In my daily work as a foundry quality engineer, I have been dealing with a persistent problem: hole-type defects in automotive crankshaft castings. Crankshafts are among the most critical components in an internal combustion engine. They must possess high strength, excellent wear resistance, fatigue resistance, and impact toughness. For this reason, they are almost always produced as nodular cast iron castings. The casting process, however, is prone to several types of internal and external defects, especially those that appear as holes. Over the years, I have analyzed thousands of defective crankshafts and have gradually developed a systematic understanding of the four major hole-type defect categories: shrinkage cavities, gas porosity, slag inclusions, and sand holes. The last category is directly linked to the concept of a sand foundry defect, which can be broadly defined as any defect caused by sand from the mold or core becoming entrapped in the casting. In this article, I present my experience and the preventive measures we have adopted in our plant.
The production process for crankshafts in our foundry uses a high-pressure green sand molding line. The main steps include sand mixing, core making for some crankshaft types, molding, melting and spheroidization of the ductile iron, pouring, shakeout, and heat treatment. A typical process flow is listed in the table below.
| Step | Description |
|---|---|
| 1. Sand preparation | Green sand with bentonite, coal dust, water, and additives is mixed to target properties. |
| 2. Core making | For crank-shafts with internal oil passages, resin-bonded sand cores are produced by hot-box process. |
| 3. Molding | High-pressure molding machines compact sand around patterns to form molds. |
| 4. Melting | Base iron is melted in medium-frequency induction furnaces. |
| 5. Spheroidization | Magnesium alloy is added to produce nodular graphite; inoculation is performed. |
| 6. Pouring | Iron is poured into molds using an automatic pouring device. |
| 7. Shakeout and finishing | Castings are separated from sand, cleaned, and heat treated. |
When I first joined the foundry, the rejection rate of the newly introduced CA4GA crankshaft was sometimes as high as 50–60%. The majority of the rejects were caused by the four hole-type defect families. The table below shows the defect distribution for the year 2011 and 2012.
| Year | Production | Rejects | Reject rate | Gas porosity | Shrinkage | Slag | Sand holes | Burns-on | Cold shut |
|---|---|---|---|---|---|---|---|---|---|
| 2011 | 106,565 | 20,567 | 19.3% | 7,816 | 8,227 | 2,057 | 1,439 | 617 | 411 |
| 2012 | 86,565 | 15,755 | 18.2% | 5,671 | 6,619 | 1,733 | 945 | 551 | 236 |
These statistics made it clear that solving hole-type defects was the most urgent task. The defects were not only causing high cost but also affecting production schedules. In the following sections, I discuss the formation mechanisms, influencing factors, and practical countermeasures for each defect type. I also emphasize that the sand foundry defect is not an isolated phenomenon; it is often intertwined with molding sand quality, core quality, and mold handling.
Shrinkage Defects
Shrinkage defects are the result of volume contraction during solidification of the cast iron. The total contraction consists of three phases: liquid contraction, solidification contraction, and solid contraction. For ductile iron, the graphite expansion during eutectic solidification is much larger than for gray iron. This expansion can compensate for some contraction, but if the mold wall moves or the liquid supply is insufficient, a shrinkage cavity or microporosity may form. The fundamental condition for a shrinkage cavity is that the casting solidifies in a layer-by-layer manner, and the last liquid to solidify cannot be fed by liquid metal from a riser or gating system.
In ductile iron, the amount of contraction can be approximated by the following equation for total volumetric change:
$$
\Delta V_{\text{total}} = \Delta V_{\text{liquid}} + \Delta V_{\text{solidification}} + \Delta V_{\text{solid}}
$$
where each term depends on temperature range, alloy composition, and cooling rate. For magnesium-treated ductile iron, the graphite expansion tends to reduce the net contraction, but excessive magnesium or rare earth residuals can increase the tendency to form shrinkage.
Root Causes in our Crankshaft Production
I identified several key factors that contributed to shrinkage defects in our 4GA crankshaft:
- Incorrect gating system ratios: The original system had a ratio of sprue : runner : ingate equal to 1 : 1.21 : 1.59. This violated the principle of simultaneous feeding and created an excessive pressure drop, leading to slow filling and premature cooling.
- Riser design problems: The riser neck was too short, and the riser became “capped” before it could feed the casting. This was observed directly on poured molds.
- Chemical composition: High silicon content in the iron increased the shrinkage tendency. Our experiments showed that when silicon exceeded 2.45%, the shrinkage defect rate increased dramatically.
- Mold rigidity: Although our high-pressure molding line provided adequate rigidity, any variation in mold hardness could cause mold wall movement.
