Ethan Clarke
There are many ways to join two pieces of plastic together, each with its own advantages and disadvantages. The most suitable method will depend on the specific type of plastic, the desired strength of the joint, and the time and resources available. Some common techniques include using glue or epoxy, drilling and inserting a metal pin, spin welding, hot plate welding, infrared welding, laser welding, and mechanical fasteners such as screws, bolts, and rivets.
Characteristics and their Values
| Characteristics | Values |
|---|---|
| Joining two pieces of plastic | Mechanical fastening with screws, bolts, or rivets |
| Solvent or adhesive bonding | |
| Spin welding | |
| Vibration welding | |
| Hot plate welding | |
| Infrared welding | |
| Laser welding | |
| Epoxy | |
| Polyester resin | |
| Styrene cement | |
| Super glue |
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What You'll Learn
Use epoxy resin and metal pins for a strong fix
Epoxy resin is a strong adhesive that can be used to bond two pieces of plastic together. When using epoxy resin, it is important to follow the manufacturer's instructions for the precise ratio of resin and hardener. For small amounts, you can use a piece of cardboard to mix the two parts. Use a toothpick or craft stick to slowly and thoroughly mix them together. For larger quantities, use a digital scale to weigh the resin and hardener.
To extend two pieces of plastic using epoxy resin and metal pins, start by drilling matching holes through the centre of the break in each piece of plastic. The holes should be large enough to fit the metal pins. Next, insert the metal pins into the holes, leaving enough of the pin exposed to fit into the other piece of plastic. Apply a thin layer of epoxy resin to one or both pieces of plastic and firmly secure the pieces together. The epoxy resin will fill any cracks or defects and create a strong bond. Leave the pieces clamped together for 24 hours to allow the epoxy to cure.
You can also insert metal pins into plastic using ultrasonics or induction welding. Ultrasonics uses a vibrating tool to transmit pulses, causing the plastic to vibrate and heat up, fusing the parts together. Induction welding involves pressing two pieces of plastic together around a metal insert. When passed through a magnetic field, the metal heats up and fuses the plastic together.
Using metal pins and epoxy resin to extend two pieces of plastic will create a strong and secure joint. This method is ideal for when gluing the pieces together may not provide enough surface area for a strong bond.
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Spin welding joins plastics using friction and pressure
Spin welding is a versatile method for joining plastics with precision and strength. It is a type of friction welding, which operates on the principle of heat-induced fusion between separate materials. Spin welding generates this friction by rotating one plastic part into a fixed second part while applying pressure between both parts, creating localised heat at the contact point. This process blends the interface between the two plastics and co-mingles their molecular structures.
The spin welding process can be used to join almost all thermoplastic polymers. It is suitable for parts that can be rotationally joined, such as round or non-round, balance-able parts. The parts are spun at high RPM (often up to 16,000 RPM) about a common centreline. The welding step consists of four main sub-steps:
- Friction between the parts begins due to rotation from a motor and downward pressure. Heat is generated until the glass transition temperature (for amorphous polymers) or the melting temperature (for semicrystalline polymers) is reached.
- Part melting begins, and some of the melted material is extruded into the "flash".
- A steady state is reached between the melt layer and the amount of material squeezed into the flash. The spinning is then stopped.
- While the joint cools, the parts are held in contact with each other under pressure to ensure a solid mating at the joint.
The heat generation rate during spin welding depends on the weld RPM, weld pressure, and weld time. Higher melting temperature materials will require more energy to melt, so they will need more welding time or higher RPMs. Spin welding machinery often includes some level of automation, with semi-automated spin welders typically featuring automatic motor controls and manual press stroke operation.
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Hot plate welding melts plastics against a heated platen
Hot plate welding is a process that joins two plastic components by melting them against a heated platen. It is one of the most widely used plastic welding methods and is used for pipelines, appliances, and injection mouldings.
The hot plate welding process can be divided into four phases: matching, heating, change-over, and welding/forging. In the matching phase, the geometry of the weld surfaces is matched to the theoretical welding plane. The weld surfaces are then heated through conduction by physical contact with the hot plate, with a temperature range of 30 to 100 °C (86 to 212 °F) above the melting temperature of the material. A constant pressure between 0.2 and 0.5 MPa is applied during this phase.
