Dimensioning requirements for injection molding assembly drawings
The injection molding assembly drawing is an important technical document for mold design and manufacturing. Its dimension markings must be clear, complete, and standardized to provide an accurate basis for mold assembly and inspection. The specific requirements are as follows. Functional dimensions must be marked first: Functional dimensions refer to dimensions that directly affect the working performance of the mold, including cavity dimensions, core dimensions, guide pin and guide sleeve matching dimensions, etc. For example, in the assembly drawing of a plastic gear mold, the cavity pitch circle diameter (φ50±0.02mm) and tooth top circle diameter (φ55±0.03mm) must be marked. These dimensions directly determine the meshing performance of the gear plastic part. At the same time, the matching dimensions of the guide pin and guide sleeve (guide pin φ20f7, guide sleeve φ20H7) must also be clearly marked to ensure guiding accuracy. Functional dimensions should be marked in the form of limit deviations, avoiding separate marking with tolerance codes to ensure that processing personnel can directly obtain the size range.
The assembly dimension chain must be complete: During mold assembly, the dimensions of each part form an interrelated dimension chain, and the dimension chain must be closed during annotation. Taking a three-plate mold as an example, the assembly drawing must indicate the thickness of the fixed plate (200mm), the thickness of the runner push plate (50mm), the thickness of the cavity plate (150mm), and the total thickness of these three (400mm) to form a complete dimension chain. For parts with a matching relationship, relative position dimensions must be marked, such as the guide post center distance (250±0.05mm) and the distance between the cavity center and the guide post center (120±0.03mm). By annotating the complete dimension chain, it can be ensured that the relative position requirements of the parts are met after assembly, avoiding interference or positioning deviations.
The marking of external and installation dimensions must be clear: The mold’s external dimensions, including length, width, and height, are crucial for mounting the mold on the injection molding machine and must be prominently noted on the assembly drawing. For example, the external dimensions of a mold are marked as “600mm × 500mm × 450mm,” clearly indicating the mold’s spatial footprint. Installation dimensions include the diameter of the locating ring (φ100mm), the nozzle aperture (φ12mm), and the location of the mounting holes on the template (4-φ16 evenly spaced, with a center circle diameter of φ400mm). These dimensions must match the corresponding parameters of the injection molding machine. When marking, the location of the mounting holes should be marked using coordinates, based on the mold center, such as “Mounting hole 1: X=+200mm, Y=+150mm,” to facilitate mold installation and positioning.
The extreme position dimensions of moving parts cannot be omitted: the push rods, sliders and other moving parts in the mold need to be marked with their maximum stroke and extreme position dimensions to ensure that there is no interference during movement. For example, the maximum ejection distance of the push rod is marked as “50mm”, and the position of the push rod after ejection is drawn with a double-dash line in the assembly drawing; the withdrawal distance of the side core-pulling slider is marked as “30mm”, and the safety distance between the slider and the template is marked (≥5mm). For moving parts with elastic elements such as springs, the free length of the spring and the working length after compression need to be marked, such as “spring: free length 100mm, working length 70mm” to ensure that the elastic force of the spring meets the movement requirements. The marking of these dimensions provides a clear basis for the debugging of the moving parts of the mold.
Standardization and readability of dimension markings are basic requirements: Dimension markings in assembly drawings should comply with national standards, use metric units, and have clear and easy-to-read numbers. For repeated identical dimensions (such as multiple push rod diameters of the same specification), they can be marked once and the quantity indicated, such as “8-φ8”. Dimension lines should avoid crossing other lines, and leader lines can be used when necessary. For dimensions of complex parts, detailed markings can be made using enlarged local drawings. In addition, the datum for dimension markings must be indicated in the upper right corner of the drawing (such as the mold parting surface as the Z-direction datum, and the template’s symmetry center as the X and Y-direction datums) to ensure that all dimensions have a clear reference basis. Through standardized dimension markings, assembly drawings can accurately convey design intent and provide reliable guidance for mold manufacturing, assembly, and maintenance.
