FORMING ANALYSIS OF POLYMER SHEETSPolymers are large synthetic molecules made by joining together thousands of small molecular units known as monomers. The process of joining the molecules is called polymerisation and the number of these units in the long molecule is known as the degree of polymerisation. Polymers is being extensively used in structural applications in the areas of aerospace, automobile manufacturing, as well as in composite materials subjected to dynamic loading.Acrylics, which are more descriptively designated as polymrthyl methacrylate, which is derived from methacrylic acid methanol, are well suited for thermoforming due to their high hot strength and wide processing temperature range of 170oC-180oC [1]. Their excellent optical properties and clarity are augmented by outstanding outdoor stability and rigidity. All common molding processes may be used, including injection molding, compression molding, and extrusion [2]. Cell cast acrylic sheets have the best optical properties and lowest shrinkage but are highest in cost. Continuous cast sheets are produced by casting a viscous, partly polymerized monomer onto a metal belt. They are nearly as good in physical properties but may show slight optical distortionsAPPLICATION: Application of polymers include refrigerator and freezer door liners complete with formed-in compartments for eggs, butter, and bottles of various types, television masks, dishwasher housings, washing machine covers, various automobile parts (instrument panels, arm rests, ceilings, and door panels), large patterned diffusers in the lighting industry, displays in advertising, various parts in aircraft industry (windshields, interior panels, arm rests, serving trays, etc.), various housing (typewriters, Dictaphones, and duplicating machines), toys, transparent packages, and much more[3].

THERMOFORMING OF POLYMER SHEET: Forming refers to group of processes which involves clamping a sheet of thermoplastic above an open mould, heating it so that it becomes soft and rubbery and then forcing the sheer to take up the contours of the mould where it cools and solidifies to shape. This principle is utilized in thermoforming processes which may be divided into three main types: (a) vacuum forming, (2) pressure forming (blow forming), and (3) mechanical forming (e.g., matched metal forming), depending on the means used to stretch the heat softened sheet. Common materials subjected to thermoforming are thermoplastics such as polystyrene, cellulose acetate, cellulose acetate butyrate, PVC, ABS, poly(methyl methacrylate), low- and high-density polyethylene, and polypropylene. The bulk of the forming is done with extruded sheets, although cast, laminated sheets can also be formed. Vacuum Forming (Non contact type forming):In vacuum forming, the thermoplastic sheet can be clamped or simply held against the RIM of a mold and then heated until it becomes soft. The soft sheet is then sealed at the RIM, and the air from the mold cavity is removed by a suction pump so that the sheet is forced to take the contours of the mold by the atmospheric pressure above the sheet. The vacuum in the mold cavity is maintained until the part cools and becomes rigid. Straight cavity forming is not well adapted to forming a cup or box shape because as the sheet, drawn by vacuum, continues to fill out the mold and solidify, most of the stock is used up before it reaches the periphery of the base, with the result that this part becomes relatively thin and weak[4].Pressure Forming (Non contact type forming): Pressure forming is the reverse of vacuum forming. The plastic sheet is clamped, heated until it becomes soft, and sealed between a pressure head and the RIM of a mold. By applying air pressure. One forces the sheet to take the contours of the mold. Exhaust holes in the mold allow the trapped air to escape. After the part cools and becomes rigid, the pressure is released and the part is removed. As compared to vacuum forming, pressure forming affords a faster production cycle, greater part definition, and greater dimensional control. A variation of vacuum forming or pressure forming, called free forming or free blowing, is used with acrylic sheeting to produce parts that require superior optical quality (e.g., aircraft canopies). In this process the periphery is defined mechanically by clamping and the depth of draw or height is governed only by the vacuum or compressed air applied [4].

Fig1: forming process: (a)- vacuum forming, (b)-pressure forming [5]

Mechanical Forming: (Contact type forming):Various mechanical techniques have been developed for thermoforming that use neither air pressure nor vacuum. Typical of these is matched mold forming. A male mold is mounted on the top or bottom platen, and a matched female mold is mounted on the other. The plastic sheet, held by a clamping frame, is heated to the proper forming temperature, and the mold is then closed, forcing the plastic to the contours of both the male and the female molds. The molds are held in place until the plastic cools and attains dimensional stability, the latter facilitated by internal cooling of the mold. The matched mold technique affords excellent reproduction of mold detail and dimensional accuracy [4].

Fig2: Mechanical forming process: (a) Heating; (b) forming

Lubricants are added to thermoplastic molding to increase internal and external lubrication during thermoforming. These agents not only reduce the internal and external coefficient of friction, they may also have positive effects on mold removal, blocking, and surface gloss. Most lubricants are made from natural raw materials: paraffin from petroleum residues, lignite carbonization, or bituminous shales; the natural fatty acids (C16 toC18 acids and mixtures of same), vegetable and animal fats. Semisynthetic lubricants are based on the conversion of alcohols to esters, metaloxides or hydroxides to salts or amines to amides. Fully synthetic waxes include low molecular PE (Polyethylene) and PP (Polypropylene) waxes or copolymers made from ethylene, VAC, acrylic, or crotonic acid [6]

DEFECTS: During forming process due to improper process parameter selection or due to any random phenomenon certain defects are occurred. Some of the defects [7] are Blisters : Probable cause are heating too rapidly , Excess moisture, uneven heatingIncomplete forming: Probable causes are sheet too cold, sheet insertion, improper clamping, and insufficient vacuum.Whitening: Probable cause is stretching below forming temperature.Webbing, Bridging, Wrinkling : Probable cause are sheet to hot, insufficient vacuum, excess draw ratioChill mark/ striation: Probable cause is mold temperature too low.

In earlier days glasses were used as wind shield for aircraft. But glass cannot sustain the bird impact, to overcome that laminated plastics were used. They also introduced an entirely new set of optical and visual effects that interfered with aircrew visual performance. Multiple imaging was one of the first issues raised by aircrew. The slope and curvature of aircraft windshields that are optimum for high speed flight cause optical degradation of pilot vision in the forward direction. The optical effects of windshield slope (or angle of incidence) and curvature are reviewed, in terms of displacement, deviation, distortion, binocular deviation, reflections, multiple images, haze, transmission loss, and reduced resolution[8].OBJECTIVE OF RESEARCH:As we seen major product are thermoformed using different process. Each process has their specific characteristics, each process produces different product. The final quality of final product depends on process parameters. The most prominent defect in contact forming is visual defect in the application of windshield/canopy of aircraft. The objective of research is to find out the causes of defects occurs during processing and provide optimum forming parameters by experimental and numerical methods using finite element method.

REFERENCES: http://www.kaysons.in/acrylic/physicalproperties.pdfhttp://en.wikipedia.org/wiki/Poly%28methyl_methacrylate%29#Processinghttp://www.hydrosight.com/applications-for-acrylic/Polymer Processing and Structure Developmentedited by Arthur N. Wilkinson, Anthony J. Ryan.Y. Dong, R.J.T. Lin, Finite Element Simulation on Thermoforming Acrylic Sheets Using Dynamic Explicit Method Polymers & Polymer Composites, Vol. 14, No. 3, 2006. pp 307-328.Polymers - Opportunities and Risks I: General and Environmental Aspects, Volume 1 By Peter Eyerer.SHINKOLITE: Trouble Shooting of Thermoforming for GP Acrilic Sheet.

Industrial Plastics: Theory and Applications by Terry L. Richardson, Erik Lokensgard.

Research scholarSupervisor

Mr. Patil Jeet P. Dr. V. M. Nandedkar