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基于去除加工原理的砂型无模铸造精密成形技术[1-5]，是一种直接铸型快速、绿色制造方法，通过现代数控加工技术和传统铸造技术之间的融合，快速制造出所需砂型[6-10]。实现了铸件生产的数字化、精密化、柔性化、自动化、绿色化[11-15]。
据统计, 由于型砂质量问题引起的铸件废品占所有废品总数的60%～70%[15-20]。砂型表面质量是砂型质量的重要要素之一，其直接影响到铸件的表面质量，甚至影响铸件的性能及质量。传统砂型的表面质量可以由砂型抗拉强度间接表征。数字化柔性挤压成形的砂型表面受到切削刀具的刮削[21-26]，容易使的砂型表面产生细小裂纹，导致砂型表面出现松散、易脱落、表面质量降低；而且铣削后的砂型表面存在着大量硬而微小的砂屑，如果直接用点触法测量，触头接触砂型表面，相当于触头在砂轮上移动，极易损坏测量设备；这时砂型的表面质量不能由砂型抗拉强度来表征。表面粗糙度也不能反映出砂型表面的松散程度及砂型的表面质量。本文采用表面性能来表征数字化柔性挤压砂型的表面质量，旨在为高效率、高精度、低成本的数字化柔性挤压成形技术提供一些理论基础。
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传统的砂型表面质量表征是表面安定性[27]。传统黏土砂表面安定性的试样无法从砂型中取得，而自制的表面性能及表面安定性试样不能真实表征传统砂型的表面质量。用抗拉强度间接表示表面性的也不多，不是通行做法。数字化柔性挤压成形技术可以对砂型进行切削，可直接从砂型中切取表面性能及表面安定性试样，从而使表面性能及表面安定性能真实表征数字化柔性挤压砂型的表面质量。
砂型的表面性能以试验前后试样质量的变化来表示：将2个圆柱形标准试样，试样尺寸ϕ50 mm×50 mm，并列放置于滚筒筛中，圆筒转动30 s后停止旋转，称量从筛孔中掉下的砂粒质量，与原试样质量的比值为砂型的表面性能。表面安定性是把圆柱形砂型试样夹在旋转试验仪上旋转，用钢丝针布刷在旋转的砂型试样表面进行刷磨，旋转30 s后称量被磨下的砂粒质量，即为砂型的表面安定性，其测试原理如图1所示。由于表面性能试验的试样在筛上易出现不规则的颠簸翻滚，从而使掉落的砂量波动较大，所以本实验选用表面安定性来表征砂型表面质量。
图 1 表面性能测试原理
Figure 1. Diagram of the surface property test
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试验材料：砂型材质为树脂砂，型砂选取硅砂、宝珠砂和铬铁矿砂，粘结材料选用碱性酚醛树脂和固化剂。试验用仪器及设备：CAMTC-SMM3000S 型号砂型数字化无模铸造精密成形机，表面性能试验仪，SHY 叶片式树脂砂混砂机碗型混砂机，百分之一电子天平。试验条件：砂温（20±2） ℃；室温（25±2） ℃；相对湿度（50±5）%。混制的砂型在数字化精确成形机上切削出砂型安定试样，如图2所示。
图 2 试样加工
Figure 2. Sample processing
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表面性能试样在耐磨测试仪上，测量不同的磨削次数。每测量一次，旋转轴旋转30圈。分别测量三个表面性能试样，按磨削次数取平均值，实验结果表1所示。
表 1 不同磨削次序型砂的表面性能
Table 1. Surface properties of sand under different grinding times
Times of grinding Surface properties/g First time 0.7075 Second time 0.4025 Third time 0.3675 Fourth time 0.3525 Fifth time 0.275 根据表1绘制磨削砂粒质量与磨削次数的曲线关系图，如图4所示。
图 4 砂粒质量与磨削次数的关系
Figure 4. Relationship between the sand quality and grinding time
实验结果如图所示，可看出，第一次磨削的砂粒质量大于后面几次的磨削质量，后面几次的磨削质量变化较小。结果说明砂型外部的表面性能与砂型内部的表面性能存在差异，无模砂型的抗拉强度不能够真实反映砂型的表面质量。砂型加工时刀具对砂型进行刮切，导致砂型表面的砂粒间会产生裂纹，降低了砂型的表面性能。
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选择宝珠砂、硅砂与铬铁矿砂分别按照型砂、碱性酚醛树脂和固化剂质量比为1000∶20∶5，使用SHY 叶片式碗型树脂砂混砂机混砂，混合时长为 15 s，将混好的树脂砂装入砂箱内，砂箱尺寸为150 mm×150 mm×100 mm，树脂砂固化12 h后，将砂块放置在数字化无模铸造精密成形上加工成表面性能试样，试样尺寸ϕ50 mm×50 mm，然后对所得砂型试样表面性能进行测量。不同种类型砂的表面性能测量结果如表2所示。
表 2 不同种类型砂的表面性能
Table 2. Surface properties of different types of sand
Type of sand Surface properties/g Ceramsite 0.09 Silica sand 0.15 Chromite sand 0.28 从表2可见宝珠砂的性能最好，铬铁矿砂的表面性能最差。这是由于型砂的角系数造成的。原砂的形状一般分为圆形、多角形和尖角形[2]。宝珠砂、硅砂与铬铁矿砂的颗粒形貌如图5所示。铬铁矿砂的砂粒为尖角形，其砂型的粘结桥容易形成压力集中。在无模切削过程中，切削表面很容易产生裂纹并且裂纹的延伸较深，导致其砂型表面容易脱落及形成表面疏松，所以其表面性能较差。宝珠砂的砂粒形状为圆形，其砂型的粘结桥受力较均匀，砂型表面不容易产生裂纹，所以其表面性能较好。而硅砂的砂粒形状介于两者之间，其表面性能也介于两者之间。
