Answers to questions related to wax mold manufacturing in precision casting?

Different industries have different requirements for precision castings. To meet these needs, precision casting technology demonstrates its wide adaptability, such as in the diversity of materials, complexity of shapes, batch production, dimensional accuracy, uniformity, stability, and surface precision. There have been new advancements and improvements in these characteristics today. For example, a precision casting made of stainless steel weighing 40g, with outer dimensions of 55mm in length, 3216mm in width, and 13mm in thickness, has the following requirements:

1. Direct use in cast state, no machining required;

2. The roundness and surface roughness of the shaft hole on the casting must meet the specifications of Japanese Industrial Standards (JIS);

3. The four holes around the shaft hole must fit with another component, and in addition to meeting JIS specifications, the casting must not deform or warp;

4. The minimum dimensional tolerance of the casting is ±0.12mm. 5. The depth of cast text, symbols, etc. is 0.13mm, requiring clear lines;

5. The minimum dimensional tolerance between planes is 0.11mm.

6. For features such as rough edges, rounded corners, surface roughness, shrinkage, and deformation that are not specifically designated, they will be executed according to production unit standards. As an example of precision castings, one can feel the current industrial production requirements for precision castings and the level that producers can achieve. From the modern lost foam casting process, it is known that meeting the above requirements is not an easy task. Therefore, if strict management, correct operation, and coordination are not implemented at every process, link, and position, it will be very difficult to achieve the above goals. This article will only provide a brief discussion on issues related to wax mold manufacturing.

 

　　

1. Wax Material

 

　　Wax material is the first step in precision casting production. As is well known, due to the development of casting production, the requirements for wax materials are also diverse. The simplest method for producers is to select from commercially available wax materials, or they can formulate their own, or commission a professional factory to produce according to requirements. However, regardless of the type of wax material, the required properties are the same, namely: melting point, solidification point, softening point, thermal conductivity, fluidity, shrinkage, sinkage, strength, hardness, toughness, release properties, coating properties, recyclability, dimensional stability, and the amount of residual ash after high-temperature burning. Based on specific casting production, a type of wax material is selected, requiring that the wax material has certain characteristics, with priorities established; at the same time, cost considerations are also an important factor. To meet the specific requirements of various castings, wax materials are divided into liquid, semi-liquid, semi-solid, and solid; those with fillers and those without; water-soluble and non-water-soluble; recyclable and non-recyclable; for wax molds, pouring systems, bonding, repair, sealing, and resin-based wax materials, etc. Just from the perspective of wax materials for wax molds, there are many varieties, and they continue to develop. Wax materials with fillers (FILLER WAX) are widely used, with common fillers being powdered polyethylene, polystyrene, organic acids, fatty acids, and starch, etc. The addition amount is 30% to 45% of the total. These types of wax materials have good thermal insulation properties, meaning they can form at lower temperatures, have good fluidity, and can clearly reproduce fine details of the wax mold. For example, three types of wax materials with added heat-hardening styrene, isophthalic acid, and a product named P2 FILLER took 59 seconds, 62 seconds, and 99 seconds, respectively, to drop from 104°C to 49°C, indicating that the type of filler has a significant impact on the performance of the wax material. The shrinkage rate of these wax materials is more than 5% smaller than that of non-filled wax materials. A closely related issue to the performance of wax materials is how the preparation, use, and handling methods of the wax material affect its performance.

 

　　

1. Prevent overheating of wax material during melting

 

　　(1) Use an indirect heating oil bath furnace and force the circulation of thermal oil, controlling the melting temperature within the range of 84 to 98°C, and keep it in a stirring state. The purpose, in addition to ensuring uniform temperature, is to prevent sedimentation for wax materials containing fillers. Excessive temperature and local overheating can degrade the performance of the wax material due to oxidation, carbonization, and other issues. Signs of overheating include changes in the color of the wax, black spots on the inner wall of the wax container, semi-transparency due to the separation of fillers, and the presence of smoke or vapors in the wax container.

 

　　(2) When reusing wax, to remove impurities and moisture, it must be heated above the melting point of the wax, usually controlled below 120°C. This ensures that the wax material has a certain flow capability to remove impurities, and the moisture contained will also evaporate due to the stirring of the mixer.

