Foundry core stripping

Triggered setting systems are used to make cores for repetition foundries. After the mixed sand is blown into the core box, the cores must be cured in the box until sufficient strength has been achieved to allow stripping without damage or distortion. Usually the core continues to harden after stripping. 

Different foundry casting techniques are used. Included are plastic-based binders mixed with sand. Various types of molds and cores are produced that include no-bake or cold-box, hot-box, shell, and oven-cured. Usual binders are phenolic, furan, and thermoset polyester. There is the foundry shell casting, also called dry-mix casting. It is a type of process used in the foundry industry, in which a mixture of sand and plastic (phenolic, thermoset polyester, etc.) is placed on to a preheated metal pattern (producing half a mold) causing the plastic to flow and build a thin shell over the pattern. Liquid plastic pre-coated sand is also used. After a short cure time at high temperature, the mold is stripped from its pattern and combined with a similar half produced by the same technique. Finished mold is then ready to receive the molten metal. Blowing a liquid plastic/sand mix in a core-box also produces shell molds. 

Two unique processes based upon novel curing mechanisms have been developed for the phenolic polyols. In the first process the cure is internally catalyzed, that is, the catalyst is mixed with the polyol, isocyanate and sand components. The catalyst, however, is unique and provides the advantageous feature of this system which is the long delay after the components are mixed and quick cure once the curing starts, as shown in Figure 1. The figure also illustrates the effect of catalyst type on time to work with the mix (WT) and on time when the core can be handled or stripped (ST). This innovative feature has allowed the foundry industry to increase their core manufacturing productivity by as much as 50-100% of what is achievable with any other core binder system. 

A. Binder for Foundry Sand. The major industrial use o furan resins is as a binder for foundry sand. Low levels of resin binder (0.8 to 27 ) are used to bond the sand as cores and molds for molten metal. By use of continuous mixers and appropriate levels of strong, acidic catalysts, set and strip times as low as 45 seconds at ambient temperature are attainable (see Figures 1 and 2). The furan resins work ideally in this application since they have sufficient thermal stability to retain the shape of the mold until the metal sets, then subsequently carbonize to allow shake-out of the sand after the metal hardens. 

---