Hermetic welding
Hermetic welding: the principles
As a reminder, welding is the permanent joining of two parts or elements of the same nature, to ensure the long-term continuity of the material. Welding can be carried out using different technologies to join metal, plastic or wood parts. It should be remembered here that assembly techniques based on adhesion (glue) and brazing (the addition of hot external material to ensure the bond), do not fall within the scope of welding as such.
The technologies used to produce hermetic welds depend on the nature of the materials to be joined.
For metal parts, the most commonly used assembly techniques are :
- electric arc welding (MIG, MAG, TIG…),
- flame welding,
- autogenous welding (fusion of the components to be joined) and oxy-fuel cutting,
- plasma welding with tungsten electrode.
For plastic parts, the most commonly used assembly techniques are :
- high-frequency welding (assembly of different layers or parts by activating the molecules of the surfaces to be welded, without the need for physical contact between the press and the material, thanks to a high-frequency electromagnetic field),
- ultrasonic welding (assembly of parts by stirring the molecules in their areas of contact with the press, using ultrasound to generate mechanical vibrations).
For wood parts, the only existing joining technique is friction welding (rubbing the parts against each other by applying a certain pressure), resulting in a molecular reaction of the lignin that creates the weld.
Depending on their intended use, metal or plastic assemblies may be required to be hermetically sealed. Hermetic welds can only be obtained for the assembly of certain thermoplastics (PVC, PET, PS…) and metals (copper, brass, steel…).
What types of plastic can be welded?
Plastics come in a variety of forms (rigid, semi-rigid, flexible, etc.) and have very different characteristics depending on whether they fall into the thermoplastic, thermoset or elastomer categories. These different categories of plastics have very different thermal and chemical characteristics, which may or may not allow them to be joined by welding.
In fact, only thermoplastics have the technical characteristics required to produce welds that are both resistant and durable. This family of materials accounts for almost 80% of the world’s plastic production, and includes all plastics that can be welded, remelted or processed without altering their mechanical properties. These manipulations are possible by subjecting thermoplastics to high temperatures, mechanical vibrations or high-frequency energy input.
The plastics that can be joined by welding and are most frequently used in many sectors of activity are the following:
- PS or Polystyrene (thermal insulation products, floating objects, etc.),
- PP or polypropylene (automotive accessories, mechanical parts, etc.),
- PET or polyethylene terephthalate (disposable packaging, textile fibers, bottles, etc.),
- PVC or Polyvinyl Chloride (floor coverings, doors and windows, pipes…).
When it comes to welding thermoplastic components to achieve a hermetic seal, the choice of welding technique is of particular importance. It will determine the possibility of combining plastics of different natures, the precision of the assembly process, the strength of the joints produced, and production rates.
How to make hermetic welds for plastics?
There are four main techniques for joining rigid or flexible plastics, as long as they belong to the thermoplastics family:
- cold welding or bonding (joining of components using epoxy resin or mastic, or adhesives consisting of particles or fibers in suspension to ensure weld strength);
- brazing (joining by heating the parts and adding material in the form of a rod at a lower temperature than the parts to be welded, using a heat stripper or hot-air gun, based on a minimum temperature of 300 degrees);
- high-frequency electrode welding (assembly of components using an electromagnetic field with a frequency of 27.12 MHz, created by a generator and diffused by one or more electrodes across the surfaces to be welded without physical contact);
- ultrasonic transducer welding (assembly of components using an electric current generated by a transducer at a frequency of between 20 and 70 kHz, producing oscillations and then vibrations through direct contact with the surfaces to be welded).
In order to assemble thermoplastic parts or assemblies with hermetic welds, it is necessary to modify the state of the material. Only high-frequency and ultrasonic welding techniques can fuse the “lips” or “edges” of the parts or components to be joined, by means of molecular agitation. This creates hermetic welds that are both highly resistant and undetectable to the naked eye.
Matrelec designs and develops high-frequency welding presses for French and European companies, enabling them to weld thermoplastic components in product or packaging form.
What's the difference between hermetic and sealed welding?
From a technical point of view, there is no significant difference between making a hermetic weld and making a tight weld in the field of thermoplastics.
The concept of weld-tightness applies to the assembly of components of a container (bag or pouch) which must contain liquids without risk of loss or leakage.
The concept of hermetic sealing refers to the assembly of components of a container (pouch or bag) designed to hold solid or liquid products under vacuum, i.e. without the potential ingress of outside air which could damage them if the preservation conditions are not respected.
Whatever your company’s needs for hermetic or airtight thermoplastic welding, Matrelec has a range of welding rails and high-frequency welding presses for high-speed assembly.
