welding technology in the medical industry

Joining plastics can be very challenging.

Especially for the medical industry, it can be difficult to find adhesives that meet biocompatibility requirements and FDA approval. In addition, many commonly used polymers, such as PP, PE and PTFE, have low surface energies, which also make them difficult to bond with adhesives.

Adhesives are often used as a quick, temporary solution in prototyping. However, when it comes to subsequent production, plastic welding is a more cost-effective solution that provides consistent, high-strength welds without the need for additional consumables.

The risks and challenges of plastic welding are often caused by improper welding process selection, weld design and testing methods, as well as part deformation and dimensional deviation. These problems can be reduced or eliminated by taking step-by-step troubleshooting analysis and improvement measures.

The development process of a plastic welding process follows these steps:

• Define the problem

• left;"> Process Selection

• Joint Design

• Test Verification

     

1. Definition Issues

When selecting a welding process, it is critical to clearly define the product material and performance requirements in the early stages of process selection. By defining these early on, you can avoid subsequent losses.

Before deciding on weld location or welding process, you should think about and answer the following questions.

1. Strength

What is the direction of the load? What kind of tensile or shear forces will be experienced?

If possible, the weld should be designed to withstand shear forces as much as possible when working, because plastic welding withstands the greatest load in the direction of shear force. Plastic welding can withstand the second-lowest tensile load and the weakest peel force. As shown in Figure 1.

 

2. Sealing

Does this product need to be sealed? If so, what is the pressure it needs to withstand? Is it airtight or watertight? Are there any test temperature requirements?

For products with sealing requirements, closed welding lines are usually designed around the chamber to be sealed, as well as corresponding grooves or narrow gaps. The molten plastic fills the grooves or gaps to achieve the sealing requirements. The plastic welding process selected will be different for different product sizes and shapes

3. Appearance

Are there any appearance requirements? Where is the "A" side? Is visible flash allowed?

Most welding processes may produce appearance defects. However, these problems can be limited by improving the jig and optimizing the welding parameters, or simplifying the surface of the part, or hiding the welding surface. If it cannot be avoided, it is necessary to add a previous process (such as drying) to prevent it, or add a subsequent process (such as flash removal) to correct it, but these measures will increase costs.

4. Particle Contamination

Are particles or loose flash allowed in the product?

For welding processes that rely on surface friction to generate heat, such as spin welding and vibration friction welding, it is difficult to avoid particle contamination and loose flash.

 5. Materials

The factors that affect the welding performance of materials include various additives such as colorants, fillers, impact modifiers, lubricants, etc. These components jointly affect the selection of welding process. For example, when using the laser welding process, the upper part is required to be free of colorants to allow the laser to penetrate and transmit; the lower part requires the addition of carbon black to fully absorb the laser energy.

In general, the filler content should be minimized to ensure that there is enough resin for welding to ensure welding strength. Therefore, it is recommended that the filler content be controlled below 33%.

In addition, the two parts must be compatible in material, that is, have the same or similar melting point and viscosity to ensure that diffusion occurs between molecules to complete the welding.

 

2. Selection of process

Comparison of common plastic welding processes:

 

Laser welding of plastics requires that a small amount of light energy is absorbed by the upper and lower parts (2um welding system), or that the upper layer transmits light energy at the weld and the lower layer absorbs light energy (1um welding system). The energy absorption efficiency is determined by the selected wavelength and the transmittance of the resin and filler at that wavelength.

(Schematic diagram of laser welding)

 

Ultrasonic welding works by generating heat through the loss of mechanical vibration energy due to the viscoelastic properties of polymers. The mold that generates ultrasonic vibrations is called a welding head. The welding head must be parallel to the welding ribs and the distance between them must be short (generally less than 6mm). In addition, sharp corners and thin cantilever structures should be avoided in and near the welding area to prevent damage.

