In view of the high rejection rate of castings caused by the unreasonable casting process design of WP10 cylinder head castings, a comprehensive analysis of the casting defects was conducted, followed by targeted process improvements. By modifying the cross-sectional area of the gating system, increasing the number and height of air vent needles, optimizing the wall thickness, and implementing other measures, the scrap rate of castings was effectively reduced. This article details the defect analysis, process improvement measures, and their implementation effects, aiming to provide reference for similar casting defect analysis and process optimization.
1. Introduction
The WP10 cylinder head, with sand box dimensions of 1200mm × 900mm × 300/300mm, accommodates 8 pieces per box, each weighing 20kg, resulting in a total pouring weight of 220kg per box. The initial pouring temperature ranges from 1400 to 1420°C. The casting wall thickness of the WP10 cylinder head generally measures 5mm, with the thickest bolt hole wall thickness reaching 30mm. During small-batch production, major defects such as porosity, sand holes, and localized sintering and sticking sand were observed, leading to a comprehensive scrap rate of over 6.2%, and a porosity scrap rate of about 2% after machining. This article focuses on analyzing the causes of these three primary defects and proposing process improvements.
2. Analysis and Improvement of Porosity Defects
2.1 Current Status and Cause Analysis of Porosity Defects
Due to the complexity and large number of sand cores in the WP10 cylinder head, which are made of coated sand, the sand cores generate a significant amount of gas, leading to porosity defects. The pouring time for each box is between 24 and 26 seconds, and many exhaust needles are not fully filled after pouring, as shown in Figure 1. After machining the upper surface, a large proportion of porosity defects are found in the middle nose bridge area of the WP10 cylinder head, as illustrated in Figure 2.
2.2 Measures for Solving Porosity Issues
2.2.1 Improvement of Gating System Process
Under modern production conditions, reactive and exudative porosity are relatively rare, while intrusive porosity is more common. When molten iron enters the mold, the mold, sand cores, coatings, binders, etc., gasify, decompose, or burn under the heat of the molten metal, generating a large amount of gas. The volume of the gas increases as the temperature rises, causing the gas pressure to continuously increase. When the gas pressure (Pgas) at a certain point on the interface is greater than the counter-pressure (ΣP) of the molten metal at that point, including surface tension, gas can enter the molten iron to form porosity. That is, when Pgas ≥ ΣP, porosity defects are likely to form.
Since the materials used in the WP10 cylinder head (mainly coated sand, coatings, etc.) have been verified in our factory, and their gas generation capacity is tested upon entry, we did not further analyze them. For thin-walled gray iron cylinder head castings, a pouring speed of 8 to 10 kg/s is recommended.
We decided to analyze the cross-sectional area of the gating system and reduce porosity defects by changing the pouring speed. Through analysis and calculation, the original casting process gating system cross-sectional area ratio was Fdirect:Fcross:Finner = 1:1.18:1.63, with a pouring time of 24 to 26 seconds per box. The original gating system had excessively small cross-sectional areas for the straight gate and cross gate, and an overly open pouring system, which easily caused the exhaust needles to not be fully filled. To reduce the open ratio, we experimented with increasing the cross-sectional areas of the straight gate and cross gate. After the improvement, the cross-sectional area ratio of the gating system was adjusted to Fdirect:Fcross:Finner = 1.04:1:1.3. As a result of this process improvement, the pouring time per box was shortened to 19 to 22 seconds, a reduction of about 5 seconds, and there was no unfilled phenomenon in the cylinder head exhaust needles.
2.2.2 Improvement of Exhaust Process
For the concentrated porosity defects in the middle nose bridge area of the WP10 cylinder head after machining, we analyzed that this location is in the middle of the cylinder head where the molten iron solidifies slowly. The gas outlet plate and gas outlet rod belong to the cold runner riser. Installing a gas outlet plate or gas outlet rod at the top or in dead corners of the casting can serve both as an overflow and exhaust and as a heat dissipation device for the cold runner riser, accelerating the cooling rate at that location, i.e., increasing Presistance and balancing the temperature difference in the wall thickness. A 10mm diameter and 100mm height exhaust needle was added to the area prone to porosity in the nose bridge, as shown in Figure 3.