图 5 砂粒形貌。（a）宝珠砂；（b）硅砂；（c）铬铁矿砂
Figure 5. Sand grain appearance: (a) ceramsite; (b) silica sand; (c) chromite sand
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选用硅砂，在型砂、碱性酚醛树脂和固化剂质量比为分别为1000∶16∶4、1000∶20∶5、1000∶24∶6、1000∶28∶7和1000∶32∶8前提下，使用SHY叶片式碗型树脂砂混砂机混砂，混合时长为15 s，将混好的树脂砂装入砂箱内，砂箱尺寸为150 mm×150 mm×100 mm，在预固化阶段的施加压强分别为0、0.05、0.1 、0.15和0.2 MPa的挤压压强，经过30 min保压和12 h固化，将砂块放置在数字化无模铸造精密成形上加工成表面性能试样，然后对所得砂型试样进行表面性能进行测量。测量结果如表3所示。绘制挤压压力与表面性能的关系图如图6所示。
表 3 不同挤压压力下不同树脂质量分数砂型的表面性能
Table 3. Surface properties of sand mold with different resin contents under different extrusion pressures
Different extrusion pressures /MPa Surface properties of sand with different
resin contents/g1.6% 2.0% 2.4% 2.8% 3.2% 0 0.203 0.123 0.113 0.090 0.077 0.05 0.193 0.120 0.123 0.063 0.037 0.1 0.194 0.110 0.102 0.060 0.053 0.15 0.193 0.103 0.093 0.063 0.043 0.2 0.160 0.107 0.073 0.063 0.060 
图 6 不同挤压压力、树脂质量分数与砂型表面性能的关系. （a）树脂质量分数1.6%; （b）树脂质量分数2.0%；（c）树脂质量分数2.4%；（d）树脂质量分数2.8%；（e）树脂质量分数3.2%
Figure 6. Relationship between the surface properties of sand mold and extrusion pressure under varying resin contents: (a) resin content 1.6%; (b) resin content 2.0%; (c) resin content 2.4%; (d) resin content 2.8%; (e) resin content 3.2%
每种砂型树脂质量分数配比，砂型表面性能随挤压压力变化的趋势如图6所示。
由图a、b、c、d可见，不同树脂质量分数的砂型，总体趋势为随着挤压压力的增大，砂型的表面性能不断提高。在挤压压力作用下，砂粒间的间距更紧密，增加了砂粒粘结桥的数量及粘结桥的接触面积。在压力作用下，砂粒之间的距离减小，砂粒并联接触方式增多，砂型在经过切削时，砂型表面产生裂纹的数量及深度大幅减小，因此，增大砂型成形时的挤压压力能够提高砂型的表面性能。由图e可见，树脂质量分数最高时，砂型表面性能随着挤压应力的增大，呈波动变化。这是由于树脂质量分数较高时，砂型具有一定的弹性特征[21]，受不同挤压压力时，砂型的回弹变形不稳定，回弹对砂型的粘结桥造成破坏，所以砂型的表面性能呈波动变化。
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选用硅砂，在型砂、碱性酚醛树脂和固化剂质量比为分别为1000∶16∶4、1000∶18∶4.5、1000∶20∶5、1000∶22∶ 5.5、1000∶24∶6、1000∶26: 6.5、1000∶28∶7和1000∶30∶7.5前提下，使用SHY 叶片式树脂砂混砂机碗型混砂机混砂，混合时长为15 s，将混好的树脂砂装入砂箱内，砂箱尺寸为150 mm×150 mm×100 mm，在预固化阶段的施加压强为0 MPa的挤压压强，经过30 min保压和12 h固化，将砂块放置在数字化无模铸造精密成形上加工成表面性能试样，然后对所得砂型试样进行表面性能进行测量。测量结果如表4所示。
表 4 不同树脂质量分数砂型的表面性能
Table 4. Surface properties with different mass fraction of resin
Mass fraction of resin/% Surface properties/g Mass fraction of resin/% Surface properties/g 1.6 0.708 2.4 0.235 1.8 0.65 2.6 0.247 2.0 0.62 2.8 0.25 2.2 0.51 3.0 0.23 绘制挤压压力与表面性能的关系图如图7所示。
图 7 不同树脂质量分数与砂型表面性能的关系
Figure 7. Relationship between the surface properties of sand mold and resin content
从图中可以看出，在不加挤压压力的情况下，随着树脂质量分数的增加，砂型的表面性能不断提高。这是由于树脂质量分数增加后，砂粒的包覆厚度增大，从而砂粒的粘结桥增多，砂型强度增加，砂型切削是产生的裂纹数量减小，所以砂型的表面性能提高了。
