Ethan Clarke
There are several ways to join two plastic pieces, and the best method depends on the specific project and budget. Common methods include mechanical fastening with screws, bolts, or rivets, and solvent or adhesive bonding. More advanced methods include ultrasonic welding, spin welding, infrared welding, and laser welding. For plastic canvas, an overcast stitch can be used to join two pieces together.
Characteristics and Values Table
| Characteristics | Values |
|---|---|
| Mechanical Fastening | Screws, bolts, rivets, self-threading or self-tapping screws, pins, washers |
| Solvent or Adhesive Bonding | Coat pieces of plastic with a special solvent, which evaporates and leaves the plastics bonded |
| Welding | Ultrasonic, spin, infrared, laser, vibration, hot plate |
| Other | Overcast stitch, styrene cement |
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Using adhesive bonding
Adhesive bonding is a common method for joining plastic pieces. This process involves coating the plastic pieces with a special solvent that evaporates, leaving the plastics bonded together. The type of solvent or adhesive you use depends on the type of plastic you are working with. For example, plastics in categories 1, 2, 4, and 5 (Polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polypropylene (PP) respectively) are very difficult to glue. However, you can use poly cement, epoxy, or cyanoacrylate to bond polystyrene, which falls under category 6.
Before applying the adhesive, it is important to clean the plastic surfaces and have them abraded. This extensive surface preparation ensures a proper bond. Additionally, some adhesives require the application of an activator as a preliminary step to prepare the surface of the plastic and create a stronger bond. After applying the activator, wait for about a minute before applying the adhesive to one surface and joining the pieces.
For larger repairs or when you are unsure of the plastic type, a two-step system like Loctite Super Glue All Plastics can be used to bond almost any type of plastic. If you require extra strength, a two-part epoxy system, such as Loctite Universal Instant Epoxy Liquid 5 Minute, can be used. This formula bonds to various plastics, including PVC, ABS, and Mylar, and provides water resistance.
It is important to note that some adhesives may require leaving the glued pieces undisturbed overnight for optimal bonding. Additionally, once the pieces are joined, it can be difficult to disassemble them. Therefore, it is essential to plan accordingly when using adhesive bonding methods.
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Mechanical fasteners
Permanent fasteners, such as snap-fits and rivets, create a strong and lasting joint. Snap-fits are one of the lowest-cost and most commonly used plastic fastening techniques, often used in high-volume plastic parts assembly. They use moulded features to fasten pieces together when pressed, as seen in cell phone cases and battery covers. Interference fits, another type of permanent fastener, involve forcing one of the parts to deform elastically, such as press-fitting a dowel pin into a hole that is slightly smaller.
Non-permanent fasteners, on the other hand, allow for the removal, reuse, and replacement of the joint. Screws, bolts, and rivets are commonly used non-permanent fasteners, offering a cost-effective and reliable solution for assemblies that need to be taken apart multiple times. When using screws, it is important to note that thread-forming screws are better suited for softer plastics, while thread-cutting screws work best on harder plastics. Additionally, flat washer heads should be used under both the nut and fastener heads to distribute the assembly force evenly.
Other factors to consider when selecting mechanical fasteners include the dimensional stability of the plastic through moisture absorption and the maximum operating temperature. If strength is a concern, it is worth noting that nearly all mechanical properties of plastic fasteners will be lower than their metal counterparts.
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Welding
To weld plastic, you will need an electric welding gun and a welding rod. First, clean and identify the plastic you will be working with. If you are unsure of the type of plastic, use a welding rod test kit to determine its composition. Choose a welding rod that matches the type of plastic—it should be made of the same material as the plastic you are welding.
Next, create your joints by clamping and taping the plastic pieces together. Place the pieces on a workbench, push them close together, and pin them to the table using C-clamps. Wrap foil tape around the pieces to keep them pressed together, avoiding the area to be welded.
Now, you can begin the welding process. Preheat your welding gun for at least two minutes, setting the temperature according to the type of plastic. The temperature typically ranges from 390°F to 570°F (200-300°C). Once the welding gun is ready, use it to gradually melt and join the plastic pieces together. Finish the weld by smoothing it out, creating a strong and cost-effective joint.
