Mechanical fastening (also called mechanical joining) is an inexpensive and practical method for joining polymers, either temporarily or permanently. Fasteners that can be used to join polymers include nuts and bolts, brackets, rivets, screws, hinges, catches, washers, and snap or press fittings.
These fasteners can be metallic or polymeric as well as integrated into the design of the parts to be joined (as with snap or press fittings). Mechanical fastening can join similar or dissimilar materials, such as when joining a polymer to a metal.
Advantages of mechanical fastening include that the parts do not require surface treatments and can easily be disassembled for inspection and repair. However, this joining technique increases the weight of parts through the addition of extra fastening materials and can leave the parts prone to large stress concentrations around the fastener holes. This process can also lead to in-service corrosion problems around the fasteners.
Mechanical fastening is widely used in the aerospace, automotive and construction industries.
Advances in chemical synthesis, surface preparation and joint design have seen the importance of adhesive bonding as a joining technique grow. Highly flexible, adhesives deliver a range of bonding strengths according to type and the materials being joined.
With adhesive bonding, an adhesive is placed between the materials that are to be joined (the adherends). The adhesive joins the parts and transmits the load along the length and breadth of the joint. Solvent bonding plasticises the polymer surfaces causing the polymer chains to interdiffuse between the materials. Solvent bonding can only be used with amorphous thermoplastics like acrylics, polycarbonates, and polystyrene resins. Adhesive bonding can be used with most plastics, although both adhesive and solvent bonding provide a relatively uniform stress distribution and high strength-to-weight ratios.
Examples include solvent cements that are designed to join acrylics together through application to one surface before the other part is held in contact while it sets into an invisible join. Epoxy resins can be used with dissimilar materials. It is supplied as a two-part adhesive comprising a resin and a hardener. The two parts are mixed together in equal quantities before being applied to the surfaces to be joined. However, the parts typically need to be clamped in place while the epoxy resin sets. Contact adhesives can join dissimilar materials onto which the adhesive is applied before they are pushed together. Contact adhesives also need to be held in place while they set.
The low cost, adaptability and high-speed of production using adhesives mean they have found multiple applications across a range of industries, from aerospace to medical.
While adhesive bonds and mechanical fastening are classed as conventional joining techniques, welding, or fusion bonding, techniques count as advanced joining processes. Polymers can be welded through the application of heat so that the edges melt and join together through intermolecular diffusion. Thermoplastics can be reformed after heating and welding rods can also be used to add additional plastic material to the join. Common welding methods for polymers include ultrasonic, laser, vibration, friction and electrical resistance welding.
Ultrasonic welding is commonly used to assemble small plastic components. Vibration welding can generate interface friction between parts on the scale of microns to millimetres and is frequently used in the automotive industry. Rotational friction welding creates plastic deformation in the parts by rapidly rotating one against the other under applied pressure. Laser welding uses a continuous or pulsed beam that excites the atoms of the parts, generating heat and melting the material. As some polymeric materials do not absorb the light, there are different techniques for welding with lasers according to the materials being joined, as follows:
- Similar absorbent materials: Placed side-by-side and welded along the line where they meet
- Transparent and absorbent materials: Transparent material placed on top of the absorbent material so that the light is stopped when it meets the absorbent material, melting it and forming a join with the transparent material above
- Similar transparent materials: Materials stacked on top of each other with an absorbent coating placed between them to prevent the laser passing through. The laser then melts the absorbent layer, joining the two materials
As each polymer joining method has its own advantages and drawbacks, there has been a trend towards using a combination of techniques to create hybrid joints and improve performance. For example, lasers can be used to optimise the curing of an adhesive to increase joint strength or using friction stir welding for pre- or post-heating of materials to optimise the joint efficiency.
Polymeric materials offer a range of properties so that they can be matched to a required application, whether ductile or brittle, soft or hard, or weak or strong. They also deliver advantages such as excellent corrosion and environmental resistance, and high levels of durability, thermal and electrical insulation, and more. These benefits as well as high design flexibility have seen polymers enter widespread use in industries such as medical, automotive, aerospace, packaging, and electronics.
While polymers can be moulded, mechanical limitations mean that there is still often a need to join polymers to similar or dissimilar materials to complete an assembly. The conditions and application that the parts are to be exposed to can determine which method of polymer joining would be preferable. This includes considerations as to whether the join needs to be temporary or permanent, whether there will be a need to be able to repair damage, and how long the join is expected to last and under what conditions.
The three types of polymer joining are adhesive bonding, mechanical fastening, and welding, although two or more processes can be used in conjunction with each other to create a hybrid joint. The effectiveness of a join has a bearing on the application and effectiveness of any polymer or composite material, so it is important to use the correct process for your given application.
1. Can different types of polymers be joined together using these methods?
Different polymers can be joined together with adhesive bonding, mechanical fastening and, in some cases, welding.
