Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C9/00—Moulds or cores; Moulding processes
  • B22C9/02—Sand moulds or like moulds for shaped castings
  • B22C9/04—Use of lost patterns
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
  • B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
  • B22C1/165—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents in the manufacture of multilayered shell moulds

Definitions

• the present invention refers to an improved process for the manufacture of ceramic molds to be used for the preparation of unidirectional and syngle crystal metal components and particularly for the manufacure of ceramic molds starting from aqueous suspensions.
• the usual method resides in the preparation of a wax pattern of the component to be manufactured, in the investment (coating) of the pattern with a certain number of ceramic layers, as to form a "green" mold, in the removal of the wax, thus allowing the formation of a cavity having exactly the size and the shape of the component to be manufactured, in the sintering of the empty mold and finally in the filling of the mold with the desired alloy.
• Such a basic method is showing several critical aspects, according to the kind of the desired product.
• binding agents and the refractories when in combination, appear as a formulation particularly fit for manufacturing quick drying coatings; the use therefore of the ones (binding agents or refractories) without the others would substantially reduce or eliminate whatsoever interesting benefit, as reported in the corresponding instructions as well as in the Foundry Trade Journal, April 21, 1989, pp 292-293.
• US-A-4,131,475 discloses ceramic molds obtained by using alumina of specific grain sizes in the baths and for spreading, which could produce fragile shells which crack on dewaxing.
• the Applicant did find that the combination of a fine grained binding agent, based on colloidal silica, with fused alumina, as the refractory, does work in an excellent way. According to the invention, the Applicant did find that is proper to use first the following primary bath:
• the primary bath contains from 2.2 to 2.8 kg of alumina per litre of binding agent, has a pH between 9.4 and 10.6, shows a viscosity (Ford B4 reservoir or cup) between 85 and 90 s and may be exploited at a temperature from 20 to 24°C.
• the secondary bath contains from 1.5 to 2.2 kg of alumina per litre of binding agent, has a pH between 9.4 and 10.6, shows a viscosity (Ford B4 reservoir or cup) between 25 and 30 s and may be exploited at a temperature from 20 to 24 °C.
• a suitable binding agent for the primary bath is the one traded as PRIMCOTE® and a suitable binding agent for the secondary bath is the one traded as FASCOTE®.
• An additional feature of the process according to the invention resides in the steps used during the manufacture of the ceramic molds.
• a wax pattern of the component part to be produced undergoes at least one immersion into the primary bath, each immersion being followed by spreading with fused alumina, having a granulometry between 0.125 and 0.250 mm, and by a drying step lasting from 1.5 to 4 h at a temperature between 18 and 22°C and with a relative humidity between 45 and 55%.
• the number of the immersions into the primary bath is at least three, preferably from four to seven.
• the mold thus obtained is furtherly treated with a plurality of immersions into the secondary bath, each immersion being followed by spreading with fused alumina, having granulometry between 0.25 and 1.00 mm, and by a drying step lasting from 1.5 and 4 h at a temperature between 18 and 22 °C and with a relative hummidity between 45 and 55%.
• the number of the immersions into the secondary bath is at least three, preberably from four to seven.
• the mold undergoes a last immersion into the secondary bath, followed by a drying step, lasting from 1.5 to 4 h, and by a final step (prolonged drying) lasting from 50 to 90 h, both the steps being carried out at a temperature between 18 and 22°C, with a relative humidity between 45 and 55%.
• the drying step after each immersion is characterized by a weight loss at least equal to 7% of the weight obtained from the respective immersion (preferably at least 9%).
• the green mold is then dewaxed, according to known techniques, and subsequently sintered.
• It can undergo a first optional treatment at a temperature between 1000 and 1100°C, according to a heating rate between 50 and 120 °C/h, followed by a cooling in the switched off oven.
• the mold undergo a sintering treatment at a temperature from 1400 to 1550°C, for a time between 3 and 6 h, with a heating rate (gradient) between 100 and 200 °C/h and with at least two homogenization stops at intemediate temperatures, each lasting from 30 to 120 minutes.
