The parting line is the contact surface between the mold assembly components. Typically, there is only one parting line, as in two-part molding; sometimes, multiple-part molding is required, which is a molding process using more than two parting lines. The rational selection of the parting line is directly related to simplifying the casting process, improving production efficiency, reducing costs, and enhancing casting quality. The chosen parting line should, as much as possible, align with the pouring position to avoid flipping the mold after closing, thereby reducing workload and preventing damage to the mold. Additionally, the selection of the parting line generally follows the principles outlined below:
(1) The entire casting or its main part should be placed in the same mold half.
The parting line is established to facilitate pattern removal and can adversely affect the dimensional accuracy of the casting. Firstly, it can lead to mismatch in the casting, caused by alignment errors during mold closing. Secondly, imperfect mold closing can increase the casting's dimensions in the direction perpendicular to the parting line. Research indicates that a closed mold always retains a certain "thickness" at the parting line, which is approximately 0.38 mm at a minimum. This parting thickness increases the dimensional deviation of the casting. Therefore, the entire casting or its main part should, as much as possible, be located in the same mold half. Critical dimensions on the casting should, as far as possible, not be intersected by the parting line. The machining locating surfaces and main machined surfaces of the casting should preferably be placed in the same mold box to minimize dimensional deviations during machining.
(2) The number of parting lines should be minimized.
Fewer parting lines make it easier to ensure casting dimensional accuracy and require fewer mold boxes. In machine molding, small parts generally have only one parting line to maximize the productivity of the molding machine. Sections from which the pattern cannot be withdrawn should utilize sand cores, rather than employing loose pieces or multiple parting lines. Although the overarching principle is to minimize the number of parting lines, using multiple parting lines can sometimes be advantageous in specific situations. For instance, using two parting lines for some castings can be reasonable for manual molding in single-part production, as it eliminates the need for manufacturing a core box, thereby reducing costs and shortening the production cycle.
(3) The parting line should be a plane whenever possible.
A flat parting line simplifies molding and the construction and manufacturing of pattern plates, making it easier to ensure casting dimensional accuracy. However, in some cases, a contoured parting line is necessary. When the shape of a machine-molded casting requires a contoured parting line, regular surfaces such as cylindrical or folded surfaces should be chosen whenever possible to facilitate the machining of the pattern match plate and ensure the precise consistency of the surface curvature between the upper and lower pattern plates for a tight mold close.
(4) Use as few sand cores as possible, and facilitate core setting, mold closing, and cavity dimension inspection.
Using fewer cores and placing the main cores, or the majority of cores, on the same mold parting surface whenever possible is beneficial for core setting, mold closing, and cavity dimension inspection. In manual molding, where pattern and core box dimensional accuracy may not be high, molders need to inspect cavity dimensions and adjust core positions during core setting and mold closing to ensure casting dimensions.
(5) Choose the maximum cross-section of the casting whenever possible to avoid excessively high mold boxes.
To facilitate pattern withdrawal, the parting line should be selected at the casting's maximum cross-section, but care should be taken to avoid making the mold box too deep. Almost all molding machines have limitations on mold box depth. For taller castings, it is advisable not to have the casting too deep within a single box; the mold box depth should be kept within the maximum allowable range of the molding equipment. Molding with deep boxes is difficult, making sand filling, compaction, pattern drawing, and core setting inconvenient. In manual molding for large castings, multiple parting lines are often used-meaning multi-part molding-to control the depth of individual mold boxes and prevent them from becoming too deep.
(6) The selection of the parting line for load-bearing parts should not weaken the structural strength of the casting.
Example: A contoured parting line scheme is unreasonable because even a slight misalignment during mold closing would alter the cross-sectional area distribution of an I-beam, potentially weakening the strength on one side. In contrast, a flat parting line scheme does not have the drawback of weakening one side due to mold mismatch.
(7) Pay attention to reducing fettling and machining work.
Based on considerations for molding, mold closing, and core making, the choice of parting line should also facilitate fettling (cleaning) and minimize the amount of required machining.
The above briefly introduces some fundamental principles for selecting a parting line. These principles can sometimes contradict or constrain each other and must be applied flexibly based on the specific characteristics of the part and production conditions. The parting line significantly influences the casting's dimensional accuracy, cost, and production efficiency. Careful analysis and comparison are necessary, leading to a rational choice based on technical requirements and production conditions. Determining which principles should take precedence for a given casting requires comparing multiple options, combining experience for correct judgment. Sometimes, certain tests or trial production runs are necessary to finalize the most suitable scheme.
The selection of the parting line is closely related to the pouring position. Typically, the pouring position is determined first, followed by the choice of the parting scheme. For specific castings, however, the choice of parting line may conflict with the initially determined pouring position. In such cases, after analyzing various parting line schemes, the pouring position may need to be readjusted; both the pouring position and the parting line should be determined concurrently. A rational casting process plan must comprehensively consider the casting's structural characteristics, available process equipment, casting quality requirements, production efficiency, and cost, integrating the selection of both the pouring position and the parting line.
