Invisible welding

Invisible soldering: the basics

Depending on the type of material to be joined, invisible welds may or may not be an achievable goal:

In metal joining, whether or not a weld is visible depends mainly on the joining technique used. While most welding technologies result in the creation of a weld bead, laser welding fuses the lips of the parts to be welded. This technology brings the metal up to its melting point, making the weld invisible to the naked eye once it has cooled.
When joining wood, the welding technique used is friction welding with the parts held together under pressure, triggering a chemical reaction that gives the lignin an adhesive character. This creates welds which, although highly resistant, are visible to the naked eye due to the very structure of the fiber-based wood.
When it comes to joining plastics, the diversity of materials means that welding operations can be carried out or not, depending on their nature. In fact, it is thermoplastics which, by their very nature, have the melting capacity to create welds. Depending on the joining technology used, the resulting weld may be either visible or invisible.

In addition to durability and strength, the quality of a weld also depends on its aesthetic appearance (its visibility or, on the contrary, its invisibility). Depending on the type of component to be assembled and the target customer, the visibility of the weld can be a sign of quality (mechanically-welded assemblies, industrial machinery…) or, on the contrary, a symbol of non-quality (swimming pool covers, plastic packaging…).

Ultimately, the invisibility of a weld depends not only on the properties and characteristics of each material, but also on the precision and efficiency of the welding technique used.

What technologies are needed for invisible welds on plastics?

Depending on the technologies used to weld plastic materials, the results may be welds that are easily visible or welds that are undetectable to the naked eye.

External heating methods for welding generally result in more or less visible welds:

Hot-air welding begins by grooving the parts to be joined via a V-groove before depositing the filler material. The resulting weld is a highly visible bead.
Rotation or vibration welding uses pressure and circular movements to join two plastic parts, making them watertight and strong. With its low-profile welds, this technology is designed primarily for round, oval and cylindrical parts.
Blade or mirror heating welding simultaneously softens the surfaces of the two parts to be joined, before they are pressurized to shape. This technology can be used to join complex geometries and curved surfaces. Cooling ensures a strong, durable joint in the form of a welded seam.

However, it is the plastic assembly processes used on the internal structure of plastic parts that make it possible to produce invisible welds:

Ultrasonic welding joins two parts by creating molecular agitation of their edges at the point of contact, using high-frequency oscillations (20 to 70 kHz) generated by a vibrating tool known as a sonotrode or welding head. This technology is designed for welding polymers with relatively low melting temperatures (ABS, PP, PE, PMM, PS…). It enables very rapid bonding of the components to be joined, in the form of a smooth, clean welding surface that is not perceptible to the naked eye.
High-frequency welding enables several parts to be joined instantaneously without contact, thanks to molecular vibrations generated by a high-frequency electromagnetic field (27.12 MHz) applied by one or more electrodes placed on an electric current generator. This type of welding is used to join high-melting polymers (PVC, PEEK, PET, PETP, PC, PA…). The weld created by this method is solid, clean, neat and, above all, invisible.

In this way, invisible plastic welding is generated by ultrasound or high frequency, thanks to molecular agitation that modifies the structure of the material, creating continuity between the two joined surfaces.

Which thermoplastics can be welded invisibly?

All thermoplastics can be joined using ultrasonic or high-frequency welding technologies. Thanks to their physico-chemical characteristics (linear molecular chains sliding on top of each other), creating molecular agitation enables homogeneous, uniform and invisible welds.

Whether semi-crystalline (opaque or milky to the naked eye) or amorphous (transparent in appearance), the following thermoplastics can be joined by invisible welds:

EVA (ethylene vinyl acetate),
PETG (polyethylene glycol terephthalate),
PVC (polyvinyl chloride),
PA (Polyamide),
PC (Polycarbonate),
PETP (Thermoplastic polyester)
PET (polyethylene terephthalate),
PEEK (Polyetheretherketone),
ABS (Acrylonitrile butadiene styrene),
PS (polystyrene),
PMM (polymethyl methacrylate),
PE (Polyethylene),
PP (Polypropylene),
…

While it is essential for most thermoplastics to be joined that the weld is invisible, this is not always the case. This depends on the material and the application objectives of each industrial sector. For example, PVC sheets need to be welded invisibly in the telecoms sector (cell phone covers, etc.), stationery (binders, diaries, etc.), promotional items (cases, displays, cases, etc.) and leather goods (pouches, wallets, etc.). On the other hand, for PETG parts used in protective equipment for industry (machine-tool hoods and casings), medical devices, orthopedic parts and cosmetics containers, the invisible nature of the weld may be optional for the former and mandatory for the latter.

It is therefore essential for players in these sectors to use high-performance welding equipment in terms of both weld quality and production volume. To meet these needs, Matrelec has designed and developed a range of welding rails and high-frequency welding presses (partially or fully automated) capable of producing invisible welds at high production rates for thermoplastics.