Injection molding mold-cavity pressure

In order to judge performance capabilities that exist within the controlled variabilities, there must be a reference to measure performance against. As an example, the injection mold cavity pressure profile is a parameter that is easily influenced by variations in the materials. Related to this parameter are four groups of variables that when put together influences the profile (1) melt viscosity and fill rate, (2) boost time, (3) pack and hold pressures, and (4) recovery of plastica-tor. TTius material variations may be directly related to the cavity pressure variation. Details on EQUIPMENT/PROCESSING VARIABLE are in Chapter 8. 

Figure 10.9. Topical pressure profiles during injection molding. and stand respectively for nozzle and mold cavity pressure. Stages are 1 - dead time, 2 - filling, 3 - packing, 4 - cooling, 5 - ejection.

Plastic temperature, °C Plastic viscosity, Pa s Injection pressure, bar Injection time, s Mold cavity pressure, bar Gates... 

It is especially important for the transition Irom injection- to holding-pressiue phase to occur at the precise moment. Figure 7-7 shows a few examples of wrong mold cavity pressure curves. If transition occurs too early, a pressure drop results. This entails the risk that it will be impossible to completely fill the cavity. If transition occurs too late, a pressure spike occurs. In the long run, such pressure spikes can result in damage to mold and machine. 

In injection molding, the pressure applied to the mold to keep it closed despite the fluid pressure of the compressed molding material within the cavity and runner system. 

Beryllium copper n. Copper containing about 2.7% beryllium and 0.5% cobalt, used for blow molds and insertable injection-mold cavities. The small percentages of Be and Co greatly increase the strength and hardness of the copper whole preserving its high thermal conductivity and corrosion resistance. Beryllium copper is easily pressure cast and bobbed into mold cavities. 

CM is the most common method of forming TS plastics. Until the advent of injection molding, it was the most important of plastic processes. CM is the compressing of a material into a desired shape by application of heat and pressure to the material in a mold cavity. Pressure is usually at 7 to 14 MPa (1000 to 2000 psi). Some TSs may require pressures down to 345 kPa (50 psi) or even just contact (zero pressure). The majority of TS compounds are heated to about 150 to 200°C (302 to 392°F) for optimum cure but can go as high as 650°C (1200°F). 

In the case of specimens molded at various injection speeds, it was found that the variation of z, did not follow any tendency. At 10% injection speed, the cavity pressure was low (about 500 MPa) at both the gate and end positions. A further increase in injection speed to 30% was enough to cause an increment of cavity pressures to 800 MPa. There was no further increase in cavity pressure even when the injection speed was increased to 80% although the x was observed to have shortened. This is an indication that the injection speed could not be used to increase the cavity pressure beyond the limit set by the amount of injection pressure used (i.e. the maximum cavity pressure at an injection pressure of 30% is 800 MPa regardless of injection speed). Thus, a further increment in cavity pressure can only be obtained by increasing injection pressure. 

Liquid-Injection Molding. In Hquid-injection mol ding (LIM), monomers and oligomers are injected into a mold cavity where a rapid polymerization takes place to produce a thermoset article. Advantages of these processes are low cost, low pressure requirement, and flexibiHty in mold configuration. Conventional systems, such as isocyanate with polyol, release Htfle or no volatiles. The generation of substantial volatiles in the mold is obviously undesirable and has represented a significant obstacle to the development of a phenoHc-based LIM system. A phenoHc LIM system based on an... 

Injection Molding. In iajection mol ding a molten thermoplastic is iajected under high pressure iato a steel mold. After the plastic solidifies, the mold is opeaed and a part ia the shape of the mold cavity is removed. Geaeral descriptioas of the process and related equipment are given ia Refereaces 29—34. 

In injection molding, the polymer is fed from a hopper into a heated barrel where it softens and becomes a viscous melt. It is then forced under high pressure into a relatively cold mold cavity where the polymer has sufficient time to solidify. Then the mold is opened and the fabricated part is ejected. The cycle of operation is then repeated. 

In this process, resin is injected into a closed mold containing the reinforcement preform. The resin can be injected either under pressure [22] or under vacuum [23]. The potential advantages of this process are (I) low mold cost, (2) inserts can be incorporated, (3) low pressure requirements, (4) accurate fiber orientation, (5) automation possibilities, and (6) versatility. The resin formulation and process variables are selected so that no significant polymerization occurs until the mold cavity has been completely filled. This is achieved by the ad-... 

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