TAILIEUCHUNG - Foseco Non-Ferrous Foundryman’s Handbook Part 11

Tham khảo tài liệu 'foseco non-ferrous foundryman’s handbook part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 188 Foseco Non-Ferrous Foundryman s Handbook Curing times Minimum curing time is 90 seconds but 2 minutes is common. The longer the curing time the thicker is the shell build-up. Increasing curing temperature does little to speed up cure rate and runs the risk of over-curing the sand adjacent to the core box or pattern plate. Core strength Surface hardness and strength is high on ejection. A resin content will give a tensile strength of 1400 kPa 200 psi . Storage properties of cured cores is excellent. Gas evolution 4-5ml g of gases for each 1 of resin. Casting characteristics Core and mould coatings are unnecessary. The surface finish of castings is excellent. Breakdown is good particularly with hollow cores. Accuracy and reproduction of pattern detail is extremely good because of the free-flowing properties of precoated sand. Some distortion particularly of moulds can occur on casting and support of shell moulds by steel shot or loose sand may be needed for heavy section castings. N defects may occur on steel castings unless low N coated sand is used. Excellent surface finish is achieved on aluminium castings. Health hazards Phenol and ammonia are released during curing so good ventilation is needed at core- and mould-making machines. Casting into shell moulds produces unpleasant and hazardous fumes and often shell moulds are cooled in an enclosed tunnel allowing fumes to be collected and exhausted outside the foundry. Reclamation When both moulds and cores are made by shell process the sand can be reclaimed thermally. Shell moulding Shell moulds are made by dumping precoated sand onto an iron pattern plate heated to 240-260 C. After a suitable time usually about 2 minutes the mould is overturned returning the uncured sand to the hopper and leaving a shell mould 20-25 mm thick which is ejected from the pattern plate. Cores are placed and the two half-moulds are glued together with hot-melt adhesive CORFIX . Shell moulds are usually cast horizontally .

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