Thermoforming heat transfer

This work was motivated by cracking of a thermoformed part while cooling on the mold, the complexity of the problem could be immediately appreciated since the effect was sensitive to very delicate changes in material composition. Due to coupling between the heat transfer and stress evolution, both problems were solved simultaneously ... 

Consequently, their use is best confined to short run or prototype use. In normal production, the improved heat transfer capability of a metal mold will more than repay the greater cost. Aluminum is most commonly used for thermoforming molds other options include cast or sprayed low melting point alloys, porous sintered metals, and copper alloys (Chapter 17). 

The main heat transfer processes that are come across in plastics processing are conduction, convection and viscous heating, with radiation only playing a role in thermoforming. Most products are much thinner than they are wide, so only one-dimensional heat flow (Fig. 5.2) will be considered. The heat flow direction is along the x axis, perpendicular to the surface of the product there are planar isotherms perpendicular to the x axis. The heat flow Q is considered across an area A of the isothermal surface. 

Secondly, the mixture produced is thermoformed in a hot-plates press at 80 °C for 5 min using a pressure of 1 ton with the aim of obtaining circular samples. The thermoformed sample of activated NR was then introduced into a circular mould. The mould was also placed in the hot-plates press where the vulcanization and foaming process were performed simultaneously. The reason for choosing a hot-plates press instead of a conventional oven was that the heat transfer is faster in a press than in an oven, which consequently allows a reduction in the production time. Nevertheless, no pressure was applied to the material. The plates of the press were only in contact with the upper and lower surfaces of the mould. Temperature and time were set up respectively to 160 °C and 30 min. During this time, NR was crossUnked and expanded due to the decomposition of azodicarbonamide in gases, mainly N2 and CO, allowing NR to fill the mould completely. Finally, the mould was water cooled and the foam was removed from it. 

Fourier number n (Np). A dimensionless group important in analysis of unsteady heat transfer in solids, such as sheets being heated or cooled in thermoforming, or cooling of extrudates and moldings, ATp = a f/x , where a is the material s thermal diffiisivity, t the heating or cooling time, and x is the a thickness or half-thickness in the direction of heat flow. 

In sheet or film extrusion haul-off rolls in a three-roll stock should be on trial and error to give the best appearance of the fiat sheet. The maximum roll temperature is normally limited by sticking of the polymer material to the roll. The larger the roll diameters, the better effectiveness and heat transfer occur. In this extrusion process, the buildup of bank of polymer in the nip of the rolls should be avoided, which leads to excessive orientation in the end product. This excessive orientation can cause brittleness in across-the-machine direction and such film or sheet is difficult to thermoform. 

These equations are then applicable to many aspects of polymer processing, such as cooling material in a mold, heating a sheet, and thermoforming. Fundamentally, they represent cases of unsteady-state heat transfer, and the solutions of Eq. (4-7) and (4-8) can be quite complex. However, generalized solutions for various shapes do exist. The basis for these is to neglect Aq and then to solve the remaining equation ... 

Heat transfer is extremely important in the thermoforming operation since the time required to reach the softening temperature can be 50%-80% of the total cycle time. Obviously, the less time needed to reach the forming temperature, the less the overall cycle time. 

Heating can be done by conduction, convection, radiation, or some combination of these. The usual mode of heat transfer in thermoforming is radiation. Radiation heat transfer in polymeric systems was discussed in Chapter 4. 

New copolymerization methods, additives, rubber modification, and blending have made of polystyrene polymer and copolymers versatile packaging materials. Developed in 1930 by BASF, polystyrene (PS) is commonly produced by the continuous bulk polymerization of styrene in the presence of ethylbezene that control product viscosity and heat transfer. PS is hydrophobic, nonhygroscopic, and easily processed by extrusion and thermoform-ing. Three types of PS are available general-purpose, impact PS, and foams. 

In the rubber industry, polyglycols are used as components of clay-containing rubber formulations to control cure rates. Folyglycols are added to thermoplastics to isprove flexibility as well as to impart antistatic properties to in rova the processing of plastic pellets or resins. Hot baths of polyglycols also act as heat transfer fluids in plastic thermoforming. Polyglycols are used as lubricants to facilitate removal of vulcanized products from molds. 

Thermal radiation is often used to heat up preforms used in blow molding or plastic sheets used in thermoforming. Furthermore, in processes such as fiber spinning considerable cooling of the outer filaments can occur through radiation heat transfer. We review here the basic ideas of radiation heat transfer. 

However, they can also be transfer or compression molded, solid state formed or thermoformed. PC is the matrix phase in most of these blends. The impact modifier can be PE, ABS or acrylic copolymer, e.g., MBS. The blends show good processability, heat resistance, ductility, HDT, high modulus, impact, tensile and flexural strength over a wide temperature range, good adhesion, solvent, chemical, and UV resistance. They can be painted, hot stamped, metallized and plated. Some blends (containing PB as impact modifier) may have poor weatherability. The blends are mainly used in the automotive industry. 

Heat can be transferred to the sheet in several ways. The most common for thermo-forming is radiation from heater elements. The sheet is moved into or through an oven with radiant heating elements. In some thermoform/fiH/seal equipment, heating is by conduction. In this case, the sheet is pulled up against a heated plate,... 

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