Cooling molds

The pressure of the extmder forces uniform plastic distribution throughout the mold. Cooling the mold solidifies the plastic with slight shrinkage. The mold is maintained closed by mechanical or hydraulic pressure while the thermoplastic is injected and solidified. 

Mold side Mold cooling water after cooling molten steel Less than 1 year... 

Forced air, water mist, or water spray cool the mold after it leaves the oven. Once the mold [. cools sufficiently for the polymer inside to solidify, it is opened. The cooling time must take. into account the resin, the wall thicknesses, and the metal from which the mold is fabricated, ff.. Very rapid cooling of the mold can introduce stresses that can weaken or distort the final part. 

In continuous extrusion blow molding, the preform is continuously produced at the same rate as the article is molded, cooled, and released. To avoid interference with the preform formation, the mold-clamping step must be rapid to capture the preform and move it to the blow mold station. There are various modifications of this that allow essentially continuous operation. 

The results of calculations for two values of the initial temperature of the reactive mass To are shown in Fig. 4.17. For To = 70°C, the curves are practically insensitive to the value of to. This is explained by the fact that the mold temperature is lower than the initial temperature of the reactive mass (polyurethane composition) and therefore the layers of polymeric material close to the surface of the mold cool rapidly down to the temperature Ts, because this temperature is strictly constant. 

Figure 9.9 Schematic diagram of an injection molding cooling problem.

Injection molding is a high-volume production technique for turning out thousands of plastic parts per hour. An extruder melts and mixes plastic as above and then forces or injects molten material into a mold that has the shape of the final object. The plastic inside the mold cools, and the mold opens and ejects the part(s). The mold closes, and the cycle repeats. See Figure 8-2. Injection molding enables the mass production of complex shapes... 

Thermoplastic sheet can be softened by warming, placed in or on a mold, pressed gently but swiftly to conform against the walls of the mold, cooled to solidify, and trimmed to separate the product from the surrounding unused sheet. 

Hollow products can be made by placing powdered plastic in a closed mold, tumbling and heating it until the plastic has coated the walls of the mold, cooling it to solidify the product, and opening the mold to remove the product. 

Another type of rubber that is used frequently is thermoplastic rubber. Components are fabricated in a process that is similar to that used for common hard plastics, such as polyethylene or polystyrene, but the final product is an elastic material with properties otherwise equivalent to those of thermoset rubbers. No chemical reactions are involved in the processing of a thermoplastic rubber. The fabrication process consists of heating the rubber compound until it liquefies, injecting the liquid into a mold, cooling the mold, and finally removing the closure from the mold. The process is reversible. Closures can be remelted and remolded into different shapes or sizes as desired. 

The important variables of transfer molding are the polymer type, melt temperature, pot hold time, transfer pressure, transfer rate, hold-up time in the filled mold, and the mold cooling. These variables are briefly reviewed in this section. 

While the heating cycle has virtually no effect on the properties of the finished part, the cooling rate determines part shrinkage, final density, brittleness, and other physical properties. Mold cooling is accomplished by the use of forced air and/or application of water spray. 

Mold cooling is typically provided by water channels in the mold, when metal molds are used. A system of fans blowing air over the top of the mold can aid in cooling, and this can be used alone for wood or plastic molds, which do not generally contain internal cooling. 

The production speed of blow-molded parts is generally limited by one of two factors extruder capacity or cooling time in the mold. Cooling of mold is accomplished by a water circuit into the mold. Flood cooling and cast-in tubes are most common in cast molds drilled holes and milled slots are the norm in machined blow molds. 

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