Silica sols (colloidal silica), also known as silicic acid solutions, are colloidal dispersions of amorphous silica aggregate particles in water. Silica sols have a wide range of water loss temperatures that not only break their bond strength, but also make their three-dimensional structure more stable and produce higher bond strength. It is a good inorganic binder for precision casting.
In the 1960s, silica sols started to be used as binders for precision casting. Since the 1990s, due to increased environmental pressure, silica sols casting has gradually replaced the widely used hydrolysis products of ethyl silicate, which were highly polluting and expensive. Polymer-reinforced, quick-drying silica sols are now widely used as binders in the investment casting industry worldwide.
Precision casting is the general term for the process of obtaining castings with precise dimensions. Precision casting is also known as lost wax casting. Its products are complex, intricate, close to the final shape of the part and ready for use without machining or with little machining. It is an advanced near-net forming process.
The process of investment casting consists of: selecting a suitable investment material (e.g. paraffin) for the investment; repeating the steps of impregnating the refractory paint (the main component is silica sol) and spraying the refractory material with refractory sand, hardening the shell and drying it; melting the internal parts to obtain a mould cavity; baking the shell to obtain sufficient strength and burning the remaining investment material; pouring in the required metal material; solidifying, cooling, obtaining a deshell and desanding to obtain a high precision finished product.
Silica sols are used for precision casting. Due to their good bonding and filling properties, the shells made are characterised by high strength, high gloss and easy demoulding and are therefore widely used in this field. Depending on the specific requirements, they are divided into surface silica sols and anti-silica sols.
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.
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.
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