Corrective Actions
First, I redesigned the gating system. The new ratio became:
$$
F_{\text{sprue}} : F_{\text{runner}} : F_{\text{ingate}} = 1 : 1.15 : 1.09
$$
This new open-type system allowed faster and more stable filling. Second, I modified the riser neck dimensions to ensure proper feeding. The distance from the riser to the casting body was set to more than 12 mm, and the ingate thickness was kept below 12 mm to avoid the connected-vessel effect. Third, we introduced a heating sleeve (exothermic sleeve) placed at the top of the riser. The sleeve reacts with liquid iron and generates heat, thereby improving the feeding ability of the riser. The effect of these changes is shown in the table below, where the monthly percentage of shrinkage among total rejects decreased significantly.
| Month | Shrinkage ratio before improvement | Shrinkage ratio after improvement |
|---|---|---|
| Jan | 43.0% | — |
| Feb | 36.5% | — |
| Mar | 38.2% | — |
| Apr | 44.0% | — |
| May | 41.0% | — |
| Jun | 49.0% | — |
| Jul | 39.2% | — |
| Aug | 47.2% | — |
| Sep | — | 10.5% |
| Oct | — | 9.8% |
| Nov | — | 9.3% |
| Dec | — | 0.0% |
In addition, I carried out a controlled experiment to verify the influence of silicon content. The results are shown below.
| Crankshaft type | Final Si range | Number of castings | Pouring temp (°C) | Shrinkage / cold shut defect |
|---|---|---|---|---|
| 376 | 2.0–2.4% | 200 | 1380–1420 | None |
| 376 | 2.45–2.7% | 200 | 1380–1420 | Cold shut 10% |
| 4GA | 2.0–2.4% | 200 | 1380–1420 | None |
| 4GA | 2.45–2.7% | 200 | 1380–1420 | Cold shut 20%, shrinkage 10.5% |
Based on these data, we fixed the final silicon content at 2.0–2.4%. This completely eliminated shrinkage defects in both crankshaft series.
Later, in an effort to reduce production cost, I tested a new type of insulating riser that integrates a filter and an exothermic material. The new riser replaced the conventional sand riser and the separate exothermic block. The expected annual saving is shown in the table below.
| Old riser iron weight (kg) | New riser iron weight (kg) | Saved iron per casting (kg) | Iron cost (yuan/kg) | Saved cost per casting (yuan) |
|---|---|---|---|---|
| 4.25 | 1.9 | 2.35 | 7.716 | 18.133 |
| New riser unit price (yuan/pc) | 5.95 | |||
| Filter cost (yuan/pc) | 1.20 | |||
| Exothermic block (yuan/pc) | 1.052 | |||
| Additional cost per casting (yuan) | 3.698 | |||
| Net saving per casting (yuan) | 14.435 | |||
| Annual saving for 160,000 pieces | 2,309,600 yuan | |||
The new riser was tested first in one out of five castings per mold, then in all five. After 400 castings without shrinkage defects, we authorized the new riser for production. Therefore, the combination of gating redesign, riser neck modification, exothermic material, and strict silicon control was successful against shrinkage.
Gas Porosity
Gas porosity in our crankshafts was almost always subsurface pinholes, located 1–3 mm below the casting surface. These pinholes were only revealed after machining. The defects were concentrated in the small end of the crankshaft. The formation mechanisms of gas porosity can be divided into several categories:
- Precipitation gas holes: Caused by gases dissolved in the liquid metal, mainly hydrogen, oxygen, and nitrogen.
- Reaction gas holes: Caused by reactions between iron and mold moisture, or between alloying elements in the iron.
For ductile iron, the most common type is reaction gas porosity, often explained by the hydrogen theory, the nitrogen theory, and the carbon monoxide theory. The reaction of magnesium with water vapor is particularly important:
$$
Mg + H_2O \rightarrow MgO + 2[H]
$$
The atomic hydrogen can dissolve into the iron and later form pores during solidification. Aluminum in the iron also reacts with water:
$$
2Al + 3H_2O \rightarrow Al_2O_3 + 6[H]
$$
Our experiments demonstrated that the aluminum content in the inoculant had a decisive effect on pinhole formation. We tested the same base iron with three different batches of ferrosilicon inoculant having different aluminum contents. The results are shown below.
| Al in inoculant | Number of ladles | Inoculant per ladle (kg) | Spheroidizer per ladle (kg) | Iron weight per ladle (kg) | Gas porosity reject rate |
|---|---|---|---|---|---|
| 1.5–1.9% | 4 | 3.0 | 12 | 730±15 | 55% |
| 1.1–1.5% | 4 | 3.0 | 12 | 730±15 | 25% |
| <1.0% | 4 | 3.0 | 12 | 730±15 | 5% |
Consequently, we specified that the inoculant must have an aluminum content below 1.0%. We also reduced the amount of spheroidizing alloy added from 1.6–2.0% down to 1.3–1.5%, and controlled the residual magnesium content in the range 0.030–0.045%. This decreased the tendency to form pinholes without compromising the nodularity.
