Alexei Volkov
There are many ways to mount a small magnet to plastic, and the best method depends on the specific use case. A common method is to use glue, which is simple and popular, but it is important to select the right type of glue. The glue should form a strong bond, be suitable for the materials, and be able to withstand different temperatures. Other methods include over-moulding, insert moulding, ultrasonic welding, snap design, and more. In some cases, a combination of methods may be required. For example, drilling a hole and using a screw to secure the magnet in place. The size and strength of the magnet are also important factors to consider, as they depend on the weight and size of the item being mounted.
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Using glue
Gluing magnets to plastic can be a challenging task, and the process may vary depending on the type of magnet and plastic involved. Here is a step-by-step guide on how to use glue to mount a small magnet to plastic:
Surface Preparation:
Firstly, prepare the surfaces of both the magnet and the plastic by cleaning them thoroughly. Use a cloth or swab dipped in isopropyl alcohol or rubbing alcohol to wipe down the surfaces, ensuring they are free of dirt, grease, fingerprints, and oil. This step is crucial, as contaminants can weaken the adhesive bond. For magnets with smooth nickel coatings, like neodymium magnets, lightly abrade the surface with fine-grit sandpaper to enhance adhesion, and be sure to remove any residual dust.
Choose the Right Adhesive:
Selecting the appropriate adhesive is critical to ensuring a strong and durable bond. Consider the specific type of plastic and magnet you are working with, as different adhesives have varying affinities for different materials. For example, some plastics, such as polypropylene and polyethylene, are challenging to bond with adhesives, and you may need to consider alternative methods or mechanical bonds. On the other hand, plastics like ABS and PVC tend to work well with a variety of adhesives.
Epoxy adhesives, such as Epoxy Resin AB adhesive, are widely used for bonding plastics and magnets in manufacturing processes. They offer a strong and temperature-resistant bond. Instantaneous adhesives (cyanoacrylate glue) are suitable for small area bonding and can be used with ABS, PVC, PC, and PMMA, but not PP or PE. If you are working with rare earth neodymium magnets, consider using AB glue. For temporary fixing or low-cost solutions, hot melt adhesive, instantaneous dry adhesive, or double-sided adhesive tape can be used.
Apply Adhesive and Assemble:
Follow the instructions on your chosen adhesive for application guidance. Typically, you will apply the adhesive to the plastic surface, carefully place the magnet in the desired position, and then allow the glue to dry undisturbed. Ensure you press and fix the magnet in place for at least a few hours, and up to 24 hours for complete curing, depending on the adhesive's instructions.
Post-Assembly Care:
Once the adhesive has cured, you can use your magnet-mounted plastic assembly. However, be cautious of high humidity environments, as these can affect the bond strength. Additionally, avoid using the assembly near steel products or other magnetic assemblies that might interfere with the magnet's function.
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Drilling a hole
Forstner drill bits are suitable for creating larger flat-bottomed holes, while brad point drill bits can be used for smaller holes. When using a brad point drill bit, it is important to select a bit that is smaller than the magnet to ensure a snug fit. If the magnet is 1/16" in diameter, for example, a brad point drill bit smaller than 1/8" would be appropriate.
To enhance the security of the magnet, consider drilling a blind pocket hole. This involves drilling a hole that doesn't go all the way through the material, leaving a thin layer of plastic at the bottom. The magnet can then be inserted into this pocket, creating a strong and discreet hold.
After drilling the hole, it is essential to use an appropriate adhesive to secure the magnet in place. Super glue, epoxy glue, or a custom glue made from acetone and plastic shavings from the drilling process can be used to create a strong bond between the magnet and the plastic.
Additionally, you can use a retaining plate or a decorative cover plate to further secure the magnet and enhance the aesthetics of the final product. These plates can be attached using adhesives, fasteners, or other suitable methods.
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Using snap-fit features
Snap-fits are a common connection method in product design, especially for plastic components. They are an effective, economical, simple, and convenient way to connect plastic parts. Snap-fit joints are created by using the flexible nature of plastic to create secure joints without the need for screws or glue.
