General Requirements for Dimensioning in Mold Design Drawings
A comprehensive guide to industry-standard practices for accurate and consistent dimensioning, including critical considerations for injection molding tolerances.
Units of Measurement for Dimensioning
There are two systems of units used in mold drawings: metric and imperial. Countries including the United Kingdom, United States, Canada, India, and Australia typically use the imperial system, while China employs the metric system. However, when the customer is based in these countries, the imperial system should be adopted to meet their specifications and ensure proper injection molding tolerances.
It's important to note that many standard components in molds (such as screws, ejector pins, and guide pillars) are manufactured using imperial units. Therefore, even when other dimensions in the mold design utilize the metric system, these standard components must retain their imperial dimensioning to maintain compatibility and appropriate injection molding tolerances.
This dual-system approach requires careful attention to unit consistency within drawings, with clear notation when switching between systems to prevent errors in manufacturing and ensure that injection molding tolerances are properly maintained across all components.
Precision Requirements for Dimensioning
The precision of dimensioning directly impacts manufacturing accuracy and ultimately affects injection molding tolerances. Different types of dimensions require specific precision levels:
Linear Dimensions
- Metric system: Two decimal places (e.g., X.XX mm)
- Imperial system: Four decimal places (e.g., X.XXXX in)
- This level of precision ensures that critical features meet required injection molding tolerances
Angular Dimensions
- Single decimal place (e.g., X.X°)
- Important for draft angles and mating surfaces
- Precise angular dimensions help maintain proper injection molding tolerances
Maintaining these precision standards is crucial for ensuring that mold components fit together correctly and that the final product meets design specifications, particularly with regard to critical injection molding tolerances that affect functionality and performance.
Dimension Datums in Mold Drawings
Datums are reference points, lines, or planes from which dimensions are measured. Proper selection of datums is essential for ensuring manufacturing accuracy and controlling injection molding tolerances. There are three primary types of datums used in mold design:
Product (Plastic Part) Datums
These datums are derived from the customer's product drawings. All dimensions related to the cavity and core are based on these product datums, which serve as the design reference. Dimensions for the mold's core and cavity in the mold design drawings must correspond exactly with the dimensions in the product drawings to ensure proper fit and maintain specified injection molding tolerances.
Mold Assembly Datums
Typically, the center of the mold base serves as the assembly datum. All dimensions related to mold base assembly, such as screw holes, cooling water channels, and guide pin locations, should be referenced from this assembly datum. This ensures that all components fit together properly during assembly and helps control injection molding tolerances throughout the mold structure.
Process Datums
These datums are determined based on the requirements for machining and measuring mold components. For example, counterbores for component holes should use the bottom surface as the datum. Process datums are critical for ensuring manufacturing feasibility and maintaining appropriate injection molding tolerances during production.
Methods for Selecting Mold Drawing Datums
In section views: The parting surface is used as the datum, with a datum symbol clearly indicated, as shown in Figure 1-17. Sometimes, product engineers may require using the plastic part drawing as the datum, in which case the plastic part datum coincides with the plastic part datum in the mold drawing to maintain consistent injection molding tolerances.
Figure 1-17: Section View Datum
Section view showing parting line as primary datum for dimensioning and injection molding tolerances control
In plan views: The situation is more complex and requires different approaches based on part geometry:
Figure 1-18: Symmetrical Part
For symmetrical parts, two center lines are used as datums to ensure balanced injection molding tolerances
Figure 1-19: Part with One Symmetry Axis
When only one axis is symmetrical, boss center is used as second datum for precise injection molding tolerances
Figure 1-20: Part with One Directional Symmetry
When no boss exists, longest straight edge serves as datum for controlling injection molding tolerances
Figure 1-21 & 1-22: Asymmetrical Parts
For fully asymmetrical parts, strategic boss or edge datums ensure proper injection molding tolerances
The correct selection of dimensional datums is a critical factor in ensuring that part design requirements are met and that manufacturing and measurement are facilitated. Proper datum selection directly impacts the ability to control injection molding tolerances throughout the production process.
