I studied the influence of the assembly jig on the formation of cold lap and misrun defects in high chromium cast iron liners produced by the lost foam casting process. The investigation was motivated by a recurring production problem: cold lap defects appeared at the lower screw hole region of liners when the runner was directly supported by a steel assembly jig. I compared two arrangements: one in which the runner was placed directly on the angle-steel jig, and another in which a refractory brick was inserted between the assembly jig and the runner. The results show that the refractory brick reduces the cooling rate of the filling liquid metal and maintains better fluidity. For high chromium cast iron liners containing hard carbides of the (Cr,Fe)7C3 type, the use of a refractory brick between the assembly jig and the runner effectively eliminates cold lap and misrun defects in lost foam castings.
Keywords: lost foam castings; assembly jig; high chromium cast iron; liner; cold lap; misrun; refractory brick; evaporative pattern casting; EPC process.
1. Introduction
Lost foam castings have become an important manufacturing route for wear-resistant components because of their excellent dimensional accuracy, near-net-shape capability, and reduced need for coring and parting lines. In the lost foam casting process, an expanded polystyrene pattern, usually assembled into a cluster, is coated with a permeable refractory coating, embedded in unbonded dry sand, consolidated by vibration, and then poured with molten metal. The molten metal progressively decomposes and gasifies the EPS pattern; the decomposition products must escape through the coating and the sand to allow the liquid metal to fill the cavity completely. This process offers considerable freedom in designing complex castings, especially components such as liners, impellers, and wear plates.
However, lost foam castings are also sensitive to process variables such as pattern density, coating permeability, sand compaction, pouring temperature, gating design, and vacuum level. Because the metal front is in direct contact with decomposing polymer gases and liquid decomposition residues, the temperature of the liquid metal front can decrease rapidly. If the liquid metal loses too much heat, its viscosity increases and its ability to merge with another advancing front is reduced. This can create cold lap defects, incomplete fusion, or misruns. These surface discontinuities are particularly detrimental in abrasive wear components because they reduce load-bearing area, create stress concentrations, and shorten service life.
The liner considered in this study is an wear-resistant component used in spiral classifiers. It operates under severe abrasive conditions, so it must be wear resistant. I selected high chromium cast iron with a chromium mass fraction of 13% to 15%. During solidification, this alloy forms (Cr,Fe)7C3-type carbides, which have a hardness of approximately HV 1500 to 1800. This hardness is much higher than that of (Cr,Fe)3C-type carbides, which usually have a hardness of HV 840 to 1100. Therefore, high chromium cast iron provides excellent resistance to abrasive wear. But in the lost foam casting process, the same metallurgical properties that give excellent wear resistance also make the alloy prone to casting defects when the filling temperature is not sufficiently high. The liquid metal has a relatively narrow solidification range, and the presence of massive carbides can make the mushy zone less fluid.
In my production practice, I repeatedly found cold lap defects at the screw hole region of the high chromium cast iron liner when the metal was poured through a middle-gating system. The defects were more severe in castings located farther away from the sprue. In the most severe cases, the lower screw hole was not completely filled, and the casting was scrapped. To understand and solve this problem, I focused on the role of the assembly jig. The assembly jig was necessary because a middle-gating system is difficult to bury in dry sand. The jig supports the horizontal runner in the air and allows the sprue, runner, and inner gates to be assembled in a stable manner. However, I suspected that the steel jig could act as a chill and extract heat from the runner before the molten metal entered the pattern cavity. This extra heat extraction could be the reason why the lower screw hole, filled at a late stage, developed cold lap or misrun.
2. Experimental Materials and Conditions
I carried out the experiments in a foundry using a medium-frequency coreless induction furnace with a capacity of 750 kg. The alloy was high chromium cast iron with chromium content between 13% and 15%. I used the standard production practice for melting, including necessary alloying additions and slag control. The pouring temperature was maintained at 1450 °C. The mould was prepared according to the lost foam casting process: expanded polystyrene patterns were assembled into clusters, coated, dried, placed in a steel flask, and embedded in unbonded dry sand with vibration. During pouring, a vacuum of 0.015 to 0.02 MPa was applied to rigidify the mould and to help remove decomposition gases.
Each flask contained 30 liner patterns. The runner was horizontal, and 15 liner patterns were placed on each side of the runner. The gating system was a middle-gating or central-gating system, in which the sprue was connected to the middle of the horizontal runner. From the runner, the molten metal passed through inner gates into each liner pattern. The liner patterns were placed vertically in the mould, which allowed several castings to be poured in a single flask. The middle-gating system is useful because it balances flow to some extent, but it complicates mould assembly. To place the runner at the correct height and to support it before and during sand filling, I designed an assembly jig from steel angle sections.