Snap-fit joints are easy to assemble as they simply snap together without the need for separate fasteners like screws or bolts. They also offer improved aesthetics as they provide a seamless appearance with no visible fasteners. Snap-fits are also cost-saving as they eliminate the need to purchase separate fasteners and reduce assembly labor costs.
However, snap-fits also have some disadvantages. They can be complex to design, requiring careful consideration of material properties, deflections, undercuts, and radii to avoid excessive stress concentrations. Undercuts can also increase manufacturing complexity and costs. Snap-fits may also break if overstressed during assembly or disassembly, and they may not be as strong as using separate screws or fasteners, especially for high load applications.
When designing snap-fits for 3D printing, it is important to start with accurate measurements of the electronic component. Digital calipers or a ruler can be used for this purpose. It is also crucial to choose plastics with good flexibility, impact resistance, and fatigue strength, such as polyoxymethylene (POM/acetal), nylons (polyamides), and polypropylene (PP).
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Ultrasonic welding
The process is ideal for small parts, such as consumer electronics, and can provide clean and precise joints without the need for drying time, making it a popular choice for various industries, including electrical, automotive, and medical fields.
When welding plastics, the interface between the two parts is designed to concentrate the melting process. One part usually has a spiked or rounded energy director that contacts the other plastic part, allowing the ultrasonic energy to melt the point of contact and create a joint. The easiest plastics to weld ultrasonically are polyphenylene oxide (PPO) and acrylonitrile butadiene styrene (ABS).
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Press fit
Choose the Right Magnet and Plastic Type:
Select a magnet type suitable for press fitting. Neodymium magnets, for instance, are commonly used but require careful handling due to their brittle nature. Consider using plastic-coated magnets, as they may be more likely to survive the press-fitting process than regular nickel-plated options. Additionally, choose a plastic with suitable mechanical and thermal properties for your specific application. Consider factors such as temperature range, humidity, and the presence of fluids. Some plastics, like nylons, can undergo significant elongation at specific humidity levels but may relax if the temperature is too high. Glass-filled plastics can provide improved high-temperature performance.
Design the Pocket or Hole:
Design the pocket or hole in the plastic to accommodate the magnet. Size the pocket appropriately, ensuring it is not too tight or too loose. Consider machining a slight lip at the edge of the hole, allowing for a snug fit against the magnet after pressing it in. Additionally, you may need to drill a blind pocket hole, creating a cushion at the bottom to securely hold the magnet in place.
Prepare the Magnet and Plastic:
Before press fitting, consider cleaning the magnet and plastic surfaces to ensure optimal adhesion. If using adjustable reamers or drilling, ensure precise sizing and alignment. You can also heat one end of a metal rod and insert the magnet, allowing it to cool and secure the magnet in place. Alternatively, heat the plastic material slightly to make it more pliable for insertion, but be cautious not to deform its structure.
Execute the Press Fit:
Gently press the magnet into the prepared pocket or hole in the plastic. Ensure that the magnet requires only a gentle push to get it in, as excessive force can damage the magnet or deform the plastic. If necessary, use a non-magnetic material, such as delrin, as a cap or bumper on the press to facilitate alignment and protect the magnet during the process.
Secure and Test the Assembly:
After press fitting the magnet, ensure its stability by testing the assembly. Consider thermal cycling tests to check the magnet's ability to stay in place over temperature variations. If needed, use a small chip of plastic or similar material behind the magnet to enhance its compliance during thermal cycling. Additionally, you can reinforce the assembly by using adhesives or epoxy, but be sure to leave an air hole to relieve pressure when using epoxy.
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Frequently asked questions
There are several methods to mount a small magnet to plastic, including over-molding, insert molding, ultrasonic welding, using glue, snap design, and more. The best method depends on your specific needs.
When picking a glue, consider the strength of the bond, whether it will discolour, how quickly it dries, ease of application, and its ability to withstand hot and cold temperatures. Seek guidance from adhesive experts before choosing a glue.
Some glues that can be used to attach magnets to plastic include UHU Max Repair and two-component glue.
Depending on the specific application, you could use a screw, silicon, or a press fit.
If the box is made of ABS, you can drill a hole and set your magnet in it. Make liquid ABS glue with acetone and the shavings from your holes. Seal on your magnet as the plastic will bond with your case.
