Heat penetration thickness

Figure 5.23 shows the dimensionless temperature as a function of dimensionless time From the above equations and figure, we can define a heat penetration thickness. 

Molds can be of any shape and can include corrugated or rib constructions to increase their stability and stiffness (large, flat walls can be difficult if not impossible). The thickness of their walls is limited to allow heat penetration. 

In the vulcanisation of thick rubber articles the temperature may be stepped up in stages to ensure adequate heat penetration to the interior before the external surface begins to cure. Such a cure schedule is termed a step cure. Step post cures are used with thick section rubber articles to ensure that volatiles are released gradually without causing damage, such as porosity, due to an over rapid release. 

Table 5.7 Penetration thickness and characteristic times in heating and cooling polymers...

Equation 6.101 describes conductive heating of a semi-infinite slab. It is valid as long as thermal penetration thickness 6( is less than the slab thickness. Essentially all temperature change (99%) takes place within the thermal penetration thickness, which is given by ... 

An additional complication Is that the solid bed Is assumed to be freely deformable. This means that the heat transfer situation Is no longer determinate. Equations 7.97 through 7.99 can only be used If the rate of deformation of the solid bed is small relative to the rate of heat conduction into the solid bed. A typical solid bed melt velocity (v y) is 0.2 mm/s. The thermal penetration thickness (4Vat, see Eq. 6.99(a)) is about 1.3 mm in one second when the thermal diffusivity (a) is 10 mVs. Thus, the rate of heat conduction into the solid bed is about one order of magnitude higher than the rate of deformation of the solid bed. Thus, the rate of deformation of the solid bed, in most cases, is relatively small compared to the rate of heat conduction into the solid bed. 

Fast heating rate Heat penetration through thick... 

The total thickness of the pouch and individual thickness of the different layers of all the retort pouches are presented in the Table 2. It has been observed that there is not much variation of thickness of pouches of individual layers from one laminate to another. The thickness of different pouches ranged from 135-110 pm for the opaque retort pouches and 150-120 pm for the clear pouches. Both imported and indigenous pouches were having uniform thickness throughout the same specimen. The OP-B pouch was having one extra nylon layer apart from polyester, aluminum foil, and cast polypropylene layer. In case of clear pouches, CP-C had only two layers that is polyester and CPP, whereas in CP-A, there were three layers, where the outer polyester layer was coated with aluminum oxide and nylon was also present. In the case of CP-B, the polyester layer was quoted with silicon dioxide. The thickness of the pouch had a direct influence on the heat penetration characteristics and product quality. Non uniform thickness can affect the machine performance, product protection, and integrity of the packages (Hemavathi, Anupama, Vani, Asha, Vijayalakshmi, Kumar, 2002). The... 

For an economical rotational production, it is necessary to machine the mold shells to a certain wall thickness, which means to machine on both sides. This will ensure an equal and fast heat penetration of the mold wall. 

A special coil configuration is used to heat thin strips of metal that caimot be heated efficiently with a coil that encircles the load, as the strip thickness is small compared to the depth of penetration. The transverse flux induction coil is positioned on either side of a strip to produce a uniformly heated strip with good efficiency in a much smaller space than conventional radiant or convective strip heating furnaces (6). 

Wood in its untreated form has good resistance or endurance to fire penetration when used in thick sections for walls, doors, floors, ceilings, beams, and roofs. This endurance is due to low thermal conductivity, which reduces the rate at which heat is transmitted to the interior. Typically, when the fire temperature at the surface of softwood is 870—980°C, the inner char 2one temperature is - 290° C, and 6 mm further inward, the temperature is 180°C or less. The penetration rate of this char line is mm/min, depending on the species, moisture content, and density (45,46). Owing to this slow... 

The gas around the arc plasma takes away a part of its heat by radiatioii. At high temperatures, the gas loses its specific gravity, becomes light weight and diminishes in momentum (< mv ). As a result, the gas is rendered incapable of penetrating through the arc plasma to quench it. The flow of gas through the thick of the arc plasma is thus impeded. 

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