Double blow processes

In the blow and blow process the gob is dropped into an open blank or parison mould. The neck is formed by top blow and then the parison or blank is blown from the base (Figure 6.2). The blank shape supported by the transfer ring is then transferred to the finishing mould where it is blown into the final shape. This blow and blow process is the major process employed—usually with IS (i.e. independent or individual section) machinery Other machines include Roirant R7, S10 and Lynch 44. IS machines may have 4, 5, 6, 8, 10 or 12 stations. For small containers double, triple or quadruple gobbing may be employed. The last can give speeds of over 400 containers per minute. 

The double-bubble process involves the extrusion and blowing of a tube of molten plastic in a downward direction. The tube is then cooled, most often using a water bath, reheated to just below the melt temperature, and reinflated. The reinflation along with the increase in haul-off speed provides biaxial orientation. Typically the next step is annealing to relieve thermal stresses and stabilize the film. The double-bubble process is most often applied to PP film, but is also used with multilayer PP or PE-based films. One of the major advantages is that this process can deliver a high-clarity film with precise shrink characteristics and very uniform flatness. 

Higher levels, into the mid-40% range, can be achieved through the double blow blowing process. 

Double blow (Fig. 33.34) is a process that produces bottles with higher heat stability than heat set-hot fill bottles covered by Yoshino patents [27,28]. 

This process represents another alternative to the double alkali process. Sulfur dioxide is absorbed in dilute sulfuric acid, oxidized to sulfate by air blowing, then precipitated as gypsum by the addition of limestone. The oxidation rate is increased by the use of iron as a catalyst in the circulating acid and is also enhanced by the low pH of the solution. The process, which was developed by Chiyoda Chemical Engineering and Construction Company, Ltd., of Yokohama, has been used quite extensively in Japan. Fourteen plants were reportedly in operation at the end of 1977 ( do, 1977). 

It is well known that a hot plate of metal will cool faster when placed in front of a fan than when exposed to still air. We say that the heat is convected away, and we call the process convection heat transfer. The term convection provides the reader with an intuitive notion concerning the heat-transfer process however, this intuitive notion must be expanded to enable one to arrive at anything like an adequate analytical treatment of the problem. For example, we know that the velocity at which the air blows over the hot plate obviously influences the heat-transfer rate. But does it influence the cooling in a linear way i.e., if the velocity is doubled, will the heat-transfer rate double We should suspect that the heat-transfer rate might be different if we cooled the plate with water instead of air, but, again, how much difference would there be These questions may be answered with the aid of some rather basic analyses presented in later chapters. For now, we sketch the physical mechan-... 

The bubble diameter is normally always much greater than the die diameter. This bubble diameter divided by the die orifice diameter is called the blow-up ratio (BUR). The BUR is usually 1.5 to 4.0, depending on the plastic being processed and the thickness required. The bubble diameter must not be confused with the width of the flattened double layer of film between the nip rolls. The width of this double layer is 1.57 times the bubble diameter and is called the blown-film width (BFW). 

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