Consistent Dimensioning Across Views
Dimensions for the same structure must be consistent across different views. This consistency is vital for maintaining proper injection molding tolerances and ensuring that manufacturing personnel can produce components that meet design intent.
For example, dimensions should be consistently标注 from the larger end, and when necessary, draft angles should be specified, such as 50±3°, as shown in Figure 1-23(a). This approach helps maintain consistent injection molding tolerances across all part features.
For ribs and holes, it's sufficient to dimension the center positions along with width, depth, and diameter, with draft angles specified separately. This method streamlines the drawing while still providing all necessary information to control injection molding tolerances.
Figure 1-23: Typical Structure Dimensioning
Consistent dimensioning methods for common features help maintain proper injection molding tolerances
Maintaining consistency in dimensioning practices across all views and drawings ensures that there is no ambiguity in manufacturing, which is essential for controlling injection molding tolerances and producing high-quality mold components that meet design specifications.
Specific Dimensioning Guidelines
Ejector Pin Holes
The position dimensions of ejector pin holes only need to be标注 on the inner mold insert drawings. They do not need to be标注 on the ejector retainer plate or template drawings, though the diameter of the ejector pin holes should still be specified. This approach reduces drawing clutter while ensuring that critical location information is available where needed, helping to maintain proper injection molding tolerances.
CNC Machined Parts
For parts that will be machined using CNC (Computer Numerical Control) equipment, it is not necessary to标注 all dimensions. Only the important reference data and inspection dimensions need to be specified. This is because CNC machines work from digital models that contain complete dimensional information. The key dimensions标注 on the drawing should focus on those that are critical for ensuring proper fit and function, as well as those that will be inspected to verify compliance with injection molding tolerances.
Moving and Fixed Insert Dimensions
The primary dimensions that should be标注 for moving and fixed inserts include: wire EDM dimensions; positions of screw holes and cooling water channels; ejector pin holes; parting surface height differences; and external form fitting dimensions. For clarity, these dimensions may be distributed across one or more drawings. This organized approach ensures that all critical features are properly dimensioned to maintain required injection molding tolerances.
Wire EDM Dimensions
Only the main dimensions need to be标注 for wire EDM (Electrical Discharge Machining) features. For complex contours, the wire EDM portion can be copied and detailed in a separate drawing, with a reference note added to the original drawing. This practice helps maintain drawing clarity while ensuring that all necessary information for accurate wire EDM processing is available, which is crucial for maintaining tight injection molding tolerances on precision features.
Standard vs. Non-Standard Mold Bases
Non-standard mold bases require标注 of template screw holes, return pins, guide pillars, and other component positions and dimensions, as well as NC inspection dimensions for the moving and fixed mold frames. In contrast, standard mold bases do not require these标注, as their dimensions are standardized and controlled by the manufacturer. When using standard mold bases, it's important to note that guide pillars, return pins, and ejector plate guide pillars are typically not ordered separately as they are included with the standard mold base. This distinction helps focus标注 efforts on critical features that affect injection molding tolerances.
Rib Electrode Machining Positions
When标注 the machining positions for rib electrodes, it is sufficient to标注 the electrode center positions. This provides the necessary information for accurately positioning the electrode during the EDM process while avoiding unnecessary detail. Proper positioning of these electrodes is essential for achieving the correct rib dimensions and maintaining appropriate injection molding tolerances for these critical structural features.
Summary of Dimensioning Best Practices
Proper dimensioning of mold design drawings is fundamental to producing high-quality molds that meet design specifications and performance requirements. By following these established guidelines, designers can ensure that manufacturing processes proceed smoothly and that the final mold components maintain the required injection molding tolerances.
Consistency in unit usage, precision levels, and datum selection across all drawings helps eliminate ambiguity and reduces the potential for manufacturing errors. Attention to detail in dimensioning specific features like ejector pins, CNC-machined parts, and electrodes directly impacts the mold's ability to produce parts within the specified injection molding tolerances.
Ultimately, accurate and consistent dimensioning practices contribute to more efficient manufacturing, reduced lead times, lower costs, and higher quality end products that meet all design and functional requirements.