1. Improvement measures
1.1 Inspection and control of molding sand performance
While maintaining the original test items and requirements of molding sand, the test items of grain size, moisture, fluidity and toughness of recycled sand are added, and the specifications are formulated, as shown in Table 1.
| Serial number | Test items | Test standard | Detection cycle |
| 1 | Grain size of recycled sand (original sand grain size) | The total number of 40-100 mesh sieve numbers shall not be less than 75%, and the percentage difference between adjacent sieve numbers shall be less than 10%, the 140 mesh sieve number shall be 5% ~ 10%, and the number below 200 mesh shall be less than 4% | Once a week |
| 2 | Mobility | The hardness difference between two samples shall not exceed 10% of the average hardness difference | Once an hour |
| 3 | Toughness (crushing index) | Not less than 75% | Once an hour |
| 4 | Raw sand moisture | 1%~2% | Once a day |
1.2 Molding sand preparation process
According to the analysis of defect causes, the compactness of sand mold surface is the key factor to prevent sand inclusion and sand sticking on the surface of steel castings. Therefore, the following process measures are taken.
(1) Strictly control the gap between the bucket and the guard plate (the gap between the bucket on the throwing wheel of the sand throwing machine and the wear-resistant guard plate inside the outer casing), and the gap shall be controlled within 1mm. Increase the gap state inspection between the bucket and the guard plate during the operation, and the frequency shall not be less than twice per shift.
(2) Reducing the air permeability of molding sand will reduce the pores between molding sand, increase the bonding bridge between sand particles and improve the compactness of sand mold surface. Therefore, the fine sand under the dust removal system is used to replace the ash mixture, which is pneumatically sent to the sand mixing system through the auxiliary material tank, and the addition amount is 0.2% ~ 1% of the sand amount.
| Type of molding sand | Equipment | Reclaimed sand /% | Fresh sand /% | Recycled sand /% | Ash /% | Bentonite /% | Starch /% |
| Surface sand 1 | BAP100B-250 | 100 | / | / | 0-2 | 1.0-3.5 | 0.2-0.5 |
| Surface sand 2 | BAP100B-250 | 90-100 | 0-10 | / | 0-2 | 1.5-4.0 | 0.3-0.6 |
| Back sand | SZG285 | / | / | 100 | 0-3 | 0.8-2.0 | / |
(3) Adjust the proportion and performance indexes of molding sand. As shown in Table 2 and table 3, the addition amount of bentonite remains unchanged, the mud content is adjusted to 10% ~ 12% (surface sand) and 9% ~ 11% (back sand), the upper limit of air permeability is 350 (surface sand) and 450 (back sand), the water content is increased to 3.0% ~ 4.0%, the sand mixing time is increased from 90 ~ 140 s to 180 ~ 220 s, and the addition amount of starch remains unchanged.
| Type of molding sand | Mud content /% | Moisture /% | Wet compressive strength / kPa | Compaction rate /% | Air permeability | Available soil /% |
| Surface sand | 10~12 | 3.0~4.0 | 110~150 | 50±3 | 170~350 | 7.0~9.0 |
| Back sand | 9~11 | 2.5~3.5 | 90~130 | 46±3 | 200~450 | 6.0~8.0 |
(4) Increase the water content of green sand raw sand (recycled sand) and control it at 1% ~ 2%.
2. Improvement effect
After the process improvement, the comprehensive properties of green molding sand have been significantly improved, the defects such as sand inclusion and sand sticking on the surface of steel castings have been greatly reduced, the scrap rate of steel castings has decreased from about 3% before the improvement to about 0.5% (Table 4), and the product quality of steel castings has been significantly improved.
| Month | Before and after improvement | Rejection rate of bolster and side frame castings /% |
| 1 | Before improvement | 1.79 |
| 2 | Before improvement | 3.27 |
| 3 | Before improvement | 3.17 |
| 4 | After improvement | 0.95 |
| 5 | After improvement | 0.45 |
| 6 | After improvement | 0.68 |
| target | / | 1.50 |
Clay green molding sand is continuously reused in the production process of steel castings. The increase of mud content in recycled sand will reduce the process performance indexes such as molding sand compaction rate, so as to increase the probability of sand inclusion and sand sticking of steel castings. In the process of producing steel castings of bolster and side frame with clay green sand, the sand inclusion and sand sticking defects of steel castings are significantly reduced and the quality of steel castings of bolster and side frame is improved by improving the proportion of green sand, improving the compactness of sand mold, and increasing the detection and control of molding sand performance indexes such as recycled sand particle size.