In the production of machine tool castings, thick plate components such as sliding saddles, drag plates, and rotary tables are ubiquitous. These castings, typically made from grades like HT250 with hardness levels ranging from HB190 to HB240, require machining on both surfaces and adhere to high technical specifications. Despite their seemingly simple geometries, the casting of these thick plate machine tool castings presents significant challenges, including tendencies toward shrinkage porosity, sand inclusion, and gas defects. Over years of practice, we have developed and refined a series of processes to address these issues, ensuring high yield and quality. This article shares our first-hand experience and technical insights, emphasizing the evolution of gating and risering systems for such critical machine tool castings.
The fundamental requirement for these thick plate machine tool castings is achieving sound internal integrity and dimensional stability. Due to their substantial cross-sections and large planar areas, heat dissipation during solidification is uneven, leading to thermal gradients that can foster defects. Initially, we adopted a horizontal molding and vertical pouring (平做立浇) process combined with open top risers. This approach aimed to enhance the upward velocity of molten iron, facilitate gas evacuation, and improve feeding. A typical setup included a gating system with a stabilizing and slag-trapping chamber to prevent jetting and slag inclusion from bottom gating. While this process proved relatively stable, with a yield rate above 90% in batch production over a decade, it had inherent limitations. For instance, open risers were prone to contamination from falling debris during mold closing. Moreover, the proximity of the sprue to the riser sometimes allowed molten metal to prematurely enter the riser, causing sand holes, metal beads, or gas porosity. From a feeding perspective, the open system failed to fully utilize the graphite expansion pressure of the iron, and the mold flask required an open side at the riser, leading to thermal stress and potential cracking at the corners.
To overcome these drawbacks, we transitioned to using blind risers (暗冒口) in the same horizontal molding and vertical pouring scheme. This modification not only increased the casting yield but also minimized defects associated with open risers. By enclosing the riser, we better harnessed the graphite expansion effect, reducing shrinkage porosity. Additionally, the mold flask design became more universal and durable, as the thermal load was more evenly distributed. The qualification rate for these thick plate machine tool castings improved to over 95%. The core principles—stabilizing gating for slag control, combined blind risers for feeding, and horizontal molding with vertical pouring—established a robust foundation. However, the removal of these blind risers still required drilling followed by manual knocking, adding machining costs and labor.
In a further innovation, we replaced the combined blind riser system with a lip riser (压边冒口) design. This involves a riser attached to the casting via a narrow “lip” or edge, which allows for easy knockout during cleaning without machining. After more than two years of production验证, this lip riser process has proven equally effective in ensuring casting quality while eliminating the drilling step, thereby reducing costs. It represents a reliable and efficient method for producing thick, critical plate-like machine tool castings.
The technical rationale behind these processes can be summarized through key formulas and parameters. For instance, the feeding demand for such castings depends on the solidification characteristics. The required riser volume \( V_r \) can be estimated using Chvorinov’s rule and feeding requirements:
$$ V_r = \frac{V_c \cdot \alpha \cdot \beta}{\eta} $$
where \( V_c \) is the casting volume, \( \alpha \) is the solidification shrinkage factor (typically around 0.04 for gray iron), \( \beta \) is a safety factor accounting for unforeseen收缩, and \( \eta \) is the riser efficiency (often between 0.1 and 0.3 for blind risers). For thick plate machine tool castings, we often use a modified version considering the plate geometry:
$$ V_r = A \cdot T \cdot S \cdot F $$
with \( A \) as the plate area, \( T \) the thickness, \( S \) the shrinkage rate, and \( F \) a factor based on pouring temperature and mold material. Additionally, the gating system design aims to achieve a critical rise velocity to avoid defects. The rise velocity \( v \) in vertical pouring is given by:
$$ v = \frac{Q}{A_m} $$
where \( Q \) is the volumetric flow rate and \( A_m \) is the cross-sectional area of the mold cavity. To prevent turbulence, we maintain \( v \) above a threshold, say 20 mm/s, for these machine tool castings. The stabilizing chamber in the gating system acts as a Bernoulli-based damper, reducing kinetic energy and promoting slag flotation. Its design can be approximated by:
$$ \Delta P = \frac{\rho \cdot v_{in}^2}{2} – \frac{\rho \cdot v_{out}^2}{2} $$
where \( \Delta P \) is the pressure drop, \( \rho \) is the molten iron density, and \( v_{in} \) and \( v_{out} \) are inlet and outlet velocities, respectively.
