Ethan Clarke
Screws can be used to fasten plastic, but it is important to be cautious when dealing with plastic as it is more sensitive to temperature changes than other materials. It is critical to pre-drill a hole before screwing into plastic, as plastic is softer than metal and the threads can be easily stripped while tightening the screw. Self-tapping screws are a good option for plastic as they form a unique thread when driven into the material. However, they are not suitable for heavy loads. For a more permanent solution, pop rivets can be used, but they require a rivet tool and cannot be removed without drilling.
| Characteristics | Values |
|---|---|
| Screw type | Self-tapping screw, drywall screw, machine screw, thread forming screw, thread cutting screw |
| Screw features | Narrow thread profile, increased pull-out resistance, lower radial stress |
| Screw selection factors | Material being screwed into, torque, clamping retention, material stiffness, flank angle |
| Screw insertion technique | Pre-drill pilot hole, lubricate with soapy water, use faucet wrench, distribute pressure with washers, avoid overtightening |
| Alternative methods | Pop rivets, plastic welding, glue or adhesives, soldering iron |
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What You'll Learn
Use self-tapping screws
Self-tapping screws are a great option for screwing into hollow plastic. They are designed to form or cut threads when screwed into a pre-drilled or moulded pilot hole. This makes them ideal for joining two pieces of plastic or even attaching plastic to other materials like metal.
There are two main types of self-tapping screws: thread-forming screws and thread-cutting screws. Thread-forming screws, also known as "plastic" screws, are perfect for soft materials like plastic, nylon, or thermoplastics. They create threads by pushing and displacing the material rather than cutting it, which minimises the risk of cracking or splitting. This displacement of material also results in a tight fit, as the displaced material rebounds and fills the space between the screw threads.
Thread-cutting screws, on the other hand, have a sharp cutting edge that cuts into the plastic as the screw is driven in, removing material as it goes. This type of screw is more suitable for stiffer plastic with a higher flexural modulus. They also offer reduced internal stresses compared to thread-forming screws and can be used with a wider range of materials, including metal and hardwoods.
When using self-tapping screws, it's important to choose the right type of screw for the material you're working with. You'll also need to ensure you have the correct pilot hole diameter and depth. The hole depth should be deeper than the screw engagement length to allow space for the removed material to move. For thread-forming screws, it's crucial to have at least three threads engaged in the material to ensure a secure hold.
To install a self-tapping screw, start by drilling a pilot hole that is slightly smaller than the diameter of your screw. This will ensure the threads can engage properly. Position the screw perpendicular to the material to ensure smooth insertion and reduce the risk of stripping the screw head. Using a power drill or manual screwdriver, apply steady pressure at a consistent speed to drive the screw into the material. The screw should be flush with the surface, and there should be no movement in the joint.
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Avoid overtightening fasteners
When screwing into hollow plastic, it is important to avoid overtightening fasteners. Overtightening can cause damage to the screw thread, preventing it from maintaining a secure fit. This can also make it difficult to remove the screw without significant effort.
To avoid overtightening, it is recommended to tighten the screw until resistance is felt. Using the correct tools, such as a torque wrench, can help to avoid applying excessive force. Torque wrenches are designed to apply a specific torque to a fastener, allowing for precise control over the tightening process. Regular calibration and maintenance of these tools are essential to ensure their accuracy and prevent damage or malfunction.
Another way to avoid overtightening is to distribute the pressure to the fasteners by using large washers under the heads and nuts of the bolts or screws. This helps to spread the load and reduce the risk of cracking or breaking the plastic.
Additionally, it is important to use the proper drive size and style for the fastener. Using the wrong size can place uneven pressure on the recess, resulting in a stripped recess or a rounded nut.
By following these guidelines, you can avoid the negative consequences of overtightening fasteners, such as compromising the integrity of the fastener and the attached materials, and ensure the longevity and reliability of your project.
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Use pop rivets
Pop rivets are a popular choice for joining plastic materials due to their convenience, stability, and superior quality. They are especially useful when a lightweight, robust, and one-sided operation is required.
When using pop rivets with plastic, it is important to consider the hardness of the plastic. Plastic has a relatively low hardness compared to metal, so using normal high-strength pop rivets may damage the surface or structure of the plastic. To prevent this, it is recommended to use large flange head pop rivets or tri-fold rivets, which have a larger head area that can disperse the stress and reduce the risk of damage.
