Casting Production Process of Transmission Box

1.Introduction and Technical Requirements of Castings

The maximum outline size of the agricultural machinery transmission box is 692mm long, 272mm wide and 312mm high, the net weight of the casting is 72kg, the wall thickness of the main body is 6-38mm, the machining allowance is 3-4mm, the material is HT250, and the chemical composition is shown in Table 1. Castings belong to medium-sized thin-walled box castings. The structure of the castings is complex, the upper part is easy to deform, the processing amount is small, and the process design and manufacturing are difficult. If one item is not done well in the production of lost foam casting that pays attention to interlocking rings, it is easy to cause waste products. Agricultural machinery box parts are used to support bearings and rotating shafts, and contain gear oil to lubricate gears, thereby realizing power transmission function. Therefore, the quality of casting is very important for the normal operation of the whole equipment of agricultural machinery. The material is required to be dense, and casting defects such as cold insulation, slag inclusion and sand inclusion are not allowed. The tensile strength is above 250MPa and the hardness is above XX. The shape of the casting is shown in Figure 1:

Table1 Chemical composition requirements of castings









< 0.10


Fig. 1 Casting shape diagram

2: the production of casting foam model

At first, the white mold was scrapped due to false filling. as shown in Figure 2.

Fig. 2 White mold with false charging

Analyzed reasons:

(1): The opening position of the mold inlet is unreasonable, resulting in too long bead travel and easy to be blocked on the way.

(2): the use of automatic gun feeding, inflation pressure is not enough.

(3): The angle between the air inlet of the material gun and the material inlet is unreasonable, resulting in insufficient power for pneumatic driving bead charging.

(4): The bead density is too small, the bead diameter is too large, and the thin wall cannot be completely enriched when filling.

(5): mold clamping gap is too small.

Improvement based on the above reasons:

(1): Air pressure size must be guaranteed to be above 0.05MPa.

(2): The angle between the feed port and the air inlet of the correction gun is about 15 degrees.

(3): the use of pressurized into the barrel beads, the pressure in the barrel at about 0.02MPa, in order to increase the feed pressure.

(4): Reduce the bead size and increase the bead density to about 25Kg/m3, and make later adjustments on this basis.

(5): The mold clamping spacing is the largest at the point where the beads just won't run out, with a field spacing of 7mm.

After the above adjustment, the success of the white model, and stable production.

3: white mold bonding and baking

The maximum wall thickness of this casting is 38mm, easy to dry, drying room temperature in 50-60 degrees, humidity in 20-30. Dry for 12 hours.

Use cold glue to bond the parting surface, and seal the parting surface with paper glue.

4: coating hanging and drying

The coating adopts self-made coating, which is coated twice and the thickness is about 1.5mm.

5: the design of the gating system

According to the single casting weight 72kg.

The gating system is shown in Figure 3:

Fig. 3 Two castings with one gate

In order to reinforce the support, wooden sticks and hot melt adhesive are used to bond and fix the model during the drying process. On the basis of completing the fixation, the amount of adhesive used is minimized to avoid sand sticking at the adhesive. The sprue 35 is x40x420 and the inner gate 8 is x80x140. The inner gate must be kept at a height of 8mm, otherwise the effect of slag avoidance of the runner will not be achieved. The inner gate is located at the thick wall of the casting, which is conducive to the smooth filling of the metal liquid, and the use of an inner gate is convenient for grouping.

In order to improve the utilization rate of the sand box, this casting is generally poured three pieces in one box and two gates in one box, as shown in fig. 4. Therefore, the other is the way that one runner corresponds to one casting. The sprue 30 is x30x420 and the in-sprue 2 is X35x6x40, as shown in FIG. 5. The reason why two in-gates are made in this way is to strengthen the fixation on the basis of convenient group type.