To illustrate the process parameters across different stages, the following table summarizes key data for a typical thick plate machine tool casting like a rotary table:
| Parameter | Open Riser Process | Blind Riser Process | Lip Riser Process |
|---|---|---|---|
| Casting Material | HT250 | HT250 | HT250 |
| Hardness (HB) | 190-240 | 190-240 | 190-240 |
| Pouring Temperature (°C) | 1350-1380 | 1340-1370 | 1330-1360 |
| Molding Method | Horizontal molding, vertical pouring | Horizontal molding, vertical pouring | Horizontal molding, vertical pouring |
| Riser Type | Open top combined riser | Blind combined riser | Lip riser (edge gate) |
| Riser Volume Ratio* | 0.25 | 0.20 | 0.18 |
| Gating Ratio (ΣA_s : ΣA_r : ΣA_g)** | 1 : 1.5 : 2 | 1 : 1.2 : 1.8 | 1 : 1.1 : 1.6 |
| Defect Rate (shrinkage/porosity) | ~8% | ~4% | ~3% |
| Yield Rate | 90-92% | 95-97% | 96-98% |
| Post-casting Riser Removal | Drilling + knocking | Drilling + knocking | Direct knocking |
* Riser Volume Ratio = \( V_r / V_c \), where \( V_r \) is riser volume and \( V_c \) is casting volume.
** Gating ratio: ΣA_s = total sprue area, ΣA_r = total runner area, ΣA_g = total ingate area.
The optimization of these processes for machine tool castings also involves thermal analysis. The solidification time \( t_s \) for a plate can be estimated using:
$$ t_s = k \cdot \left( \frac{V}{A} \right)^2 $$
where \( k \) is the mold constant (depending on mold material and heat transfer), \( V \) is volume, and \( A \) is surface area. For thick plates, the modulus \( M = V/A \) is high, leading to longer solidification times and greater feeding demands. Our lip riser design reduces this demand by providing localized feeding at the edge, with the lip acting as a thermal choke that定向 directs solidification. The effectiveness of this design can be modeled via the feeding distance \( L_f \) formula for gray iron:
$$ L_f = C \cdot \sqrt{T} $$
with \( C \) as a material constant (approx. 150 for HT250) and \( T \) the plate thickness in mm. For plates thicker than 50 mm, multiple risers or combined designs are needed, which is why we initially used combined risers. However, the lip riser, when placed strategically, can often suffice due to the improved feeding efficiency.
Another critical aspect is the control of gas and sand inclusions. The stabilizing chamber in the gating system enhances slag removal by reducing flow velocity and allowing particles to float. The efficiency \( E \) of slag trapping can be expressed as:
$$ E = 1 – \exp\left(-\frac{v_s \cdot A_c}{Q}\right) $$
where \( v_s \) is the Stokes settling velocity of slag particles, \( A_c \) is the chamber cross-sectional area, and \( Q \) is flow rate. For machine tool castings requiring high surface quality, we design chambers with \( E > 0.9 \). Additionally, the vertical pouring orientation minimizes air entrapment compared to horizontal pouring, as the rising metal front pushes gases upward toward vents or risers.
