Chloe Zhang
Joining metal and plastic can be challenging due to their dissimilar properties, but several techniques are available. Mechanical fastening, such as riveting, is a common method that provides reliable joints without fusing surfaces. Welding methods like ultrasonic welding and laser welding can also be used, but the process differs for metals and plastics. Adhesives, such as structural MMA adhesives and cyanoacrylate adhesives, are another option but require careful surface preparation to ensure optimal adhesion and durability. The choice of joining technique depends on the specific application and desired outcome.
Characteristics and Values of Joining Metal and Plastic
| Characteristics | Values |
|---|---|
| Goal | Enhance product design flexibility to use differing materials in an efficient and functional manner based on their specific properties |
| Metal-to-polymer assemblies | Combine the strength and ductility of metal with the physical-chemical resistance and light weight of polymers |
| Metal use case | Sections where high stiffness and strength are needed |
| Plastic use case | Provides unique chemical properties |
| Joint contribution | Maximize the contribution of each material to ensure optimal operational performance while maintaining a weight- and cost-effective approach |
| Joining technique | Several methods like mechanical fastening, ultrasonic welding, laser welding, heat stacking, and adhesives are used for hybrid joints |
| Mechanical fastening | Clamping components like screws and rivets are used without fusing joint surfaces |
| Riveting | A reliable joint that may require a heating cycle before fastening so rivets shrink upon cooling |
| Ultrasonic welding | Ultrasonic vibration is applied to weld metals and plastics, with the direction of oscillation differing for metals and plastics |
| Laser welding | Offers unique manufacturing opportunities for joints that were previously difficult or impossible |
| Adhesives | Structural MMA adhesives like Araldite 2050 and cyanoacrylate adhesives like Permabond 910 are used; surface preparation and environmental factors are critical considerations |
| Surface preparation | Metals may require treatments like abrasion and degreasing, while moisture, pollution, and plasticizers can affect the surface energy of plastics |
| Challenges | Joining dissimilar materials can be difficult, and the behavior of joints is not always fully understood |
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What You'll Learn
Mechanical fastening
There are two main categories of mechanical fasteners: permanent and non-permanent. Screws, for example, are non-permanent fasteners as they can be removed, replaced, and reused. They are reliable and inexpensive, and they do not require molded-in threads or threaded inserts, reducing molding and assembly costs. Non-permanent fasteners allow for disassembly and reassembly of the joint, which is advantageous for maintenance and repairs. However, they increase product cost due to increased assembly time and can be more challenging to handle and align.
On the other hand, permanent fasteners such as snap-fits are integrated into the parts through direct molding and are more robust, with less potential for working loose. Snap-fits, along with tabs, are among the lowest-cost and most commonly used plastic fastening techniques for high-volume plastic parts assembly. They use features molded into the plastic part assembly to fasten pieces together.
When using mechanical fasteners to join metal and plastic, it is important to consider the specific requirements of the application, such as cost, quality, manufacturability, and plastic assembly techniques. The type of fastener should be chosen based on technical factors that impact performance, including corrosion resistance, material compatibility, and conductivity. For example, when using thread-locking liquids to secure metal fasteners, it is crucial to test for chemical compatibility with the thermoplastic material to avoid potential damage.
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Welding
To weld plastic, you must first clean the surface with soap and water, and sand it with 80-grit sandpaper. Secure the plastic pieces together by tack welding the ends, melting the ends of the joint to ensure they are secure. Next, preheat your welding gun for at least 2 minutes, setting it to the correct temperature for the type of plastic you are working with. The temperature range for welding plastic is between 200 and 300 °C (392 and 572 °F). Once the welding gun is preheated, insert a welding rod into the speed nozzle and hold the gun downward at a 45-degree angle. Move the tip of the welding gun slowly over the area, melting the plastic and feeding the welding rod into the nozzle with your free hand. Finish the weld by smoothing it out, creating a strong and cost-effective fix.
Ultrasonic welding is another method that can be used to join both metals and plastics. This technique is characterised by low energy input and requires the clamping and positioning of the workpieces between the welding tool (sonotrode) and an anvil by static force. No microstructural changes occur in the metal, and the weld achieved depends on how the ultrasonic energy (vibration) is delivered to the weld.
Laser welding is another method that offers unique manufacturing opportunities. It can be used to fabricate and process joints that are difficult or impossible to achieve with other welding methods. Laser welding can be used to achieve stable metallic, chemical, and covalent bonds between metal and polymer hybrid components. The metal-to-polymer joint interface is heated by a laser beam, melting the plastic material in a narrow region adjacent to the interface.
