温度・粘度
Predry
Japanese: predry（石膏型予熱）
A process in which the plaster mold is dried at low temperatures prior to casting. If moisture remains inside the mold, there is a risk that the mold will burst due to steam during firing. This is carried out in three stages at 90°C, 230°C, and 480°C.
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Predry (plaster mold preheating) is a process used in glass casting (such as lost-wax casting and box casting) to completely remove moisture from the plaster mold by gradually heating it at low temperatures before filling the mold with glass and performing the final firing (§10.1). If the plaster mold is heated to the high-temperature range of the main firing (800°C or higher) while moisture remains inside, there is a risk that the moisture will rapidly expand as steam, causing the mold to burst. Since mold rupture is a serious accident that can lead to both glass damage and electric furnace failure, predry is an essential prerequisite for safe firing.
The standard three-stage program for predry (§10.1) consists of the following sequence: (1) holding at 90°C (pre-stage for evaporation of crystalline water) → (2) holding at 230°C (elimination of bound water) → (3) holding at 480°C (passing through the α-cristobalite phase transition point). The hold time for each stage is adjusted according to the size and thickness of the mold; for large, thick molds, this process may take anywhere from several hours to one day. Predry is often incorporated as stages A through B of the main firing program or performed as a separate firing the previous day. Complete drying can be confirmed by a significant reduction in the weight of the plaster mold before and after firing (due to moisture loss).
Related Concepts
Chikaoka-style Homemade Refractory Plaster (chikaoka-plaster) · G-2 (Noritake) (g2-noritake) · Lost-wax casting (lost-wax-cast) · Thermal diffusivity κ (thermal-diffusivity) ・Understand the safe preparation process for cast firing, including the flow-cast zone.
This entry is written in Japanese by Rei Chikaoka and translated for readers outside Japan. If something reads oddly, the original is the source of truth.