A METHOD OF OPTIMIZING A THERMOFORMING PROCESS
Publication Date: 2003-Aug-05
The IP.com Prior Art Database
Rodolfo Jose Salmang Frohard: INVENTOR [+4]
A computer automated apparatus for measuring the melt strength and predicting the performance of a resin during the thermoforming process, comprising a "bubble burst test" apparatus modified with a load cell in the shaft of the linear motor pushing the plug that transmits all the force required by the plug during its stroke and simultaneously measures the total force, linked to a high speed data acquisition system that provides all the process parameters.
Thermoforming is achieved by heating and stretching a polymeric sheet until it duplicates the surface of a mould. Stretching may be achieved by means of a mechanical plug or by means of air pressure or vacuum, or combinations thereof. Evaluating the performance of different resins in thermoforming processes has been very difficult and sometimes subjective due to the nature of the process itself, i.e. isolating the results and main effect of each step of the process is very difficult.
In a plug assisted thermoforming process, typically used in producing deep draw articles, the sheet is stretched initially by a plug, which may provide up to 90 % of the total volume deformation, and then air pressure (or vacuum) generates the last 10% or more of the required deformation, until the sheet touches the mould walls and cools. The stretching of the sheet begins with the plug contacting the sheet and pushing it into the mould cavity. The sheet "sticks" to the plug in the initial place of contact while the remaining non-contacting sheet is stretched. It should be noted that plug temperature has a great influence on this process and although plug temperature is not typically measured, it must reach a level of equilibrium to obtain consistency in the final articles. Cooling of the sheet occurs as a result of increasing the surface area and reducing the thickness while stretching. As a result of cooling, the melt strength of the resin increases to a point where the friction force between the plug surface and the sheet is overridden by said melt strength and the sheet begins to slide over the plug. This marks a transition point from friction dragging to slippage. The friction force is also affected as expected by the normal force on the contacting surface, and hence influenced by the air pressure in the mould cavity.
In a blow-stretch thermoforming process, the sheet is stretched first by means of air pressure (positive, vacuum or combinations thereof) up to over 100% if required, and then allowed to retract into a mould that has been driven into the "bubble", i.e. blown sheet. The sheet is stretched by air differential pressure, and then contacts the mould in a few confined areas (edges) during the end of its travel stroke. Simulations of this process are quite precise, but do not allow to draw conclusive evaluations to totally predict the thermoforming performance of different resins. Specifically, a method for measuring mechanical behavior of polymers in a simulated thermoforming process is disclosed in “Improving our Industry through Technical Developments: the next step: A European programme sponsored by the EEC”, presented by Bernhard Hegemann from the University of Stuttgart. However, this method is a simulated thermoforming process and thus does not consider all the parameters which influence the performance of the materi...