Table 1 summarizes the main experimental conditions used in my study.
| Parameter | Condition |
|---|---|
| Casting alloy | High chromium cast iron, Cr 13%–15% |
| Pattern material | Expanded polystyrene foam |
| Pattern cluster | 30 liner patterns per flask |
| Arrangement | 15 patterns on each side of runner |
| Gating system | Middle gating, sprue connected at center of runner |
| Pattern position | Vertical |
| Mould medium | Unbonded dry sand |
| Vacuum pressure | 0.015–0.02 MPa |
| Pouring temperature | 1450 °C |
| Assembly jig | Angle-steel frame supporting the horizontal runner |
| Variable investigated | Refractory brick placed between jig and runner |
The first process, which I call Process I, used the assembly jig directly under the runner without any insulating material between the steel jig and the runner. The second process, which I call Process II, used a refractory brick between the assembly jig and the horizontal runner. In both processes, all other factors were kept as constant as possible: the same batch of patterns, the same coating, the same pouring temperature, and the same vacuum level. I poured both types of moulds in the same foundry environment to ensure that the comparison was meaningful.
3. Assembly Jig and Gating Design
The assembly jig was necessary for practical reasons. In a conventional lost foam casting cluster, the gating system is usually arranged with a vertical sprue and horizontal runner. If a bottom-gating system is used, the inner gates are located at the lower part of the pattern, which makes it difficult to place the runner and sprue in the flask while maintaining dimensional accuracy. A middle-gating system, on the other hand, requires that the horizontal runner be positioned above the lower portion of the pattern but below the upper portion. The runner must therefore be held at a specific height while the pattern cluster is being assembled, coated, dried, and buried in sand. A steel assembly jig can support the runner reliably and prevent the cluster from collapsing during vibration.
In my design, the assembly jig consisted mainly of angle steel welded together to form a stable frame. It had an upper horizontal surface on which the runner could rest. The runner was laid horizontally on the flat upper plane of the jig. The sprue was connected to the runner at its middle point. The inner gates were connected to the two sides of the runner, and then the liner patterns were attached to the inner gates. This arrangement allowed me to assemble the entire cluster in a robust way. The assembly jig was especially useful when the cluster was transferred to the flask and when dry sand was poured and compacted around the pattern cluster.
Although the assembly jig solved the mechanical problem of supporting the runner, I recognized that it also introduced a thermal problem. The angle-steel jig is made of carbon steel, which has a much higher thermal conductivity than dry sand or the refractory coating. When molten iron flows through the horizontal runner, the runner coating and the metal runner walls can be in direct contact, or at least in close contact, with the steel jig. Heat can be conducted away from the molten metal through the steel jig into the surrounding air or into the sand at the bottom of the flask. In the lost foam casting process, this may seem small, but the filling time is short and the temperature margin is limited. A localized chill from the steel jig may be enough to cause a serious filling problem in thin sections.
I therefore formed the hypothesis that the direct contact between the steel assembly jig and the horizontal runner was contributing to the cold lap defect. To test this hypothesis, I introduced a refractory brick between the assembly jig and the horizontal runner. The refractory brick has a much lower thermal conductivity than steel and therefore acts as a thermal insulator. It reduces the heat extraction rate from the runner. I expected that this would keep the molten metal hotter for a longer time and would allow the metal front to fill the thin section around the lower screw hole before it solidified.
4. Defect Observations and Comparison
The results of the two production trials were very clear. In Process I, where the runner was placed directly on the steel assembly jig, many of the liner patterns located farther away from the sprue showed cold lap defects. In severe cases, the lower screw hole of the liner was not filled completely, producing a misrun. The defect was consistently located at the lower screw hole region, not at the upper screw hole. This regularity indicated that the cause was related to the metal flow path and the local heat loss. The metal front that traveled to the lower screw hole had already lost a significant amount of heat to the EPS pattern, to the coating, and to the steel assembly jig through the runner. By the time it reached the lower thin section, it had become too cool to fuse with another front.
In Process II, where a refractory brick was placed between the assembly jig and the runner, the same liner castings were produced without cold lap defects or misruns. The screw holes were fully formed and sound. This result was reproduced consistently enough to confirm that the refractory brick had a decisive influence.
Table 2 compares the observations between Process I and Process II.
| Casting condition | Defect observed | Location | Severity |
|---|---|---|---|
| Process I: no refractory brick | Cold lap, severe misrun | Lower screw hole of liners distant from sprue | Frequent and production-limiting |
| Process II: refractory brick under runner | No cold lap or misrun | None | Sound castings |
This simple comparison strongly suggested that the assembly jig contributed to the cold lap defect through a thermal mechanism. I then analyzed the heat transfer process more carefully to explain why the refractory brick was so effective.
5. Heat Transfer Analysis of the Runner–Jig Interface
The assembly jig acts as an external heat sink in the lost foam casting process. When molten high chromium cast iron flows through the horizontal runner, the runner is initially at room temperature or near room temperature. The steel jig is also at room temperature. The temperature difference between the molten metal and the steel jig is large, approximately 1400 °C or more. According to Fourier’s law of heat conduction, the heat flux through a solid layer is proportional to its thermal conductivity and temperature gradient. For one-dimensional steady conduction, the heat flux can be expressed as
$$ q = k \frac{\Delta T}{d} $$
where \(q\) is the heat flux, \(k\) is the thermal conductivity, \(\Delta T\) is the temperature difference across the layer, and \(d\) is the thickness of the layer.
