Thermoforming of polypropylene

There are two main types of thermoforming, (1) vacuum forming and (2) pressure forming. In vacuum forming, the plastic sheet is heated until it becomes soft and pliable, the heater is removed and the mold is rapidly evacuated. The reduction in pressure on the mold side allows deformation of the sheet, by atmospheric pressure, into the mold cavity. On contacting the mold, the sheet cools rapidly and retains the shape of the mold. The force available to deform the sheet is limited to that provided at atmospheric pressure and this type of thermoforming is thus limited to thin gauge, low-modulus polymers. 

Pressure forming can be used to form high-modulus polymers in a wide range of sheet gauges. In this process, the heated, softened sheet is forced into the mold by means of a positive pressure (up to 0.3 MPa) applied above the sheet. 

Both vacuum forming and pressure forming may be used in conjunction with a mechanical plugging device which is used to enhance the material deformation capabilities of the process and to improve the 

A number of polymers may be formed by the thermoforming process, some more successfully than others. Amorphous polymers, such as polystyrene (PS), upon which the industry has traditionally relied, are by far the easiest to thermoform. These materials are, however, being replaced at a rapid rate, particularly in the packaging field, by semicrystalline polymers such as polypropylene (PP). 

PP s semicrystalline nature imparts to it an excellent combination of physical, thermal and chemical properties which makes it a premium material in packaging applications. Its commercial exploitation in this sector has, however, been impeded by several material-related properties which make it difficult to thermoform. The main problems are (1) low melt strength leading to sagging of the sheet during heating and which in turn results in parts with poor material distribution, and (2) a narrow temperature processing window within which the sheet may be formed. 

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Macauley, N. (1996) Extrusion and Thermoforming of Polypropylenes, PhD. Thesis, The Queen s University of Belfast. 

Thermoforming and Extrusion. Improved equipment and polymers have increased the capabiUty to extmde and thermoform polypropylene however, consumption of polypropylene in these areas has not grown dramatically. Drinking straws are commonly extmded from polypropylene, however most larger diameter tubes, such as pipes and conduits, are predominantly extmded from other thermoplastics. Extmded sheet is thermoformed into food containers and trays polypropylene is used when microwavabiUty is desired. 

Autobar has worked with Italian supermarket chain, Iper, on the manufacture of PLA thermoformed trays and containers for fresh foods in place of polypropylene. The packaging consists of 12 sizes, categorised as small , medium , large and maxi , that offer a range of dimensions. 

By using NatureWorks PLA instead of polypropylene, Autobar was able to reduce the wall thickness of the containers, from 460 microns down to 330 microns. The downgauging allows Autobar to use less material, which helps reduce the production costs, without compromising the quality of the thermoformed container. 

Products A wide range of polypropylene products (homopolymer, random copolymer and impact copolymer) can be produced to serve many applications, including injection molding, blow molding, thermoforming, film, extrusion, sheet and fiber. Impact copolymer produced using this process exhibits a superior balance of stiffness and impact resistance over a broad temperature range. 

Talc is always an attractive subject of such studies due to its platelet structure. In thermoforming and compression molding processes of three resins (PP, HDPE, and PPS), each containing 20% talc, the talc particles were always parallel to the specimen surface, regardless of the resin used. Crystallites grew in a direction normal to the surface of talc particles and thus were perpendicular to the specimen surface. But in the case of unfilled HDPE, crystallites grew parallel to the specimen surface. There was no difference in crystallite growth direction in the case of polypropylene with and without talc. 

Marlex . [Phillips Phillips Petrol. Chem. SA V] PolyethyleM or polypropylene resins for inj., blow, or rota-tiotial molding thermoforming of sp-pliance parts, chemical equipment industrial parts, containos, toys, htrase-wares, chemical tanks, monofilament filmai lics. 

From the three basic categories of polypropylene, namely, homopolymers, heterophasic copolymers, and random copolymers (with ethylene), there are specialty resins with enhanced capabilities for specific applications. Producers of large blow-molded or thermoformed parts can thus utilize grades with high melt strength to fabricate heat-resistant under-the-hood automotive parts. 

Scherer, R. (1995) Thermoforming of unidirectional continuous fibre-reinforced polypropylene laminates and their modeling, in Polypropylene Structure, Blends and Composites, Vol. 3, Chap. 8, (ed. J. Karger-Kocsis), Chapman Hall, London, pp. 293-315. 

Table 6 Comparison of some relevant properties in thermoforming between 8-nucleated PP and other classes of polypropylene. 0 means identical to PP-homopolymer without nucleating agent + means good, ++ very good, - bad, - very bad. The data provided for PP block and random copolymers deal with classical resins. Data taken from Wolfschwenger et al. [49]...

Harrison P, Long AC, Clifford MJ, Gil RG, Ward IM, Hine PJ (2002) An investigation into the thermoformability of hot compacted polypropylene sheet. In proceedings automotive composites and plastics conference (ACP), Basildon, UK... 

First, the market needs for a thermo formable TPO were characterized to define specific performance requirements. This was accomplished via review of published data and Voice of the Customer (VOC) interviews with key stakeholders in the custom thermoforming value chain including sheet extruders, custom thermoformers and original equipment manufacturers (OEM s). The output of this process was the need for development of polypropylene based thermo formable sheet that could replace fiber glass, metal, and wood. While many performance requirements are unique for each application, four specific performance requirements were common. 

Sheet Extrusion and Thermoforming. Sheet for thermoforming and analogous operations is usually formed by extmding the melt through a slot die onto a set of poHshed chill roUs. The sheet is usually approximately 150 cm wide. After rapid cooling, the web is coiled or cut into sheets. Polystyrene, PVC, polyethylene, polypropylene, and filled polypropylene are prepared in sheet form by extmsion. 

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