Clamping pressure

Low clamping pressures required—thus allowing production of very large mouldings. 

An alternative way of looking at this equation is that the clamping pressure, based on the projected area of the moulding, is given by... 

In practice the clamping pressure will also depend on the geometry of the cavity. In particular the flow ratio (flow length/channel lateral dimension) is important. Fig. 4.42 illustrates typical variations in the Mean Effective Pressure in the cavity for different thicknesses and flow ratios. The data used here is typical for easy flow materials such as polyethylene, polypropylene and polystyrene. To calculate the clamp force, simply multiply the appropriate Mean Effective Pressure by the projected area of the moulding. In practice it is... 

Example 4.6 The mould shown in Fig. 4.35 produces four cup shaped ABS mouldings. The depth of the cups is 60 mm, the diameter at the is 90 mm and the wall thickness is 1.0 mm. The distance from the sprue to the cavity is 40 mm and the runner diameter is 6 mm. Calculate the clamp force necessary on the moulding machine and estimate how the clamp force would change if the mould was designed so as to feed the cups through a pin gate in the centre of the base (as illustrated in Fig. 4.38). The clamp pressure data in Fig. 4.42 should be used and the taper on the side of the cups may be ignored. 

Product size is limited by the size of available equipment and a process s available pressure such as melt and clamping pressure. The ability to achieve specific shape and design detail is dependent on the way the process operates. Generally, the lower the processing pressure, the larger the product that can be produced. Other restrictions are the... 

Modem shuttle machines can be set up with two complete moulds so that approximately 2/3 of the cure time is under full clamp pressure in the main station. The clamp pressure is then reduced and the closed mould is shuttled out to continue the cure in the side station where it is opened for product removal, mould cleaning and insert loading. While one mould is in the side station the other is injected and receives the first part of its cure. With a special mould opening device on the shuttle table the machine becomes as effective as a two-station machine. 

Chang et al. [183] presented a similar design in which two discs (with orifices in the middle) were used to compress the sample material. Pressurized air (without any moisture) was then passed through the orifices of fhe discs toward the sample DL, which then flowed peripherally to the atmosphere. The two discs were compressed in order to see how the permeability of the DL changed as a function of the clamping pressure. The permeability coefficient was solved using Darcy s law thus, only the viscous in-plane permeability was taken into account. Other, similar techniques can be found in the literature [215-217]. 

A brief report85 has appeared on the pressure dependence of the HFEPR of CuGe03. Measurements were made at 4.2 K in the frequency range 358-462 GHz in fields of up to 16 T. A clamped pressure cell with sapphire pistons was... 

The local clamping pressure on the MEA can be much higher than the nominal clamping pressure. The ribs of flow channels on the flow plate reduce the effective contact area with the MEA. In a cross-flow arrangement, the contact area is only 25% of the electrochemical active area of the MEA. Figure 2 shows a checker board pattern formed on a pressure sensitive film inserted into a single cell assembly. The pattern shows the non-uniformity of the clamping pressure distribution on the active area of the MEA. 

Figure 2. A checker board pattern of clamping pressure distribution on the active area of cell measured by Pressurex pressure sensitive film the flow channels in bipolar plates are arranged in a cross-flow pattern red indicates higher pressure.

The pressures required for injection at high speeds can range up to 245 MN m-2 (35000 lb in-2), so that high clamping pressures are needed to stop the mould opening during filling. This in turn means that equipment must be substantial (and costly) in order to withstand repeatedly the pressures involved. 

Low clamping pressures only are needed, and this in turn means that quite large products can be made on inexpensive plant. By varying the raw materials it is possible to produce either rigid or flexible articles, microcellular or otherwise, and (if required) to include fillers or reinforcing sections. Thermosets other than polyurethane can be used—like epoxides and polyesters (with the latter, the process is known also as resin transfer moulding ). 

Regardless of the heating source, elevated-temperature curing epoxy adhesives necessitate that the parts be held with clamps, pressure pads, or fixturing methods to apply constant and uniform pressure throughout the curing cycle. Reduction in viscosity due to elevated temperatures could result in excessive adhesive flow during cure. This could result in joints that are starved of adhesive or movement of the parts, if not firmly fixtured. 

Thick, uneven glue line Clamping pressure too low No follow-up pressure Curing temperature too low... 

No appearance of adhesive Clamping pressure Increase pressure. 

Adhesive bond line too thin Clamping pressure too high Lessen pressure... 

Since the in-mold pressures in RIM are generally relatively low [50 to 150 psi (0.4 to 1.1 MPa)] a variety of tooling constructions have been used. These include machined steel or aluminum, cast aluminum or kirksite, sprayed metal or electroplated shells, and reinforced or aluminum filled epoxy (Chapter 17). With mold pressures usually below 100 psi (0.7 MPa), mold-clamp-pressure requirements can accordingly be low when compared to injection and compression molding. 

Less clamping pressure required than in most other processes. 

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