Molds projected area

There are many cases in which the volume of material for all the cavities is small and the projected areas of the cavities are large. In this example, the choice of machine size is governed by the tonnage of the clamp required to keep the mold from opening during the fabricating process (3). 

Since the longest flow path may exceed the radius of the projected area that causes mold separating pressures, we must also find the radius of equivalent projected area, Rp, to compute a more accurate mold clamping force. However, to perform the calculations to predict velocities and pressure fields, we assume a disc geometry of radius R and thickeness h, schematically depicted in Fig. 8.41. 

Figure 8.40 Schematic diagram of an injection molding item with its projected area and its lay-flat representation.

In this process, the polyurethane mix is poured into a mold that can be placed into a compression molding press with heated platens. The material is allowed to gel, and a top plate is placed on the material. The molding press is fully closed. The pressure applied is normally on the order of 1.50 to 2.0 MPa (250-300 psi) based on the projected area of the molding e.g., a parts plan area is 300 x 400 mm in size. Its area is 0.300 x 0.400 m2 = 0.12 m2 therefore, a pressure of 1.500 x 0.12 = 0.18 MPa on the mold is required. The molds must be capable of withstanding the direct heat and pressure of the press. Aluminum or steel molds are preferred for this process. 

The preceding discussion has shown that a significant portion of the electrical energy In injection molding is used in the operation of the clamping system. The amount of clamping force is determined by the projected area of the parts and runners. The relationship is ... 

Stack mold also features two or more daylights in the open position. Two daylights are the normal form (Figure 17.5) but up to four are also used more could be used. The purpose of the stack mold is to increase the number of cavities in the mold without increasing the projected area so the clamp force required from the IMM remains the same. Similar cavities and cores between each of the daylights are normally used. 

Other developments have not been as dramatic, but they have nevertheless contributed significantly to the process efficiency. The electric screw drives were replaced by hydraulic motors with a gear box on the reciprocating screw unit. In the future the hydraulic motor may in turn be replaced by a direct hydraulic drive (19). The Introduction of stack molds has also increased productivity. In a stack mold, two cavity plates are placed on top of each other so that the projected area is the same and twice as many parts can be formed with the same clamping tonnage. This technique works best with thin wall, flat parts with short cycle times. 

The blowing pressure is usually below 100 psi except for acetal, which can go as high as 150 psi. These pressures allow one to go to aluminum tooling. The clamping force is set at 1.25 times the blowing pressure times the projected area. Other mold materials used are zinc alloy, steel, and beryllium copper. 

Clamping force n. In injection and transfer molding, the force applied to the mold to keep it closed, and opposing the pressure exerted by the injected plastic acting upon the projected area of cavities and runners. Per square centimeter of cavities and runners, at least 3.5 kN of clamping force is required. 

Projected area n. In molding, the area of a cavity, or all the cavities, or cavities and rurmers, perpendicular to the direction of mold closing force and parallel to the parting plane. In injection molding and blow molding, this area must be safely less than the quotient of the force apphed to hold the mold closed divided by the maximum melt pressure or blowing pressure within the mold. In transfer molding, it must also be about 15% less than the cross-sectional area of the pot. 

Molds may be single cavity or multiple cavity, and press tonnage must be adequate to provide as much as 300 kg/cm for phenolics, less for polyesters, of projected area of the molded part or parts at the mold parting surfaces. Overall cycles depend on molding material, part thickness, and mold temperature, and may be about 1 min for parts of 3-mm thickness to 5 or 6 min for parts of 8-cm thickness (Fig. 6.7). 

The injection molding clamp force is directly proportional to the projected area of the part, or to the empirical pressure or viscosity factor. Clamp force can be related to melt flow length and part thickness [3] by... 

Clamping pressure is the pressure needed to hold the clamp closed during injection and packing stages of injection molding. The clamping pressure times the projected area of the part must not exceed the tonnage... 

Three different pressures can be distinguished in the injection unit injection, hold, back [46]. Pressure is required by both injection unit and clamping system. This force in tonnage should be high enough to prevent the mold from material injection at maximum pressure and speed. The applied force is proportionally related to the projected area of a part including the areas of runner and sprue. It aids the holding pressure and allows for the part to fill and pack properly. 

Clamping Capacity n The largest rated projected area of cavities and runners that an injection or transfer press can safely hold closed at full molding pressure. 

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