Most people are familiar with the concept of welding as a procedure to join two metallic pieces. The image that people have of the said procedure is of an electric arc producing intense light and spatter.
However, welding is not exclusive to metals. Welding can be used to join two or more plastic pieces together. Not just any plastic, though.
And not the traditional welding methods used for joining metallic pieces are used for welding of plastic. Let’s elaborate.
In different industries, joining plastic pieces permanently is sometimes necessary. Examples abound, including some toys, car parts, outdoor furniture, and more. The welding process involves the fusing of the plastic material.
Welding is done with thermoplastics only. This type of plastic has the property of melting with heat. This property facilitates the welding process.
The principle is simple. The plastic pieces are heated until they melt. These parts must be of compatible thermoplastic materials.
Then, a filler material is added as in some metal welding techniques such as MIG and stick welding. In some cases, however, no filler material is needed.
Why Use Plastic Welding?
Often, people ask why plastic welding is necessary. Creating one-piece plastic pieces using molds is not as difficult as with metals. Hence, why to weld plastic pieces?
So, even if one has one-piece plastic parts, they may break under certain conditions. In some cases, repairing the broken part is the most cost-effective solution.
A good example is a broken car bumper. You’d save some money by welding the broken part rather than buying a new one. The same is true with high-end lawn plastic furniture and other items.
How Plastic Welding Works
Plastic welding creates bonds at the molecular level between compatible materials. This results in superior strength. There are several plastic welding methods. However, all of them have the following stages in common:
What differentiates one welding method from the other is how the materials are heated up. Pressing and cooling may vary from one welding machine to another. But these mechanical procedures can be common to most welding methods.
Hence, the most common plastic welding methods include:
- Ultrasonic Welding. In this technique, ultrasonic motion is applied to the plastic pieces that will be welded. This motion and the pressure applied create frictional heat. At some point, the heat melts the joint areas, and the two pieces create molecular bonds;
- Vibration Welding. Like in the previous method, frictional heat is used to melt the materials. However, linear or orbital motion is used to generate the required vibration;
- Spin Welding. In this method, rotational motion is used to generate frictional heat. This will melt the materials at the joint area.
These methods are also known as contact welding methods since they rely on frictional heat to melt the materials. There are also non-contact methods, which include:
- Hot-Plate Welding. In this method, the two plastic parts to be welded are pressed against a hot plate. When the plastic melts, some pressure is applied, such that the two parts are welded;
- Infrared Welding. This method uses infrared (IR) rays to heat the plastic parts to be welded;
- Laser Welding. In this case, a laser melts the plastic. The laser can deliver the right amount of energy very precisely. This is why this method is considered one of the best in the industry. Almost all types of thermoplastics can be welded with this method.
How Plastic Laser Welding Works
This welding method utilizes a laser that passes through one of the pieces to be welded. The laser is focused on delivering all its energy on the point of contact between the two plastic pieces.
The second plastic piece must be capable of absorbing this energy. This absorption will produce heat.
The energy will be high such that the heat melts both plastic pieces along a weld seam. Below, we describe in more detail the different elements that are necessary for this welding method.
The first necessary element is a layer of a material that allows a laser to pass through. This is not a problem with thermoplastics since most of them transmit IR radiation properly. Hence, an IR laser with a wavelength of either 980 or 880 nanometers is often used.
This poses a problem since thermoplastics transmit IR lasers instead of absorbing them. To turn the second layer of material into laser-absorbing, an additive is used.
In most cases, carbon black will do the job. But some pigments can create an absorbing surface too.
Also, the melting temperatures of the materials to be welded must be the same or quite similar.
Good Surface Contact
Since the heat generated by the absorption of the laser must melt both plastic layers, good physical contact is a must. Hence, both parts are clamped together to ensure good contact that allows proper heat transfer by conduction.
Advantages of Plastic Laser Welding
For the welding procedure, IR diodes are typically used as radiation sources. The electrical power can vary widely, going from 1 W and up to 100 W. It depends on the type of material that will be welded.
In all cases, laser welding is preferred because it produces a much stronger joint than adhesive bonding. The finish is also of superior quality.
The laser welding procedure doesn’t produce any gas or liquid that will damage the materials. This allows for a very clean finish.
Moreover, except for some additives to dope the second layer, no consumables are needed. This makes the process more straightforward. Moreover, this welding procedure will work with practically any geometry, even if it is quite challenging.