Bonding, adhesive pressure equipment

Pressure equipment. Pressure devices should be designed to maintain constant pressme on the bond during the entire cure cycle. They must compensate for thickness reduction from adhesive flow-out or thermal expansion of assembly parts. Thus, screw-actuated devices like C clamps and bolted fixtures are not acceptable when constant pressure is important. Spring pressure can often be used to supplement clamps and compensate for thickness variations. Dead-weight loading may be applied in many instances however, this method is sometimes impractical, especially when heat cure is necessary. 

Films. Both structural and nonstructural adhesives are commonly available in film form. Adhesives applied in the form of dry films offer a clean, hazard-free operation with minimum waste and excellent control of film thickness. However, the method is generally limited to parts with flat surfaces or simple curves. Optimum bond strength requires curing under heat and pressure, which may involve considerable equipment and floor space, particularly for large parts. Film material cost is high in comparison to liquids, but waste or material loss is the lowest of any application method. 

After the adhesive is applied, the assembly must be mated as quickly as possible to prevent contamination of the adhesive surface. The substrates are held together under pressure and heated if necessary until cure is achieved. The equipment required to perform these functions must provide adequate heat and pressure, maintain constant pressure during the entire cure cycle, and distribute pressure uniformly over the bond area. Of course, many adhesives cure with simple contact pressure at room temperature, and extensive bonding equipment is not necessary. 

Ease of fabrication is one of the many advantages of polyethylene foam. It can be skived to precise thickness, cut and shaped to form custom parts, and joined to itself or to other materials without major investment in complex equipment. It can also be vacuum formed. Expanded polyethylene will adhere to itself by the use of heat alone. Hot air, or a plate heated to approximately 350°F (177°C) can be used to simultaneously heat the surfaces of two sections of foam to be joined. Upon softening, the two pieces are quickly joined together under moderate pressure, and an excellent bond formed, with only a short cooling period required. Release of the melted foam is aided by a coating of fluorocarbon resin or silicone dispersion on the heating surface. The foam may also be bonded to itself and to other materials by the use of solvents or commercially available adhesives (6). 

Assembly of silicon chips onto substrates with anisotropically conductive adhesives uses specialized equipment, initially developed for ffip-chip solder and TAB inner lead bonding. Heat and pressure are transmitted to the adhesive through a thermode attached to a robotic arm or a high-precision linear translator. Equipment requirements are more demanding than for solder assembly, as no self-alignment can occur. A minimum placement accuracy of 0.0005 in. is required. Coplanarity between the substrate and die is critical one study reports maintaining coplanarity to within 0.00004 in. [19]. The pressure required to achieve interconnection depends on the size of the die, the type of conductive particle used, and the viscosity of the adhesive at the bonding temperature. 

Laminates are composites made by combining two or more natural or artificial materials to maximise the useful properties of the components and minimise the weaknesses of individual components. A laminate consists of one or more sheets of fibres of one or more materials permanently bonded together by heat, pressure, welding, or adhesives. Different laminate stractmes that are used in cut/stab resistant personal protective equipment (PPE) are discussed below. 

Spruce (Picea abies Karst) lamellas were used to investigate the influence of the heat treatment of wood on the shear strength of the phenol-formaldehyde adhesive bond. Lamellas were heat treated at two temperatures 180°C and 220°C. The process of heat treatment in a vacuum, developed by Rep and co-workers [36], was used. Prior to heat treatment, all lamellas were planed to dimensions of 350 mm x 100 mm X 18 mm, and oven dried at 103°C. The treatment was performed in a vacuum chamber (Kambic, Laboratory Equipment d.o.o., Semic, Slovenia), where an absolute pressure of 5 kPa was achieved. The lamellas were heated to the desired temperatures, which took about 1 h, and then treated for 3 h at a constant temperature. The lamellas were then left to slowly cool down to room temperature. The lamellas were oven dried before and after heat treatment to determine mass loss. Mass loss (ML) after heat treatment was estimated according to the formula ... 

Pneumatic and hydraulic presses are excellent tools for applying constant pressure. Steam or electrically heated platen presses with hydrauUc rams are often used for adhesive bonding. Some units have multiple platens, thereby permitting the bonding of several assembUes at one time. Large bonded areas, such as on aircraft parts, are usually cured in an autoclave. The parts are first joined then covered with a rubber blanket to provide uniform pressure distribution. The entire assembly is then placed in an autoclave, which can he pressurized and heated. This method, which is widely used in the aerospace industry, requires capital-equipment. ""... 

Adhesive bonding is also usefiil for the repair of both metals and composites patches can be applied as temporary or permanent repairs to reinforce damaged stractures and bonded in place using special equipment to apply heat and pressure (see Repair methods). In these circumstances, it is not always possible to use the same pre-treatment and bonding... 

Heating equipment. Many structural adhesives require heat as well as pressure. Most often the strongest bonds are achieved by an elevated temperature cure. With many adhesives, trade-offs between cure times and temperature are permissible. But generally, the manufacturer will recommend a certain curing schedule for optimiun properties. 

In the last 40 years, many suppliers tried to find methods of clean and easy application of adhesives, glues and sealants. The main idea was to use adhesives in dry form, to avoid the stickiness and dirty application of the old glues. A need for adapted application equipment came from this. The main invention was the pressure sensitive tapes and labels, and then the hot melts supplied as dry solid granulates. There was also the preformed tape sealants, the structural adhesives supplied as film adhesives used in aircraft bonding figure 92, the heat sealable tapes and films, the hot melt sticks and lately the glue sticks for office use. 

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