There are several types of joining of aluminum composite panels: gluing, welding, mechanical joining, riveting, screwing, profile joining. Let’s take a closer look at each of them.
Gluing
Aluminum composite surfaces can be bonded with any metal adhesive or with suitable universal adhesives. When bonding composite materials to other materials, it is necessary to use highly elastic adhesives (e.g. polyurethane-based one-component adhesives). When joining large surfaces of materials of different chemical groups, preliminary tests are recommended to clarify the response of the surfaces, since differences in linear tensile coefficients may lead to undesirable deformations.
In case polyethylene is used as filler of the composite material, butt-gluing of the panels is practically unfeasible due to the low surface energy of this polymer. It is possible to use modern special adhesives for polyethylene and polypropylene, but the best solution is still other joining methods: welding or mechanical fastening. If the intermediate layer is filled with polyurethane or mineral filler, adhesives suitable for butt joints can be used, but it should be noted that the adhesive bond may be destroyed under high mechanical shear, bending or shear loads.
If there are no great demands on the strength of the product or when fixing small parts, double-sided adhesive tapes with maximum adhesion can be used. Split joints can be conveniently made using assembly aids such as self-adhesive Velcro and snap fasteners.
Welding
Welding of composite materials can be performed by the most popular method for joining polymeric materials: hot air welding (t = 260 – 270 °C) using welding wire (solder). With the help of welding machines, the inner layer of the composite material and the welding wire are heated and firmly joined. The decisive factors for a quality joint are: the necessary preparation of the surfaces to be welded, the quality of the welding wire, the impurity-free hot air, the required temperature and pressure and the speed of the welding process.
The most convenient technique of hot air welding is the accelerated welding method with the use of a special wire-fiber pulling device. This method provides uniform heating of the surfaces necessary for a strong bond, pressure and high productivity of the operation.
To achieve optimum joint quality, the ends of the composite material to be welded should be beveled prior to welding. The bevel angle depends on the profile and width of the thread. Once the edges have been prepared in this way, welding should be carried out within 24 hours. This requirement is due to the possible reaction of the putty with oxygen, which may impede welding or reduce its strength.
For joining composite panels with polyethylene filler, it is recommended to use polyethylene soft solder, the surface of which is sanded before soldering.
After welding, an overlap is formed on the joint surface. It is removed with a profiled knife after the weld has cooled completely. The blade of the knife must be kept with the maximum inclination towards the surface of the welded parts.
Note: there are composite materials whose inner layer is not a solid filler, but a thin polymer (PE) profiled sheet with longitudinal air sections. These materials cannot be joined or butt welded due to the negligible contact surface.
Mechanical connection
The mechanical joining methods for composite materials in lightweight structures are similar to those for plastics and sheet metal. It is advisable to use plastic, aluminum and stainless steel fasteners that are not subject to corrosion. If the material of the screws, bolts or rivets is unstable to environmental influences and over time may spoil the appearance of the product or weaken the strength of the connection, these elements should be varnished or insulated with gaskets before installation.
When panels are used for construction purposes and in the manufacture of large advertising structures, the question of fastening must be approached responsibly. In these cases, not only the appearance of the product, but also the safety of people and property depends on the strength of the joint. Special self-tapping screws have been developed for composite panels. The screw tip is shaped like a hardened steel drill bit. It is this part of the fastener that bears the main load of drilling a hole not only in the composite, but also in the supporting structure material, e.g. steel purlins. The rest of the screw is made of stainless steel or galvanized steel. Stainless steel bolts are more durable and reliable than galvanized bolts, which are very sensitive to moisture and aggressive media. During the screwing process, the zinc layer is severely damaged, which further reduces the service life of the screw (up to 2 – 5 years). The service life of the stainless steel screws is comparable to the durability of the composite panels themselves.
Screws for composite materials have the following characteristics:
- The diameter of the part of the bolt to be fastened in the bearing base is smaller than the diameter of the part of the sub washer. This is necessary to securely fasten the screw into the thin aluminum layer of the panel.
- The diameters of the threads are also different: the central thread has a smaller diameter than the thread of the lower washer. This provides additional sealing.
The connection is sealed with an EPDM (ethylene propylene diene diene terpolymer) washer, the most weather resistant, durable synthetic material suitable for outdoor use.
Composites, like other materials, expand and contract under the influence of temperature fluctuations. When installing panels in the street, it is especially important to take this factor into account. The coefficient of linear expansion of the composite is based on the properties of the aluminum coating and has values of 0.023 – 0.025 mm/m°C depending on the manufacturer. To avoid irreversible deformations, the diameter of the mounting holes and the profile openings must be correctly calculated according to the possible maximum and minimum operating temperatures. The shape of the profile must guarantee a reliable fixing of the panel in all temperature conditions and under maximum mechanical loads (wind, vibrations, etc.).
Riveting
Composite panels can be joined by rivets suitable for aluminum. When the structure is used outdoors or in humid areas, it is recommended to use aluminum or stainless steel blind rivets and to leave the necessary clearance for the free movement of the sheet during expansion. Countersunk rivets do not allow material movement and are therefore not suitable for outdoor use. To prevent the thin aluminum layer of the composite material from bending towards the filler during riveting (in the case of a non countersunk hole), cushioning pads can be used to protect the material from excessive and uneven pressure of the rivet head on its surface.
Bolted joint
When mounting composite panels indoors, metal or wood screws with heads of various shapes can be used. To create a clean connection and avoid increased local internal stresses when using countersunk screws, it is necessary to enlarge the hole beforehand with a countersinking tool. At constant ambient temperature, it is not necessary to leave gaps for free movement of the material.
It is recommended to fix the panels in exterior conditions using strong facade screws with watertight joints. The screws should not be overtightened. Easily removable plastic decorative washers and plugs can be used to conceal the fasteners, the color of which can be matched to the color of the panel. Outdoor use requires taking into account the dimensional variations of the panels and complying with the relevant installation standards.
Profiles
Strong joints in the manufacture of advertising structures can be created using a wide variety of profile systems. Plastic and aluminum profiles in P-, U-, L- and H-shape, as well as folding frames with snap-in mechanism, are commonly used indoors.
Outdoor conditions require a more careful approach. The panel in the profile must be installed with a gap that allows free movement of the material. In the case of large structures, to increase their resistance to various types of loads and for safety reasons, it is advisable to fix the profile at several points with screws and press its shelves to the surface of the panel (if the thickness and material of the profile allow it).
Several specialized profile systems are available on the market for the installation of ventilated facades.
Attention. When installing constructions composed of several (or numerous) identical panels, it is necessary to install each panel in the same direction. This will be easy to do if the protective film with markings is not removed beforehand. Failure to observe this rule may result in identical elements appearing different from each other at different viewing angles and in bright light.
