Maya Ellison
Joining plastic to plastic can be challenging due to the variety of plastics and glues available. The most common adhesives for gluing plastic are super glue, plastic bonder, epoxy glue, and polyurethane glue. The choice of adhesive depends on the type of plastic being used, as different plastics have unique characteristics and require specific glues for effective bonding. Some plastics may require solvent-based bonding, while others may need an epoxy-based adhesive. Additionally, the surface energy of the plastic affects its ability to be glued, with low surface energy plastics being particularly difficult to bond. To overcome this issue, surface preparation methods like primers or ion beam etching can be used. Joining plastic can also be achieved through welding techniques such as ultrasonic welding, spin welding, vibration welding, laser welding, and infrared welding, each offering unique advantages and suitability for specific plastic types or applications.
Characteristics and Values Table for Joining Plastic to Plastic
| Characteristics | Values |
|---|---|
| Plastic Type | Polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), Polyvinyl Chloride (PVC), Low-Density Polyethylene (LDPE), Polypropylene (PP), Polystyrene (PS), Polycarbonate, Acrylic, Nylon, Mylar, Delrin, Phenolic |
| Adhesive Type | Super Glue, Cyanoacrylate Glue (CA Glue), Epoxy, Poly Cement, Solvent-Based Bonding, Epoxy-Based Adhesive, Polyurethane Glue, Plastic Bonding Systems, PVC Primer and Glue |
| Surface Preparation | Clean with Soap and Water, Use Plastic Cleaner, Soak in Isopropyl Alcohol, Sand Surface, Use Primer or Activator Pen, Roughen Surface |
| Application Techniques | Use Paintbrush or Needle Tip for Precision, Clamp Pieces Together, Use Applicator Bottle for Solvent Cement, Press Pieces Lightly, Remove Air Bubbles, Hold Firmly with Clamp or Tape |
| Setting and Curing | Leave for 10 Minutes to 24 Hours, Cure Fully Overnight for Best Results |
| Additional Considerations | Ventilation or Respirator when Using Primer and Solvent Cement, Avoid Excess Adhesive, Clean Up with Damp Paper Towel, Scrape Dried Adhesive with Razor Blade |
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What You'll Learn
Vibration welding
The process involves bringing two plastic workpieces into contact under pressure and applying a reciprocating motion (vibration) along the common interface to generate heat. This motion can be achieved through linear vibration welding, which uses a one-dimensional back-and-forth motion, or orbital vibration welding, which moves the pieces in small orbits relative to each other. Linear vibration welding is more common due to its simpler and relatively cheaper machinery requirements.
The vibration creates friction, which melts the workpieces, and they become welded when the vibration stops and the interface cools. The welding process consists of four steps: solid friction, transient flow, steady-state flow, and solidification. During the transient flow step, the melt layer thickness grows, causing a decrease in frictional forces and heat input to the system. This leads to a lateral flow of molten material. In this phase, the melting rate of the material matches the flow of the extruded material at the lateral surfaces. The material flow and thickness of the melt layer become constant, determining the quality of the weld. At the desired 'melt-down' thickness, the vibration is stopped, and the weld is allowed to cool.
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Induction welding
The process of induction welding involves placing a small amount of thermoplastic compound in the joint between the two pieces to be welded. This compound, which can take the form of tape, wire, or a moulded gasket, is made from either the same polymer as the pieces being joined or a compatible polymer. The compound contains a magnetic powder, usually iron, iron oxide, or stainless steel, which facilitates heating. The faster the compound heats up, the faster the weld is completed.
An induction coil attached to a high-frequency power supply, such as a radio frequency generator, provides the heat required for the welding process. When the generator is switched on, alternating current (AC) power passes through the coil, producing eddy currents that heat the magnetic powder in the compound. As the compound heats up, it softens and melts, and slight pressure applied to the pieces being joined results in a joint being formed.
One of the advantages of induction welding is its speed. The thermoplastic compound melts in a matter of seconds when heated and cools almost instantly when the current is turned off, forming a solid joint almost instantly. Additionally, induction welding does not distort the materials being welded. Since the heat is directed at the compound rather than the parts being joined, the parts are less likely to suffer stress caused by heat. The use of a thermoplastic compound also results in strong joints as it completely fills any gaps between the two pieces, creating a molecular bond with the materials being joined.
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Mechanical fastening
There are several types of mechanical fasteners, including screws, rivets, pins, sheet-metal nuts, latches, nails, and hinges. When using mechanical fasteners, it is important to pay attention to the fastener's head. For example, countersunk heads should be avoided for plastics as they produce undesirable tensile stress in the mating parts. Instead, bolt or screw heads with a flat underside, called pan or cheese heads, are preferred as they produce less harmful, compressive stress in plastics. Flat washers can also be used under both the nut and fastener heads to help distribute the assembly force over larger areas.