• a heating rate grade
• homogenization stops at intemediate temperatures, each lasting from 30 to 120 minutes.
• the sintered mold is cooled in the switched off oven.
• binding agents were prepared according to the following Table 1: TABLE 1 BATH Blank According to the invention Primary Secondary Colloid SiO2 (% b.w.) 30 30 25 Granulometry (micrometer) 25 10 10 pH (20 °C) 10 10.6 10.6 Viscosity (m.Pa.s) to 20 °C 2.5 6.7 5.5 Na2O (% b.w.) 0.30 0.48 0.41 Specific gravity (g/cm3) 1.20 1.18 1.15
• binding agents hereinabove were loaded with a fused alumina having a granulometry lower than 200 mesh, in amounts equal to 2.2 kg per litre of binding agent (as to the blank) and equal to 2.5 and 1.8 Kg per litre of binding agent respectively as to the primary bath and secondary bath according to the invention. There were then prepared a few wax patterns, subdivided into two groups, for unidirectional and single crystal components.
• the first group (blank) was coated with eight layers in the reference bath, whereas the second group was coated with three layers in the primary bath and with five layers in the secondary bath.
• Both the groups were spreaded with fused alumina, according to a a sequence comprising three fine spreadings (granulometry between 0.125 and 0.250 mm), two middle spreadings (granulometry between 0.250 and 0.50 mm) and three gross spreadings (granulometry between 0.50 and 1.00 mm). The last gross spreading was followed by an immersion into the bath.
• Each layer was dried under controlled conditions, with a relative humidity equal to 50% (at 21°C) and with an air change, in the drying chamber, equal to 12 change/h.
• the drying time, for the blank was 24 h between layer and layer, and 15 days for the final drying, thus accounting for an overall time of 24 days, whereas the group according to the invention required 3 h between layer and layer and 3 days for the final drying, thus accounting for an overall time slightly higher than four days.
• a first advantage coming from the invention is residing in a 6 times shortage of the manufacturing time for the green molds.
• the average weight loss of the molds according to the present invention, between the second and the seventh layer was 24 g/layer, against an average weight increase (coming from suspension accumulation and spreading material) equal to 260 g/layer, namely an average weight loss equal to 9.2%. From the other side, the average weight loss of the blank group was considerably lower, about 4%.
• a dry green mold according to the invention had a weight of 2.06 kg and a thickness of 4.6 mm, whereas an equivalent comparison mold had a weight of 2.56 kg and a thickness of 5.5 mm. Both the molds were steam-dewaxed in an autoclave according to a known method.
• Both the groups of molds were then pre-heated from room temperature to 1050°C, according to a heating rate (gradient) of 60°C/h, then cooled in the oven.
• gradient heating rate
• the unidirectional and single crystal components obtained from the molds according to the present invention did not show any macroscopic surface blemish and had shape and size in conformity with the strict tolerances required by the single crystal components.
• the pieces obtained from the comparison molds were showing a high faultiness, coming from a breaking of the mold and from shape distortion.
• wax is understood as defining whatsoever material fit for the preparation of patterns, like for instance waxes, paraffines, polystyrene and so on.

Landscapes

• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Compositions Of Oxide Ceramics (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)
• Oxygen, Ozone, And Oxides In General (AREA)
• Mold Materials And Core Materials (AREA)
• Inorganic Compounds Of Heavy Metals (AREA)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

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Cited By (2)

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• 1991
  • 1991-09-02 IT ITRM910655A patent/IT1249688B/it active IP Right Grant
• 1992
  • 1992-08-26 AT AT92114510T patent/ATE138299T1/de not_active IP Right Cessation
  • 1992-08-26 EP EP92114510A patent/EP0530658B1/en not_active Expired - Lifetime
  • 1992-08-26 DE DE69210918T patent/DE69210918T2/de not_active Expired - Fee Related
  • 1992-09-02 JP JP4257579A patent/JPH05277628A/ja active Pending
  • 1992-09-02 BR BR929203429A patent/BR9203429A/pt active Search and Examination
  • 1992-10-13 TW TW081108108A patent/TW221973B/zh active

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