Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2003-04-28
2004-11-02
Sells, James (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S073100, C156S082000, C156S272200, C156S379600, C156S497000, C156S498000, C156S499000, C156S580000
Reexamination Certificate
active
06811632
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus of joining of thermoplastic materials.
BACKGROUND OF THE INVENTION
While metals are currently being used for many applications, and new metal alloys are being developed, more parts are being designed and manufactured using plastic and composite materials. Likewise, existing parts that are currently made with metals, are being redesigned and manufactured with plastic materials.
Plastics offer several advantages over metals. Engineering plastics have a higher strength to weight ratio than many metals. As a result, they require less energy to move due to decreased inertia, which results in energy savings and faster moving parts. Likewise, while most metals are isotropic, composite materials can be designed to be anisotropic. This allows designers to use less material in unloaded areas while strategically placing material in areas under high loads and stresses resulting in even higher strength to weight ratios.
Plastics are tough, viscoelastic materials. Because of this, most plastics can undergo high amounts of deformation before yielding. Durability is a definite strength when dealing with plastics.
Plastics are more resistant to the environment than most other classes of materials. This is why many chemical containers are made of plastics because they are chemical resistant, lightweight, and will not break if dropped. Fillers and additives, such as chlorine and UV stabilizers, can be added to raw plastic materials to improve their environmental resistance.
Plastics are also easily processed. Thermoplastic materials must simply be melted, molded into a desired shape, and then allowed to cool. Injection molding is a simple process that has revolutionized several industries by making it possible to produce complex parts in large volumes, at low cost. Costs are reduced further as scrap material can be easily recycled, sometimes even as part of the production process.
Despite the continuous improvements and development of new plastic materials, the processes used to fabricate final products are often inadequate.
One of the areas undergoing the greatest advances in the past few years is the area of joining processes. While, several new processes have been developed to meet the need for producing large volumes of quality plastic parts, there is still a need for a process for joining plastic materials, particularly for structural and high performance applications. Unfortunately, plastics can be difficult to join due to their low surface energies, poor wetability, and the presence of release agents from previous processing steps. A good joining method should be able to meet basic requirements. These requirements may include:
1. Reproducibility of joint efficiency
2. Ability to join materials in various joint geometries
3. Suitability for small and large bonding areas
4. Minimal surface preparation required
5. Minimal use of expensive specialty equipment
6. Potential for production applications
7. Retention of joint integrity in a variety of environments and load systems
Modern joining methods address these process requirements in a variety of ways. Modern joining methods fall into two general categories: mechanical fastening and bonding. Mechanical fastening utilizes a separate, mechanical device that holds the materials together at the joint area. Bonding joins parts through adhesives or fusion (welding). Adhesive bonding includes the use of adhesives or solvents to chemically join parts. Welding includes thermal, friction/mechanical, and electromagnetic methods that melt or soften to fuse the material together at the joint. While all of these methods have advantages in joining plastics, and all have been used successfully in commercial applications, there is still a need for a method for joining plastics. As discussed in greater detail below, existing methods still are deficient for the joining the plastics.
Mechanical Fastening
Mechanical fastening involves attaching or joining parts by using external materials or components to mechanically hold the plastic parts together. Screws, rivets, clips, and brackets are some examples of mechanical fasteners, which can be metal or plastic, depending on the application. A disadvantage of mechanical fasteners is the concentration of stresses that develop at the localized fastening areas. Although straightforward and relatively simple to form, mechanically fastened joints are typically of low performance, when compared with other methods
Adhesive Bonding
Adhesives
Adhesive bonding is a mature process. Adhesive research is very advanced and has made the joining of virtually any material possible. Adhesives effectively join a variety of plastics, and are relatively simple, requiring neither expensive equipment nor extensive training of personnel.
Because most adhesives rely on chemical reactions between materials to effectively bond parts, there are disadvantages associated with this process. The performance of adhesive joints depends greatly on part surface preparation, especially with thermoplastics, to ensure that all release agents have been removed prior to bonding. Surface preparation and long cure times reduce the applicability potential for this process. Another disadvantage is that different materials require different adhesives to properly bond them together. The interfacial bond between the two surfaces exhibits different properties than the rest of the material due to the fact that an adhesive typically behaves differently than the base material under the same applied stresses. The chemicals used in adhesives can also present environmental and health hazards. In addition, some plastics, are relatively chemically unreactive and do not effectively react with chemically reactive adhesives.
Solvents
Solvent bonding is also effective for some plastics. After the surfaces to be joined are softened by contact with the solvent, they are held together until the molecules in the plastic interlock across the bond line and the solvent evaporates. This process is simple and inexpensive, but may require long waiting times and can cause stress cracking due to the action of the solvent on the plastic. Times required to make a strong bond can be as long as four days, which significantly slows the manufacturing process. Solvents can also be very volatile, creating safety and health hazards. In addition, some plastics, (polypropylene, polyethylene, nylon) are basically insoluble to common solvents.
Welding Processes
Welding processes are the most widely used joining processes in high production applications because they are fast and versatile. Welding requires heating and some degree of softening or melting to form a bond. Thermoplastic polymers lend themselves to these processes because they can be quickly heated, formed, and then cooled to retain a new configuration or form. However, care must be taken not to degrade the plastic parts as repeated heating or overheating will eventually result in degradation and diminished properties.
As mentioned earlier, the three general classes of welding methods are thermal, friction/mechanical, and electromagnetic. Some of the more widely used processes from each of these classes will be discussed.
Thermal Methods
Thermal methods use heat generated by the tooling to soften or melt the surfaces to be joined. Hot gas welding, extrusion welding, hot tool welding, and infrared heating are thermal methods used for welding plastics.
Hot gas welding is a well established process, as it has been used to weld plastics for over 30 years. Its name explains the process well. A heated gas is used to soften the joint surfaces and a softened filler rod is used to fill the joint area and bond the surfaces together. The gas is typically just air, but an inert gas such as nitrogen must be used with some plastics to prevent oxidation. For a butt joint, the edges to be joined are typically beveled to increase the surface area for the filler rod to fuse with. This filler rod is heated and fed into the joint, similar to many fusion welding processes used for joinin
Johns Clark Joseph
Nelson Tracy Wendell
Sorenson Carl David
Brigham Young University
Sells James
Sonntag James L.
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