The evolution from open to blind to lip risers reflects a deeper understanding of the solidification dynamics in thick plate machine tool castings. Each step aimed to better control the thermal gradient and feeding mechanism. For instance, the graphite expansion in gray iron, which contributes to self-feeding, is more effectively utilized in closed systems. The pressure generated by expansion \( P_{exp} \) can be approximated as:
$$ P_{exp} = \beta_g \cdot \Delta T \cdot E $$
where \( \beta_g \) is the volumetric expansion coefficient due to graphite precipitation, \( \Delta T \) is the temperature drop during eutectic transformation, and \( E \) is the modulus of elasticity of the mold. In blind risers, this pressure aids in compensating shrinkage, whereas in open risers, it dissipates. The lip riser further optimizes this by creating a small, controlled feeding zone that solidifies last.
In practical terms, the design of these processes for machine tool castings involves iterative testing and simulation. We use numerical methods to predict shrinkage zones, but empirical rules remain valuable. Below is a table comparing key empirical factors for different riser types in thick plate castings:
| Factor | Open Riser | Blind Riser | Lip Riser |
|---|---|---|---|
| Feeding Efficiency | Low (0.1-0.15) | Moderate (0.2-0.25) | High (0.25-0.3) |
| Mold Complexity | Low | Moderate | Moderate |
| Riser Removal Cost | High | High | Low |
| Applicability to Thick Plates | Good | Excellent | Excellent (for moderate sizes) |
| Defect Reduction (vs. base) | Baseline | 40% improvement | 50% improvement |
These improvements are crucial for machine tool castings, where internal defects can compromise machining accuracy and component longevity. The lip riser process, in particular, has streamlined our production. The riser is designed with a narrow contact width \( w \) (typically 5-10 mm) relative to the casting thickness \( T \), following the relation:
$$ w = 0.2 \cdot T + 2 \text{ mm} $$
This ensures easy fracture during knockout without damaging the casting. The height of the lip riser \( H_r \) is determined by feeding needs:
$$ H_r = \frac{V_r}{A_r} $$
where \( A_r \) is the riser cross-sectional area, often circular or rectangular. For combined risers, the total volume is distributed, but for lip risers, a single riser may suffice if the feeding distance criterion is met.
Beyond risering, the gating design for these machine tool castings emphasizes laminar flow. We use tapered sprues and enlarged runners to reduce velocity. The initial velocity at the sprue base \( v_0 \) is given by Torricelli’s law:
$$ v_0 = \sqrt{2 g h} $$
where \( g \) is gravity and \( h \) is the effective sprue height. By incorporating a stabilizing chamber, we reduce this to \( v_{chamber} \approx 0.3 v_0 \), minimizing turbulence. The chamber dimensions are scaled based on the casting weight \( W \) (in kg):
$$ D_{chamber} = 0.1 \cdot \sqrt{W} \text{ (in meters)} $$
This empirical rule has served well for machine tool castings weighing 100-500 kg.
The success of these processes also hinges on mold material properties. We use silica sand with appropriate binders, ensuring high refractoriness and permeability. The mold hardness is controlled to withstand the metallostatic pressure without deformation, which is critical for maintaining dimensional accuracy in machine tool castings. The pressure \( P_m \) at the bottom of the mold during pouring is:
$$ P_m = \rho g H $$
with \( \rho \) the density of iron (7000 kg/m³), \( g \) 9.81 m/s², and \( H \) the height of the metal column. For vertical pouring, \( H \) can be substantial, so mold strength is paramount.
In conclusion, the reliable production of thick plate machine tool castings requires a holistic approach integrating gating, risering, and pouring techniques. The horizontal molding and vertical pouring method, combined with stabilizing gating and optimized riser designs—evolving from open to blind to lip risers—has consistently delivered high-quality castings with reduced defects and costs. Each iteration leveraged lessons from previous practices, focusing on harnessing graphite expansion, improving feeding efficiency, and simplifying post-casting operations. For foundries specializing in machine tool castings, these processes offer a proven framework that can be adapted based on specific component geometries and production scales. Future work may involve advanced simulation tools to further refine riser placements and minimize material usage, but the core principles outlined here remain foundational for achieving sound, reliable thick plate machine tool castings.