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Riveting of metal parts, also known as rivet connection, is a mechanical term that involves using axial force to deform the shank of the rivet inside the rivet hole, forming a rivet head and thereby connecting multiple parts.

Riveting of plastic parts involves plastic components as the main body, while the connected parts can be plastic components, metal parts (e.g., metal sheets), electrical components (e.g., PCBs), fabrics (e.g., mesh cloth), etc. Unlike metal riveting, which requires additional rivets or rivet posts, plastic riveting directly uses plastic structures like columns or ribs grown from the plastic body. These structures pass through the connected parts, and the protruding columns or ribs are heated, softened, and molded under the pressure of the riveting head. Once cooled, the riveting is completed.

Riveting Processes Based on Heating Methods:

Hot Melt Riveting:

This is a contact-type riveting method. Some techniques involve placing a heating tube inside the riveting head to heat the metal riveting head. This results in a larger metal riveting head and lower heating efficiency. Currently, common technology uses high-frequency pulse heating principles to allow the metal hot riveting head to self-heat, eliminating the need for heating blocks or tubes to conduct heat. This increases heating efficiency and results in a smaller metal riveting head, making it suitable for more applications.

Hot Air Riveting:

The hot air riveting process mainly uses hot air as the heating source to heat and form the rivet column. The entire process includes two stages:

In the first stage, hot air uniformly heats the rivet column to a malleable state. Stable temperature and uniform airflow are crucial for effectively heating the rivet column.

In the second stage, the cold riveting head presses the softened rivet column to form a firm rivet head. Since the rivet column has been fully heated and softened, the formed rivet head can securely fasten the parts to be riveted. In hot air cold riveting, the fit between the rivet column and the holes on the parts to be riveted should not be too loose. If the gap is too large, the softened plastic may fill the gaps during the riveting process, leading to insufficient rivet head size.

Ultrasonic Riveting:

This is also a contact-type riveting method. The process is as follows:

Choosing the Right Riveting Process: Pros and Cons

Common Advantages:

• Simple plastic part structure, reducing mold costs.
• Simple assembly process, no need for additional materials or fasteners, high reliability.
• Multiple rivet points can be riveted simultaneously, greatly improving assembly efficiency.
• Suitable for connecting not just plastic parts but also metal and other non-metal parts, especially in constrained spaces.
• Riveted parts are suitable for long-term mechanical vibration and extreme environmental conditions.
• Simple operation, energy-saving, fast, with easy visual inspection of product quality.

Common Disadvantages:

• Requires additional riveting equipment and tooling.
• Not suitable for high-strength or long-term load applications.
• Permanent connection, not suitable for detachable or repairable applications.
• Once failed, it’s difficult to repair, requiring consideration of redundancy in the design phase if necessary.

Specific Pros and Cons Comparison:

Comparison Item	Hot Melt Riveting	Hot Air Riveting	Ultrasonic Riveting
Riveting Strength	The metal hot melt head directly acts on the rivet post, melting while pressing, resulting in high stress. The post-riveting strength is unreliable and sensitive to vibrations.	The entire rivet post is heated, resulting in low stress. The post-riveting strength is high and not sensitive to vibrations.	The ultrasonic welding head directly acts on the rivet post with high-frequency vibrations, resulting in high stress. The post-riveting strength is unreliable.
Fixing Effect	The pressing head acts on the rivet post, softening and pressing simultaneously. The root of the rivet post cannot completely soften, leading to gaps in product assembly and flawed fixing effects.	Excellent. The entire rivet post softens and quickly forms under pressure, fully filling the assembly gaps.	The welding head acts on the rivet post, softening and pressing simultaneously. The root of the rivet post cannot completely soften, leading to gaps in product assembly and flawed fixing effects.
Riveting Speed	Small rivet point: 6-10s, Large rivet point: 50-60s	6-10s heating, 2s cooling	Less than 5s
Equipment Flexibility	Heating and riveting are integrated, customized according to the product, making changeover complex.	Heating and cold riveting can be adjusted independently. The air nozzle can be customized according to the rivet post, and the rivet point temperature is independently adjustable.	If it’s an integrated welding head, the depth or amplitude of the rivet point cannot be independently controlled.
Rivet Point Appearance	Bright and beautiful surface, easy to draw wires.	It can achieve brightness, mostly matte surface, no wire drawing.	Bright and beautiful surface.
Material Adaptability	The glass fiber may precipitate, affecting appearance.	Can rivet almost all common thermoplastic materials and glass fiber materials.	It cannot or difficult to rivet glass fiber materials.
Impact on Product	Contact heating riveting, the heat from the hot melt head can affect components or the product surface near the rivet post.	Non-contact heating riveting, does not damage components or product appearance.	Vibration heating riveting, vibrations can harm components.
Equipment Cost	Low	Medium	High

Other Aspects:

Hot Melt Riveting:

The advantages are clear. The rivet head simultaneously heats the rivet column and forms the rivet head. This allows for a very compact equipment design, especially suitable for small components with closely spaced plastic rivet columns.