After sufficient heating, the change-over phase begins. The parts are retracted from the hot plate, and the plate is quickly moved away. The parts are then brought together, and the change-over should be as short as possible to prevent the melted region from cooling off.
The welding/forging phase starts when the two molten surfaces are pressed together, creating intermolecular diffusion of the plastic molecules. The weld strength is provided by the entanglement of these diffused molecules. The necessary welding pressure depends on the melt viscosity and wall thickness of the parts and typically ranges from 0.025 to 0.05 MPa. This pressure is maintained as the melted material cools and resolidifies.
To ensure good-quality welds, the components should be firmly positioned in holding fixtures before initiating the welding cycle. This ensures that the surfaces to be heated are flat and free from distortion. Some common variants of conventional hot plate welding include high-temperature and non-contact versions, which help address the issue of material sticking to the hot plate between weld cycles.
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Use bolts, screws, or rivets to mechanically fasten plastics
Joining two pieces of plastic with bolts, screws, or rivets is a common method of fastening plastic parts. This technique is reliable, cost-effective, and allows for the joined pieces to be taken apart multiple times.
When using bolts or screws, it is important to consider the type of screw and the amount of torque applied. Self-threading or self-tapping screws can be used to join plastic parts. To ensure optimal clamping force without damaging the threads, the torque applied should be lower than when screwing into metal. The screw should be rotated counterclockwise until it "falls" into the thread, and then engaged for a quarter turn. Flat-headed screws, such as pan or cheese head screws, are preferable to countersunk heads as they produce less harmful compressive stress in plastics. Flat washers can also be used to distribute the assembly force over larger areas.
Rivets are another option for fastening plastics. Nylon rivets, for example, are softer than metal rivets and less likely to damage plastic during assembly. They are also non-conductive. Peel and bulb-style rivets distribute the load across a larger area, reducing the stress induced in the plastic assembly.
When joining plastic and metal parts, it is important not to over-tighten fasteners, as this can lead to part failure. Threadlockers can be used to prevent unintended screw loosening, but some formulations can cause stress cracking in plastics, so they must be chosen carefully.
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Use styrene cement to partially melt and weld plastics
Styrene cement is a popular method for bonding two pieces of plastic. This method involves using a solvent to partially melt the plastic surfaces, allowing them to be welded together. The result is a strong, permanent bond that is often stronger than the surrounding material.
To use styrene cement, start by scraping or sanding off any existing glue or residue from the plastic surfaces. This ensures a clean surface for the weld. Then, apply the styrene cement to both surfaces. The cement will partially melt the plastic, creating a thin film of dissolved plastic along the joint. Join the two pieces of plastic and hold them together until the weld sets. Some styrene cements, such as the Plastruct Brand Plastic Weld, set in just a few minutes.
It's important to work in a well-ventilated area when using styrene cement, as the solvents can give off pungent and potentially harmful fumes. Always refer to the safety information provided with the product and take the necessary precautions, such as wearing a mask.
In addition to styrene cement, there are other methods for joining two pieces of plastic. One method involves drilling matching holes through the break in each piece of plastic and inserting a metal pin. The pieces can then be glued together with epoxy, creating an internal splint that strengthens the joint. Another method is to use a product like Flex-I-File Plast I Weld, which is a liquid solvent cement applied with a squeeze bottle or applicator.
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Frequently asked questions
There are several ways to join two pieces of plastic, including spin welding, vibration welding, hot plate welding, infrared welding, laser welding, and plastic overmolding. Joining two pieces of plastic can also be done through mechanical fastening with screws, bolts, rivets, or other adhesives. The best method will depend on the specific project and budget.
First, sand and clean both surfaces with isopropyl. Drill small holes in each piece of plastic and insert metal sewing needles. Then, bond the two pieces with epoxy adhesive, clamp them together, and leave for 24 hours.
Epoxy is the strongest glue for bonding two pieces of plastic. However, depending on the type of plastic, you might have some luck with styrene cement.