In Process I, the heat flux from the runner to the environment through the steel assembly jig is controlled by the thermal resistance of the steel jig. If the thickness of the steel angle is \(d_{\mathrm{steel}}\), the thermal resistance is
$$ R_{\mathrm{no}} = \frac{d_{\mathrm{steel}}}{k_{\mathrm{steel}}} $$
In Process II, a refractory brick with thickness \(d_{\mathrm{brick}}\) and thermal conductivity \(k_{\mathrm{brick}}\) is inserted between the runner and the steel jig. The total thermal resistance becomes
$$ R_{\mathrm{with}} = \frac{d_{\mathrm{steel}}}{k_{\mathrm{steel}}} + \frac{d_{\mathrm{brick}}}{k_{\mathrm{brick}}} $$
The heat flux in Process I is therefore
$$ q_{\mathrm{no}} = \frac{\Delta T}{R_{\mathrm{no}}} $$
and the heat flux in Process II is
$$ q_{\mathrm{with}} = \frac{\Delta T}{R_{\mathrm{with}}} $$
The ratio of the heat flux with the refractory brick to the heat flux without the refractory brick is
$$ \frac{q_{\mathrm{with}}}{q_{\mathrm{no}}} = \frac{R_{\mathrm{no}}}{R_{\mathrm{no}} + R_{\mathrm{brick}}} $$
Because \(R_{\mathrm{brick}}\) is always positive, this ratio is always less than 1. In other words, the refractory brick always reduces the heat loss through the runner support. The exact value of the ratio depends on the thickness and thermal conductivity of the steel jig and the refractory brick.
I used the thermal properties mentioned in the original production note: carbon steel with about 0.5% carbon has a thermal conductivity of approximately 31 W/(m·°C) at high temperature, while a typical refractory brick has a thermal conductivity of approximately 11.1 W/(m·°C) at about 1400 °C. For the purpose of estimation, I assumed a steel thickness of 10 mm and a refractory brick thickness of 65 mm. Table 3 gives the properties used in this estimate.
| Material | Thermal conductivity (W/m·°C) | Assumed thickness (mm) | Thermal resistance (m²·°C/W) |
|---|---|---|---|
| Carbon steel assembly jig | 31 | 10 | 0.000323 |
| Refractory brick | 11.1 | 65 | 0.005855 |
The total thermal resistance in Process I is simply 0.000323 m²·°C/W. In Process II, the total thermal resistance is 0.000323 plus 0.005855, which equals 0.006178 m²·°C/W. The heat flux ratio is therefore
$$ \frac{q_{\mathrm{with}}}{q_{\mathrm{no}}} = \frac{0.000323}{0.000323 + 0.005855} \approx 0.052 $$
This calculation shows that, under ideal flat-contact conditions, the refractory brick reduces the heat extraction rate to only about 5% of the value obtained with the bare steel jig. Of course, the real situation is more complex because of imperfect contact, radiation losses, and the fact that the runner is not a flat plate. Nevertheless, the order of magnitude is instructive. The refractory brick dramatically reduces the chilling effect of the steel assembly jig.
The total amount of heat lost through the runner support during filling can be written as
$$ Q_{\mathrm{loss}} = q \, A_{\mathrm{contact}} \, t_{\mathrm{fill}} $$
where \(A_{\mathrm{contact}}\) is the contact area between the runner and the support, and \(t_{\mathrm{fill}}\) is the time during which molten metal is flowing through the runner. Because the heat flux is much smaller with the refractory brick, the total heat loss is correspondingly smaller.
The temperature drop of the molten metal caused by this heat loss can be estimated from the heat balance:
$$ \Delta T = \frac{Q_{\mathrm{loss}}}{m c_p} $$
where \(m\) is the mass of metal that is affected, and \(c_p\) is the specific heat capacity of the high chromium cast iron. If \(Q_{\mathrm{loss}}\) is reduced by a factor of nearly 19, then the corresponding temperature drop is also reduced by a comparable factor. In a process where the available superheat may be only 50 to 100 °C above the liquidus temperature, saving even 20 to 30 °C at the metal front can make the difference between a sound casting and a cold lap.
The heat transfer analysis therefore confirms that the assembly jig is not thermally neutral. In lost foam castings, any metallic insert that touches the gating system should be examined as a possible chill. In this case, the steel assembly jig was in contact with the runner for a substantial length, and the contact area was enough to produce a noticeable temperature drop.