 

　　(3) For wax rods used under insulation in a heating cylinder, the heating temperature only needs to be 2 to 3°C higher than the injection temperature.

 

　　(4) When wax material is removed in a high-pressure kettle (pressure tank), the vapor temperature can exceed 150°C. This condition is a major factor causing the degradation of wax material performance, so efforts should be made to lower the dewaxing temperature and pressure to maintain the performance of the wax material.

 

　　2. Maintain ambient temperature

 

　　In the wax mold manufacturing room, whether in winter or summer, the indoor temperature should be maintained between 20 to 25°C, keeping the dimensions of the wax mold in a stable environment, while coordinating with the environmental conditions of the slurry coating shell process to ensure the quality of the shell.

 

　　(1) Heating and insulation of wax material

 

　　The wax material enters the mold under pressure from the wax barrel and cools and solidifies in the mold. During this process, the temperature difference is the main factor affecting the shape, size, and stability of the wax mold. Therefore, when using the wax barrel to keep the wax material warm during wax pressing, the uniformity of the wax barrel temperature is extremely important. Due to the poor thermal conductivity of the wax material, it is difficult to achieve uniformity using the heat from the wax barrel. An uneven temperature in the wax barrel will not produce a sound wax mold. To address this, a heating and insulation tank is usually used to heat the wax barrel, which is typically set between 58 and 62 °C, slightly higher than the injection temperature. It must be heated uniformly for at least 8 hours before use. The insulation tank is an oil bath heating device, where the heat transfer oil circulates under the pressure of an oil pump, heating the wax barrel to a uniform temperature. The temperature controller of the insulation tank can be set to the required temperature as needed. When the wax barrel is uneven in temperature during wax pressing, defects such as incomplete filling, cold separation, granular surfaces, flow marks, and mesh patterns may occur in the wax mold.

 

　　(2) Wax Mold Forming Methods

 

　　Different forming methods are generated based on the size, shape, and technical requirements of the castings. Different forming methods are matched with different wax materials. From the perspective of wax pressing dynamics, there are manual, electric, pneumatic, and hydraulic options; from the pressure perspective, there are low pressure, medium pressure, and high pressure; from the state of the wax material, there are liquid, semi-liquid, semi-solid, and solid; from the mold type, there are metal molds, rubber molds, plaster molds, etc.; from the casting perspective, there are industrial products, artworks, and so on.

 

　　

II. Wax Pressing Forming Machine (Wax Press Machine)

 

　　In the wax pressing machine, the wax material is pressed into a wax mold, meaning that the properties of the wax mold are largely constrained by the parameters of the wax pressing machine, which directly relates to the design, structure, and performance of the wax pressing machine. For example, for a vertical high-pressure semi-solid wax pressing machine, before performing the wax pressing operation, it is necessary to check the wax material temperature, injection pressure, injection time, cooling time, and mold temperature. Then, operations are carried out according to the specifications of the process card. The performance of the wax pressing forming machine is mainly reflected in the process parameters it can adjust, including manual or automatic operation; wax barrel temperature adjustment, injection port temperature adjustment; mold clamping force adjustment, mold clamping stroke adjustment; injection pressure adjustment, injection speed adjustment, injection time adjustment; return speed adjustment, etc. For example, for some castings, each end of the wax mold has two hole rings. Due to the weak connection between the hole rings and the main body, micro-cracks visible to the naked eye can easily occur at the connection. Later, by adjusting the injection speed and holding time, this defect was eliminated. Therefore, it can be seen that the adjustable process parameters of the wax pressing machine, their adjustment range, adjustment methods, and correct control are extremely important for manufacturing qualified wax molds. The central part of the wax pressing machine is the wax cylinder. Its size is <100mm ×400mm, used to place wax rods. Fully preheated wax rods are placed in the wax cylinder, which is electrically heated, maintaining the temperature at the injection temperature. When the injection port of the mold is placed at the injection port of the wax cylinder (the injection port has an independent heating and insulation system), the oil pump can be started to drive the upper piston downwards, pressing the wax material into the mold; after maintaining pressure for a certain time, the mold is opened, and the wax mold is removed. The main parameters of the entire process are: ① The temperature of the semi-solid wax material is controlled slightly above the injection temperature, usually taken as 55-60 °C. To ensure uniform temperature in the wax cylinder, it must stay in the insulation tank for more than 8 hours; ② The injection pressure is adjustable, usually between 214-218MPa; ③ The clamping force given by the wax pressing machine to the mold is adjustable, usually between 310-315MPa; ④ The injection speed is implemented through injection time, which varies depending on the shape of the wax mold and is closely related to the shrinkage of the wax mold, thus affecting the quality of the wax mold.

