TAILIEUCHUNG - Foseco Non-Ferrous Foundryman’s Handbook Part 10

Tham khảo tài liệu 'foseco non-ferrous foundryman’s handbook part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 168 Foseco Non-Ferrous Foundryman s Handbook Typical hardening curve for self-hardening sand Figure Tw work time Ts strip time Tc casting time Tmax time to achieve maximum strength. the mould or core must be formed Fig. . If the work time is exceeded the final strength of the mould will be reduced. Work time is typically about one third of the strip time and can be adjusted by controlling the type of catalyst and its addition rate. The work time and strip time must be chosen to suit the type and size of the moulds and cores being made the capacity of the sand mixer and the time allowable before the patterns are to be reused. With some binder systems the reaction rate is low at first then speeds up so that the work time strip time ratio is high. This is advantageous particularly for fast-setting systems since it allows more time to form the mould or core. Stripping is usually possible when the sand has reached a compression strength of around 350 kPa 50 psi but the actual figure used in practice depends on the type of binder system used the tendency of the binder to Resin bonded sand 169 sag before it is fully hardened the quality of the pattern equipment and the complexity of the moulds and cores being made. It is advisable to strip patterns as soon as it is practical since some binder chemicals attack core box materials and paints after prolonged contact. The properties of chemical binders can be expressed in terms of Work time bench life which can be conveniently defined as the time after mixing during which the sand mixture has a compressive strength less than 10 kPa at this stage it is fully flowable and can be compacted easily Strip time which can be defined as the time after mixing at which a compressive strength of 350 kPa is reached at this value most moulds and cores can be stripped without damage or risk of distortion Maximum strength the compressive strength developed in a fully hardened mixture figures of 3000-5000 kPa are often achieved It is

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