Wax pattern (also known as "investment pattern") production is the first and most important process in manufacturing, and is the primary condition for obtaining high-quality castings. Defects in wax patterns waste labor costs, production time, and valuable resources; unqualified wax patterns entering the shell-making process, or even the pouring process, result in defective products. This increases factory production costs and delivery times, affecting the company's reputation and economic benefits. Therefore, attention should be paid to and solutions found for wax pattern defects.
Currently, low-temperature wax-based molding materials or medium-temperature wax materials are widely used for pressing wax patterns. Generally, wax patterns have defects such as pores, shrinkage cavities, and cracks.
I. Pores
1. Defect Characteristics
Smooth-surfaced cavities are present on the local surface of the wax pattern.
2. Causes
(1) Excessive gas is entrained during wax material preparation, and insufficient degassing treatment is performed.
In production, the preparation of low-temperature wax materials usually involves four processes: wax melting, wax flaking, wax paste mixing, and degassing. A spiral blade mixer is often used to mix the wax material (or "wax paste"). During the mixing process, a large amount of gas is inevitably entrained; and the more the mixer rotates, the more gas is entrained. Especially if the wax material is not sufficiently degassed after mixing, a large amount of entrained gas remains in the wax material.
(2) Improper location of the wax injection port, leading to gas entrainment during the wax injection process.
Improper placement of the wax injection port causes turbulence during the injection of wax into the mold cavity, leading to excessive gas entrainment.
(3) Improper mold design, hindering cavity venting.
The distance the wax material travels to fill the mold cavity is too long, and the cavity venting is poor.
3. Preventive Measures
(1) Strictly adhere to the wax material preparation process and operating procedures.
When preparing paraffin-stearic acid wax material, pay attention to the order of adding materials, i.e., add stearic acid first, and after it is completely melted, add paraffin or recycled wax, heating to a fully molten state, with a temperature ≤90℃. When starting the mixing process, the mixer speed can be higher (e.g., 400 r/min). After the wax temperature rises, the mixer speed can be reduced (200-300 r/min). After the wax is properly mixed, a degassing treatment should be performed for at least 0.5 hours to allow the gas trapped in the wax to fully escape.
(2) Improve the position of the wax injection port to avoid turbulence in the mold cavity.
The wax injection port should preferably be located on the inner gate or on a surface with machining allowance, and its size should match the injection nozzle of the wax injection machine; the injection port should ensure that the wax fills the mold cavity smoothly and over the shortest distance, without generating turbulence.
(3) Improve the mold design to facilitate the removal of gas from the mold cavity.
Improving the mold design facilitates the removal of gas from the mold cavity; if necessary, add vent holes.
II. Shrinkage
1. Defect Characteristics
Depressions caused by localized shrinkage appear in the thickest parts of the wax pattern.
2. Causes
(1) Insufficient feeding in the thickest parts (depressions) of the wax pattern.
The injection pressure of the wax pattern is one of the main parameters of the molding process. Low injection pressure leads to a larger shrinkage rate in the thickest parts (depressions) of the wax pattern. When sufficient feeding is not achieved, shrinkage occurs in these areas.
Holding pressure time is another process parameter of the molding process. After the wax fills the mold cavity, insufficient holding pressure time increases the shrinkage rate in the thickest parts (depressions) of the wax pattern. When sufficient feeding is not achieved, shrinkage occurs in these areas.
(2) Insufficient wax injection volume and insufficient feeding.
If the wax injection port is small, or improperly positioned, or the injection pressure is low, the amount of wax injected into the mold cavity is insufficient. When the thickest parts (depressions) of the wax pattern do not receive sufficient feeding, shrinkage occurs in these areas.
(3) Excessive wall thickness difference in the wax pattern, which is unfavorable for feeding.
The structural design of the wax pattern is unreasonable, with excessive wall thickness differences, which is unfavorable for feeding.
(4) The wax temperature is too high during injection, or the mold temperature is too high, or both are too high. High wax temperature, high molding temperature, or both, increase the shrinkage rate of the wax material; when sufficient compensation for shrinkage is not achieved, shrinkage cavities occur.
(5) The shrinkage rate of the wax material is relatively large.
In production, paraffin-stearic acid wax is used, usually 58-62°C (i.e., No. 58-62) white paraffin and first-grade triple-pressed stearic acid, each accounting for 50% when newly prepared. The shrinkage rate of No. 58 white paraffin is greater than that of No. 62 white paraffin. Paraffin-stearic acid wax has a higher shrinkage rate than paraffin-low molecular weight polyethylene wax.
3. Preventive Measures
(1) Increase the shrinkage compensation ability of the wax material.