In the original design, after placing a stone wool pad on the exhaust core head structure, the stone wool pad was squeezed into the sand mold. Due to squeezing during insertion into the sand mold and manufacturing deviations of the stone wool pad itself, poor sealing easily occurred nearby, leading to porosity defects at those locations. The sealing method of the stone wool pad was changed to a plane sealing method with the upper sand mold pressed down to reduce sealing imperfections caused by extrusion deformation and stone wool pad deviations, thereby reducing porosity defects.
3. Analysis and Improvement of Sand Hole Defects
3.1 Current Status and Cause Analysis of Sand Hole Defects
Sand holes in the WP10 cylinder head are mainly of two types: one is regularly and fixedly located below the exhaust needle, as shown in Figure 4; the other is scattered sand holes irregularly distributed on the bottom, top, and sides of the casting, as shown in Figure 5.
3.2 Measures for Solving Sand Hole Issues
3.2.1 Increasing the Height of Exhaust Needles
Through analysis of the sand holes at the roots of the exhaust needles, it was found that due to the low height of the exhaust needles on the upper mold plate, the drilling machine had an excessive stroke when drilling the exhaust holes, causing it to drill deeper, and sand remained at the intersection of the exhaust needles, which was difficult to blow away during cleaning, as shown in Figure 6. To address this issue, the height of all exhaust needles on the upper mold plate was increased from 250mm to 295mm, and the drilling stroke was reduced by 30mm. This shifted the sand intersection up by 30mm, making it easier for the upper box sand blowing personnel to blow away the sand and reduce sand hole defects at this location.
3.2.2 Adding Automatic Sand Blowing Device
During the molding process of the main machine, some scattered sand adhered to the inner surface of the upper sand mold (as shown in Figure 7), which easily formed scattered sand holes after pouring. Since there were no sand blowing personnel for the upper box, it could not be guaranteed that all sand would fall off when the sand box was turned over. Therefore, an automatic sand blowing device for the upper mold was added before the closing process to automatically blow off the scattered sand on the upper mold and reduce sand hole defects, as shown in Figure 8.
3.2.3 Optimizing the Bottom Machining Allowance
After machining, small sand hole defects were found on the bottom surface of some castings, and some defects on the bottom surface of the castings involved the combustion chamber and sealing. These defects were small in size (within 0.5mm), but they affected the performance of the diesel engine. Such defects could not be repaired. To reduce the fine sand hole defects on the bottom surface of the castings, the bottom machining allowance was optimized, and the structure of the bottom mold core was modified by gradually increasing the machining allowance by 0.5mm within the combustion chamber area to remove the defective areas during machining.
4. Analysis and Improvement of Sintering Defects
4.1 Current Status and Cause Analysis of Sintering Defects
The sintering defects in the WP10 cylinder head mainly occur near the inner gate, where the sand core surrounds the main bolt hole area. The wall thickness of this casting part is 30mm, and this area of the sand core is exposed to heat for a long time after pouring, as shown in Figure 9.
4.2 Measures for Solving Sintering Issues
Through dissecting and analyzing the areas prone to sintering, the dimensions were first analyzed. Theoretically, the thickness of the bolt hole area was 35mm, with a unilateral wall thickness of 6.3mm after processing. The measured dimensions of the processed casting matched the theoretical dimensions. However, according to the product design requirements, the casting wall thickness should be 5mm ± 1.0mm, and this wall thickness was at the upper limit. Therefore, it was decided to optimize the structure of this area by increasing the thickness of the sand core prone to sintering byhave revolutionized numerous industries by enabling computers to learn from and make predictions based on vast amounts of data. One of the most profound impacts has been in the healthcare sector, where AI-driven tools are now being used to diagnose diseases, predict patient outcomes, and personalize treatment plans.
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