Molds closure

Figure 3 Percent unfills by elastomer type for 5-s mold closure.

Figure 6 Percent excess flash by elastomer type for 11-s mold closure.

The possibilities with RTM are many and the size of moldings can vary from small to very large. For large components it becomes necessary to take measures to reduce mold closure forces. Large RTM parts, therefore, are usually of low volume fraction because the cost of sufficiently stiff molds and facilities for high closure forces tend to be out of reach for production. 

Once the charge is properly placed in the mold, it is closed quickly to contact the top surface of the charge. Mold closure is a critical factor delay in closure time may cause some precure owing to the gelation of the top surface, occurring before the part is molded, and faster closure rates trap air, which induces surface imperfections, which is undesirable. A 10-sec closure time has been suggested as optimum to avoid any precure. 

Injection Molding, Closures, Rotational Molding, Compression Molding, and Tubes... 

To avoid the fiber separation problem, the initial precharge thickness should be increased and/or the speed of mold closure should be increased. However, if the initial precharge thickness is increased, the flow path should be increased. This change in flow path may generate a preferential fiber orientation state and the corresponding material anisotropy. 

In Fig. 3.24, the process window is presented in terms of the mold temperature and molding pressure or the speed of mold closure. Practical problems that may occur are also listed when the process conditions are outside the process window. According to the material characteristics and the part size (especially the thickness and planar dimensions), the process parameters should be optimized with respect to the process window. For process optimization, both analytical models and numerical simulations may be utilized. 

Mold closure pushes resin Into preform Preform deforms... 

Partial mold closure 4 Resin injection into gap... 

Check mold closure for obstruction on parting line surface... 

PP films are commercially available with different coatings (acrylic and PVEXT) to improve heat sealing, hanier, and optical characteristics, as well as to lower the coefficient of friction. Also, metallized PP films are available to provide extremely low values of permeability to gases and vapor." PP films are well suited for bag-in-box applications in cereals, crackers, soup mixes, and stand-up pouches. PP is also an excellent material to fabricate injection-molded closures for HDPE, PET, and glass bottles. 

Set desired process parameters including platen heating temperatures, molding force, mold closure speeds, and cycle times. 

Figure 14.11 Schematic view of semipositive and flash mold closure designs.

Figure 14.12 Pictures of a sample part with flash made from two different mold closures. (A) Flash from semipositive mold closure. (B) Excess material from flash mold closure.

For economic efficiency, overall cycle time must be minimized. Note that the molding time is the time under full pressure and does not include other steps such as loading, mold closure, degassing, and part ejection, all of which contribute to the overall cycle time. Compression molding time must be adequate... 

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