Thermoforming is restricted to relatively simple shapes having limited depth of draw. It is easy to thermoform articles having section thickness less than 1.0 mm, whether it is large area articles like refrigerator liners or smaller articles like thin wall disposable cups, which are otherwise difficult to injection mould.

In thermoforming, the mould and machine costs are quite low as compared to injection moulding. The thermoforming moulds as compared to injection mould are lighter in weight and easy to install on the machine. They are easy to fabricate and that to in a shorter period of time. In thermoforming the starting material is sheet and after forming a sheet, secondary trimming operation is carried out to get the finished article. This will generate nearly more than 35% of the sheet scrap.

The major thermoplastic raw materials used for thermoforming of thin wall containers are:

Characterstics of major thermoplastic materials used for thermoforming of thin wall containers

Out of all the above thermosplastic materials, high impact polystyrene is easy to thermoform. ABS sheets also thermoform better but they should not be overheated otherwise on forming they may blister off and may give rough surface. Unfilled polypropylene, because of its partly crystalline structure and definite melting point is comparatively difficult to thermoform while filled polypropylene having better stiffness property relatively thermoform better. Multilayer sheets also can thermoform good.
The choice of each material depends upon the end-use requirement and its cost. The most important properties required particularly in the case of rigid packaging are:

	Stiffness and ductility of the material.
	Easy processability - thermoforming / Injection moulding etc.
	Maximum wall thickness permissible in the article or the weight of the article.
	Cooling cycle during forming and moulding.
	Permeability of gases and water vapour or the required shelf-life of the product to be packed.
	Compatibility of the product to be packed with the plastic material.
	Heat resistance of the plastic materials.
	Ease at which the plastic containers can be printed by dry offset process for four or six colours printing.
	Whether plastic containers are sterilisable.
	Economic consideration.

No single thermoplastic material can satisfy all these properties. Out of all the thermoplastics, high impact polystyrene can satisfy most of the properties except impermeability of oxygen and water vapour. Polypropylene is to an extent better than HIPS in impermeability of water vapour and heat and chemical resistance but not that good in case of oxygen impermeability.

The introduction of five layer multilayer sheets like HIPS/EVOH/HIPS or HIPS/EVOH/PP or HIPS/PVDC/PP or PP/EVOH/PP etc. help in improving the impermeability of gases. The extruded block copolymer or GPPS/HIPS sheet give glossy surface on the matt surface of HIPS and also can give two colour sheet - bright colour on the outside and white colour in the inside of the cups thermoformed from such sheet.

Thin wall cups, tubs etc.

There is an increasing demand for thin wall disposable cups, tubs etc. for packaging of food, especially milk products. The preferred materials are Polystyrene, PVC and Polypropylene. The selection of the processing technique depends upon important properties of the materials like stiffness, permeability of gases and water vapour, transparency, etc. The selection of the material also depends upon the price of the material, minimum wall thickness permissible in the cup, weight of the cup, cooling cycle during forming and the desired quality of the cup. The stiffness of the material is expressed by the modulus of elasticity of the material. By the thermoforming process, it is possible to bring down the wall thickness and weight of the cup to a minimum possible limit with acceptable impact and stiffness property. In stiffness property, PVC works out as a better material and polystyrene is comparable to it while polypropylene is inferior to both of them. The cooling time-cycle depends upon the wall thickness and weight of the cup.

The thin wall thermoformed cups first came on the market in the year 1950. The cups were produced on a roll fed sheet thermoforming line. In the year 1960 it was brought out that in line sheet extrusion and thermoforming equipment will help in producing thin wall cups from transparent general purpose polystyrene, which otherwise is not possible because of its brittleness. But the 1974 oil crisis accelerated the development of in line sheet extrusion - thermoforming equipment. The oil crisis increased the prices of the plastic raw-materials and the cost of the energy and the need of the hour was to conserve energy. This led to the development of in line sheet extrusion - thermoforming equipment which were desplayed first time by the world's three leading manufacturers in K-75 Dusseldorf fair. These lines save the evergy by 30% and utilise the material efficiently and gave out cups at a very high output rates. It also overcame the problem of uniformly heating the sheet particularly in polypropylene because of its low thermal conductivity. All the products which can be thermoformed on sheet fed lines can be formed on in line thermoforming process. Therefore, in-line sheet extrusion thermoforming technique offers the best economics as there is a saying in the energy cost;(as high as 30 to 40%) the output rates are very high, the scrap generated is recycled and there is hardly any loss in the material. As setting time of the line is long and more scrap is generated the line is good only if it run is long and frequent changes of the moulds are not desired.

Trays

The trays for confectionery products are thermoformed from HIPS/PP/APET thin gauge sheets. They can be transparent, coloured or translucent and mostly used as insert trays.
The roll fed sheet lines are popular all over the world for thermoforming of trays. These lines are suitable for all the materials like Polystyrene and Polypropylene. These lines have an advantage that they can use multilayer sheets.
Material like OPS sheets pressure formed at 110°C for trays for salad or for cake packaging.
The roll fed sheet therforming line for thin wall cups, tubs or confectionery trays can handle materials like HIPS,PP,PET, Multilayer sheets etc. The sheet reels normally handed having width ranging from 150 mm to 495 mm and can take up the material rolls up to 1200 mm dia. The machine forms the sheet by plug assist technique in case of cups and tubs and straight thermoforming in case of trays having multlicavity female moulds. The sheets are heated for thermoforming by infrared ceramic sandwich type heaters, so that they are brought to the forming temperature fast and can give forming cycle as high as 35 to 40 cycles per minute. In case of PP as it takes longer heating-up time a preheater is provided prior to sandwich heating system in order to get faster forming cycle. The heater unit is equipped with rows of ceramic heater elements and they are controlled iln longitudinal direction, through thermocouple. This keep the temperature fluctuation at the mimimum over the entire area. The maximum depth of draw which can be taken on forming is 100mm.

The mould is so designed that the cup, the tub and the trays can be formed and punched in place. The mould may be cooled by chilled water. There are machines available which can give output of 10000-100000 cups or tubs or trays per hour. Roll fed sheet thermoforming lines are more flexible than in-line sheet and thermoforming lines as the moulds can be changed frequently and short run for any shape and size is possible, only the scrap generated required to be recycled.

Solid phase pressure forming

Polypropylene, because of its low melt strength at thermoforming temperature is better thermoformed just below its crystalline melting point of 167°C - which is around 160°C + 2°C. The process involves female tooling with plug assisted pressure forming. PP pellets, usually of MFI 2 to 3 g/10 mts. (230°C/2.16 kg) are extruded into 0.25 to 2.0 mm thick sheet and then fed through the heating unit to the pressure former and thermoformed.

Dual ovenable tray

The dual ovenable tray are CPET tray and can withstand temperature from - 40° to + 220°C. It is thus excellent for use in freezer, micro waves and conventional electrical ovens.
While PP tray can withstand temperature from - 20° to + 120°C and therefore it is good for use in freezer and microwave oven and not conventional electrical oven.
The APET tray can withstand temperature from - 40°C to +65°C, therefore it is neither used for microwave nor for conventional ovens.

Conclusion

Today a number of food processing industries have come up in the country and they need the right type of packaging materials. The dairy industry is expanding at a faster rate and the demand for the plastic packing materials is increasing by many times. The indigenous machine manufacturers have also come out with thermoforming machines needed to form the thermoplastic sheets for the packaging industry. The thermoplastic sheet manufacturers have started developing multilayer sheets. Therefore, during the coming decade our country will witness the excellent development taking place on the thermoforming front.