However, there are also significant disadvantages. 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 because it becomes increasingly difficult to transfer surface heat to the center and bottom of the rivet column, potentially causing a cold core phenomenon and insufficient gap filling between the rivet column and the connected part. Additionally, products made using hot melt riveting tend to have relatively high residual stress and lower pull-out strength. Therefore, it is not suitable for products with high positioning and fixation requirements.

Hot Air Riveting:

Since the plastic rivet column is uniformly heated in a high-temperature hot air environment, the plastic rivet column softens completely from the inside out, effectively reducing internal stress after forming. In the second step, the cold riveting head presses and forms the completely softened plastic material, which can quickly fill over 90% of the assembly gap between the connected part and the rivet column, achieving a very good fixing effect.

Ultrasonic Riveting:

The riveting strength and fixing effect are similar to hot melt riveting. However, because ultrasonic riveting generates heat through friction, once the rivet point is formed, the ultrasonic generator stops working. Unlike hot melt riveting, the ultrasonic welding head does not carry heat, reducing the likelihood of filamentation. Ultrasonic riveting also takes the shortest time.

When using ultrasonic riveting, the rivet column should not be designed on planes with significant height differences, as this can cause amplitude differences at various rivet points, leading to uneven heating rates and potentially loose or degraded columns. The distribution distance of columns is also limited if using a single welding head. In contrast, hot melt or hot air riveting processes allow rivet columns to be designed on different planes and can achieve multi-point riveting at once, even over considerable distances.

Material Adaptability:

Riveting is only suitable for thermoplastic plastics, which can melt within a specific temperature range. Thermosetting plastics harden at a certain temperature and are difficult to rivet using the three methods described above. Therefore, people often choose thermoplastic plastics for riveting, and product structures frequently involve thermoplastic plastics.

Thermoplastic plastics are further divided into amorphous (also known as non-crystalline) plastics and crystalline (also known as semi-crystalline) plastics.

Non-crystalline Plastics:

These have disordered molecular arrangements and a distinct temperature (Tg, glass transition temperature) at which the material gradually softens, melts, and flows. Such plastics are suitable for all three riveting processes.

Semi-crystalline Plastics:

These have ordered molecular arrangements with a clear melting point (Tm) and recrystallization point. Before reaching the melting point, semi-crystalline plastics 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 more suitable for hot melt riveting.

Semi-crystalline plastics 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. Semi-crystalline plastics often have high melting points, requiring sufficient ultrasonic energy to melt the plastic. Therefore, they are more challenging to rivet than non-crystalline plastics. Achieving higher riveting quality for semi-crystalline plastics requires considering more factors, such as higher amplitude, suitable joint design, welding head contact, welding distance, and welding fixtures. To concentrate ultrasonic energy, the rivet column’s top should be designed for minimal initial contact with the welding head.

Additional material properties that affect ultrasonic riveting include hardness (higher hardness generally improves ultrasonic riveting), melting point (higher melting points require more ultrasonic energy), and purity (higher purity of raw materials enhances riveting effects, while impurities in recycled materials reduce performance).

Plastics with Fillers (e.g., Glass Fibers):

Plastics with fillers have significant differences in melting points between the plastic and the fillers. For hot melt riveting, temperature control within ±10° is crucial; high temperatures cause glass fibers to precipitate from the plastic, leading to adhesion and rough surfaces, while low temperatures cause cracks and cold forming. For ultrasonic riveting, greater vibration energy is needed to melt the plastic. High filler content results in residue and detachment at the riveting points, reducing riveting strength and reliability.

When filler content is below 10%, it may not significantly affect material properties. Fillers (e.g., glass fibers) are beneficial for riveting soft materials like PP, PE, and PPS. Filler content between 10%-30% reduces riveting strength, while content above 30% significantly impacts riveting performance.

Common Rivet Columns and Rivet Heads

1. Semi-Circular Rivet Head (Large Profile)

1). Suitable for rivet column diameters (D1) less than 3mm, preferably greater than 1mm to prevent breakage.

2). The height of the protruding part of the rivet column (H1) is generally (1.5-1.75) * D1.