Although this is relatively easy with mechanical fastening and, given the right adhesives, with adhesive bonding, welding can be more difficult because of the different melting points and properties of different polymers. However, covalent bonding, cross-linking, or grafting can be used to create strong, permanent bonds between different types of polymer.
2. How do environmental factors affect the choice of polymer joining method?
Environmental factors and service conditions need to be considered when joining polymers. Environmental conditions can cause the polymer itself to degrade, cause the adhesive to lose its effectiveness or cause fatigue on a fastening or a weld.
3. What are the main considerations when choosing an adhesive for polymer bonding?
There are a number of considerations when choosing an adhesive for polymer joining. These include:
- Materials: Knowing what materials you are bonding allows you to determine whether the adhesive is fit for purpose. Most adhesives will work with dissimilar materials but some are more difficult than others, including low surface energy (LSE) plastics, which require a specialist product and/ or specialist surface preparation.
- Time: It is important to know how much time you have to apply the adhesive (before it begins to dry) and also how long it will take for the adhesive to cure and reach ‘handling strength.’ The faster an adhesive cures, the quicker it will reach handling strength, but fast curing could also mean that you have to work faster to apply the adhesive to the parts to be joined.
- Characteristics: The characteristics of the adhesives are also important, including strength, flexibility, viscosity, hardness, colour, UV stability, potential health risks, and temperature and chemical resistance. It may be difficult to find an adhesive that perfectly suits your requirements for all of these in-service factors, but adhesive manufacturers can help with this be adjusting their adhesives to deliver different properties.
- Cost: Cost may not be the most important factor in choosing an adhesive, but it is still important to consider, especially as many cheaper adhesives may be inferior and end up costing you more in the long-run.
4. How does the strength of a polymer joint compare to that of the base material?
When judging the strength of an adhesive you can assess the tensile strength and the shear strength of the bond. Tensile strength is the ability to withstand forces that are pulling in opposite directions while shear strength is the ability to withstand forces that are pulling parallel to the bond. Stronger adhesives demonstrate high strengths and the strength of an adhesive can be measured using the megapascal (MPa) rating system. The adhesive strength is tested with a tensometer to determine the MPa of adhesives. Strong adhesives tend to be over 3 MPa, but some adhesives can reach an MPa as high as 30. The higher the strength of an adhesive, the less chance there is of the bond coming apart. However, if the adhesive is too strong , with a greater strength than the substrate, the substrate itself will fail.
5. What safety precautions should be taken during the welding of polymers?
As with all welding, there are a number of safety precautions that should be followed when you are welding polymers:
- Safety Instructions: The first step is to be aware of any safety instructions associated with the use of your welding equipment.
- Material Safety Data: It’s not just the equipment that will have safety information that you need to assess. The plastic that you are welding will have a material safety data (MSD) sheet, which you should check to assess any hazardous ingredients, fire and explosion hazard information, preventative measures and first aid advice. These sheets can be obtained from your plastic manufacturer.
- Employee Training: All personnel engaged in the welding of polymers should be trained on the use of the welding equipment, including how to clean it after use and what to do if there is a malfunction or problem.
- Storage: Welding equipment and machinery should be properly stored between use. This includes making sure any hot air hand tools are kept away from flammable materials or body parts and that the tool cannot roll from the surface it is placed upon.
- Cleaning: The plastic parts being welded should be clean of contaminants such as grease or oil as they can pose a fire hazard or prevent the part from being weldable. The work area should also be tidied up after use to prevent any accidental spillages. Air filters should be cleaned of dust and contaminants and the tool should be cleaned of melted plastic after use. Of course any damaged or frayed parts should be replaced or repaired.
- PPE: It is important for operators to wear the correct personal protective equipment (PPE), so as to limit your exposure to heated surfaces or tools as well as airborne plastic and other debris. This could mean wearing long sleeved garments, gloves and protective eyewear. There may also be a need to use a ventilator or a respirator. Polypropylene, polyethylene and polycarbonate all generate the carcinogen benzene during laser cutting, while polystyrene releases styrene and PVC emits hydrogen chloride. Many plastics also give off ultra-fine particles and carcinogenic polycyclic aromatic hydrocarbons. Hence the need for organisations to install the correct ventilation systems to control exposure. You can find out more about the health and safety implications of welding and cutting plastics, here.
6. Can mechanical fastening provide a permanent solution for joining polymers?
Mechanical fastening can provide a permanent solution for joining polymers, such as when using rivets. While many mechanical fastening methods are easy to disassemble, rivets offer a more permanent connection.
7. How do I determine the best joining method for my polymer application?
The first thing to do is to consider the types of polymer being used as well as its properties and the intended application. From here you can investigate the feasibility and cost of different joining methods.
Each method brings its own advantages and disadvantages, including whether the join is permanent or temporary and how it will perform in different conditions. Generally-speaking, mechanical fastening methods are easier and less expensive than other techniques, but adhesives or welds will tend to deliver a stronger join.