6. Flow-Front Temperature and Fluidity
The cold lap defect in lost foam castings is controlled not only by the bulk metal temperature but also by the temperature and velocity of the advancing metal front. When the molten metal enters the liner pattern through the inner gate, it first encounters the EPS foam at a temperature well below the decomposition temperature. The metal must supply the heat required to raise the foam to its decomposition temperature, to decompose the polymer chains, to vaporize the decomposition products, and to heat those products to the metal temperature. This heat is taken from a thin boundary layer at the liquid metal front, so the front can become much cooler than the incoming bulk metal.
The energy consumed by the EPS pattern can be represented in simplified form as
$$ Q_{\mathrm{front}} = Q_{\mathrm{heat}} + Q_{\mathrm{decomposition}} + Q_{\mathrm{gas}} $$
where \(Q_{\mathrm{heat}}\) is the heat needed to raise the EPS from room temperature to the decomposition temperature, \(Q_{\mathrm{decomposition}}\) is the heat of polymer decomposition, and \(Q_{\mathrm{gas}}\) is the heat needed to raise the decomposition gases to the metal temperature. In addition, there is heat loss to the sand, to the coating, and to the assembly jig. In high chromium cast iron, the heat extraction by a metallic support can be substantial because the support is a continuous metallic path extending outside the mould.
For a liquid metal front to remain fluid, it must have enough superheat above the solidus or liquidus temperature to overcome the viscous resistance and the pressure of the decomposition gases. The viscosity of the liquid metal depends strongly on temperature. This dependence can be approximated by an Arrhenius-type relation:
$$ \mu(T) = \mu_0 \exp\left( \frac{E}{RT} \right) $$
where \(\mu_0\) is a constant, \(E\) is the activation energy for viscous flow, \(R\) is the universal gas constant, and \(T\) is the absolute temperature. As the temperature decreases, the viscosity increases rapidly. In high chromium cast iron, the presence of alloying elements and carbide-forming elements increases the tendency for early solidification when the temperature falls below the liquidus. Once solid particles form in the advancing front, the effective viscosity of the slurry increases even more dramatically. This is why a small additional heat loss caused by the assembly jig can lead to cold lap.
The critical condition for sound fusion at the meeting point of two advancing fronts can be stated as
$$ T_{\mathrm{merge}} \ge T_{\mathrm{critical}} $$
where \(T_{\mathrm{critical}}\) is the minimum temperature needed for the two liquid streams to fuse into one continuous solid structure. If \(T_{\mathrm{merge}}\) is below \(T_{\mathrm{critical}}\), the two streams will not fully coalesce, and a cold lap will remain. In the lost foam casting process, the metal front temperature at the merging point is equal to the pouring temperature minus all cooling losses along the flow path:
$$ T_{\mathrm{merge}} = T_{\mathrm{pour}} – \Delta T_{\mathrm{runner}} – \Delta T_{\mathrm{pattern}} – \Delta T_{\mathrm{jig}} – \Delta T_{\mathrm{gas}} $$
The refractory brick reduces \(\Delta T_{\mathrm{jig}}\). This directly increases \(T_{\mathrm{merge}}\), bringing it above the critical temperature and eliminating the cold lap defect.
7. Quantitative Effect of the Refractory Brick
To make the analysis more quantitative, I considered the thermal resistance calculation described earlier. The reduction in heat flux through the runner support is not the only important effect; the refractory brick also changes the temperature distribution in the runner and the surrounding sand. The brick has a lower thermal diffusivity than steel, so the surface temperature of the runner remains closer to the molten metal temperature during the short filling time.
The thermal diffusivity of a material is defined as
$$ \alpha = \frac{k}{\rho c_p} $$
where \(\alpha\) is thermal diffusivity, \(k\) is thermal conductivity, \(\rho\) is density, and \(c_p\) is specific heat capacity. Steel has a high thermal diffusivity, so it conducts heat away from the surface very quickly. Refractory brick has a lower thermal diffusivity, so it stores heat in its surface layer more slowly and transfers less heat to the surrounding environment. This is beneficial for molten metal flow in lost foam castings.
The benefit can be expressed in terms of a thermally affected layer. During a filling time \(t\), the penetration depth of the thermal disturbance is proportional to
$$ L_{\mathrm{thermal}} \sim \sqrt{\alpha t} $$
Because steel has a larger value of \(\alpha\), the thermal disturbance from the runner surface penetrates deeper into the steel jig, and the heat flux away from the molten metal is higher. In contrast, the refractory brick limits the thermal disturbance to a shallow layer near the contact surface. The heat extraction from the molten metal is therefore much smaller.
I also considered the possibility of an air gap between the runner and the refractory brick. In practice, the runner may not make perfect contact with the brick. An air gap would add another thermal resistance and further reduce heat extraction. This only strengthens the conclusion that the use of a refractory brick is beneficial. On the other hand, if the runner directly contacts the steel jig, the contact may be relatively good because the runner is pressed against the flat steel surface by its own weight and by the weight of the pattern cluster. The chilling effect is therefore more pronounced.
