Mold powder entrapment

Unfortunately, there is yet no clear explanation of the critical condition for each of the above entrapment phenomena to occur in real processes. Among these phenomena, the second has been most actively investigated, probably because of the ease of carrying out model experiments [27,30,32] The description in this section similarly focuses on this mode of powder entrapment. 

A number of researchers have considered steady flow in the mold and conducted model experiments using water and silicone oil. The flow field in the mold can be illustrated schematically as in Fig. 8.17a. The silicone oil is pushed away from the narrow face of the mold by the reversing flow reflected from the face [27]. Silicone oil droplets are pulled from the silicone oil layer into the water layer in the presence of high shear stress. Such a situation may not be realistic in the real processes 

It is therefore more plausible to assume that the mold powder entrapment occurs under unsteady conditions, as shown in Fig. 8.17b. Existing studies on the slab casting mold have shown that the steel flow in the mold is essentially unsteady due to uneven flows discharging out of the ports of the immersion nozzle [37-39]. Mold powder entrapment under unsteady flow conditions seems to be closely associated with the Kelvin-Helmholtz instability, hereinafter abbreviated as KHI [40,41]. Some researchers have suggested that KHI is one of the causes of the onset of mold powder entrapment and that a critical velocity difference can be predicted by the original KHI theory or an infinitely extending interface [33,42]. However, it is not 

The original KHI theory [40,41] implies that the critical velocity difference responsible for the onset of mold powder entrapment is influenced by the interfacial tension and the densities of molten steel and mold powder. In addition to these parameters, the kinematic viscosity of mold powder must be taken into consideration. As a first step, particular attention is paid, in the description here, to the effect of the kinematic viscosity of the mold powder on the entrapment, and model experiments carried out using salt water and silicone oils. 

In order to determine the critical velocity difference for the initiation of KHI, it is necessary to measure the velocities of salt water flow and silicone oil flow near the silicone oil/salt water interface. A particle imaging velocimetry (PIV) technique is described as it is considered the most adequate for this purpose. 

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S KHI-Induced Mold Powder Entrapment in Continuous Casting Mold... 

Figure 8.16 shows a schematic of the current continuous casting mold practice. The entrapment of mold powder into molten steel strongly influences the quality of the steel product. The mechanism of entrapment has therefore been extensively investigated [26-33]. Considerable effort has also been devoted to preventing it [34-36]. The following three types of entrapment phenomena have been suggested [28] ... 

Fig. 8.17 Entrapment of mold powder due to reversing flow, (a) Steady entrapment, (b) Unsteady entrapment...

Watanabe K, Nakata M, Suzuki M, Shiomi T (1998) The effect of mold powder properties on powder entrapment. CAMP-ISIJ 11 861... 

The semipositive mold (Figure 2.2c and d) combines certain features of the open flash and fully positive molds and makes allowance for excess powder and flash [4]. It is also possible to get both horizontal and vertical flash. Semipositive molds are more expensive to manufacture and maintain than the other types, but they are much better from an applications point of view. Satisfactory operation of semipositive molds is obtained by having clearance (0.025/25 mm of diameter) between the plunger (top force) and the cavity. Moreover, the mold is given a 2-3° taper on each side. This allows the flash to flow on and the entrapped gases to escape along with it, thereby producing a clean, blemish-free mold component. 

In order for thermoplastics to be molded into useful articles, they must first be plasticated, i.e. melted, and then held in the mold until resolidified. For ease of handling and to avoid excessive air entrapment, thermoplastics are usually supplied as cylindrical pellets about 1/8 inch in diameter and 1/8 to 1/4 inch in length. Powdered polymers are used in the processing of PVC and in rotational molding. The incorporation of minerals as fillers and remforcements creates certain problems, since they are much harder and... 

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