Keywords: masked heating, roll-fed, thermoforming, thickness distribution.

In thermoforming, an extruded plastic sheet is heated above its softening temperature and stretched into a female or male mould, by applying pressure or a vacuum, eventually assisted by direct mechanical loading (e.g. a plug). Once the part is sufficiently cold, it retains the mould details and can be extracted. During the deformation stage, the hot sheet is initially stretched as a free bubble and then pushed against the mould cold surface, which prevents it from undertaking any further deformation [1,2]. Consequently, thermoformed parts exhibit a thickness distribution that may compromise their mechanical performance under service conditions.

Thickness distribution (or its effects) can be minimised via careful part design, selection of an appropriate thermoforming technique, or optimisation of the sheet temperature field prior to the deformation stage [3]. In fact, local sheet temperatures affect the polymer's viscoelastic response and, consequently, its local stretching ability. Thus, portions of sheet subjected to higher deformations should be cooler than those that will form the shallower regions of the part.

Sheet temperature fields can be produced by setting individually the various radiant heaters of the heating panel [4,5,6]. Methodologies for determining their temperatures in order to produce a specific sheet temperature profile after a certain heating time have been developed [7,8]. However, very often the actual dimensions of the ceramic heaters are too large to produce the required detailed (and frequently intricate) temperature field. For example, in roll-fed thermoforming one aims at ensuring an overall uniform temperature across the sheet surface but, simultaneously, the thickness distribution of each individual part would be improved if a specific local sheet temperature profile was produced. One possible solution consists in fixing screens between sheet and heating panel, thus applying an old procedure used for thermoforming large parts. The screens will affect the spatial radiative heat transfer between both surfaces which, in addition to individual heaters setting, may yield the required sheet temperature fields.

In this work, the potential effect of coupling differential sheet heating (individual heaters plus masks) to modify the thickness distribution of parts produced by roll-fed plug-assisted thermoforming was investigated.

Modelling the heat transfer in roll-fed thermoforming was performed considering the interplay between heaters and portions of sheet as its advances discontinuously below the heater bank towards the forming station, with and without screens. In terms of heat transfer, this is a multi-step process with the temperature rise beginning before each fraction of the sheet is directly exposed to the heaters. The commercially available finite element software T-SIMR [9] was used to simulate sheet deformation during thermoforming (using the sheet temperatures at the end of the heating stage as input) and predict the resulting thickness distribution.

Computational results shown that, if the mesh of the shield is chosen adequately, non-uniform sheet temperatures are indeed produced. The practical effect of this was illustrated computing the thickness distribution (along the arc length) of a typical conical cup, produced from a sheet with uniform and non-uniform temperatures. In the two cases, due to the large sheet area in contact with the plug and plug-sheet interaction (hindering sheet deformation as the plug advances downwards), parts are thicker in the centre and edges and thinner around the corner. However, this effect is minimized when the portion of sheet in contact with the plug is set at higher temperature (non uniform temperature profile), as it deforms more easily. The hotter sheet centre will become thinner and the edges thicker, yielding parts with a more uniform thickness distribution.

1

J.L. Throne "Technology of Thermoforming", Hanser Publishers, Munich, Vienna, New York, 1996.

2

H.G. DeLorenzi and H.F. Nied, in "Modelling of Polymer Processing", Isayev, A. I. (Ed.), Hanser Publishers, Munich, Vienna, New York, 117-171 1991.

3

F.M. Duarte and J.A. Covas "On the Use of the Heating Stage to Control the Thickness Distribution in Thermoformed Parts", Inter. Polym. Proc., 19, 186-198, 2004.

4

D. Weinand and G. Menges, "Computer-aided heating of plastics by infrared-radiation", SPE-Tech. Papers, 45, 421-424, 1987.

5

H.F. Nied and H.G. DeLorenzi, "Solution of the inverse thermoforming problem using finite element simulation", PPS 6, Paper P03-10, Nice, 1990.

6

H.F. Nied, "Current challenges for improved optimization and control of thermoforming process", PPS 17, Paper 374, Montreal, 2001.

7

F.M. Duarte and J.A. Covas, "Heating thermoplastics sheets for thermoforming: solution to direct and inverse problems", Plast., Rubber Compos. Process. Appl., 26, 213-221, 1997.

8

F.M. Duarte and J.A. Covas "IR sheet heating in roll fed thermoforming. Part 1- Solving direct and inverse heating problems", Plast., Rubber and Compos., 7, 307-317, 2002. doi:10.1179/146580102225006530

9

T-SIM, Computer Simulation of Thermoforming, version 4.32, Accuform, Czech Republic.

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