3). The diameter of the rivet head (D2) is generally around 2 * D1, and the height (H2) is about 0.75 * D1. Specific numbers should be based on volume conversion S_head = (85%-95%) * S_column.

4). This type is most commonly used, generally for situations with low strength requirements, such as PCB boards and plastic decorative parts.

2. Semi-Circular Rivet Head (Small Profile)

1). Suitable for rivet column diameters (D1) less than 3mm, preferably greater than 1mm to prevent breakage.

2). The height of the protruding part of the rivet column (H1) is generally 1.0 * D1.

3). The diameter of the rivet head (D2) is generally around 1.5 * D1, and the height (H2) is about 0.5 * D1. Specific numbers should be based on volume conversion S_head = (85%-95%) * S_column.

4). This type has a shorter riveting time than the large profile semi-circular rivet head, generally used in situations with low strength requirements, such as FPC soft ribbons and metal spring pieces.

3. Double Semi-Circular Rivet Head

1). Suitable for rivet column diameters (D1) between 2-5mm.

2). The height of the protruding part of the rivet column (H1) is generally 1.5 * D1.

3). The diameter of the rivet head (D2) is generally around 2 * D1, and the height (H2) is about 0.5 * D1. Specific numbers should be based on volume conversion S_head = (85%-95%) * S_column.

4). This type has a slightly larger rivet column than the semi-circular head type. To shorten riveting time and achieve better riveting results, a double semi-circular head method is used, generally for situations requiring higher fixing strength.

5). The centers of the rivet column and the mold hot riveting head must align to obtain a neatly shaped rivet head.

4. Annular Rivet Head

1). Suitable for rivet column diameters (D1) greater than 5mm.

2). The height of the protruding part of the rivet column (H1) is generally (0.5-1.5) * D1, with larger diameters taking the smaller value. The inner diameter is 0.5 * D1 (to avoid shrinkage on the backside of the column).

3). The diameter of the rivet head (D2) is generally around 1.5 * D1, and the height (H2) is about 0.5 * D1. Specific numbers should be based on volume conversion S_head = (85%-95%) * S_column.

4). As the diameter of the rivet column increases, to shorten the riveting time and achieve better results while avoiding shrinkage defects on the backside, hollow rivet columns are used, generally for situations requiring higher fixing strength.

5). Hollow rivet columns are evenly heated inside and out, making it easier to achieve a neatly shaped rivet head.

5. Flat Rivet Head

1). Suitable for rivet column diameters (D1) less than 3mm.

2). The height of the protruding part of the rivet column (H1) is generally 0.5 * D1.

3). The diameter (D2) and height (H2) of the rivet head should be based on volume conversion S_head = (85%-95%) * S_column.

4). The connected part needs to have sufficient thickness for countersinking; otherwise, the connection will be unreliable and lack sufficient fixing strength.

5). Flat rivet heads are suitable for situations where the formed rivet head should not protrude from the surface.

6. Ribbed Rivet Head

1). The base diameter of the rivet column (D1) should be less than 3mm, with a top diameter (D3) of (0.4-0.7) * D1.

2). The height of the protruding part of the rivet column (H1) is generally (1.5-2) * D1, and H1 should be less than the length of the rivet column (L).

3). The diameter of the rivet head (D2) is generally around 2 * D1, and the height (H2) is about 1.0 * D1. Specific numbers should be based on volume conversion S_head = (85%-95%) * S_column.

4). Use ribbed rivet heads when needing a larger contact area for the rivet head and insufficient space exists to design a hollow rivet column.

7. Flanged Rivet Head

1). The base diameter of the rivet column (D1) should be less than 3mm, with a top diameter (D3) of (0.3-0.5) * D1.

2). The height of the protruding part of the rivet column (H1) is generally (1.5-2) * D1, and H1 should be less than the length of the rivet column (L).

3). The diameter of the rivet head (D2) is generally around 2 * D1, and the height (H2) is about 1.0 * D1. Specific numbers should be based on volume conversion S_head = (85%-95%) * S_column.

4). Flanged rivet heads are suitable for situations requiring crimping or wrapping of the connected part.

Tips: Looking to explore various rivet types and their specific applications? Check out our detailed guide on types of rivets.

Notes:

If the rivet column is on an inclined surface or is high from the base, design it as follows:

Since riveting is a permanent connection and difficult to repair once failed, we can design redundancy into the structure if necessary. For example, double the number of rivet columns and holes, use the yellow rivet columns first, and if repair is needed, use the white rivet columns for a second chance at repair.