Alexei Volkov
Joining plastic to plastic can be challenging due to the material's smooth and non-porous surface. The type of plastic is the most important factor in determining the best adhesive to use. Common adhesives don't work on all plastics, and some adhesives can even damage plastic. There are a variety of adhesives available, including solvent-based bonding, epoxy-based adhesives, cyanoacrylate glue, and poly cement. Some advanced plastic joining methods use heat to manipulate melt temperatures, such as infrared welding and laser welding. Overmolding is another method that improves the design and performance of plastic components by dampening sound and vibration. The best method depends on the specific type of plastic and the project's requirements and budget.
| Characteristics | Values |
|---|---|
| Joining methods | Mechanical fastening with screws, bolts, or rivets, solvent or adhesive bonding, ultrasonic welding, infrared welding, laser welding, plastic overmolding |
| Factors influencing joining method | Project requirements, budget |
| Types of adhesives | Cyanoacrylate glue (CA), solvent-based bonding, epoxy-based adhesive, plastic cement, epoxies, poly cement, acrylic solvent adhesive, ABS solvent adhesives, epoxy cement, two-part primer and PVC solvent cement, glue for flexible plastics |
| Types of plastics | Polyethylene, polypropylene, polystyrene, polycarbonate, acrylic, ABS plastics, PET (Polyethylene Terephthalate), HDPE, PVC, LDPE, PP, PS |
| Considerations when choosing an adhesive | Type of plastic, surface preparation, curing time |
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What You'll Learn
Choosing the right glue
Joining plastic to plastic can be challenging due to the smooth, non-porous nature of plastic surfaces. The right glue depends on the type of plastic being used.
First, identify the type of plastic you are working with. The recycling symbol on the plastic will help you identify this. For instance, clear, brittle plastics like imitation glass or CD cases are usually polystyrene, while a plastic bucket or milk crate is likely to be made of polyethylene or polypropylene.
Once you have identified the type of plastic, you can choose the right glue. For instance, polystyrene can be bonded using poly cement, epoxy, or cyanoacrylate. Polycarbonate works best with epoxy, while acrylic requires an acrylic solvent adhesive. Polyethylene and polypropylene require an adhesive marked specifically for use with these plastics.
For hard plastics, you can use a two-part cyanoacrylate glue like the Loctite Plastic Bonding System, which is water-resistant, sets quickly, and dries clear. For flexible plastics, Loctite Vinyl, Fabric & Plastic Flexible Adhesive is a good option.
In addition to choosing the right glue, surface preparation is important. Clean the surface with soap or a plastic cleaner, or with isopropyl alcohol, and dry thoroughly. Roughen the surface with sandpaper or steel wool to improve adhesion. Apply the glue with a small paintbrush or needle, and seal the pieces tightly with a clamp, tape, or elastic while the glue dries. Curing times can range from a few minutes to 24 hours.
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Plastic welding
There are several different methods of plastic welding, each with its advantages and disadvantages. The chosen method depends on the type of welded material and their shape, as well as the required weld strength. Here are some of the most common plastic welding techniques:
- Hot gas welding uses a specially designed heat gun with additional plastic welding rods or sheets to create malleable and easy-to-join pieces of plastic. The equipment is portable and easy to use, but it has a slow heating rate compared to other methods.
- Laser welding is a fast and accurate process that uses a concentrated heat source while subjecting the workpiece to pressure. It can be automated through robots, creating clean welds with little to no post-weld processing. However, the equipment is initially expensive, and the process is not suitable for plastics with a thickness above 0.5 inches (12.7 mm).
- Ultrasonic welding uses high-frequency, low-amplitude mechanical vibrations (15kHz – 40kHz) to generate heat. The vibrations create friction, which melts the plastic polymers and creates a strong molecular bond. This method is fast, safe, and suitable for plastics prone to generating fumes when used with other techniques.
- Spin welding is a rotational friction welding process that uses pressure and friction between a stationary and moving plastic. This process is quick and straightforward, making it compatible with most plastics.
- Vibration welding joins plastic materials by heat generated through pressure and friction between rubbing parts, with one plastic part remaining stationary while the other moves back and forth.
