Q. Write short note on laminating.
Explain the Laminating processes.
What do you mean by lamination of plastics, explain it.
Ans. Laminations of Plastics : There are two popular methods of lamination plastics :
(a) High pressure laminating.
(b) Low pressure laminating or reinforced plastic moulding.
(a) High pressure laminating : The process involves joining of layers of
fibrous reinforcing materials with thermosetting resin binders by the application of heat and pressure both. The pressure usually ranges between so kg/cm2 to 24 kg/cm2. The layers are generally of some fibrous material like paper, cotton fabric or mat, asbestos, glass, nylon or wood veneer. Laminates with special properties are mad by using graphite, mica and alumina, etc. the resins commonly used are phenol formaldehyde, urea, melamines, silicones, epoxies or their combinations.
The process consists of preparing varnish solution from resin by dissolving them in suitable solvents followed by impregnating or coating the fibrous sheets with this varnish. These sheets are then dried, trimmed to size and heated and pressed between metal plates to form the laminated sheets. Tubes are made by rolling the impregnated fibrous material around mandrels and heating till they are cored. Rods are made by rolling and heating the impregnated material inside cylindrical moulds, followed by grinding to size. Moulded shapes are formed by cutting the impregnated sheets to required shapes and sizes, followed by heating pressing as usual.
(b) Low pressure laminating : It is also known as reinforced plastic
moulding. In this process, pressures of only up to 32 kg/cm2 are applied. Mostly thermosetting resins are used in laminating, although in the same cases thermoplastic resins are also used. The reinforced materials used are glass fibers, cotton, asbestos, nylon, paper and other fibers. They are used in various forms like cloth, performs or mate etc. The commonly used resins are phenolics, polyesters, epoxies, furnaces and silicones. Pigments are carbonate, asbestos and aluminium silicate, are used for obtaining better surface finish, better physical properties, reducing shrinkage and lowering cost. The following are few of the several methods used for low pressure laminating.
1. Contact moulding : It is the simplest of all the methods. In this process,
layers of reinforcing material are coated with catalysed resin and placed one over the other on a prepared form. In order to accelerate the process, particularly in large moulds, the resin and reinforcement material are sprayed instead of being coated by hand operation. The resin is then allowed to be cured by heat or at room temperature. The main advantages of this process are low cost, simple operation, no size limits of products, readiness to adopt changes in design and minimum equipment required.
2. Pressure bag moulding : In this method the resin impregnated work
piece is placed in the mould. A tailored to size rubber bag covers the work piece from above. To the top of the mould is secured a pressure plate. Air or steam pressure is applied between the bag and the plate to inflate the bag against the work piece. Its main advantages include easy formation of cylindrical shapes, dense and voidless products, undercuts can be made, cores and inserts can be used and advantages of contact moulding are still retained.
3. vacuum bag moulding : In this, the resin impregnated material lay-up placed over the form mould, followed a covering provided by a bag made of a film of cellophane or polyvinyl. Exhaust povts are provided in the mould through which vacuum is drawn from under the bag. This enables the atmospheric pressure to act on the bag to force the lay-up against the form. The main advantages of this method are less air entrapment, less voids, better surface finish, higher glass loading and still retaining the advantages of contact moulding.
4. Autoclave forming : It is a modification of the above two methods. Either of the pressure bag or vacuum bag arrangement, after laying-up and covering, can be placed in a steam or hot air autoclave to provide an increased pressure and heat to provide higher glass loading, effective air removal and have a dens and void free moulding.
5. Vacuum injection process : It consist of holding the reinforcing material between two matching moulds and drawing the resin up, from a container placed of the bottom, through the reinforcing material, by drawing the vacuum through the top mould.
6. Matched dies : Reinforced plastic mouldings are also formed in rigid or flexible punches and dies. Mostly steel dies are used, but for small production they can be made from reinforced plastics also.