Figure 2. This is a 70-metric- ton core made with Bangni resin sand.
图2 用邦尼树脂砂制成的 重70吨的砂芯
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.
Tis glass-like film also is difficult to wet with resin;
during mixing of reclaimed sand with added resin, it is dif- ficult to adhere the resin uniformly to the sand grain surface, and the readherence properties are not good. Terefore, the strength of self-hardening sand prepared from reclaimed sand is decreased significantly.
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 酸盐。加热到510℃以上，碱金属盐转变为碱金属氧化 物，此种氧化物又可以与硅砂作用，在砂粒表面上形成 玻璃状碱金属复合硅酸盐薄膜。这种薄膜是碱金属氧化
May 2013
FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 65
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74