Hot air rivets

Riveting is used to create non-detachable form-fit connections. The connection is achieved by remoulding the thermoplastic material. In this process, only spot or segment-type joining seams are usually realised. In contrast to the welding of two components, no tight welds can be achieved. A special variant of this process is the joining of plastics with metals, textiles and cardboard.

The process is frequently used for wooden decorative parts in which a chrome trim is embedded.

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Riveting of plastics

Properties:

• Joining the components by forming or reshaping
• Production of a point or segment-type joint seam
• Overlapping of the joint seam required
• One joining partner must have a through hole

Advantages:

• Compounds of dissimilar and identical plastics
• Joining plastics with metals, textiles or cardboard
• Cost-effective assembly and joining technology

In contrast to ultrasonic riveting, thermal riveting can be used for sheet metal/plastic joints without any problems, as no vibrating tools touch the sheet metal and therefore jeopardise the joint. Thermal riveting achieves a very high joining quality, particularly with welding-critical plastics such as POM or PA-GF, which tend to become brittle with ultrasonic riveting. The process is also of great benefit where contamination of the parts due to the splitting off of brittle particles must be avoided at all costs for safety reasons.

Hot air rivets

Hot-air riveting works without contact during the warm-up phase. A continuously circulating stream of hot air heats the rivet tang. Here, the rivet head is formed with a cold die in a separate station. The downstream cooling phase under the influence of pressure and forced moulding results in less relaxation of the plastic. The heating temperature is usually higher than 300 °C. In order to achieve consistent process conditions during heating, comprehensive control and regulation of all parameters is essential.

Air volume monitoring has a decisive influence. The air outlet nozzle is another important element that determines the air flow. It is designed and customised depending on the application. As a result, hot-air riveting also involves only minor relaxation processes, so that strong joints with a very good press fit are achieved. In principle, all thermoplastics can be processed using this method.

Basic principle

Second act process:

• Contactless warm-up phase of the rivet stud
• Moulding of the rivet tang in a separate cold stamping station
• Cold stamp cools the rivet head

Advantages:

• High rivet quality
• No material adhesion
• homogeneous heating of the rivet tang

Disadvantages:

• long process times, machine cycle times >20 seconds

Basic sequence:

• Place article in holder and sliding table moves to the rear (working area)
• Component/dome is plasticised on the right-hand side
• Article holder moves to the left side
• Component/dome are moulded with cold stamp
• Pick-up moves forwards for removal

Generally, the systems for riveting plastic parts are designed as two-stroke systems. One way of reducing the overall cycle time for the riveting process is to use rotary indexing tables. In the example above, a plastic pass-through is to be riveted to a metal sheet. In the heating station, the required heat is introduced into the rivet studs by applying heat. In a second station, the cold stamping unit, the homogeneously heated rivet studs are formed under the influence of pressure and moulding pressure.

The prerequisite for a high-quality welded joint is a professional and application-specific joint design. We will be happy to provide you with advice and assistance in designing your plastic parts for welding.

Due to the different material properties of thermoplastics and the resulting boundary conditions during plasticising, the individual process variants have been developed. The advantage of these processes is that plastic-metal connections or dissimilar materials can be produced with a clearance-free press fit. Strength values in the range of the material characteristics of the plastic are achieved. The further development of hot-air riveting in particular has recently led to a wide range of applications in the field of hybrid components.

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