- Infrared welding is a fast, non-contact method that introduces infrared radiation into the plastic parts, which is transformed into heat and melts the surfaces. The parts are joined by using pressure to hold them together until bonded.
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Plastic overmoulding
The process begins by placing a substrate material into an injection moulding tool. The overmould material is then placed into, onto, or around the substrate. When the overmould material solidifies, the two join together as a single part. This process is utilised for various reasons that vary according to project requirements.
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Mechanical fastening
When using mechanical fasteners, it is important to consider the type of plastic being used. The plastic must be strong enough to withstand the strain of fastener insertion and the subsequent high stress around the fastener. For example, when joining plastic and metal parts, care must be taken not to over-tighten fasteners, as this can prevent relative movement and lead to part failure. To avoid this, metal inserts can be used for joints that need to be regularly disassembled.
The design of the fastener's head is also important. Flat-headed bolts or screws, known as Pan or Cheese heads, produce less harmful compressive stress in plastics and are therefore preferable to countersunk heads, which can cause undesirable tensile stress. Flat washers can also be used under both the nut and fastener heads to distribute the assembly force over a larger area.
When using rivets, it is recommended to use those with large heads and washers under the flared end to minimize compressive stress and shear in the joint area. Calibration of rivet-setting tools is also important to ensure correct length and minimize stress.
Other mechanical fastening methods include interference fits, which involve forcing one part to deform elastically, and snap fits, which are molded features that fasten when two or more components are pressed together. Ultrasonic assembly is another technique that uses high-frequency mechanical vibration to melt mating surfaces and create permanent, aesthetically pleasing joints.
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Gluing acrylic to acrylic
Choose the Right Adhesive:
The key to successfully gluing acrylic is to use an acrylic solvent adhesive. Cyanoacrylate glue, commonly known as super glue, can also be used, but it may not always provide a strong bond and can sometimes be visible, especially on clear acrylic. Other adhesives like epoxy or poly cement are not recommended for acrylic as they are designed for other types of plastics.
Surface Preparation:
Before gluing, it is important to clean the acrylic surfaces thoroughly. Use soap or a plastic cleaner to remove any dirt, oils, or residue. You can also soak the acrylic in isopropyl alcohol and then dry it completely. To further improve adhesion, you can roughen the surface slightly using sandpaper or steel wool. Just be gentle to avoid scratching the acrylic too deeply.
Apply the Adhesive:
When applying the adhesive, use a small paintbrush or the tip of a needle for precision. Apply the glue evenly across the entire surface area that will be in contact. Adhesives generally form stronger bonds when applied over larger surface areas. Try to avoid using too much glue to prevent it from squeezing out and creating a mess.
Clamping and Curing:
After applying the adhesive, bring the two acrylic pieces together and clamp them tightly to ensure good contact and prevent sliding. You can also use tape or elastic bands to hold the pieces together while the glue dries. Check the curing time for your specific adhesive, as it can range from a few minutes to 24 hours.
Hiding the Glue Line:
If you are concerned about the appearance of the glue line, try to hide it behind an opaque element or use an adhesive that dries clear. You can also score a line on the acrylic with a laser to guide your glue application and help with the alignment of additional layers.
Alternative Methods:
If gluing acrylic to acrylic proves challenging, you may consider alternative methods such as plastic welding or overmolding. These methods can provide stronger and more seamless bonds but may require specialized equipment and expertise.
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Frequently asked questions
Mechanical fastening with screws, bolts, or rivets and solvent or adhesive bonding are some simple methods of joining plastic to plastic.
Some advanced methods include plastic overmolding, ultrasonic welding, infrared welding, and laser welding.
The different types of adhesives used for joining plastic to plastic include cyanoacrylate glue, epoxy-based adhesives, solvent-based adhesives, and plastic cement.
The factors to consider when choosing an adhesive include the type of plastic, the surface preparation, and the curing time. Different types of plastics require different types of adhesives, and the surface may need to be roughened to improve adhesion. Curing times can range from a few minutes to 24 hours.