Mold Venting
Another major factor was inadequate venting of the mold. I added vent holes and a vent riser at the locations where the iron front converges. This allowed the gases generated by the sand mold to escape quickly. The improvement was straightforward and inexpensive, and it significantly reduced the gas porosity reject rate.
Sand Moisture and Effective Coal Dust
The green sand properties played an important role in gas porosity. I found that the optimal sand parameters for our conditions were:
| Parameter | Target range |
|---|---|
| Moisture content | 2.5–2.8% |
| Compactibility | 30–33% |
| Effective coal dust | 2.5–2.8% |
When the effective coal dust was below 2.5%, batch porosity often appeared. When it was above 2.8%, the moisture increased, which also promoted gas holes. This demonstrates that even the sand foundry defect of gas porosity is closely related to sand composition.
Additional Measures
I also took steps to reduce the initial gas content of the base iron:
- Ensured that metallic charge materials were clean and free from rust, oil, and aluminum.
- Baked ladles and treatment ladles for at least 48 hours.
- Avoided holding molten iron in the furnace for more than 12 hours.
After implementing these measures, the proportion of gas porosity rejects dropped from 38% to about 5% of total rejects. The remaining few cases were attributed to occasional excursions in sand parameters or residual magnesium due to our transfer ladle process.
Slag Inclusions
Slag inclusions, also called slag holes or slag spots, are formed when non-metallic inclusions become trapped in the casting. The inclusions originate from multiple sources:
- Oxides and dross from the melting process.
- Products of spheroidization and inoculation reactions.
- Erosion of furnace lining and ladle refractory.
- Secondary oxidation during pouring and mold filling.
In the gating system, the runner (cross gate) is the primary location for slag trapping. The design must ensure that the runner is always full, the flow velocity is low, and the cross-section is wide and shallow. In addition, filters can be placed in the runner to trap inclusions. The table below compares the performance of different filter types in our trials.
| Filter type | Pore size (ppi) | Number of castings | Pouring temp (°C) | Iron flow state | Slag defect rate |
|---|---|---|---|---|---|
| Straight-hole ceramic filter | 10 | 100 | 1400–1420 | Slight splashing | 5% |
| Straight-hole ceramic filter | 20 | 100 | 1400–1420 | No splashing | 3% |
| Foam ceramic filter | 10 | 100 | 1400–1420 | Slight splashing | 2% |
| Foam ceramic filter | 20 | 100 | 1400–1420 | No splashing | 0% |
From these results, I decided to use a 20-ppi foam ceramic filter. This filter not only traps coarse inclusions but also adsorbs fine inclusions on its large internal surface. The foam filter also smooths the metal flow and reduces secondary oxidation. In our redesigned gating system, the filter was placed in the runner close to the ingates, with a cross-sectional area ratio of:
$$
F_{\text{sprue}} : F_{\text{runner before filter}} : F_{\text{filter}} : F_{\text{after filter}} : F_{\text{ingates}} = 1 : 1.1 : 3\!-\!4 : 1.75\!-\!2 : 1.2
$$
This design ensured that the liquid metal velocity was not excessively slowed and that laminar flow was maintained.
Improving Iron Cleanliness
In addition to filters, I improved the melting practice:
- Returned scrap and gates were blasted to remove adhering sand.
- Molten iron was held at 1530°C for 15 minutes to allow inclusions to float.
- A high-quality slag coagulant was used to remove slag from the furnace and ladle.
- For treated iron, slag removal was performed three times before pouring.
I also lowered the sulfur and manganese levels in the iron. The target values are shown below.
| Element | Target range |
|---|---|
| Residual Mg | 0.015–0.030% |
| S | ≤0.03% |
| Mn | 0.3–0.5% |
Finally, the pouring temperature was raised from 1380–1400°C to 1400–1425°C. This increased the time available for slag to float to the top of the casting before solidification. These combined actions reduced slag defect rejects to zero in the first eight months of 2013.
Sand Holes
Sand holes are a classic sand foundry defect caused by loose sand particles from the mold or core becoming trapped in the casting. They are usually visible on the casting surface or just below it. The mechanism is that the dynamic pressure of the liquid metal exceeds the surface strength of the sand mold, causing sand grains to be eroded and swept into the metal. The dynamic pressure can be expressed as:
$$
P = \frac{\rho Q v}{g S} = \frac{\rho v^2}{g}
$$
where $\rho$ is the density of the metal, $v$ is the velocity, $S$ is the cross-sectional area of the stream, and $Q$ is the volumetric flow rate. If this pressure is larger than the high-temperature strength of the sand surface, erosion occurs.
