Mold life

Part trim equipment Generally expected mold life (parts) 3,000 Yes 1,000 3,000 With optimum shear edges, minor trimining only 150.000+ 150.000+ ... 

Metal molds and cores are used in permanent mold casting. The process works best in continuous operation so that the mold temperature can be maintained within a fixed operating range. The operating temperature of the mold is one of the most important factors in successful permanent mold casting. Mold cavities are machined from solid blocks of graphite. Mold life is the major cost factor in permanent mold casting. 

Although the temperature rise due to exothermic curing reaction and mold heating is not significant in RTM for typical resin systems (typically 100°C above the room temperature, or less), this may shorten the mold life especially if the mold is nonmetaUic (such as composites). 

Blow molds should be provided with stops on surfaces outside the cavity area to prevent damage to the pinch-off edges during setup or when clamping the mold without plastic. These stops protect the mold against excessive clamp force they also ensure parallel clamping and increase the mold life. 

Figure 5.2 shows the resulting choices for the material of the injection mold and the resulting mold life time, measured on the basis of the qualitatively achievable injection cycles. 

As a simplification for a design, an 80 to 85% utilization of the maximum possible mold life should be used. By way of example, this means that with a sales expectation of 800,000 components in the overall life cycle (start of series up to the end of the intended end of spare parts production), the mold should have a total output of about 1 million components. The difference of 200,000 components is a safety margin, which should prevent that an additional mold has to be manufactured at a... 

Another consideration in raising the mold temperature is the longevity of the mold itself It has been dem-onstratedl" that operating the mold at extreme temperatures, even in the absence of thermal degradation of the plastic, affect the mold life and maintenance costs detrimentally. The balance of cycle time reduction and cost increase is unfavorable, especially for resins such as fluoropolymers that require high temperatures. 

For maximum mold life, deflocculants should be used in the minimum quantity required to obtain proper slip characteristics. Also, when sodium carbonate and sodium silicate are employed in combination as a deflocculant, the amount of carbonate should be reduced to the absolute minimum permissible. Silicate in general has a less eroding effect on mold surface... 

Permanent mold casting is similar to sand casting except that a reusable split metal mold (cast iron, steel, bronze) is used instead of compacted sand. A thin refractory coating (parting agent) of sodium silicate or graphite is often used to ease removal from the mold and to increase mold life. 

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