Injection molding layout is a key step in mold design, determining the number and arrangement of cavities. It directly impacts mold size, molding efficiency, and consistent part quality. Scientifically and rationally planning maximizes mold space utilization, reduces production costs, and facilitates mold manufacturing and maintenance while ensuring part precision. Planning must adhere to the four principles of balance, economy, process adaptability, and maintainability. It comprehensively considers factors such as part structure, production batch size, and injection molding machine parameters to determine the optimal cavity layout.
The principle of balance is at the core of injection molding layout. It requires consistent filling conditions, cooling, and demolding across all cavities to ensure uniform part size and performance. When arranging cavities, the distance from each cavity to the gate must be equal, and the runner length and cross-sectional dimensions must be consistent, ensuring that the melt fills each cavity simultaneously under the same pressure and velocity. For example, for circular parts, a uniform circumferential distribution is employed, with the center-to-center distance between adjacent cavities controlled to within ±0.1mm. For rectangular parts, a matrix arrangement is employed, ensuring symmetrical distribution of cavities within each row and column. Furthermore, cooling channels must be symmetrically arranged around each cavity, with consistent channel diameter and spacing to ensure a temperature difference of no more than 3°C between cavities, preventing differential shrinkage due to uneven cooling. Furthermore, the ejector mechanism must be properly positioned for each cavity, with the same ejection point location and number, and synchronized ejection speeds, to prevent deformation of the part due to uneven force.
The principle of economy dictates that, while still meeting production needs, mold manufacturing costs and energy consumption should be reduced through optimized mold placement. The number of cavities should be determined based on the injection molding machine’s clamping force, shot volume, and mold plate dimensions to avoid wasting equipment resources. For example, for an injection molding machine with a clamping force of 500 tons, an appropriate number of cavities (e.g., 8) can be calculated based on part weight (e.g., 50g per part) and runner weight (approximately 50g). This allows the total shot volume to be controlled within 70%-80% of the machine’s maximum shot volume (to ensure filling pressure and avoid material waste). A compact layout should be employed to minimize the mold’s length and width. For example, eight cavities could be arranged in a 4×2 matrix, with a center-to-center distance set at 1.5-2 times the maximum part dimension. This would keep the mold plate dimensions within 600×500mm, reducing steel consumption and processing costs. At the same time, the volume of the runner system needs to be minimized, and a balanced runner design should be adopted to reduce waste generation. For example, the use of a hot runner system can eliminate the cold runner, thereby increasing the raw material utilization rate by 20%-30%.
The principle of process adaptability requires that the mold layout be matched to the injection molding process parameters to ensure a stable and controllable molding process. For large parts or plastics with poor flowability (such as PC and POM), a single-cavity or small-cavity layout is recommended to shorten the runner length and reduce melt flow resistance. For small parts or plastics with good flowability (such as PE and PP), a multi-cavity layout (such as 16-cavity or 32-cavity) can be used to improve production efficiency. When parts incorporate complex structures such as side core pulls and inclined ejectors, sufficient space must be reserved for the installation of the relevant mechanisms. For example, side core pulls must be installed 50-100mm from the side of the cavity to avoid interference with adjacent cavity mechanisms. Furthermore, the layout of the exhaust system must be considered during mold layout. In multi-cavity molds, the exhaust slots of each cavity should be symmetrically distributed on the same side of the parting surface to facilitate centralized exhaust and subsequent cleaning, ensuring smooth exhaust of gases within the cavity and preventing defects such as bubbles and burns in the plastic part.
Maintainability requires that the layout plan facilitate mold processing, assembly, and subsequent maintenance, reducing maintenance costs during production. Cavities should be arranged to avoid irregular shapes and adopt a regular geometric layout (such as circular or rectangular). This allows for the use of standardized tooling and fixtures, improving processing efficiency and precision. For example, circular cavities can be machined in a single clamping cycle using an indexing head, while matrix cavities can be drilled in batches using a jig boring machine. Fragile components (such as sprue bushings and ejector pins) should be positioned in easily removable areas of the mold. For example, sprue bushings should be positioned in the center of the mold for easy replacement and cleaning. Ejector pins should be positioned away from mold plate reinforcement ribs and bolt holes to facilitate inspection and replacement. Furthermore, multi-cavity molds should be numbered (e.g., 1-8) next to each cavity to facilitate tracking of the corresponding cavity for defective parts during trial production and production, allowing for quick identification and repair.
The layout of special plastic parts requires targeted design based on their structural characteristics, allowing for flexible adjustment of layout principles. For parts with inserts, the layout must consider the insert’s installation space and positioning accuracy. The cavity spacing should be increased by 20%-30% compared to standard parts to facilitate insert placement by operators or automated equipment. For parts with high aesthetic requirements (such as automotive instrument panels), single- or dual-cavity layouts are recommended to minimize surface quality impact caused by pressure fluctuations during multi-cavity filling. Buffer structures should also be placed around the cavities to reduce surface damage from melt impact. For combined parts (such as upper and lower covers), a male-female mold layout can be employed. The cavities of the two mating parts are symmetrically arranged within the same mold to ensure dimensional compatibility and minimize assembly errors. The layout of special parts requires CAE simulation analysis (such as Moldflow) to verify filling balance and cooling uniformity. Based on the simulation results, the location and number of cavities should be optimized to ensure the feasibility and rationality of the layout plan.
