Cooling phase

For the material within the mold cavity, the holding pressure phase is concluded as soon as the sprue solidifies-in other words, as soon as it is sealed. From this point on, no more material can enter the cavity. During the injection and holding pressure phases, the material in the cavity has already begun to cool against the relatively cold mold wall. The outer layers have soldified very quickly. The cooling time therefore starts as early as injection. 

Cooling takes longer in the middle of the molded part. The surplus heat of melt remaining there must be dissipated through the outer layers to the mold wall. Now it becomes apparent that plastics are poor conductors of heat. The thermal conductivity of plastics is approximately 100 times poorer than that of steel. 

Heat transfer can be effectively defined with the help of mathematical equations. This makes it possible to perform advance calculations of the cooling time, during which the molded part must remain in the mold until it is sufficiently cooled. 

The choice of the proper cooling time is also especially important from the profitability aspect. Apart from the comphcated equations for defining the cooling process-as applied in computer simulations of injection molding-there are also simple equations which make it possible to calculate cooling times with a pocket calculator or by hand. 

The cooling time equation has been derived at as a simple approximation for defining heat-transfer within the molding during the cooling process in the injection molding tool  

For this idealized blow molding operation the objectives are the following  

Determine the temperature/time curves for both the polymer and the mold. 

Determine the necessary cooling time in the mold before it can be removed. 

Determine the effects of the following variables on the cooling time  

In order to proceed with the solution the following assumptions are made  

---

In the cooling phase (assuming t. is non-/ero), the velocities are periodically rescaled to change the system temperature from the run temperature to the final temperature Tj in increments of the temperature step AT. The cooling period for rescaling the velocities. P... is defined by ... 

The use of heating and cooling phases are optional and not often used in Monte Carlo simulations. However, they may be useful for some applications. 

The cooling cycle starts when all parts of the refrigerator are at about 1.3K. At this temperature, the 3He is completely adsorbed by the pump. The pump temperature is now raised to about 25 K by means of an heater. At 25 K, the 3He is desorbed, and its pressure increases over the saturation pressure at 1.3 K. Consequently, 3He condenses in the part of the tube T internal to the copper support C and drops down into the evaporator E. In this phase, the latent heat of condensation and the enthalpy variation are delivered to the 4He bath. The cooling phase starts when all the 3He is condensed in E and the power on the pump heater is switched off. The pump starts cooling towards the bath temperature, reducing the pressure on liquid 3He in E. The adsorption heat of the 3He vapour is delivered to the 4He bath by L. 

In the dual slope method, referring to the Fig. 12.1, the support (Sp), with the thermometer (Th) and the sample (S), is heated by the heater (H) with a constant power I during the first phase of an acquisition. In this step, the sample temperature Ts rises from 1 to T2. Then, the power P is reduced to a value P to bring the sample back to a temperature T( 7, . During these phases, the temperatures rrise and TfM are both recorded. The two following equations describe the heating and the cooling phase respectively ... 

The heat-insulating properties of polymers cause some problems in the first phase of moulding, the hot temperature of the mould is transmitted slowly within the thermoplastic. If the part is rather thick, the softening and shaping of the core is very long. There is the same drawback for the cooling phase. 

B. COOLING PHASE. During the period when cooling water is flowing through the jacket, only one energy equation for the jacket is required if we assume the jacket is perfectly mixed. 

After the forming process, the drying process follows. In earlier times bricks were dried outside and consequently people were dependent on the weather. Nowadays brick production is a continuous process and the bricks are dried in drying chambers. Hot air, partly from the cooling phase of the ovens, is transported to those chambers and ventilators ensure a proper air circulation. 

During the cooling phase-10 to-30 TAT during the freezing phase, compared with ... 

Obviously all these expressions are also valid for cooling, then (Tc < T). They are useful in process design, for calculating cycle times, where heating and cooling phases are often time-consuming. Of course, their use requires knowledge of the overall heat transfer coefficient U. Its determination is described in Section 9.3. 

Fig. 28 Scanning electron micrograph of high density polyethylene first isothermally crystallised at 128 °C and then rapidly cooled to room temperature. The sample was etched with hot p-xylene to remove the material crystallising in the cooling phase. Scale bar represents 20 pm. From Gedde and Jansson [154] with permission from Elsevier, UK...

Figure 5.8 is referred to the performance of the PID controller. It can be recognized that the temperature tracking error is always below 0.5 K and is characterized by a very similar time history of the control input with respect to the previously tested schemes (Fig. 5.5). By comparing the results in Fig. 5.8 with those in Fig. 5.4, the adaptive model-based scheme presents better performance than the linear PID control, especially during the heating and cooling phases, i.e., when the reference temperature is not constant.

Contrarily to the flatness and horizon problem which have a very similar origin, the unwanted relic problem comes from a very different cause. During the expansion, the Universe becomes colder because of the redshift that radiation experiences. It can happen that during this cooling phase, some phase transition occur, during which some stable objects called topological defects are created. 

---