 

　　

III. Wax Mold Defects

 

　　Due to various reasons, the wax molds produced by the wax pressing forming machine may have some defects, mainly including the following types: 1. Incomplete Filling

 

　　The main reason is that the temperature of the wax material is too low, the injection speed is slow, and the mold temperature is low, causing the wax material to cool quickly during the flow process. This is reflected in the corners and edges or the thin-walled parts of the wax mold not being filled completely, with rounded corners appearing, similar to insufficient pouring in metal castings.

 

　　2. Bubbles

 

　　One type is surface bubbles, which are subcutaneous bubbles visible to the naked eye, with individual distribution or dense distribution; the other type is deep bubbles in the wax mold, usually larger bubbles that are concentrated and not visible to the naked eye, but cause local bulging in the wax mold, appearing in the central part and the last cooling part of the wax mold. This is a result of the release of internal pressure and gas expansion.

 

　　3. Surface Wrinkles

 

　　Due to insufficient temperature of the wax material, low injection speed, improper coordination between the movement of the wax material and the mold temperature; or due to damage or uncleanliness of the inner surface of the mold; or improper placement of the cold metal block, leaving traces of movement on the surface of the wax mold. Deeper wrinkles are similar to the cold separation defects of metal castings; slight wrinkles can be polished away. Additionally, seams may sometimes be visible around the core and holes, slightly recessed, which is actually the result of two streams of wax failing to fuse well at the junction, caused by insufficient temperature of the wax material and inadequate pressure.

 

　　4. Surface Roughness

 

　　Due to insufficient injection pressure or inadequate injection speed, the contact density between the wax material and the inner surface of the mold is insufficient, failing to accurately reflect the surface state of the mold, with severe cases also accompanied by wrinkles.

 

　　5. Shrinkage

 

　　The surface of the wax mold is recessed, mainly due to insufficient injection pressure and time, or the wax material temperature being too high, insufficient cooling time, and sometimes excessive release agent may also cause a decline in the properties of the wax material. Surface shrinkage involves a larger area and is more difficult to repair.

 

　　6. Deformation

 

　　6、变形

 

　　After the wax mold is removed from the die, in addition to a reduction in size, deformation may occur due to improper handling during removal; it is common for the mold to warp due to changes in the temperature field while the wax mold is still cooling. Therefore, the wax mold just removed from the die should be carefully placed, usually laid flat on a larger surface, and if necessary, a metal block shaped to fit the deformed area should be embedded in the wax mold to prevent deformation, allowing it to cool fully. It may also be due to the wax material being too soft, or the components of the die being unsuitable, etc.

 

　　7. Flash (Seam)

 

　　This is the most common defect, which occurs at the joint of the die, at the connection between the core and the core seat, where very thin wax sheets escape. The main causes are insufficient die precision, damage or contamination at the die parting surface or internal component joints, insufficient clamping force, or excessive injection pressure. Alternatively, the wax temperature may be too high. The flash must be completely removed before the wax mold can be used.

 

　　8. Adhesion

 

　　Adhesion of the wax mold to the die is caused by the absence of a release agent, or both the wax and the die being too hot, or damage to the die, or uncleanliness of the die.

 

　　IV. Conclusion

 

　　The manufacturing of wax molds, along with subsequent assembly, shell making, wax removal, pouring, and finishing, emphasizes the importance of operational processes. The coordination between each link must adhere to the process cards and relevant technical management documents for each procedure. Additionally, these documents should be continuously improved to meet the needs of normal production. Staff at each position must have a thorough understanding of the contents and requirements in the relevant process cards and execute them effectively; otherwise, despite having various technical documents, the results will be meaningless. There should be clear guidelines for defect prevention, material utilization, equipment maintenance, tool and fixture manufacturing and inspection, waste analysis, repairs, and scrapping, with designated responsibilities, ensuring that the process system is effectively implemented throughout the entire production activity.