The Sheet-forming Process

Head box Fourdrinier formation table Press section 

Models have been developed for this drainage process which are based upon theories of filtration. The Kozeny-Karmen equation is the most common rate expression used as a model for this filtration process. It can be expressed as  

There are a number of weaknesses in this approach, amongst 

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In the sheet-forming process, stainless steel, bronze, nickel-base alloys, or titanium powders are mixed with a thermosetting plastic and presintered to polymerize the plastic. Sintering takes place in wide, shallow trays. The specified porosity is achieved by selecting the proper particle size of the powder. Sheet is available in a variety of thicknesses between 16 x 30 mm and as much as 60 x 150 cm. A sheet can be sheared, roUed, and welded into different configurations. 

For production of woodfree uncoated and coated fine papers up to 40 kg starch per ton of paper are applied. 3 to 10 kg starch is added at the wet end, with the aim of internal strength improvement and retention increase. The major share of the starch is added to the sheet in surface treatment. A mass balance on a typical fine paper machine has shown, that more than 90% of the added starch is retained in the final paper product. Losses occur mainly during the sheet forming process in the wire section due to insufficient retention. Starch which is not held back in the paper is discharged with the process effluents to the waste water treatment plant, where a complete biodegradation process follows. 

Sheet forming processes, such as vacuum forming, do have effects on the product. The designer should be aware that these will affect the performance of one s product and one should learn how to modify the design to minimize any deleterious effects. Probably the most serious problem encountered in formed film or sheet products results from the fact that the materials are made from film or sheet at temperatures well below the melt softening point of the plastic, usually near the heat distortion temperature for the material. Forming under these condition when the draw down ratio is exceeded for a specific plastic can result in over stretched orientation of the material, the production of frozen-in stresses, poor product reproducibil-... 

Over the past few years, however, techniques have been developed to enable continuous reinforcement of thermoplastics. The simplest way is to put a cloth and a plastic sheet on top of each other in a heated press and to carry out impregnation under pressure. More difficult is the forming of an end-product from the sheet produced with conventional sheet-forming techniques the position of the fibres will be distorted in an unacceptable way. As in nearly all processing techniques, the modern finite-element methods with advanced computers are able to present solutions to this problem in principle they can predict the position of the fibres in the sheet-forming operation, so that optimum reinforcement is realised in the end product. 

This is an extension of the cold forming process. It uses thermoformed B-stage thermoset reinforced plastic (RP) skin to improve surface and other characteristics to a cold molded thermoplastic. The mold is closed and the fast, room temperature curing RP plastic system hardens. The finished product has the smooth TP-formed sheet. 

Thermoforming has close similarities with vacuum forming, except that greater use is made of air pressure and plug assisted forming of the softened sheet. The process is invariably automated and faster cycle times are achieved than in the vacuum forming process. Only thermoplastic sheet can be processed by this method. 

As briefly described above, the objective of the Sheet Forming Control Problem (SFCP) is to control the sheet thickness at 15 different points as uniformly as possible around different targets (yo). Thus, there are 15 controlled variables (CV s), which correspond to the thicknesses in the cross-direction, with the same relative weight (w). Moreover, this process has 9 manipulated variables (MV s) and 3 disturbance variables (DV s). The steady-state gain model, the sets within which the input and output variables are constrained and the relative weights of the output variables are given in the following equations ... 

The final chapter (Chapter 10) focuses on web and sheet forming processes. It demonstrates how the statistical techniques can be applied to evaluate process and control performance for quality assurance and to acquire fundamental insight towards the operation of such processes. 

A unique characteristic of the process data for sheet forming processes is the presence of two independent variables, space n and time k. In most cases the target of the process is to maintain the uniformity of Y for all n and k. For some applications a predefined constant CD profile ay... 

Gram polynomials are orthogonal and defined uniquely for discrete data at equidistant positions much like the spatial data collected in sheet forming processes. For N data positions, discrete-point scalar components of the mth-order polynomial vector pm = [Pm,i---Pm,n---Pm,jv] are defined as... 

Performance objective for sheet forming processes is to maintain uniformity or flatness in (Y - The target matrix represents y T for... 

Sheet forming processes have univariate MD and multivariate CD controllers. Process dynamics for both are dominated by gain and time delay. Most of the appreciable dynamics arise from the design of signal filters and... 

The vacuum forming process is shown schematically in Figure 11.12. The plastic sheet is clamped in place mechanically and heated. A vacuum is then placed beneath the hot elastic sheet, and this makes atmospheric pressure push the sheet down onto the contours of the cold mold. The plastic material cools down, and after an appropriate time the cooled part is removed. 

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