Thermoforming heaters

Products are affected dimensionally by the difference between their forming temperature and their product-use temperature. Thus, a plastic s coefficient of thermal expansion and contraction has a significant effect on service conditions. The thermoforming pressure, time, and temperature variations that can exist will affect the final dimensions. Of these factors, evenness in heating throughout the sheet thickness before forming is usually the most important control. Type of heater has a direct effect on obtaining uniform heat... 

In the plastication step of the process, melting is critical in controlling cycle time. Also, during thermoforming, the heating of the sheet is the most time consuming step of the whole process. For example, if we consider the melting of an infinite slab, at an initial temperature of To, as presented in Fig. 6.61, the heat supplied by thehot wall, set at a heater temperature of Th, will create a layer of molten polymer of thickness, X (t). 

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,... 

In practice, the thermoformer oven temperature is set to match the theoretical best temperature for absorption of the particular polymer, and the heating time is determined by trial and error. Obviously, one should discuss the heating temperatures with the equipment manufacturer and the plastic supplier for recommendations. The heaters usually used in thermoformers are either calrod type or quartz lamps. Less frequently, other types of heaters are used. 

What is the relationship between the discussion of the optimum temperature for radiative heaters for thermoforming and plastics identification using IR spectrophotometry ... 

In the thermoforming process, the temperature of the ceramic or metal radiant heaters is typically 400 °C (673 K). These can be treated as black bodies at an absolute temperature T, for which the radiation heat flux from an area A is... 

In this process, PLA sheet is heated to soften the polymer, forced either pneumatic and/or mechanically against the mold, allowed to cool, removed from the mold, and then trimmed. The radiant heater of the thermoforming line for PLA must be adjusted to very low temperatures. Preheating is not absolutely necessary however, it has the general advantage that the sheet is homogeneously preheated. 

Heat is usually applied in various amounts and in different locations, whether in a metal plasticating barrel (extrusion, injection molding, etc.) or in a metal mold/die (compression, injection, thermoforming, extrusion, etc.). With barrels a thermocouple is usually embedded in the metal to send a signal to a temperature controller. In turn, it controls the electric power output device regulating the power to the heater bands in different zones of the barrel. The placement of the thermocouple temperature sensor is extremely important. The heat flow in any medium sets up a temperature gradient in that medium, just as the flow of water in a pipe sets up a pressure drop, and the flow of electricity in a wire causes a voltage drop. 

Murphy, C., Radiant Heater Panels Improve Thermoforming, MP, pp. 78-82, May 1987. 

Sandwich heating (two-sided heating) n. A method of heating a plastic sheet for thermoforming in which the sheet is positioned between two radiant heaters. 

Strip heaters present most of the same construction varieties as the band heaters. Strip heaters are commonly used to apply surface heating to dies and molds. Sometimes they act as a radiant heat source in thermoforming or plastic-bending applications. 

Radiant heaters are nonconductive sources that work exceptionally well in plastic thermoforming and bending and welding applications by replasticizing the polymer before the final production processes. The more conamonly accepted industry styles of radiant heaters are ceramic infrared units. Quartz-faced and black body... 

Tubular heating elements are extremely durable, resistant to contamination, and quite economical. They can be used on dies, radiant, thermoforming, and hot runner manifold applications. Tubular heaters provide temperatures up to 649°C and offer tremendous design flexibility. 

The sheet is softened to an extent such that to stretch, enable, or ease without tearing. The softening behavior purely depends on the thermomechanical behavior in the form of orientation, stress, and crystallinity of the polymeric material. Therefore, the sheet memory is partially removed because of comparatively low temperatures over short periods of time. Polymer with amorphous nature is thermoformed above its glass transition temperature. The forming temperature is slightly above the melting temperature of their crystalline domains. The polymeric sheet is heated by contact, convection, or radiant heater. Then, the heated sheet... 

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. 

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