There are several different welding techniques you can use, depending on the type of repair needed. These include hot plate welding, extrusion welding, ultrasonic welding, laser welding, and pendulum welding. Hot plate welding involves placing the plastic pieces on a heated surface until they melt, then pressing them together until they cool and solidify. Extrusion welding is used for large plastic structures and involves melting plastic welding rods with a heated extrusion gun and applying the melted plastic to the joint. Ultrasonic welding uses mechanical vibrations to create frictional heat that bonds the plastic pieces. Laser welding employs a laser beam to melt the plastic, followed by the application of pressure to bond the joints. Pendulum welding is a slow technique suitable for confined spaces and short seams, where both the plastic and welding rod are heated evenly.
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Solvent bonding
The choice of solvent depends on the solubility of the thermoplastic and the processing temperature. Polymers are generally more soluble in solvents with similar solubility parameters to their own (liquid or solid). Increasing the temperature lowers the free energy of mixing, promoting dissolution and interdiffusion bonding.
The tensile shear test using a lap joint configuration is the most common method for testing solvent bonds. This method is particularly suitable for thin specimens as it mitigates distortion during testing.
While solvent bonding can be an effective way to join thermoplastics, it is important to note that it may not work for all types of plastics. Some plastics may require a solvent-based bonding agent, while others may need an epoxy-based adhesive. Additionally, solvent-based adhesives can sometimes warp and change the surface of the plastic, so it is crucial to choose the appropriate adhesive for the specific type of plastic being joined.
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Stitching
Joining two pieces of plastic canvas together can be done using an overcast stitch. This is a simple process that can be used to create 3D pieces like a phone pouch, a badge, a keyring, or a project bag. Here is a step-by-step guide:
- Secure the end of the yarn by holding the last inch to the back of the canvas.
- Pass the needle through each hole from the back to the front two or three times per hole to cover the tail. You may need to pass the needle through a few more times at the corners to fully cover the canvas.
- Lay the two pieces you want to sew together on top of each other, ensuring that the threads and holes of the canvas are lined up.
- Anchor the thread in the back of the stitches and pass the needle through each set of holes. This is the same process as overcasting a raw edge, but you are sewing through two or more layers. You can do this on a straight edge or a diagonal.
- To join two pieces of canvas to make a bigger piece, overlap the two pieces so that the threads and holes line up.
- Sew through both layers of canvas simultaneously. This method will create a visible join, so it is best to do this in a place that won't be too noticeable.
There are also other types of stitches that can be used for plastic canvas projects, such as the backstitch, continental stitch, and Scotch stitch. It is recommended to master the basic stitches before attempting more complicated patterns.
In addition to stitching, there are other methods to join plastic pieces, such as snap fits, heat welding, solvent bonding, and mechanical fasteners. These methods may be more suitable depending on the specific application and requirements.
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Frequently asked questions
Some simple ways to join two plastic pieces include using self-threading or self-tapping screws, rivets, pins, and threadlockers. You can also use adhesive bonding, where you coat the pieces of plastic with a special solvent that evaporates, leaving the plastics bonded.
Advanced plastic joining methods include using heat, adhesion, and other procedures. Infrared welding, for example, uses intensified infrared beams to instantly heat and join plastics. Laser welding uses the heat and directionality of a laser beam to join plastics. Spin welding uses surface friction to create a circular weld joint.
Ultrasonic welding combines vibrational pulses and pressure to join fairly rigid plastics. The extensive vibration and pressure cause the plastic materials to rub vigorously, creating heat and helping to melt the plastic together.
To join two pieces of plastic canvas together, you can use an overcast stitch. First, secure the end of the yarn by holding the last inch to the back of the canvas and passing the needle through each hole from the back to the front two or three times per hole. Then, hold the two pieces together and line up the matching holes, passing the needle through both holes two or three times. Continue along the join and weave the end of the yarn underneath the backs of stitches in several directions to secure it.