Table 4 summarizes the key thermal differences between the steel jig and the refractory brick in the context of lost foam castings.
| Property | Steel assembly jig | Refractory brick |
|---|---|---|
| Thermal conductivity | High, about 31 W/(m·°C) | Low, about 11.1 W/(m·°C) |
| Thermal diffusivity | High | Low |
| Heat extraction rate | Large | Small |
| Chilling effect | Strong | Weak |
| Effect on metal front temperature | Causes undesirable temperature drop | Maintains fluidity |
| Suitability for lost foam castings | Poor unless insulated | Recommended for metallic supports |
The comparison in Table 4 is consistent with the experimental observations. The refractory brick reduced the cooling rate of the filling iron enough to allow the metal front to rise through the thin section around the lower screw hole. The two streams that met above the screw hole were still hot enough to fuse completely, so no cold lap was formed.
8. Why the Lower Screw Hole Was Most Vulnerable
One of the most interesting observations in this study is that defects always occurred at the lower screw hole, not at the upper screw hole. This location is directly related to the middle-gating system and to the filling sequence inside the pattern cavity.
In the middle-gating system, the inner gate enters the pattern cavity at approximately the middle height of the liner. When the molten metal first enters the pattern, it does not immediately fill the upper part of the cavity. Instead, it has to flow downward to fill the lower part of the liner. This downward flow is against the direction of buoyancy of the decomposition gases. The metal front must push into the lower portion of the EPS pattern, decomposing the foam as it advances. At the same time, the decomposition gases, which are hot and tend to rise, create a local pressure at the downward-moving front. This pressure opposes further flow and slows the filling speed. The combination of heat consumption and gas backpressure is more severe during downward filling than during upward filling.
By the time the metal front reaches the lower screw hole, it has already traveled through the lower section of the pattern. The screw hole region contains a complicated geometry: there is a relatively thick boss around the hole, but there is also a thin surrounding wall. The thin wall has a smaller local modulus, meaning it solidifies more quickly. According to Chvorinov’s rule, the solidification time of a local section is proportional to the square of its volume-to-surface-area ratio:
$$ t_s = B \left( \frac{V}{A_s} \right)^2 $$
where \(B\) is a mould constant, \(V\) is the volume of the section, and \(A_s\) is the cooling surface area. A thin wall has a small \(V/A_s\) ratio, so it solidifies much faster than a thicker section. In the lower screw hole region, the thin wall is the most difficult part to fill. It may become filled only after the thick boss has carried the metal upward and then returned downward from above. When two fronts meet, one from below and one from above, the temperature is low and the thin wall is already partially solidified.
The upper screw hole, in contrast, is located in the upper part of the pattern. It is filled later in the overall sequence, but by that time the metal front has already heated the surrounding sand and the pattern has been largely decomposed in that region. More importantly, the upward filling is assisted by the buoyancy of the decomposition gases. The metal front can move upward more easily, and the gases can escape through the coating and sand. Therefore, the upper screw hole does not suffer from the same cold lap problem.
The assembly jig adds extra cooling to the runner. In Process I, the metal entering the inner gates was already cooler than intended because it had lost heat through the steel jig. This initial temperature deficit was amplified during the downward filling from the inner gate to the lower screw hole. The metal front arrived at the lower screw hole with very little superheat, so it could not overcome the high gas pressure and the rapid solidification of the thin section. The result was a cold lap or a misrun.
In Process II, the refractory brick reduced the heat loss in the runner. The metal entered the pattern cavity with a higher temperature. Although it still lost heat while filling the lower part of the cavity, it retained enough superheat to pass through the thin wall around the lower screw hole. The two streams met at a higher temperature and fused properly. This explanation is consistent with the observed elimination of defects.
9. Practical Implications for Lost Foam Castings
The results of this study have several practical implications for the production of high chromium cast iron liners and similar wear-resistant parts by the lost foam casting process.
First, any steel tooling that comes into contact with the gating system should be treated as a potential chill. In lost foam castings, the gating system is often supported by steel frames, clamps, or fixtures. These fixtures are designed for mechanical stability, but they are rarely considered in the thermal design of the casting process. This study shows that a simple steel fixture can be enough to cause cold lap defects in a high chromium cast iron part. The thermal interaction between the fixture and the molten metal should be considered during process design.
Second, a refractory brick is a simple and economical solution. The brick is easy to place between the assembly jig and the runner. It does not require a complicated redesign of the gating system. It does not reduce the strength or stability of the assembly jig because the brick can support the runner just as well as the steel surface, provided it is thick enough and placed on a stable support. The refractory brick also reduces thermal shock on the runner and may reduce coating cracking at the contact point.
Third, the pouring temperature must be selected with the entire flow path in mind. For high chromium cast iron, a pouring temperature of 1450 °C is normal, but the effective temperature at the end of the flow path can be much lower. If the gating system contains long horizontal runners and the runner is supported by a steel fixture, extra superheat may be necessary. However, increasing pouring temperature too much can cause sand burning, metal penetration, or shrinkage defects. The use of a refractory brick is therefore preferable to simply raising the pouring temperature, because it preserves the desired temperature distribution without overheating the alloy.
