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​Application of silica sol in investment casting

Complex parts of forging remold, no cutting and high surface finish,high precision,which are difficult to machine,created a broad application prospect for investment casting process.However,products of military and aerospace industry have high demands on the surface quality,dimensions of investment casting parts.Traditional use of liquid sodium silicate can't meet the requirements.And silica sol is alkaline stable colloidal dispersion system,discontinuous amorphous silica particles as dispersed phase,taking water as dispersion medium,made from dilute sodium silicate solution with special process.Among which,SiO2 could mix with organic polymer and work as adhesive by alkalization and acidification,silicon melt modification.

Currently,we use silica sol binder to manufacture ceramic shell mold.Advantages are:Preparation technology is simple,coating facilitate storage,dip coating used slurry can continue to use after adjusting composition, reduce the material consumption,dip coating process performance is stable,easy to operate, canceled ammonia dry when using ethyl silicate hydrolysis liquid,working environment improved,hot air drying whose humidity can be controlled, high shell mold strength, to ensure the quality of ceramic shell mold and precision casting parts,using water-based silicon solvent at the same time, good safety.Thus silica sol binder is rapidly and widely used by many countries.Disadvantages:invasion is no good than  ethyl silicate, liquidity of coating is bad.Bubbles are not easy eliminated.Meanwhile,volatilization of water is no faster than alcohol,and it will greatly effected by wetness and temperature of environment.So the production cycle is relatively long.

1.Characteristics of silica sol

Silica sol is milk white transparent colloid.Common used silica sol is sodium type silica sol(SiO2 30%、Na2O<0.5%).It is the same as liquid sodium silicate and hydrolyzed ethyl silicate.taking the role of bonding by colloidal silicate when making ceramic shell mold.The basic composition is the amorphous SiO2.One of the important features of silica sol is its stability.The stability is related to pH value, pH 8.5 ~ 10 is most stable.PH of 2 ~ 4 is relatively stable, the most unstable pH is about 7.Preventing mixed with acidic or alkaline substances is one of the main factors to maintain stability of coatings when mixing.Electric medium of silica sol will affect the charge distribution of layer structure and reduce the stability.Condensed threshold concentration of silica sol to KCl、BaCl2、Al2(SO4) is 450、125、3.3 mg/L.coagulation ability ratio of K+、Ba2+、Al3+ is 1:3.6:136.4.So the purity of wetting agent and nucleating agent must be high when ingredient.Kinematic viscosity is an important parameter to measure the stability of silica sol.The larger kinematic viscosity is, the worse the stability will be.There are many methods to make silica sol,such as acidification method,dialysis method,ion exchange method,gel glue solution method,hydrolysis method, etc.No matter which method,we must ensure SiO2 grow up under condition of alkali.

2.Choice of silica sol

Performance indexes of silica sol include SiO2 content,density,the content of stabilizer,PH value,SiO2 particle size,etc.Special silica sol needs surface modification or additional surfactant.In order to ensure the quality of shell mold,recommended technical index by the casting department of aviation is:SiO2-24% ~ 31%, Na2O<0.40%, pH-8.5 ~ 10, SiO2 particle size 10 to 20 nm, kinematic viscosity ≤6×10-6 m2/s,Cl-1< 0.03%.Appearance should be milky white or light color,no foreign inclusions,stable period should be not less than 1 year.From the point of practical application,using a single silica sol as binder,it is more suitable to choose Ludox HS - 30,HS - FS,SM - 30,D30,Syton 2 X,Nalcoag 1030,ect.There are also many domestic silica sol manufacturers,but with a single variety.In recently years,silica sol is gradually close to international business category.So the performance tends to be stable in investment casting industry.To control the quality of silica sol, we should be in strict accordance with the relevant industry standards,and suggest that the best is designated supply for a long time.

3.Preparation of silica sol

For high temperature alloy, steel, titanium alloy, aluminum alloy and copper alloy casting parts,selection of refractory powder must be accessible to the silicon sol bond,and and should not cause slurry gel when mixture of coating at room temperature.In order to improve dipping and coating characteristics,we still need to add auxiliary materials such as suspending agent, mineralizing agent, penetrating agent, flow ping agent in alumina, zirconia, mullite, aluminum ore and quartz glass powder with 0.075mm particle size.When adding powder,we must stirring while adding water to prevent local powder suct too much water,causing silica gel.Coating performance will be determined by its viscosity and density after selecting formula.

The performance will change without doubt with digging usage of coating.Due to decreased water and improved viscosity,we need to add little silica sol and deionized water acording to calculated amount to ensure the content and viscosity of original coating SiO2.When using silica sol binder in winter,if indoor temperature is too low, viscosity will be on the high side when mixing coating,silica sol binder is prone to bubbles and will affect the shell quality.

4.Dry of silica sol when coated.

Dry conditions in the process of coating system is an important factor to ensure quality.Shell mold grying process is that water passed to coating surface from shell inside by capillary action and then evaporate.So it will be affected by a series of factors like wetness of air,environmental temperature,air velocity,drying time,shell mold material.Environment temperature is the primary factor, the range should be controlled in ±1.5 ℃;Wetness is the second factor,air relative wetness should be 40% ~ 60%.

In drying process,moisture will be evaporated from the shell mold surface and absorb heat,making decreasing temperature of wax mold and smaller sizes,which will be easy to cause surface defects.Drying process should be as soon as possible, in order to prevent the impact of the shell temperature changes.Now we normally use hot air drying with controllable temperature,it can shorten the drying time, but also to ensure the quality of shell mold.