When pressing wax molds, the injection pressure is generally selected to be 0.2-0.5 MPa. Appropriately increasing the injection pressure not only increases the density of the wax material in the mold but also reduces the shrinkage rate of the wax material. Appropriately increasing the holding pressure time, generally (3-10) s or longer, can also reduce the shrinkage rate of the wax material and increase the shrinkage compensation ability of the wax material.
(2) Increase the amount of wax injected to increase the shrinkage compensation ability.
Appropriately increase the cross-section of the wax injection hole to increase the amount of wax injected per unit time; or change the position of the wax injection hole to ensure sufficient shrinkage compensation in the thicker parts of the wax mold.
(3) Improve the wax mold structure to make the wall thickness as uniform as possible.
Where possible, make the wall thickness of the wax mold uniform; if necessary, place pre-made cold wax blocks (also called "wax cores") in the thicker parts of the wax mold before injecting the wax material to form the wax mold. Cold wax blocks can be placed locally or entirely depending on the size and shape of the wax mold. The cold wax blocks should be positioned in the mold using conical protrusions. The height of the protrusions should be determined according to the size of the part, generally controlled at 2-3 mm. The wax material used for the cold wax blocks should be the same as the wax material of the wax mold.
(4) Control the temperature of the wax material and the mold during wax injection. When injecting wax, for low-temperature waxes, a wax temperature of T=48–52℃ and a mold temperature of T=18–25℃ are generally suitable; for medium-temperature waxes, a wax temperature of T=52–60℃ and a mold temperature of T=20–24℃ are generally suitable.
(5) Select waxes with a smaller shrinkage rate.
For example, using 62℃ white paraffin wax instead of 58℃ white paraffin wax reduces the shrinkage rate. Increasing the paraffin content in the wax by 5–10%, i.e., to 55–60%, can also reduce the shrinkage rate. Alternatively, using a wax blend of 60℃ white paraffin wax (95% content) and low-molecular-weight polyethylene (5% content) results in a shrinkage rate of approximately 1%, which is relatively low.
III. Cracks
1. Defect Characteristics
Localized cracks in the wax mold often appear on the parting line of the wax mold.
2. Causes
(1) High shrinkage rate and poor plasticity of the wax material.
When using 58℃ refined white wax or white paraffin wax mixed with first-grade triple-pressed stearic acid at a 50% ratio each, the free shrinkage rate of the wax mold is approximately 2%. When the stearic acid content in the wax is greater than 80%, its toughness and plasticity are poor, easily causing cracks in the wax mold.
(2) The temperature of the mold, or the molding room, or the cooling water is too low.
A mold temperature that is too low, or a molding room temperature that is too low, or a cooling water temperature that is too low, leads to excessively rapid cooling of the wax mold. When the shrinkage of the wax mold is hindered, cracks are easily produced; or if the cooling time of the wax mold in the mold is too long (i.e., the demolding time is too long), hindering the shrinkage of the wax mold, cracks are easily produced.
(3) Unreasonable wax mold structure/mold design.
An unreasonable wax mold structure/mold design, uneven wall thickness of the wax mold, and too small or sharp corners in the transition areas between thick and thin sections, can lead to cracks in the weak points when the shrinkage of the wax mold is hindered.
(4) Improper operation.
Improper demolding methods, or extracting the core pin too late, can cause cracks in the wax mold.
3. Preventive Measures
(1) Select waxes with lower shrinkage rates.
For low-temperature waxes, using 62℃ paraffin wax instead of 58℃ paraffin wax can reduce shrinkage. Alternatively, increasing the paraffin content in the wax by 5-10%, i.e., to 55-60%, can also reduce shrinkage. Or, using a wax blend of 60℃ paraffin wax (95% content) and low-molecular-weight polyethylene (5% content) results in a lower shrinkage rate of approximately 1%.
(2) Control the temperature of the molding, mold-making room, and cooling water.
For low-temperature waxes, the molding temperature should be controlled at 18-25℃, and for medium-temperature waxes, the molding temperature should be controlled at 20-24℃. The temperature of the mold-making room should be the same as the molding temperature, and the cooling water temperature should also be the same as the room temperature; if necessary, air conditioning should be installed in the mold-making room, and the temperature should be controlled at 20℃.
(3) Improve wax mold design.
Improve the wax mold design to ensure uniform wall thickness; if the wall thickness is uneven, use rounded corners (the radius of the rounded corner should be "1/5 to 1/3" of the sum of the two adjacent wall thicknesses); if necessary, add reinforcing ribs to reduce the stress during wax mold shrinkage to a level that prevents cracking.
(4) Strictly follow operating procedures.
Improve the demolding method, and if necessary, add a demolding device to prevent cracks in the wax mold during the demolding process.
Strictly control the wax mold cooling time, generally 10-60 minutes. After the wax mold cools, the core pin that cools with the wax mold should be removed promptly so that it does not hinder the shrinkage of the wax mold.