Fourth, the filling rate and vacuum should be adjusted to minimize pattern decomposition gas pressure. In lost foam castings, the decomposition gas pressure at the metal front depends on the gas generation rate, the coating permeability, the sand permeability, and the vacuum level. If the gas pressure is too high, the metal front slows down and cools. The negative pressure of 0.015 to 0.02 MPa used in this study was sufficient when the refractory brick was present. Without the brick, the same vacuum could not overcome the combined negative effects of a colder front and gas pressure. This suggests that insulation of the gating system is as important as process parameters such as vacuum and filling time.
Fifth, local thin sections should be filled as early as possible or should be made more resistant to premature solidification. The screw hole region could be modified by adding a small transition radius, increasing the wall thickness, or changing the orientation of the pattern so that the thin section is filled from the bottom in an upward direction. I found that an upward filling direction is more favorable because the decomposition gases can escape upward. In this liner, the lower screw hole was inevitably filled by a downward-moving front because of the middle-gating system. The refractory brick addressed the thermal problem without changing the geometry.
The comparison between the two processes is summarized below:
| Aspect | Process I: no brick | Process II: with brick |
|---|---|---|
| Runner support | Direct steel contact | Steel plus refractory brick |
| Heat loss to support | High | Very low |
| Metal front temperature | Lower | Higher |
| Fluidity in thin sections | Insufficient | Sufficient |
| Cold lap | Present | Absent |
| Misrun | Present in severe cases | Absent |
| Scrap rate | High | Low |
This table makes it clear that the refractory brick is not merely a secondary improvement; it is the decisive factor that eliminates the defect. The lost foam casting process has many interacting variables, but in this case the thermal effect of the assembly jig was the dominant variable.
10. Extended Discussion of Cold Lap in Lost Foam Castings
Cold lap defects in lost foam castings are often attributed to insufficient pouring temperature, slow filling, or poor coating permeability. However, my investigation shows that local heat sinks within the mould assembly can be equally important. The assembly jig is an example of an unintended heat sink. It is not part of the casting, but it touches the runner and conducts heat out of the metal stream. In a cast metal mould or a permanent chill, this effect is intentional. In a lost foam casting mould, it is unintended and usually harmful.
The role of the assembly jig is especially important in lost foam castings because the filling time is relatively long compared to conventional sand casting. In a conventional sand mould, the metal flows quickly through well-defined channels and fills the cavity before much heat can be lost. In lost foam casting, the metal front must decompose the pattern progressively, so the flow is slower. The longer fill time means that there is more time for heat to be conducted through metallic fixtures. Even a moderate heat flux can remove a large amount of heat over a filling time of 30 to 60 seconds.
The heat loss through the assembly jig is similar to adding a chill to the runner. In this case, the chill is not located at a riser or a heavy section, but at the runner itself. The consequence is that all castings fed by that runner are affected. Castings near the sprue may receive enough fresh hot metal from the sprue to minimize the temperature drop. Castings farther from the sprue receive metal that has been cooling along the entire length of the runner. They are therefore more sensitive to the assembly jig chilling effect. This is why the defect was observed in castings located farther from the sprue.
In Process I, the lower screw hole defect was more likely in castings at the ends of the runner because the metal had already traveled through a long part of the horizontal runner before entering the inner gate. The metal had also lost heat to the runner coating, to the sand, and to the steel assembly jig along the entire path. By the time it reached the farthest pattern, it had lost a significant portion of its superheat. The additional heat loss caused by the jig was enough to push the metal front below the critical temperature for complete fusion.
In Process II, the refractory brick reduced the heat loss from the runner along its entire length. The metal entering all inner gates was hotter. This did not require a higher pouring temperature. Instead, it preserved the thermal energy that had already been supplied to the metal. The effect was distributed to all patterns in the cluster, including those farthest from the sprue.
I also considered whether the refractory brick could affect the mechanical stability of the assembly jig. The brick is placed between the jig and the runner, so the runner is no longer in direct contact with the steel support. The contact area between the runner and the brick may be smaller than the contact area between the runner and the steel jig. However, the brick is rigid and can bear the weight of the cluster without deformation. In my trials, I did not observe any instability or misalignment caused by the refractory brick. The brick maintained the runner at the correct height and did not interfere with the assembly of the sprue or inner gates.
Another consideration is the thickness of the refractory brick. A thicker brick provides greater thermal resistance, but it also raises the runner height and may make the assembly less stable. The brick thickness I used was sufficient to eliminate the defect. If the brick is too thin, its thermal resistance may not be large enough. If the brick is too thick, the gating system may no longer fit in the available flask depth. Therefore, the brick thickness should be chosen based on the specific rigging dimensions.