Fig. 4 Burying Process of One Box with Three Pieces

Fig. 5 A casting with one gate

6: modeling process

The casting does not have a structure that is not easy to fill sand, and it is easier to fill sand. River sand 20-40 mesh, XXXXXXXXX sand box, 150mm bottom sand, lay the top of the bottom sand and put it into the casting to start sand filling. The exciting force is about XX, and the vibration is about 60s.

Then tear off the self-adhesive on the upper part of the gate, cover with thin plastic cloth, cover with about 50mm of cover sand, put mud strips on the upper part of the gate cup, and sit on the gate cup, but prevent the mud strips from extending into the white mold and causing defects in the pouring and flushing mud strips.

7: casting defects

(1): Cold barrier defect

The reason is that the pouring personnel are not skilled, which leads to the pouring speed failing to keep up with the pouring process, and the cold barrier defect appears at the thin wall of the final filling at the distal end of the inner gate. In the subsequent pouring process, the molten metal is kept pouring at a slow, fast and slow pace, and this defect does not appear again. as shown in Figure 6.

Fig. 6 Cold Shot Defect

(2) There is no water vapor cooling defect caused by dry white mold drying.

This defect is similar to the cold isolation caused by insufficient temperature, but its area is relatively large. Due to the initial production of castings, every link of the process is not well mastered, and the running-in period causes this defect. On the basis of keeping the humidity and temperature of the drying room normal, the drying time of the first coating is kept from the original 12h to 5h, and the time of the second coating is increased from the original 12 hours to 18 hours to ensure that the coating is dry. as shown in Figure 7.

Fig. 7 Cold Shot Defect

(3) Casting unpacking

The coating of the casting at the production site is self-matched, and the proportion of the binder in the coating is not correct, resulting in the coating tension becoming larger and the bubble top in the coating is not broken. Correct the addition ratio of the binder, andReduce the aggregate mesh of the coatingThis issue was then resolved. as shown in Figure 8.

Fig. 8 Casting unpacking

(4) Coating slag

Since the casting is formed by the upper and lower molds, the bonding area is large, so cold glue is used for bonding, and then paper tape is used to repair the seam. The bonding process and bonding results are shown in FIG. 9.

During the bonding process, the casting did not do a good job in the treatment of the parting surface, and the joint was not good, resulting in the coating seeping into the joint when the casting was painted, resulting in slag inclusion defects and scrapped.


Fig. 9 Paint slag inclusion diagram

(5) Slag inclusion at the gate

The height of the inner gate close to the casting is designed to be 8mm, but in the later production operation, the workers did not notice the slag retaining effect under this size, and the height of the inner gate was made to exceed 8mm, and the slag retaining effect of the runner decreased, resulting in slag inclusion defects at the inner gate of the casting. as shown in Figure 10.

Fig. 10 Slag inclusion defect diagram

(6) Slag inclusion in mud strips

The lower part of the post-molding pouring cup needs to be sealed with mud strips to prevent gunfire. After tracking production, it was found that due to improper use of mud strips, the mud strips went out of bounds and rushed into the cavity during pouring. The shape of the defect is similar to the mud strip. as shown in Figure 11.

Fig. 11 Slag Inclusion Defect of Mud Strip

(7) Sticky sand at the inner gate

The sprue adopts the method of brushing paint first and then bonding. In the bonding process, due to the unskilled operation of the workers, the joint between the inner gate and the casting is not tight, and there are cracks after the coating is replenished. The poured castings often stick sand here, which is difficult to clean.

Fig. 12 Sanding at Inner Gate

(8) Sanding at hardwood strips


In order to realize two pieces of one gate, this process requires a lot of wood strips on the fixed model, and the upper mouth of the white mold is easy to deform, requiring wood strips to be used as lacing bars.

Fig. 13 Sanding Defect of Wood Strip

8 Summary

Casting seems simple, but it is not easy to do. Each link is closely linked, and if one item is not done well, it will lead to unqualified castings. Therefore, for enterprises that have just started the lost foam production line, it is very important to train workers and establish a reasonable management system.

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