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Vacuum Casting Process

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  1、preface

  Ever since Fused casting refractories been developed at 20th last century in America and France, it become to indispensable materials for building glass furnace. Fused casting refractories started from 50th last century, from normal casting, Inclined casting, No shrinkage cavity casting to reduction method and oxidation style, quality of fused refractories gradually improved and help to improve glass furnace’s working life and glass quality. Casting method for making bricks is very important in glass furnace. Traditional casting requests sand mold based on bricks’ quality, which directly effect precision of bricks’ measurement and bricks surface, and that always cause crack on bricks surface and hollow inside of bricks, all that effect qualification rate. Besides, disposable casting mold waste lot of sand, which bring big problem for environment. To solve all that problem, we research and improve casting method by inventing vacuum casting, it greatly improve precision and bricks surface, also reduce sand consumption and production cost, saving energy and environment.

  2、Vacuum Casting and its basic principle for improving AZS bricks’ quality.

  AZS bricks alteration mechanism.

  AZS bricks, mainly composited by corundum phase, baddeleyite phase (eutecticum) and glass phase. 13% of glass phase has function of buffering thermal stress. Bricks’ anti-erosion feature mainly up to: Density, growth and development of corundum phase and baddeleyite phase, the number of cracks.

  When bricks been casting, temperature of fused liquid is different from casting mold, air come together with liquid can not be vented, with the addition of cold shrinkage and thermal expansion inside of bricks, micro porosity can not be eliminated, which has been trouble the AZS industry for many years,

  Vacuum Casting is to let liquid touch mold wall in vacuum condition inside of mold, plastic film which cover mold wall will be burn and in inhaling condition, under atmospheric pressure, instead of floating, micro bubble will move to mold wall, that make bricks more densification. Research shows a brick 300X400X600 in vacuum casting has 3~6 kg more weight than sand mold casting, its volume-weight increased by 2%, bricks surface looks more fine and smooth, can not even find micro porosity.

  As we all know, developing temperature and time is vital for complete eutectic crystal structure of AZS bricks. The bricks in vacuum casting is in mold under vacuum conditions, like hot water in vacuum bottle, temperature decline slow, which create time and condition for crystal development, let baddeleyite finish its bead-like structure and corundum eutectoid finish its growth. Vacuum casting longer 1~3 days in finishing time, stress cause by cold shrinkage and thermal expansion is well released which avoid cold crack, so that bricks quality get improved.

  Vacuum casting cavity is a complete cavity covered by plastic film, thermal insulation sand keep even in compactness and thickness all around, after casting, bricks solidified without crack. But in mold assembly stage, compactness and thickness shall be different, cooling speed’s inconformity will make bricks surface and cemented side seam inconformity, which will cause crack on bricks.

  The reason AZS bricks get cracks is due to solidification imbalance between sand mold expansion and bricks shrinkage. Vacuum casting use barometric pressure as power source to form mold, save agglomerant, no expansion, sand will flow after release vacuum, which create excellent condition for bricks’ shrinkage. Besides, vacuum condition provide good condition for temperature delivery, bricks in whole mold are in same temperature, solidification balance is higher, thus avoid cracks on bricks and improve quality.

  3、Vacuum Casting Technology:

  Vacuum Casting is a physical forming methods, its fundamental is: fill dry sand into box which come with air exhaust, shake it to solid, then cover the mold surface and box by plastic film. Vacuumize box and forming it by pressure gap between inside and outside of box, then withdraw mold and seal box, casting fused liquid while remaining box in vacuum status. Process are as below:

  3.1Molding

  Put the wooden mold on molding base with exhaust chamber, when vacuumize exhaust chamber, the air hole on mold will help plastic film to stick on mold.

Sand Box Sketch

  3.2Heating film to cover mold

  The plastic film has certain intensity and thickness which is expandable under heating. After heating, plastic film cover mold surface.

Sketch of film heating

Sketch of film covering

  3.3Use sand box with compaction

  Put sand box on mold with film cover and fill fine sand into box, then shake it to high density.

  

Sketch of Sand box sitting on film cover.

Sketch of filling sand box

  3.4Plastic film covering and Vacuumize box

  Covered by plastic film to seal sand box. Top box open casting head and trim sand to flate, vacuumzie box after covered by plastic film.

Sketch of covering and vacuumization

  3.5Withdraw mold and close box for casting

  Withdraw mold after vacuum sand box, close the sand box (seal it by cream), casting through casting head and cavity.

Sketch of withdraw mold

Sketch of casting head and cavity

  3.6 Open box and clean sand.

  After casting, bricks will be solidification in setting time. Relive vacuum and reserve heating in 7~12 days, open sand box to clean sands, take out bricks. Sands can be recycled after cleaning.

Bricks’picture withdraw from box.

Pictures of Casting site

  4、Quality evaluation of AZS bricks by vacuum casting

  Vacuum casting bricks reach and exceed national standard after testing by NRQS.

项目 Item  单位 Unit  指标
AZS33-Y 前卫 Chief Way
AZS33PT-Y
AZS41-Y 前卫Chief Way
AZS41PT-Y
化学成分
Chemical composition
Al2O3       % 余量
Balance
50.14   42.92
ZrO2 32~36 33.74 40~44 43.42
Fe2O3   0.063   0.068
TiO2   0.068   0.12
CaO   0.064   0.060
MgO   0.030   0.045
K2O   0.049    
SiO2   ≤   16.5 14.00 13.0 11.98
Na2O 1.50 1.49 1.30 1.30
Fe2O3+TiO2+
CaO+MgO+
Na2O+K2O+B2O3
3.00   2.50  
Fe2O3+TiO2 0.30   0.30  

体积密度(致密部分)

Volume Density (Dense Part)

g/cm3 3.70 3.80 3.60 4.27

显气孔率(致密部分)

Apparent porosity 

% 2.0 0.6 1.3 0.6

静态下抗玻璃液侵蚀速度(普通钠钙玻璃,1500℃×36h)

Anti-Erosion speed in Static status

mm/24h 1.60 三相界面
(mm/24h)
1.45 1.30 三相界面
(mm/24h)
1.12
液下1/2处
(mm/24h)
0.03   液下1/2处
(mm/24h)
0.03

玻璃相初析温度

Temperature of glass phase initial separated out

1400 1400 1400 1410

气泡析出率(普通钠钙玻璃)

Bubble separated rate (common soda-lime glass)

% 2.0 0.7 1.0 1.0

玻璃相渗出量(1500℃×4h)

Glass phase seepage

% 提供实测数据

热膨胀率(1000℃)

Thermal Expansivity

  提供实测数据
容重1)Capacity   > PT、QX   Kg/m3 3400   3550  
ZWS 3550   3800  
WS 3600   3850  
注:1)适用于单重大于50kg的制品

  Advantage by this casting method:

  1、Surface clean and smooth.

  2、Precise Measurement

  3、Seldom cracks

  4、 Edge crack almost been eliminated.

  5、More dense

  Outstanding feature:

  1、Rate of finished products increased by 10%

  2、Save quartz sands, save sodium silicate, save manpower and material resources

  3、Decrease production cost

  4、Improve bricks quality and expand furnace working life and save money.

  Conclusion:

  1、Vacuum Casting is considered as big breakthrough in AZS filed, greatly support the development of glass industry. It can not solve all quality issue, but indeed, it widely improve quality in environmental friendly way by reusing sand.

  2、Vacuum negative pressure casting is overall model, which reduce error caused by assembling model, increase its surface compact layer, enlarge glass furnace life and benefits customers and society.