Friction spot welding has been used to weld dissimilar metals, and its feasibility for welding metal-to-polymer joints is an area of interest. A process called friction lap welding has been developed by engineers at the Joining and Welding Research Institute in Osaka, Japan, which successfully joined aluminium and nylon parts.
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Laser welding
The laser welding process involves using a laser beam to heat the metal surface and melt the plastic near the joint interface. This creates small bubbles in the plastic, which strengthen the joint. The specific steps may vary depending on the materials being joined and the desired outcome. For example, if the plastic sheet has high transparency, it should be placed on the upper side. A shielding gas may be used to protect the top plastic sheet surface from heat and contamination.
When using laser welding, it is important to consider the compatibility of the materials being joined. Plastics have much lower melting temperatures than metals. Therefore, it is crucial to select plastics with similar melting points to ensure they mix well in the weld pool and provide greater mechanical strength. Additionally, the laser beam must be able to travel through the top material to the joint interface, and the bottom layer must be able to absorb the laser beam to create the necessary heat.
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Adhesives
There are a variety of adhesives available for bonding metal to plastic. Cyanoacrylates, acrylics, epoxies, urethanes, elastomers, and silicones all reliably bond plastics to metals. For instance, Master Bond offers a range of epoxy adhesives for bonding plastic substrates to metal substrates in high-tech industrial applications. Their Supreme 3HT-80 product is a high-strength, one-component epoxy featuring a fast cure time and a wide service temperature range.
When choosing an adhesive, it is important to consider the environmental temperature as it can vary considerably. A flexible adhesive can absorb stresses from the different expansion and contraction rates of the materials. Additionally, the surface of the metal and plastic should be properly prepared to ensure good adhesion. Mold releases, plasticizers, particulates, chemicals, and oxidized layers can all affect the surface energy and adhesion of the materials.
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Heat stacking
Heat staking is a process used in plastic assembly where at least one of the parts is made from thermoplastic. It is a common method for joining metal parts to plastics or for joining two dissimilar plastic components. Heat staking combines the use of heat and pressure to soften a specific area of the plastic, allowing it to be remoulded into a new shape or profile that forms over or around the connecting part. Once cooled, the plastic's inherent strength holds the mating part in place, resulting in a solid, hardware-free bond.
Heat staking is a straightforward, effective, controllable, and repeatable process. It offers several advantages over other joining techniques. Firstly, it eliminates the need for extra hardware, which can be prone to defects, breakage, and other quality control issues. Fasteners tend to be more expensive and labour-intensive than heat staking. Heat staking can be performed in a single machine cycle and motion, making it faster and more efficient than fastener-based technologies.
Additionally, heat staking is safe to use with sensitive electronics and offers superior pull strength. Unlike ultrasonic plastic assembly, which uses high-frequency vibrations to generate heat for bonding, heat staking precisely controls temperature and pressure to reform the plastic at the contact point. This localised nature of heating protects nearby electronics from damage. Heat staking machines operate within tight temperature windows, making them ideal for complex and delicate applications in industries such as medical devices, automotive, telecommunications, and consumer products.
The versatility of heat staking makes it suitable for a wide range of applications. For example, it is used in the automotive and aerospace sectors for assembling LED Stamped Arrays or LED Flex Circuits into plastic housings. In the medical device industry, heat staking is employed for attaching adhesive patches to wearable glucose monitoring and medicine dispensing devices, providing a softer and more comfortable feel compared to ultrasonic welding.
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Frequently asked questions
There are several ways to join metal and plastic, including:
- Mechanical fastening: This method involves using clamping components such as screws and rivets to form a joint without fusing the surfaces.
- Welding: Ultrasonic welding and laser welding can be used to join metals and plastics, but the process differs for each material.
- Adhesives: Some adhesives are specifically designed for bonding metal to plastic, such as structural MMA adhesives like Araldite 2050 and Araldite 2051.
Mechanical fastening is a reliable method for joining metal and plastic and can form a strong joint without the need for fusing the surfaces. However, it requires additional mechanical operations such as drilling holes and making screw threads. It may also not be suitable for applications where the thickness of the joint is a critical factor.
When selecting an adhesive, it is important to consider the type of metal and plastic being joined, as well as the environmental conditions the joint will be subjected to. Each type of metal and plastic expands and contracts at different rates with changes in temperature, which can affect the performance of the adhesive. The surface energy of the plastic and the treatment of the metal surface can also impact the adhesion and durability of the bond.