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（1）本文采用砂型表面性能来表征无模数字化挤压成型砂型的表面质量，能够直接反映砂型加工后型腔的表面质量。得到了砂型材质对砂型型腔表面质量的影响规律。
（2）不同砂型种类的砂型表面性能不同。砂粒的角形系数对砂型的表面性能有较大的影响。砂粒的角形系数越小砂型表面性能越好。
（3）随着挤压压力的增大，砂型的表面性能不断提高。随着树脂质量分数的增大，砂型表面性能不断提高。本研究为无模成形砂型表面质量的测量提供了新方法，为提高无模成形砂型表面质量提供了依据。
Effect of sand-mold material and extrusion forming process on sand-mold surface properties
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摘要: 以数字化柔性挤压成形砂块为研究对象，通过设计单因素试验进行了砂型种类、粘结剂质量分数及挤压压力对型腔表面质量影响规律的研究，进而得出高精度树脂砂型挤压成形的最优参数组合。结果显示：无模砂型外部与砂型内部的表面性能存在差异。不同砂型种类的砂型型腔表面性能不同，沙粒的角形系数对砂型型腔表面性能有较大影响。随着砂型挤压力的提高，砂粒之间的距离减小，砂粒并联接触方式增多，砂型在经过切削时，砂型表面产生裂纹的数量及延伸深度大幅减小，砂型型腔表面性能不断提高。随着树脂质量分数的增大，砂粒的包覆厚度增大，从而砂粒的粘结桥增多，砂型强度增加，砂型切削时产生的裂纹数量减小，砂型型腔表面性能不断提高。本文为真实获得砂型表面质量提供了方法，有助于无模铸造精密成形技术的推广。Abstract: Considering the digital flexible extrusion sand mold as the research object, the surface quality of sand mold was studied by designing a single-factor experiment, and then the optimal parameter for the high-precision flexible forming of sand mold was obtained. The results show that there are differences in surface properties between the outside and inside of the sand mold, and the different types of sand mold had different surface properties. The angle coefficient of sand has a great influence on the sand mold surface properties. With the increase in the extrusion force, the distance between sand grains decreases and the parallel connection mode of sand grains increases. When the sand mold was cut, the number and extension depth of cracks of the sand mold were greatly reduced; thus, the sand mold surface properties increased. With the increase in the resin content, the coating thickness of sand grains increases, the bonding bridge of sand grains increases, the sand mold strength increases, the number of cracks of sand mold decreases, and the surface properties of sand mold increase. In this paper, a new method to obtain the surface quality of sand mold is provided, which can help popularize the precision forming technology of pattern-less casting. The method of sand-mold near-net forming with digital flexible extrusion makes the extrusion unit array pack form the sand-mold cavity. Moreover, in the digital precision forming technology without pattern casting, by filling the mold with molding sand, holding pressure, and hardening, the sand-mold near-net forming is obtained as a preform. This technology saves a lot of molding sand and reduces the amount of cut molding sand