Cooling mold cavity

Even though most of the literature on processing specifically identifies or refers to thermoplastics (TPs) as in this book, some thermosets (TSs) are used (TS polyesters, phenolics, epoxy, etc.). The TPs reach maximum heat prior to entering cool mold cavities, whereas the TSs reach their maximum temperature in hot molds (Fig. 6-3). 

Contact forming is restricted to roll-fed thin sheet. The sheet to be heated is brought in contact with a heated plate by vacuum. When the sheet reaches its forming temperature, which is the same temperature as the heated plate, the plate and sheet are pressed against the female or negative mold. A combination of vacuum and air pressure pushes the sheet from the plate to the cool mold cavity (Fig. 16.10). When the sheet is cool, the mold drops away and the sheet moves to a trimming fixture. 

We used a specially designed mold with interchangeable mold cavity inserts making it possible to vary the specimen thickness. The mold cavity insert has separate heater cartridges but no forced cooling. Mold cavity depths of 3.1mm, 2.6mm and 2.1mm were used in order to understand the role of plastic/insert thickness ratio... 

Injection molds are constmcted of metal precision-machined with internal cooling, multiple cavities, and multifaces, and with devices for extraction... 

Freezing action Because of the heat exchange between the flowing TP melt and the mold walls, the flow may freeze (solidify) before the product is completely filled. Products that have alternate sections with thick and then thin walls can cause problems in flow and cooling that make them difficult to fill. In some cases the plastics that have been selected for the end use requirement are too viscous to flow properly in a mold cavity, and this makes the manufacture difficult. 

Time, pressure, and temperature controls indicate whether the performance requirements of a molded product are being met. The time factors include the rate of injection, duration of ram pressure, time of cooling, time of piastication, and screw RPM. Pressure requirement factors relate to injection high and low pressure cycles, back pressure on the extruder screw, and pressure loss before the plastic enters the cavity which can be caused by a variety of restrictions in the mold. The temperature control factors are in the mold (cavity and core), barrel, and nozzle, as well as the melt temperature from back pressure, screw speed, frictional heat, and so on in the plasticator. 

The sequence of events during the injection molding of a plastic part, as shown in Fig. 3.40, is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front of the screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected. 

Figure 3.41 presents the sequence of events during the injection molding cycle. The figure shows that the cycle time is dominated by the cooling of the part inside the mold cavity. The total cycle time can be calculated using... 

The mold cavity. The central point in an injection molding machine is the mold. The mold distributes polymer melt into and throughout the cavities, shapes the part, cools the... 

As expected, the largest portion of the cycle time is the cooling of the blow molded container in the mold cavity. Most machines work with multiple molds in order to increase production. Rotary molds are often used in conjunction with vertical or horizontal rotating tables (Fig. 3.59 [14]). 

Heating and cooling often take place while the polymer melt flows, making viscous dissipation an influencing factor during the process. However, since most plastic parts are thin, the conduction often occurs only across the thickness and the viscous heating is a result of shear within the narrow gap of a die or mold cavity. For such cases, the equations reduce to,... 

Figure 7.8 Single-crystal casting of an airfoil. Casting is directionally solidified from a water-cooled chill. A pigtail grain selector ensures that only one grain, or crystal, enters the blade section of the mold cavity.

Filling the Mold Cavity to Form the Product. The mold cavity is designed and machined to form the shape of the finished product. This is itself a complete art and science, based partly on experience, and increasingly on computerized engineering principles. Some major considerations are fast uniform flow, avoidance of degradation, minimization of orientation/anisotropy, fast cooling/ solidification, shrinkage and dimensional tolerances, and of course final properties of the product. 

Hot Runners. When the molten plastic is pumped into the water-cooled mold, the cooling system solidifies both the plastic product in the mold cavities and also the plastic material in the runners. Later the solid runners must be separated, reground, and reused. This is an extra burden on the process. An alternative is to avoid cooling the runners, and actually keep them hot, so that the molten polymer in them remains ready for the next shot into the mold. 

The term IM is an oversimplified description of a quite complicated process that is controllable within specified limits. Melted or plasticized plastic material is injected by force into a mold cavity (Figure 4.1). The mold may consist of a single cavity or a number of similar or dissimilar cavities, each connected to flow channels or runners which direct the flow of the melted plastic to the individual cavities (Chapter 17). The process is one of the most economical methods for mass production of simple to complex products. Three basic operations exist. They are the only operations in which the mechanical and thermal inputs of the injection equipment must be coordinated with the fundamental behavior properties of the plastic being processed. These three operations also are the prime determinants of the productivity of the process since manufacturing speed will depend on how fast we can heat the plastic to molding temperature, how fast we can inject it, and how long it takes to cool (or solidify) the product in the mold. 

The processes use an inert gas that is usually nitrogen with pressures up to 20 to 30 MPa (2,900 to 4,400 psi). Within the mold cavity the gas in the melt forms channels. Gas pressure is maintained through the cooling cycle. In effect the gas packs the plastic against the cavity wall. Gas can be injected through the center of the IMM nozzle as the melt travels to the cavity or it can be injected separately into the mold cavity. 

A TP is melted into a tube that is generally referred to as a parison. While still in a heated ductile and firm plastic melt state, the parison is clamped between the cavity halves of a cooled mold, so that the open top and bottom ends of the parison are trapped, compressed and sealed by the mold faces (Chapter 17). Air pressure enters the parison. Air pressure causes the parison to expand like a balloon, so that it takes up... 

In the continuous extrusion design process, the parison is continuously extruded between the open mold halves from an accumulator head. When the required length of parison has been produced, the mold is closed, trapping the parison that is severed usually by a hot knife from the die. Figure 6.7 provides a simplified schematic of a continuous BM process. Land or pinch-off areas on the mold compress and seal the upper and lower ends of the parison to make an elastic airtight part. Compressed air is introduced through the blow pin into the interior of the sealed parison that expands to take up the shape of the mold cavities. The cooled mold chills the blown object that can then be ejected when the mold opens. 

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