silicate binder system is only suitable for aluminium or other low pouring-temperature alloy castings.
• After molds and cores have been produced, they can only be hardened by removing the free water in the binder. At present, this system is used only for making cores in Europe. The hardening method is to blow hot air, below 200C, through the core, causing it to be dehydrated and thus hardened. Furthermore, other materials should be prevented from coming into contact and reacting with silicate, so the used sand from shakeout will not contain any other reaction products. Silicate is a binder with good application prospects. How-
ever, at the moment it is used only for producing aluminium castings. For application in steel casting, shakeout, reclama- tion and recycling still present problems and need further study and development.
Alkaline phenolic resin—Alkaline phenolic resin is
prepared by chemical modification and is based on A-stage phenolic resin, a kind of modified resole phenolic resin. Tis resin is strongly alkaline and contains KOH and NaOH, hence it is called alkaline phenolic resin. Te alkaline phenolic resin self-curing sand process was
developed by Borden Chemical Co. in the UK and was pat- ented in 1981. Te hardening agent used for curing the sand is not an acid, but an organic ester. Tis self-curing process is also called the a-Set process. Alkaline phenolic resin self-curing sand has the fol-
lowing advantages: • Gas and smoke emitted during sand milling, molding and pouring are less than that for furan resin sand hardened with acid, resole phenolic resin sand and phenol-urethane resin sand hardened by amine;
• Because the sand still has some plasticity during strip- ping, the stripping properties are good; sand does not easily adhere to the pattern and the mold surface is smooth and clean, which allows the pattern stripping draft to be smaller.
• Alkaline phenolic resin self-curing sand has a second-hard- ening characteristic, which occurs after pouring. After second hardening, the sand has good thermal stability and almost no veining occurs on the surface of thick section castings.
• After pouring at high temperature, alkaline phenolic resin self-curing sand is easy to break down; this is beneficial to prevent cracking for complicated steel castings and improves the shakeout property of molds. Te main problem with the alkaline phenolic resin self-
curing sand process is the significantly reduced strength of recycled, reclaimed sand. One of the products of the alkaline phenolic resin
cross-linking reaction is alkaline metal carbonate. When heated above 510C, this alkaline metal carbonate changes to alkaline oxide and can react with silica sand to form a glass-like, combined alkaline silicate film on the sand surface. This film is formed by the reaction between alkaline metal oxide and silica sand. It has a very strong adhesive bond to the sand and is very difficult to remove during reclamation.
知，要解决硅酸盐粘结砂的再生、回用问题，必须遵 循以下两条原则。
（1）铸型或芯子不能经受太高温度的作用，不能 使水玻璃膜失去其中的牢固结合水和化合水，更不能 使其与砂粒熔合，目前，只宜用于生产铝合金铸件或 某些浇注温度更低的合金铸件。
（2）铸型、芯子制成后，只能借助于脱除自由水 使之硬化。目前，欧洲还只是用于制芯，采用的硬 化方法是在制芯后吹200℃以下的热空气使之脱水硬 化，而且，应该避免任何材料与硅酸盐发生反应，落 砂得到的旧砂中应不含任何反应产物。 由此可见，硅酸盐是一种前景很好的粘结剂，铸造 行业对其应予以高度的关注，但是，目前还只能用于 生产铝合金铸件。用于生产铸钢件，落砂和砂再生、 回用的问题仍然存在，这当然是有待今后进一步进行 探讨、研究课题。
2、关于碱性酚醛树脂
碱性酚醛树脂是以甲阶酚醛树脂为基础，经化学改 性处理而制得的，是一种改性的甲阶酚醛树脂。这种 树脂的碱性很强，其中含有KOH、NaOH等强碱性成 分，所以通常称之为碱性酚醛树脂。 碱性酚醛树脂自硬砂工艺由英国Borden公司开 发，1981年获得专利。配制自硬砂时，所用的硬化剂 不是酸，而是有机酯。这种自硬砂工艺也称为α-Set 工艺。
碱性酚醛树脂自硬砂工艺主要有以下优点：
• （1）混砂、造型、浇注时散发的烟气少于用酸性 硬化剂的呋喃树脂砂、甲阶酚醛树脂砂和以胺为 硬化剂的尿烷树脂砂；
• （2）由于起模时型砂仍然保持一定的塑性，故起 模性能好，型砂不易粘附在模具上，砂型表面比 较光洁，模样的起模斜度也可较小；
• （3）碱性酚醛树脂自硬砂具有二次硬化的特性， 二次硬化后，砂型的热稳定性较好，厚壁铸件表 面上也很少出现脉状纹缺陷；
• （4）浇注后，在高温的作用下，碱性酚醛树脂 自硬砂较易溃散，有利于防止形状复杂的铸钢件 产生裂纹，同时也可使浇注后铸型的落砂性能改 善。
碱性酚醛树脂自硬砂工艺存在的问题是，再生砂 的循环使用会导致自硬砂的强度显著降低，这是影响 此工艺推广应用的最大障碍。
碱性酚醛树脂发生交联反应的产物之一是碱金属碳 February 2013
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