in the process of the digital precision forming technology without pattern casting. As the preliminary process of the sand-mold digital precision forming without pattern casting, the technology of sand-mold near-net forming with digital flexible extrusion effectively shortens the development cycle of castings. The basic research on sand mold efficiently achieves high-quality and near-net forming of sand molds with digital flexible extrusion. The research improves the digital level, eco-friendliness, and efficiency level of pattern-less casting technology.
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Key words:
- digital flexible forming /
- pattern-less casting /
- milling process /
- resin sand /
- sand surface properties
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图 5 砂粒形貌。（a）宝珠砂；（b）硅砂；（c）铬铁矿砂
Figure 5. Sand grain appearance: (a) ceramsite; (b) silica sand; (c) chromite sand
图 6 不同挤压压力、树脂质量分数与砂型表面性能的关系. （a）树脂质量分数1.6%; （b）树脂质量分数2.0%；（c）树脂质量分数2.4%；（d）树脂质量分数2.8%；（e）树脂质量分数3.2%
Figure 6. Relationship between the surface properties of sand mold and extrusion pressure under varying resin contents: (a) resin content 1.6%; (b) resin content 2.0%; (c) resin content 2.4%; (d) resin content 2.8%; (e) resin content 3.2%
图 7 不同树脂质量分数与砂型表面性能的关系
Figure 7. Relationship between the surface properties of sand mold and resin content
表 1 不同磨削次序型砂的表面性能
Table 1. Surface properties of sand under different grinding times
Times of grinding Surface properties/g First time 0.7075 Second time 0.4025 Third time 0.3675 Fourth time 0.3525 Fifth time 0.275 
下载: 导出CSV
表 2 不同种类型砂的表面性能
Table 2. Surface properties of different types of sand
Type of sand Surface properties/g Ceramsite 0.09 Silica sand 0.15 Chromite sand 0.28
下载: 导出CSV
表 3 不同挤压压力下不同树脂质量分数砂型的表面性能
Table 3. Surface properties of sand mold with different resin contents under different extrusion pressures
Different extrusion pressures /MPa Surface properties of sand with different
resin contents/g1.6% 2.0% 2.4% 2.8% 3.2% 0 0.203 0.123 0.113 0.090 0.077 0.05 0.193 0.120 0.123 0.063 0.037 0.1 0.194 0.110 0.102 0.060 0.053 0.15 0.193 0.103 0.093 0.063 0.043 0.2 0.160 0.107 0.073 0.063 0.060
下载: 导出CSV
表 4 不同树脂质量分数砂型的表面性能
Table 4. Surface properties with different mass fraction of resin
Mass fraction of resin/% Surface properties/g Mass fraction of resin/% Surface properties/g 1.6 0.708 2.4 0.235 1.8 0.65 2.6 0.247 2.0 0.62 2.8 0.25 2.2 0.51 3.0 0.23
下载: 导出CSV
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