 

(Ultrasonic welding diagram)

(Weldability of commonly used plastics in the medical industry)

Hot plate and infrared welding, heat the welding surfaces of the two parts separately, then put the two parts together, and apply pressure to promote molecular diffusion. In hot plate welding, the surface of the part is in direct contact with the heating tool, so it is suitable for welding plastics with higher viscosity to avoid the material sticking to the hot plate. In infrared welding, the surface of the part should avoid white, and fillers should be added to increase energy absorption efficiency and reduce production cycles.

 

(Hot plate welding diagram)

Rotary and vibration friction welding use surface friction to generate heat to achieve welding. During these movements, the joint design must adapt to the friction movement/vibration direction. For rotary friction, this means that the weld line design must be circular. For vibration friction, the joint design must have enough lateral clearance to accommodate a certain vibration amplitude. In addition, it is very important that the supporting wall under the weld bar is rigid enough not to deform under the load to achieve frictional heating.

 

(Schematic diagram of rotational friction (left) and vibration friction (right))

The working principle of high-frequency welding is to expose a polymer with high dielectric loss to an electromagnetic field, and the direction of the magnetic field changes alternately at a high frequency (usually 27MHz). Applicable to welding thin materials with a thickness of 10-50mm. The materials that can be used for this process are very limited. Among them, polyvinyl chloride (PVC) is the easiest.

 

(Schematic diagram of high-frequency welding)

 

3. Joint Design

In plastic welding, joint strength depends on the flow of polymer chains at the melt interface and the degree of molecular chain entanglement at the joint. To achieve this, a reasonable weld design is very important. It can melt and collapse to produce melt flow. In addition, a well-designed weld helps to increase the heating rate, prevent leakage, hide flash, and help align the upper and lower parts. Common joint designs.

 

(Common ultrasonic joint design)

Different welding processes generate heat in different ways and have different heating rates. Good joint design can actually increase the heating rate. For example, ultrasonic welding increases the heating rate by using a joint called a "triangular energy guide rib." Rotary friction welding, with a shear weld design that has a higher positive pressure, heats faster than a lap weld design.

Grooved welds can help improve sealing. With this type of joint, the melt can stay in the groove and in the gaps in the sidewalls, which reduces voids and improves sealing. However, in some cases, air can be trapped in the grooves, which can cause leaks when the trapped air creates enough pressure to form pores in the melt.

During welding, a certain amount of melt is squeezed out of the joint, which can be controlled by the joint design. Adding flanges on both sides of the joint can hide flash and even control the direction of melt flow. The sidewalls can also help align the upper and lower parts.

 

4. Test Verification

The quality of welding results must be evaluated with the correct test methods and fixtures. Weld strength and airtightness are easy to test if some simple guidelines are followed.

1. Strength Test

The direction of the applied load is crucial to the test results. The location and direction of the applied load should be consistent with the actual load in service.

Ensure that the joint fails before the bulk material fails. If the bulk material fails during the test, the test cannot provide data on the weld strength and the test plan should be redesigned.

Test the entire joint rather than cutting out a section for strength testing. Local testing can be inaccurate because it ignores some potential areas of higher stress. After the initial crack is initiated, the remaining parts of the weld will fail in the tensile test. If only a local test is performed, this failure mode will be ignored and the weld strength will be erroneously overestimated.

If the entire joint is difficult to fix for the tensile test, consider using a burst pressure test.

2. Sealing performance test

The defined test pressure and acceptable leakage rate are related to the final use environment and requirements. Commonly used air attenuation leak testers are divided into air detection and water detection. Different water temperatures can be set for watertight detection.

Do not artificially increase the clamping force of the fixture during the leak test. Larger clamping forces cause compression of the weld joint, which will produce an artificially low leak rate.

 

V. Conclusion

For the medical industry, welding provides a better plastic joining solution than adhesives. This simple guide will help you make an initial selection of welding processes. Finally, if something should weld, but it doesn't. Is it possible to improve it by modifying it? At this time, you need to consult a more professional plastic welding expert. We are happy to help answer this question.