Maya Ellison
When working with plastic rivets, it's important to understand the different types of plastics and riveting methods available. The three main riveting processes are hot melt or hot air riveting, cold riveting, and ultrasonic riveting. Each process has its advantages and disadvantages, and the choice of process depends on factors such as the type of plastic, the size and thickness of the parts, heating requirements, and the desired speed of assembly. Hot melt riveting, for example, is ideal for thermoplastic amorphous plastics, while semi-crystalline plastics are more challenging to rivet due to their higher melting points. Cold riveting is a clean, non-contact heating process that is suitable for precision applications, but it may be less efficient for large-scale production. Ultrasonic riveting is the fastest method, but it may not be suitable for larger rivet columns or parts with significant height differences. Understanding these factors will help ensure successful plastic riveting and prevent issues such as filamentation, cold core phenomena, and insufficient gap filling.
Characteristics and Values Table for Melting Plastic for Rivets
| Characteristics | Values |
|---|---|
| Type of plastic | Thermoplastic plastics with a glass transition temperature (Tg) that allows softening and melting within a specific range |
| Type of riveting | Hot melt riveting, cold riveting, ultrasonic riveting |
| Tools | Hot air rework station, soldering iron, dremel, drill, pop rivet gun, pliers, automotive brake shoe lining riveter, hot glue gun |
| Techniques | Grinding or drilling out rivets, using a backing washer, applying even pressure to rivets, using industrial adhesive |
| Disadvantages | Residual heat can cause plastic to stick to the rivet head, high residual stress and low pull-out strength in products, difficulty transferring heat to larger rivet columns, potential for cold core phenomenon and insufficient gap filling |
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What You'll Learn
- Hot melt riveting is best for compact, high-efficiency applications
- Semi-crystalline plastics are more suitable for hot melt riveting
- Hot air riveting allows multi-point riveting and is suitable for thermoplastic plastics
- Ultrasonic riveting takes the shortest time but is not suitable for significant height differences
- Cold riveting is clean, non-contact, and ideal for precision applications
Hot melt riveting is best for compact, high-efficiency applications
Hot melt riveting is a contact-type riveting method that uses a heated riveting head to soften the protruding part of the rivet column. This is inserted through the reserved hole in the plastic part, and the riveting head transfers its heat to the rivet, causing it to soften and become malleable. Once the rivet column is softened, pressure is applied to shape it, forming a permanent rivet head. After cooling, the connection is complete.
Hot melt riveting is best suited for compact, high-efficiency applications. It is a very fast and efficient process, especially for small plastic parts. It is a clean, vibration-free process with minimal heat spread, reducing the risk of damage to nearby components. This makes it ideal for precision bonding where high accuracy is required. It is also suitable for applications where a smaller, compact rivet head is required.
Hot melt riveting is faster than some traditional methods as it reduces the need for external heating elements. It is also suitable for multi-point riveting, even over considerable distances. However, it may require specialized equipment for precise heat control. The process needs careful control of temperature to prevent overheating or incomplete forming.
Hot melt riveting is particularly useful for semi-crystalline plastics, which have a clear melting point and recrystallization point. These plastics have ordered molecular arrangements, and before reaching the melting point, they remain solid. When the temperature reaches the melting point, the molecular chains begin to move, and the plastic starts to melt. If the heat decreases, the plastic quickly solidifies. Due to the dual function of heating the rivet column and forming the rivet point, semi-crystalline plastics are well-suited for hot melt riveting.
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Semi-crystalline plastics are more suitable for hot melt riveting
Hot melt riveting is a contact-type riveting method that uses a heated riveting head to soften the protruding part of the rivet column. Once the rivet column is softened, pressure is applied to shape it, forming a permanent rivet head. After cooling, the connection is complete. This method is suitable for applications where a smaller, compact rivet head is required. It is also faster than some traditional methods as it reduces the need for external heating elements. However, hot melt riveting may require specialized equipment for precise heat control.
Hot melt riveting is only suitable for thermoplastic plastics, which can melt within a specific temperature range. Thermosetting plastics, on the other hand, harden at a certain temperature and are difficult to rivet using hot melt riveting. Thermoplastic plastics are further categorized into amorphous (non-crystalline) plastics and semi-crystalline plastics. Amorphous plastics have a disordered molecular structure and soften at a distinct glass transition temperature (Tg). They are suitable for all three riveting processes: hot melt riveting, cold riveting, and ultrasonic riveting.
Semi-crystalline plastics, on the other hand, have an ordered molecular structure and a clear melting point (Tm). They are more challenging to rivet, especially with ultrasonic methods, due to their higher melting points and difficulty in absorbing ultrasonic energy. However, semi-crystalline plastics are more suitable for hot melt riveting compared to amorphous plastics. This is because semi-crystalline plastics have a dual function of heating the rivet column and forming the rivet point. They have a regular, spring-like molecular structure that easily absorbs high-frequency ultrasonic vibration energy, making it difficult to generate heat at the rivet joint.
Additionally, semi-crystalline plastics have strong intermolecular forces, resulting in tough plastics with excellent chemical resistance. They also have a very low coefficient of friction and good toughness. However, achieving higher riveting quality for semi-crystalline plastics requires considering factors such as higher amplitude, suitable joint design, welding head contact, welding distance, and welding fixtures. Overall, while semi-crystalline plastics pose some challenges, they are indeed more suitable for hot melt riveting compared to amorphous plastics.