I also observed that the refractory brick helped to reduce the temperature loss during pouring interruptions. In lost foam casting, the pouring stream is sometimes interrupted accidentally or the pouring rate fluctuates. When the metal flow stops temporarily, the metal remaining in the runner continues to lose heat. The steel jig accelerates this heat loss. The refractory brick slows the cooling of the metal in the runner, giving the operator more flexibility to recover from minor pouring fluctuations. In Process I, the cold lap defects were more severe in castings poured at the end of the pour, when the pouring rate may have slowed. In Process II, the additional insulation made the process more forgiving.
11. Metallurgical Considerations for High Chromium Cast Iron
High chromium cast iron has a solidification behavior that is different from that of ordinary gray iron or ductile iron. The chromium content promotes the formation of chromium-rich carbides. In particular, the (Cr,Fe)7C3 type carbide is hard and provides excellent abrasive wear resistance. However, these carbides also have a low thermal conductivity and a high carbide fraction in the microstructure. The presence of massive carbides in the semisolid state reduces the feeding ability of the alloy.
During solidification, high chromium cast iron behaves like a long-freezing-range alloy in certain respects. The primary austenite dendrites form first, followed by the eutectic carbide network. When the temperature falls into the mushy zone, the dendrites obstruct the flow of the remaining liquid. If the metal front is forced to move through thin sections while dendrites are already present, the effective viscosity is very high. This can prevent complete filling and cause cold lap defects.
The relation between solidification and flow can be described by the fraction of solid \(f_s\). The viscosity of the partially solidified slurry increases rapidly as \(f_s\) increases. A simple empirical expression is
$$ \mu_{\mathrm{slurry}} = \mu_{\mathrm{liquid}} \left(1 – \frac{f_s}{f_{\mathrm{max}}}\right)^{-n} $$
where \(f_{\mathrm{max}}\) is the maximum solid fraction at which flow stops, and \(n\) is an empirical exponent. As the metal front cools, \(f_s\) increases, and the flow resistance increases dramatically. In the lower screw hole region, the thin wall cools quickly, so \(f_s\) can reach a high value before the two metal fronts meet. The result is a cold lap defect.
The refractory brick reduces the cooling rate and therefore reduces the rate at which \(f_s\) increases. This keeps the metal front in a more fluid condition for a longer period. The high chromium cast iron is then able to fill the thin section completely and fuse at the meeting point.
The microstructure of high chromium cast iron also plays a role in defect formation. If the metal front becomes partially solid before fusion, the resulting interface may contain carbides, oxides, and gas pores. These microstructural discontinuities are visible as a cold lap on the surface of the casting. Even if the part is not rejected by visual inspection, a cold lap can dramatically reduce the mechanical strength of the component. In a wear-resistant liner, a cold lap can lead to premature spalling or fracture under impact or abrasive loading. Therefore, eliminating cold lap defects is not only a cosmetic issue but also a reliability issue.
In my study, the (Cr,Fe)7C3 carbide type was present in the final microstructure of both Process I and Process II castings. The difference was not in the type of carbide but in the absence of surface discontinuities in Process II. This confirms that the alloy composition was correct for the wear application; the defect was caused by the thermal environment of the lost foam casting process rather than by the alloy itself.
12. The Role of Gases in the Filling Process
Another important factor in lost foam castings is the behavior of gases produced by the decomposition of the EPS pattern. When molten metal enters the cavity, the EPS pattern is heated rapidly. The polymer decomposes into a mixture of gases and condensed liquid products. These products must pass through the porous refractory coating and the dry sand before they can be evacuated by the vacuum system. If the gas production rate is higher than the evacuation rate, the gas pressure at the metal front increases. This pressure opposes the metal flow and makes the filling process slower and more turbulent.
The gas pressure at the metal front can be expressed in simplified form by Darcy’s law:
$$ Q_g = \frac{k_{\mathrm{perm}} A_g}{\mu_g} \frac{\Delta P_g}{L_g} $$
where \(Q_g\) is the volumetric gas flow rate, \(k_{\mathrm{perm}}\) is the permeability of the coating and sand, \(A_g\) is the cross-sectional area available for gas flow, \(\mu_g\) is the gas viscosity, \(\Delta P_g\) is the gas pressure difference, and \(L_g\) is the thickness of the permeable layer. In the screw hole region, the gas must escape through a relatively long and complex path. The local gas pressure can become high, especially if the coating is damaged or blocked by condensed decomposition products.
In Process I, the metal front was already cold when it reached the lower screw hole. The cold front had higher viscosity and could not easily overcome the gas pressure. The gas pressure also slowed the front, giving it more time to lose heat and to solidify. This created a self-reinforcing problem: lower temperature caused higher viscosity, higher viscosity caused slower filling, slower filling caused more heat loss, and more heat loss caused even lower temperature. Eventually, the front stopped moving, and a misrun formed.
In Process II, the higher metal front temperature reduced the viscosity and allowed the metal to push through the gas pressure more easily. The faster filling reduced the amount of time available for heat loss. This broke the vicious cycle and allowed the casting to fill completely. The refractory brick therefore did not directly reduce the gas pressure, but it gave the metal enough thermal energy to overcome that pressure.