When joining plastic and metal parts, the tightening torque for the inserted screw is important. Fasteners should not be tightened too tightly, as this can prevent relative movement and lead to possible part failure. If a fastener needs to be removed multiple times, metal inserts are recommended. These can be molded in place, forced, glued, or expanded into holes, or inserted ultrasonically.
Another consideration when using mechanical fasteners is the type of plastic being joined. For example, threaded fasteners work best on thick sections, with thread-forming screws preferred for softer materials and thread-cutting screws working better on harder plastics. Push-on locknuts and clips may be better suited for thinner sections.
In addition to traditional mechanical fasteners, there are also plastic-specific fastening techniques such as snap fits and tabs. These are low-cost and commonly used for high-volume plastic parts assembly, with features molded into the plastic part assembly to fasten pieces together. Interference fits are another type of plastic fastening technique where a fastener is installed by forcing one of the parts to deform elastically.
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Choosing the right glue
Plastics in categories 1 (Polyethylene Terephthalate/PET), 2 (High-Density Polyethylene/HDPE), 4 (Low-Density Polyethylene/LDPE), and 5 (Polypropylene/PP) are challenging to glue. For these plastics, look for adhesives designed for polyethylene or polypropylene. Category 3, Polyvinyl Chloride (PVC), commonly used in plumbing, can be glued with a two-part primer and PVC solvent cement. Polystyrene (PS), marked with a 6, can be bonded with poly cement, epoxy, or cyanoacrylate. Category 7, a miscellaneous group, includes polycarbonate and acrylic, with epoxy being the best choice for polycarbonate.
For flexible plastics used in items like shoes or toys, opt for adhesives specifically formulated for such materials, like Loctite Vinyl, Fabric & Plastic Flexible Adhesive. Hard, high-density plastics, such as those used for buckets or containers, require adhesives like the Loctite Plastics Bonding System, a cyanoacrylate adhesive that provides a strong, water-resistant bond. When bonding materials like polycarbonate, acrylic, Nylon, or stainless steel, Loctite Plastic Bonder, a two-component acrylic formula, offers exceptional strength and impact resistance.
In addition to choosing the right adhesive, surface preparation is crucial. Clean the plastic surface with soap or a specialised cleaner, and dry it thoroughly. Sanding the surface with grit sandpaper, steel wool, or emery cloth creates a rough texture that enhances the glue's bond. Some plastic bonding systems include an activator to prepare the surface and improve adhesion.
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Surface preparation
Joining plastic to plastic requires a good understanding of the different types of plastics and the most suitable adhesives for them. The first step in preparing the surface of plastic for jointing is to identify the type of plastic you are dealing with. Different types of plastics require different types of plastic glue. The recycling symbol, typically consisting of three arrows forming a triangle, is often the easiest way to identify a plastic type. For instance, plastics marked with a 6 or "PS" are polystyrenes, and plastics in categories 1, 2, 4, and 5 are very difficult to glue.
Once the plastic type is identified, the surface must be cleaned thoroughly. This can be done by washing the plastic with soap, using a specialized plastic cleaner, or soaking the plastic in isopropyl alcohol. Ensure that the plastic is dried thoroughly after cleaning. To minimize oil residue, avoid touching the plastic surface with bare hands.
The next step is to sand the surface to create a rough texture that the glue can bond to. 120 to 200 grit sandpaper can be used for this purpose, but only a brief rub is needed. Alternatively, steel wool or emery cloth can be used to achieve a similar effect. The Loctite two-part plastic bonding system offers an easier solution, as it includes an activator that helps roughen the surface.
After preparing the surface, it is important to choose the right adhesive for the specific plastic type. For instance, flexible plastics commonly used in household items such as shoes or toys require adhesives specially formulated for these materials, such as Loctite Vinyl, Fabric & Plastic Flexible Adhesive. On the other hand, for bonding harder plastics, a two-part cyanoacrylate adhesive like the Loctite Plastics Bonding System is recommended.
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Frequently asked questions
There are several methods to join plastic to plastic, including plastic overmolding, mechanical fastening with screws, bolts, or rivets, solvent or adhesive bonding, and welding.
Different types of welding used to join plastic to plastic include ultrasonic welding, spin welding, vibration welding, hot plate welding, infrared welding, and laser welding.
The most common adhesives used to join plastic to plastic are super glue, plastic bonder, and epoxy. Other adhesives include solvent cement, poly cement, cyanoacrylate (CA) glue, and plastic welds with a primer/activator pen.
It is important to identify the type of plastic before choosing an adhesive. Different types of plastics require different types of adhesives. The recycling symbol, which consists of three arrows forming a triangle with a number or code in the center, can help determine the type of plastic. Additionally, the surface energy of the plastic can impact the ability to form a strong bond. Low surface energy plastics can be challenging to bond and may require surface preparation techniques.