Pop rivets are commonly used in the assembly of plastic shells, such as routers, computers, televisions, and audio equipment. They can also be used to fasten the plastic casings of household appliances, such as washing machines, refrigerators, and microwave ovens, to their internal structures. This ensures the solidity of the appliance shell while reducing assembly cost and time, making them ideal for mass production.
The installation process of pop rivets is tool-controlled, preventing over-tightening of the material. Unlike screws or bolts, pop rivets do not exert excessive pressure on the plastic once installed. Additionally, pop rivets do not require expensive equipment or highly skilled operators, making them a simple, fast, and cost-effective solution for joining plastic materials.
However, it is important to note that pop rivets require a rivet tool for attachment and cannot be removed without drilling holes. They may also loosen over time, especially if subjected to strain.
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Pre-drill holes
When screwing into hollow plastic, it is critical to pre-drill a hole before inserting the screw. This is because plastic is a soft material, and without a pre-drilled hole, the screw can tear into the plastic, causing spider cracks. Spider cracks can cause the plastic to rip and tear, especially when exposed to cold temperatures.
Pre-drilling a hole for a screw will ensure a nice, clean connection. It is also important to note that the type of screw used is crucial. Self-tapping screws are designed to pre-drill the hole as they are screwed in, but they can still cause tears and spider cracks in the plastic. Therefore, it is recommended to pre-drill a hole before using a self-tapping screw.
Thread-forming screws for plastic do not have a pointed tip and require a pre-drilled hole. The screw thread deforms the plastic material, causing it to flow around the screw threads. The overlapping amount of thread engagement and the thread profile angle are critical factors in this process.
When selecting a screw, it is important to consider the material's stiffness and choose a screw with a suitable flank angle. The drive-to-strip torque ratio and clamping retention are also critical factors to consider when choosing the correct threaded fastener.
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Repairing stripped screw holes
Using Glue
One way to repair a stripped screw hole in plastic is to use glue. Apply a small amount of glue, such as Gorilla Glue or CA glue, to the inside of the stripped hole with a toothpick. You can also add a small amount of baking soda to the glue to create a makeshift cement. Allow the glue to dry and expand overnight. This method may need to be repeated several times for full effectiveness.
Using a Zip Tie
Another quick fix for a stripped screw hole is to insert a zip tie into the hole and then tighten the screw as usual. This method is especially useful if you need to fix multiple holes.
Using Plastic Welding
For a more permanent solution, plastic welding can be used to fill the stripped screw hole. Start by attaching a piece of cardboard to the back of the hole to prevent the material from running through. Then, hold a plastic welding rod about 1/2 inch (13 mm) above the hole and use a welder to melt the plastic into the hole. Once the hole is filled, turn off the welder and let the plastic solidify. You can then use a soldering iron to trim and smooth the edges.
Using a New Piece of Plastic
If you have access to the same type of plastic as the part you are repairing, you can cut a small piece to fit into the stripped screw hole. Drill a bigger hole in the new piece of plastic, glue it in place, and then drill a new screw hole. This method ensures that the new plastic matches the original part.
Using a Rivet
If you are repairing a stripped screw hole in a metal surface behind the plastic, you can use a rivet to create a solid attachment point. A riv-nut or blind rivet set into the metal can provide a tight and releasable connection.
It is important to note that when working with plastic, it is sensitive to temperature changes and overtightening fasteners can crack or break the material. Always be cautious and consider the specific type of plastic and screws you are working with.
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Frequently asked questions
Yes, but you must be careful. It is recommended to pre-drill a hole before screwing into plastic, especially if you are using a self-tapping screw.
Self-tapping screws are a good option for plastic, but you must pre-drill a hole. You can also use machine screws, but these require a fastener-sized faucet and a faucet wrench.
To avoid cracking, it is important to distribute the pressure to fasteners by using large washers under the heads and nuts of the bolts or screws. Do not overtighten the fasteners.
If you strip the screw hole, you can use gorilla glue to fill the hole. Coat the inside of the hole with a thin layer of glue using a toothpick. Wait overnight for the glue to dry and expand, then drill out if necessary.
