Mechanical sand, also known as liquid metal penetration sand, is a mechanically mixed adhesive layer of metal and sand formed on the surface of the casting by drilling liquid metal or metal oxide into the sand gap on the surface of the mold cavity through capillary penetration or gas phase penetration. Metallic luster can be seen when cleaning the sticky sand layer. The mechanical sand surface is spongy and firmly adhered to the surface of the casting, which mostly occurs in the parts where the heat action is strong on the surface of the sand mold and the sand core and the sand mold compactness is low, such as near the pouring gate, the thick section of the casting, the internal Angle and the groove.

Mechanical sand not only affects the appearance quality of castings, and even causes scrap, so enough attention should be paid to the mechanical sand of castings to improve the product yield. Based on many years of production experience and reference to the relevant materials, the author discusses the causes of casting mechanical sand and its prevention measures.

If the structure of the casting and pattern gives the sand shape a sharp Angle, overhanging or protruding shape, or a flat and thin sand table or sand core, this design will produce mechanical sand adhesion. Because the casting of this type of structure will cause the local heat concentration of the sand mold and sand core, its normal performance is damaged, the surface of the sand mold and sand core collapses in advance, and the molten metal or metal oxide is easy to penetrate and produce mechanical sticky sand. In this regard, sand types can be used in the parts where the heat is concentrated, such as zirconium sand or olivine sand, to improve its ability to be heated for a long time or be exposed to high temperature irradiation.

2. Appearance

(1) The process design of the pattern is poor, so that the sand ramming is not compacted or the compactness is uneven, which will cause the surface of the cavity to be rough or loose, and have a direct impact on the production of mechanical sand. The following errors often occur in the process design of the pattern:

One is the improper selection of parting surface, resulting in a high upper profile, or the formation of a deep cavity in the sand mold.

The second is that the position of the pattern on the mold bottom plate is improperly arranged, so that the depression is difficult to pound, which may be due to the pattern is too close to the wall of the box, or the pattern is arranged too closely. Because the pattern arrangement is too crowded, it will cause the casting to produce swelling sand, even if there is no obvious swelling sand, mechanical sand will occur. Due to the same source of defects, mechanical sand and swelling sand will appear on the same casting.

Third, the position of the straight runner, riser and cross runner is not arranged properly, and there will be depression parts that are not easy to ram, which is related to the layout of the sprue system.

Fourth, when the drawing inclination is <1.5°, even if the compactness of the entire sand mold is appropriate, the surface of the sand mold will be torn, and it is easy to penetrate metal or metal oxides.

(2) The process structure of the pattern is poor, resulting in high metal hydrostatic pressure. Due to the increase of static pressure, the load of the sand mold is increased, so it is necessary to use sand mold or more compact modeling method to resist the penetration pressure of metal liquid on the sand mold.

3. Sand box

The design and structure of the sand box are not reasonable, resulting in uneven sand compactness or too soft ramming.

The general causes of these problems are as follows:

(1) The improper position of the box belt prevents the ramming of some depressed parts, resulting in local sand mold too soft, causing swelling sand or mechanical sand.

(2) Because the sand box is too small, the sand is not solid.

(3) Improper placement of the box belt, so that the sprue and riser cannot be placed in a suitable position, so that the sand between the riser or sprue and the pattern is not tight. And because of the inevitable heat concentration, so that the mechanical sand more serious. Therefore, if the position of the runner or riser cannot be changed, the box belt must be cut off or removed altogether.

(4) The upper box is too high. Mechanical sand is usually related to the permeability resistance of the cavity surface and the static pressure of the liquid metal, and the size of the liquid metal pressure is related to the density of the metal and the height of the upper box. If starting from the need for casting feeding, the upper box should have a height.

4, gating system

(1) The improper location of the spout leads to local overheating of the sand mold, which causes the premature destruction of the cavity surface, and the metal liquid or metal oxide formed at high temperatures is easier to penetrate the cavity surface.

Examples of this type of situation are as follows:

First, the straight runner or riser is too close to the mold surface to form a hot zone. If the sand type in this hot zone is not rammed, the mechanical sand sticking defect will be very serious.

Second, too much liquid metal on the surface of the cavity will heat the surface enough to destroy it. If the initial solidified shell in the cavity is remelted by the later inflow of metal liquid, the sticky sand is more serious.

Third, when pouring, any factor that can cause the liquid metal pressure to be too high may cause the casting to produce mechanical sand. The situation is particularly serious when the liquid metal is not only under high pressure, but also at high temperature.

(2) The cross section ratio of the straight runner, the cross runner and the inner runner is improper, so that the metal liquid is continuously oxidized during pouring, increasing the amount of metal oxides, and leading to the melting effect of mold sand. During the pouring process, the pouring system should always be filled with liquid metal, otherwise, any part of the pouring system can oxidize the liquid metal.

(3) The size of the riser neck is too large, which will cause the sand around it to overheat, which is a more common problem. This problem is often caused by the riser neck too long, in order to prevent premature solidification of the metal liquid in the riser neck, it is necessary to increase the size of the neck. If feeding is not possible because of improper design of the riser neck, it is necessary to shorten the riser neck to prevent solidification, rather than increase its size. The smaller size of the riser neck can reduce the heat of the sand mold.