Factors Affecting Sand Holes
In my analysis, the main causes of sand holes in crankshaft castings were:
- Insufficient green sand strength.
- Insufficient core strength.
- Poor mold handling and core setting practice.
I performed a series of tests to correlate green sand parameters with the occurrence of sand holes in 4GA crankshafts. The results are listed below.
| Test | Compactibility % | Moisture % | Wet compressive strength (MPa) | Effective bentonite % | Number of castings | Sand hole rejects |
|---|---|---|---|---|---|---|
| 1 | 35 | 2.86 | 0.17 | 6.9 | 200 | 11 |
| 2 | 35 | 2.88 | 0.19 | 7.3 | 200 | 4 |
| 3 | 35 | 2.85 | 0.21 | 7.5 | 200 | 2 |
| 4 | 35 | 2.87 | 0.18 | 7.0 | 200 | 6 |
| 5 | 35 | 3.0 | 0.16 | 6.8 | 200 | 16 |
| 6 | 35 | 2.95 | 0.15 | 6.6 | 200 | 21 |
It is evident that higher wet compressive strength, achieved by adequate effective bentonite content, significantly reduced sand hole defects. However, too much moisture increased the defect rate. The optimum parameters for our foundry are summarized in the table below.
| Parameter | Optimum range |
|---|---|
| Moisture | 2.8–3.0% |
| Compactibility | 32–35% |
| Wet compressive strength | 1.9–2.1 MPa |
| Effective bentonite | 7.5–8.0% |
Core Strength
For the 376 crankshaft, which contains an internal oil passage core, sand holes were also caused by weak cores. We used a hot-box phenolic resin-coated sand with a required tensile strength above 3.5 MPa. The core-making process parameters were optimized as follows: core box temperature 290–320°C and curing time 110–120 seconds. We also found that the core sand had a shelf life of no more than two days, and the finished cores could be stored for no more than one day. The following table shows the effect of storage time on core sand properties.
| Storage time | Weather | Tensile strength (MPa) | Gas evolution (ml) | Ignition loss |
|---|---|---|---|---|
| 1 day | Rainy | 3.8 | 13.37 | 2.2 |
| 2 days | Sunny | 3.7 | 12.80 | 2.1 |
| 3 days | Sunny | 3.0 | 14.47 | 2.4 |
| 4 days | Sunny | 2.8 | 14.72 | 2.4 |
After adopting this storage discipline, core-related sand holes were eliminated.
Molding Process Control
The molding process itself can generate a sand foundry defect even when the sand properties are adequate. I implemented the following controls:
- The pattern plates were re-chromed to maintain a smooth surface and prevent sand sticking.
- Sprues and pouring cups were cleaned before mold closing.
- A thin layer of surface hardener was sprayed on the mold cavity to increase surface strength and prevent drying.
- Pin-and-bush alignment of the mold was checked regularly to avoid damage during closing.
- Mold hardness was maintained above 90 on the Brinell scale (using a mold hardness tester).
These actions reduced the sand hole reject rate to negligible levels. The remaining sporadic sand holes were traced back to temporary fluctuations in sand system parameters, which were corrected by improved process monitoring.
Conclusion
Through the systematic investigation described above, I have demonstrated that the four major hole-type defects in crankshaft castings—shrinkage, gas porosity, slag inclusions, and sand holes—can be effectively controlled by a combination of process design, material control, and disciplined production management. The key lessons are:
- Shrinkage defects require correct gating ratios, proper riser design, and control of alloy composition, particularly silicon and magnesium.
- Gas porosity is minimized by reducing residual magnesium and aluminum, optimizing green sand moisture and effective coal dust, and improving mold venting.
- Slag inclusions are prevented by clean melting, adequate slag removal, proper runner design, and the use of high-efficiency foam ceramic filters.
- Sand holes, a typical sand foundry defect, are avoided by maintaining adequate green sand strength and core strength, and by careful mold handling.
In our foundry, the combined reject rate for crankshafts fell from 18.2% in 2012 to less than 3% in the first half of 2013. This remarkable improvement was achieved without major capital investment in new equipment. Instead, the focus was on understanding the physical and chemical mechanisms behind each defect and applying proven engineering solutions. I believe that the same methodology can be applied to any foundry facing similar challenges.
Ultimately, every casting process is unique. The optimum parameters depend on the casting geometry, the molding system, the alloy, and the available equipment. However, the fundamental principles remain the same: control the flow of metal, control the gas evolution, control the solidification sequence, and control the mold material. Only by considering all these factors together can we produce high-quality crankshaft castings free from hole-type defects and ensure customer satisfaction.