This interaction between thermal and gas effects is common in lost foam castings. A defect that appears to be caused by gas entrapment may actually be caused by low metal temperature. Conversely, a defect that appears to be caused by cold metal may be aggravated by high gas pressure. The best solution is to address both factors. In this study, the refractory brick solved the thermal factor, while the existing vacuum and coating permeability were sufficient to handle the gas factor once the metal front was hot enough.
13. Recommendations for Production Control
Based on my results, I recommend several measures for the production of high chromium cast iron liners by the lost foam casting process.
The first recommendation is to place a refractory brick between the assembly jig and the horizontal runner whenever the runner is supported by steel. This applies not only to the central support but also to any secondary supports or clamps. Every steel contact point is a potential heat sink. A layer of refractory material should separate the metal-carrying gating system from the steel fixture.
The second recommendation is to monitor the pouring temperature and filling time carefully. The pouring temperature must be high enough to allow for the temperature losses caused by pattern decomposition, runner contact, and sand conduction. I used 1450 °C in this study, which was adequate when the refractory brick was present. If the pouring temperature drops during the pour, the insulating effect of the refractory brick becomes even more important.
The third recommendation is to avoid placing steel inserts or chill materials near thin wall sections of the casting. In the liner, the screw hole is a thin-wall feature. Any local chill in that area will increase the chance of cold lap. The assembly jig was not directly under the screw hole, but it chilled the runner and indirectly chilled the metal that eventually entered the screw hole. The effect was less direct but still severe.
The fourth recommendation is to design the gating system so that the metal path to thin walls is as short as possible. In the middle-gating system, the lower screw hole was far from the inner gate. A bottom-gating system might reduce the problem because the metal would enter the pattern at the bottom and fill upward. However, a bottom-gating system is more difficult to assemble in lost foam castings. If a middle-gating system must be used, the refractory brick is a simple way to compensate for the longer downward flow path.
The fifth recommendation is to ensure good coating permeability and adequate vacuum. The coating should be thick enough to prevent metal penetration but permeable enough to allow gas evacuation. The vacuum should be adjusted to give a firm mould without creating excessive pressure gradients. In my trials, the vacuum of 0.015 to 0.02 MPa was effective. I did not need to change the vacuum to eliminate the cold lap defects; the refractory brick alone was sufficient.
The sixth recommendation is to inspect the screw hole region for cold lap defects after every production trial. Cold lap defects can be hidden by surface grinding or shot blasting. A careful visual inspection and, if necessary, a liquid penetrant test should be performed on the first castings from a new process. This allows the process engineer to detect problems before mass production begins.
14. Conclusions
In this work, I studied the influence of the assembly jig on the cold lap and misrun defects of high chromium cast iron liners produced by the lost foam casting process. I compared two production processes: one with the runner placed directly on a steel assembly jig, and one with a refractory brick placed between the assembly jig and the runner. The following conclusions can be drawn from my investigation.
- The steel assembly jig used to support the horizontal runner acts as a metallic chill. It conducts heat away from the molten iron flowing through the runner and reduces the temperature of the metal front entering the pattern cavity.
- In the absence of an insulating layer, the metal front reaches the lower screw hole with insufficient superheat. The thin wall around the screw hole solidifies prematurely, and two advancing fronts fail to fuse. This produces cold lap defects, and in severe cases, misruns.
- Placing a refractory brick between the assembly jig and the runner introduces an additional thermal resistance that greatly reduces the heat loss. In a simple one-dimensional calculation, the heat flux through the refractory-brick assembly is only about 5% of the heat flux through the bare steel jig under ideal contact conditions.
- The refractory brick preserves the metal front temperature and improves the fluidity of the high chromium cast iron. This allows the molten metal to fill the lower thin section completely and to fuse properly.
- For high chromium cast iron liners with (Cr,Fe)7C3 carbides, the use of a refractory brick between the assembly jig and the runner effectively eliminates cold lap defects and misruns in lost foam castings.
- Metallic fixtures used in lost foam castings should always be considered in the thermal design. Any steel surface that touches the gating system should be separated from the molten metal by an insulating refractory material if possible.
- The lost foam casting process is highly sensitive to localized heat extraction. A small thermal change, such as the insertion of a refractory brick, can have a decisive effect on casting soundness.
In conclusion, my study demonstrates that the assembly jig is not merely a mechanical support in lost foam castings; it is also a thermal factor that must be controlled. The simple addition of a refractory brick solved a long-standing production problem without requiring major changes to the gating system or the alloy composition. This finding is directly applicable to other lost foam castings with complex thin sections, high alloy content, and metallic support fixtures. I recommend that foundry engineers inspect all supports and fixtures in the lost foam casting assembly for possible heat-sink effects. By insulating these supports with refractory bricks, it is possible to maintain a more stable metal front temperature and to produce high-quality high chromium cast iron liners free from cold lap and misrun defects.