(4) Improper design of the pouring cup or straight runner, so that the pouring of air caused by liquid metal oxygenation, which is usually caused by turbulence, with the continuous accumulation of oxide film on the surface of the liquid metal will cause mechanical sand.

5. Molding sand

(1) Uneven distribution of raw sand particle size will result in low sand compactness, and the grain size distribution of raw sand has a direct impact on the compactness of sand.

(2) The fluidity and formability of molding sand are poor.

(3) The resin coating on the shell sand is too thin, which will reduce the local strength of the shell and the local sand is not covered by the resin, often because of this simple reason, the casting will be mechanically sanded.

(4) Insufficient carbon materials or deoxidized substances in the sand will produce too much metal oxide, so that the oxide wets the sand and is easy to penetrate.

6. Core making

(1) The sand of the unpainted sand core is too coarse or the particle size distribution is poor, which has an effect on the production of mechanical sand.

(2) The sand core is not rammed, and the nature of the sand type is not rammed, which has a significant impact on the mechanical sand.

(3) The surface of the sand core is rough or porous, which will cause mechanical sand adhesion. Molten metal or metal oxides infiltrate this rough or porous surface and penetrate into the core.

(4) The sand core absorbs moisture during storage. For the core, too much water is more troublesome. Because there is too much water in the sand core. Some cores appear to be dry, but are actually still high in moisture.

(5) The sand core is handled carelessly during handling, or when the paint is still in the wet state, resulting in damage or abrasion of the coating. The result is that there is not enough paint on the core to prevent metal or metal oxides from penetrating into the core. The paint damage is like opening a vent hole in the sand core, causing the metal liquid to boil and produce mechanical sand.

(6) The immersion of the sand core coating is too shallow, which will directly cause mechanical sand adhesion.

(7) The high temperature strength of the sand core coating is low. Due to the lack of clay in the paint, or too much solvent, so that the binder content is reduced, resulting in a decrease in the high temperature strength of the paint and cause mechanical sand.

(8) The core sand mixing is poor, so that the strength of individual parts of the sand core is too low, resulting in collapse during pouring, resulting in mechanical sticky sand.

(9) When the sand core is improperly cleaned and trimmed, it will directly cause mechanical sand adhesion. Strengthen the inspection of the sand core, such defects should be able to avoid.

(10) The sand core is not dried again after dipping and spraying paint. Like the moisture absorption and moisture return of the sand core in the sand mold, the coating is easy to flake and peel off.

(11) The core box is not clean, which will make the core sand adhere to the core box, causing the surface of the sand core to be rough. The core with good quality has a dense surface layer, and the core with a dense surface layer cannot be obtained if the core box is not clean.

7. Styling

(1) The softness and compactness of ramming are not uniform. In most cases, ramming compactness is as important as uniformity of compactness. However, sometimes uneven compactness is more harmful than the overall softness of the sand.

(2) Bad sand repair. The patching surface of sand mold is rough and loose, which will cause mechanical sand sticking. Excessive repair or repair parts of the water is too high, will also cause metal boiling and cause mechanical sand.

(3) The parts of poor sand repair are rough and loose, and mechanical sand will occur.

(4) Uneven or insufficient sand paint.

(5) The surface dry type of paint is not evenly dried or not fully dried.

(6) Too much release fluid will weaken the surface strength of the sand mold, and promote the boiling of liquid metal to produce mechanical sand in the casting.

(7) The beauty of the sand paint is too low, the paint will be inhaled by the sand, and the coating will peel, chip off or flake off, resulting in mechanical sand.

(8) The surface of the pattern is covered with insufficient sand, which is a common negligence in operation.

(9) Hot and cold materials (sand core, sand mold, core support, cold iron, etc.) come into contact with each other. When hot and cold materials meet, water condenses. Water can cause boiling mechanical sand, and at the same time greatly increase the formation rate of oxides, then the infiltration of metal oxides occurs. This is the general reason for the production of mechanical sand, but also the main reason for the production of high pressure molding mechanical sand.

8. Metal composition

(1) It is easy to form metal oxides with good fluidity, and metal with good fluidity is more likely to cause mechanical sand than metal with high surface tension or viscosity.

(2) The alloy contains low melting point components, such as lead in lead bronze, which is easy to cause mechanical sand. Because lead is still in a fluid state when it is much cooler than the parent metal.

(3) Alloy that needs to be poured at high temperature. Due to the good fluidity of the alloy, it is easy to produce mechanical sand. In addition, due to the high pouring temperature, the formation rate of oxides is accelerated, so it has a tendency to form oxidizing mechanical sand.

9. Pouring

(1) When the pouring temperature is too high, it will not only improve the fluidity of the liquid metal, but also make the liquid metal rapidly oxidize, thus accelerating the occurrence of oxidizing mechanical sand.

(2) The ladle is lifted too high and the upper box is too high, which will form too high metal indenter, and promote metal or metal oxide to enter the pores of the sand.