Plastic welding: What you need to know about it

 Plastic welding is a manufacturing and recovery method used in the casting of plastic parts. It’s a process that works by heating certain parts or points of the parts. Therefore, the plastic softens and begins to liquefy, allowing bonding and solidification again after cooling. That is, when the plastic cools, a chemical bond is produced between the two parts, which causes the melting of these parts. Typically, a thermoplastic welding rod is used to act as an alloy element between the parts.

However, when it’s necessary to join thermoplastic parts, there are some methodologies for this to happen. Due to the complexity of the internal geometry, it’s not possible to make one piece at a time. In this way, the piece is molded in two phases, with two molds, for later welding.
 

Union against plastic welding

The welding process is quite different from simply joining with some type of adhesive. “In the union, we would have an adhesive that theoretically joins the elements, in this case, the two pieces. But we have a
big problem when we talk about plastics. Many times, I may want to join a thermoplastic A with a thermoplastic B, but they are not compatible with each other”, warns Eduardo García Vargas, professional training instructor and consultant in the areas of thermoplastic transformation in Senai São Paulo.

According to the expert, "welding arose from the moment when the union of plastic materials was necessary when there was no glue or adhesive at room temperature." In addition, the adhesive has limitations and can be hydrolyzed or even loosened if it’s too rigid during a turn.

When welding, it’s possible to heat the polymer to the melting point where it will be possible to join one surface with another. Therefore, the possible techniques are diverse. That is to say: each one has its application, particularity and limitation. "However, in Brazil, we do don’t first-class equipment for this type of application. In addition, many techniques aren’t yet widely used in our country and cutting-edge equipment is generally imported," Vargas said.
 

Types of Welding

There are several methods of plastic welding and are used for different purposes. Therefore, the type of equipment and welding material varies, including the type of thermoplastic to be used. See the most common types:

Ultrasonic welding - Made using  vibratory ultrasonic energy
Vibration welding– It uses vibrations and the mechanical movements are longitudinal, low frequency, with linear or orbital friction;
Hybrid welding – It consists of the combination of vibration welding and infrared welding;
Rotation welding – With the heat generated by the friction of the pieces, rotation welding occurs. This process is used in cylindrical pieces; 
Hot melt welding or hot plate – Two electrical resistors heat a metal plate. With heat, the plastic material softens and can melt; 
Dielectric welding – Electric current causes friction at the molecular level (due to the vibration of electrons in molecules), generating heat and making it possible to melt in the contact area;
Hot gas welding – Welding is done by applying a hot gas to the surface with a welding rod made of the same material to be welded (which is consumed during the process); 
Infrared welding – This is a mechanical contactless welding process, which uses the concentrated radiant heat that comes from a contouring tool that’s between the parts to be welded, which allows the union between them; 
Hot plate welding – This type of welding uses a heated plate that, when in direct contact with the parts, makes the union between them; 
Laser welding – This technique is ideal for large volumes of production, allowing the manufacture of parts with excellent finishes, with small burrs and generated particles.