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Hot air riveting allows multi-point riveting and is suitable for thermoplastic plastics
Hot air riveting is a process that uses hot air to heat and form rivets. It is a non-contact heating method that is suitable for thermoplastic plastics. Thermoplastic plastics have a distinct glass transition temperature (Tg) at which they gradually soften, melt, and flow. This makes them ideal for riveting as the softened plastic can be moulded under pressure.
Hot air riveting offers several advantages over other riveting methods. Firstly, it allows for multi-point riveting, meaning that multiple rivet points can be riveted simultaneously, even over considerable distances. This is in contrast to single-welding head methods, which have limited distribution distances for the columns. Additionally, hot air riveting is a clean and uniform heating process that does not require direct contact with the rivet, reducing the risk of pollution, vibrations, and displacement. The absence of additional heating elements simplifies the process and improves efficiency.
However, hot air riveting also has some disadvantages. Temperature control is critical to achieving uniform heating, and it may be less efficient for large-scale production compared to direct heating methods. Additionally, if the fit between the rivet and the holes in the plastic parts is too loose, the softened plastic may fill the gaps during the riveting process, resulting in an insufficient rivet head size.
Hot air riveting is particularly useful in precision applications where cleanliness and accuracy are crucial, such as in the electronics and medical device industries. It is also suitable for connecting not just plastic parts but also metal and other non-metal parts, especially in constrained spaces where other connection methods may be challenging to adapt.
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Ultrasonic riveting takes the shortest time but is not suitable for significant height differences
Ultrasonic riveting is a highly efficient and energy-saving technique used to join thermoplastics to each other or to non-plastics. It is particularly useful when fusion joints are not possible, when metal parts need to be inserted into plastic housings, or when the joint is not visible. Ultrasonic riveting generates heat through friction, and the rivet point is formed, the ultrasonic generator stops working. This reduces the likelihood of filamentation, a common issue with hot melt riveting.
Ultrasonic riveting also has the shortest cycle time of all riveting methods. However, it is not suitable for all applications. For instance, it is not recommended for significant height differences in the rivet column design. This is because amplitude differences at various rivet points can lead to uneven heating rates, causing loose or degraded columns.
Ultrasonic vibration can improve the riveting quality by reducing the riveting force and friction. This promotes the flow of rivet material, increases the diameter of the nail rod, and improves shear strength. The improvement effect is directly proportional to the amplitude. However, at the end of the rivet upsetting process, cold work hardening can occur, increasing yield strength but reducing plasticity. This makes continuous riveting challenging and can lead to inadequate installation quality.
Ultrasonic riveting is widely used in the automotive industry for interior trim and airbag parts. It is also used in the manufacture of housings, household goods, electronic consumer goods, and toys. While it offers advantages in speed and energy efficiency, it is important to consider its limitations, such as the sensitivity to height differences in rivet column design.
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Cold riveting is clean, non-contact, and ideal for precision applications
Cold riveting is a mechanical fastening process performed at room temperature, where a rivet, typically a ductile metal pin with a head, is inserted into aligned holes in two or more materials. The rivet is then deformed at the opposite end, creating a second head that secures the materials together. This process is clean and non-contact because it does not involve heating the rivet, which distinguishes it from hot riveting.
Cold riveting is ideal for precision applications due to its reliability and consistency. It provides a durable, sound, and reliable joint, especially with its anti-loosening capability, even under incessant vibration. This makes riveting a preferred method over welding in many cases, including bridge construction. Cold riveting is also suitable for smaller rivet columns, where it can effectively fill the assembly gap between the connected part and the rivet column, achieving a good fixing effect.
The aerospace industry, for example, favours cold riveting for numerous critical applications due to its stringent safety requirements and performance demands. Different types of rivets are used in various aerospace applications, such as flush-head rivets for skin-to-frame attachments, countersunk rivets for wing skin panels, and titanium rivets for control surface hinge brackets.
Cold riveting is also advantageous in applications where high residual stress and pull-out strength are not required. It is important to note that hot melt riveting can cause higher residual stress and lower pull-out strength, making it unsuitable for products with high positioning and fixation requirements. Therefore, cold riveting is a preferred method when precision and clean, non-contact processes are necessary.
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Frequently asked questions
Thermoplastic plastics are suitable for riveting. These plastics melt within a specific temperature range. Within thermoplastic plastics, amorphous plastics and semi-crystalline plastics can be used for riveting.
Hot melt riveting involves heating the plastic rivet column in a high-temperature hot air environment. The rivet column softens and pressure is applied by the welding head to compress and form the rivet, creating a secure connection between the parts.
If the rivet head does not cool completely, residual heat can cause the plastic to stick to the rivet head, resulting in filamentation. The rivet head needs frequent replacement. It is not suitable for larger rivet columns as it becomes difficult to transfer surface heat to the center and bottom, potentially causing a cold core phenomenon and insufficient gap filling. Products made using hot melt riveting tend to have high residual stress and lower pull-out strength.
Hot melt riveting is a clean, non-contact heating process that avoids pollution, vibrations, and displacement. It is ideal for precision applications where cleanliness and efficiency are crucial, such as in electronics and medical devices. It also allows for multi-point riveting and can be used on different planes.
To avoid damaging the plastic, it is recommended to use a Dremel to grind off the back of the rivet slowly. Drilling can generate heat and cause the plastic to melt. Using a towel with an ice cube on the other side of the rivet can help keep it cool and prevent melting.
