Injection Molding Machine Hole Forming Technology
A comprehensive guide to precision hole formation in injection molding, including blind holes, through holes, cross holes, and dimensional considerations, with special focus on micro injection molding applications.
1. Blind Hole Forming
Blind holes are cavities that do not penetrate completely through the molded part. In micro injection molding, the precision required for blind hole formation increases significantly due to the smaller scales involved. The design and production of blind holes demand careful consideration of several factors to ensure proper filling, cooling, and ejection.
In micro injection molding processes, blind hole formation presents unique challenges related to material flow and pressure distribution. The depth-to-diameter ratio is particularly critical, as it affects both the molding process and the structural integrity of the final part.
Figure 2-10 illustrates the basic structure of a blind hole in an injection molded component. The core pin design must account for proper material flow around the pin while preventing excessive pressure buildup that could cause deformation or core shift, especially important in micro injection molding where tolerances are extremely tight.
Bottom wall thickness (A) is a critical parameter in blind hole design. As shown in Figure 2-11, when the hole diameter (B) is less than 1.5mm, the recommended bottom thickness (C) should be 2A. For larger diameters greater than 1.5mm, the bottom thickness should equal the hole diameter (C = B). This ratio helps prevent bottom bulging, a common defect in blind hole molding that becomes even more problematic in micro injection molding applications.
Proper venting is essential in blind hole formation to allow trapped air to escape, particularly in micro injection molding where even small air pockets can ruin a precision part. The location and size of vents must be carefully calculated based on the hole dimensions and material properties.
Figure 2-10: Blind Hole Formation
Core design for blind hole formation showing recommended dimensions and material flow paths, critical for micro injection molding precision.
Figure 2-11: Hole Formation Bottom Wall Thickness
Bottom wall thickness specifications: A - hole diameter; B - wall thickness. For A < 1.5mm, C = 2A; for A > 1.5mm, C = B.
2. Through Holes and Cross Holes Forming
Through holes and cross holes represent common features in injection molded parts, each presenting unique challenges in both conventional and micro injection molding processes. The formation of these holes requires careful core design, proper material selection, and precise process control to ensure dimensional accuracy and part integrity.
Figure 2-12: Through Hole Formation
a) Single core pin: Basic design for simple through holes, commonly used in both standard and micro injection molding applications.
b) Two core pins: Utilized for longer holes where a single core might deflect under pressure, essential for maintaining precision in micro injection molding.
c) Tapered core: Facilitates easier ejection and provides better alignment, particularly useful in micro injection molding where part removal can be challenging.
d) Supported core: One end fixed, the other guided to prevent deflection, critical for maintaining dimensional accuracy in micro injection molding processes.
Figure 2-13: Cross Hole Formation
Cross holes create additional complexity due to intersecting core pins. In micro injection molding, the challenges multiply as the small scale increases the risk of core interference and material flow issues.
The design must account for proper material flow around both cores, adequate venting at the intersection, and sufficient support for both cores to prevent deflection during injection pressure. The intersection area often requires special attention in micro injection molding to ensure proper filling without creating weak points.
Figure 2-14: Core Lengths for Different Molding Methods
a) Single core: Used for simple holes where sufficient support is available from one end.
b) Two cores: Employed when hole length exceeds recommended limits for single core support.
Compression molding: 4-28 ratio guidelines
Transfer molding: 4-68 ratio guidelines
Injection molding: 4-68 ratio for transfer, 4-158 for injection processes, with even stricter ratios in micro injection molding.
Figure 2-15: Converting to Stepped Holes for Excessive Depth
When hole depth exceeds recommended limits, converting to stepped holes provides an effective solution, especially valuable in micro injection molding where deep, narrow holes present significant challenges.
Stepped holes reduce the effective depth-to-diameter ratio of each section, improving material flow and reducing core deflection. This approach maintains the required functionality while making the molding process more manageable, particularly critical in micro injection molding where tooling precision is paramount.
Key Considerations in Through and Cross Hole Molding
- Core deflection increases with length, requiring additional support in longer holes, especially critical in micro injection molding
- Cross holes create potential weak points at intersections that must be reinforced
- Proper venting is essential at hole ends and intersections to prevent air traps
- In micro injection molding, core alignment becomes exponentially more important due to smaller tolerances
- Material selection impacts hole quality, with certain polymers flowing more easily around cores in micro injection molding applications
3. Hole Forming Dimensions
The dimensional relationships in hole formation are critical factors in ensuring moldability, part strength, and functional performance. In micro injection molding, these dimensional considerations become even more precise, as small variations can significantly impact part functionality. Proper dimensioning prevents defects such as incomplete filling, core deflection, and excessive wall thickness variations.
3.1 Relationship Between Hole Diameter and Depth
The relationship between hole diameter and depth represents one of the most fundamental design considerations in injection molding hole formation. This relationship becomes particularly critical in micro injection molding, where the small scale magnifies the challenges of material flow and core stability. Table 2-14 outlines the recommended guidelines for this important relationship.
| Hole Type | Diameter (D) | Maximum Recommended Depth | Considerations for Micro Injection Molding |
|---|---|---|---|
| Blind Hole | D < 1.6mm | 2D | Reduced to 1.5D due to material flow challenges in micro injection molding |
| Blind Hole | D > 1.6mm | 4D | Reduced to 3D in micro injection molding applications |
| Through Hole | D < 1.6mm | 4D | Requires additional support in micro injection molding |
| Through Hole | D > 1.6mm | 8D | May require stepped design in micro injection molding for depths exceeding 6D |
| Cross Hole | All sizes | 50-75% of standard hole depths | Significantly reduced in micro injection molding due to intersection challenges |
A critical guideline from Table 2-14 is that when hole depth exceeds 4 times the diameter, a through hole design is recommended over a blind hole. This recommendation is particularly important in micro injection molding, where deep blind holes increase the risk of incomplete filling and core deflection due to the high pressure requirements and reduced material flow characteristics at small scales.
3.2 Recommended Limit Dimensions for Holes
Hole dimensions must be carefully controlled within recommended limits to ensure both manufacturability and functionality. Table 2-15 provides these critical limit dimensions, with special considerations for micro injection molding applications where tighter tolerances are typically required.
| Dimension Type | Minimum Value | Maximum Value | Micro Injection Molding Adjustments |
|---|---|---|---|
| Minimum Hole Diameter | 0.5mm | - | Can be as small as 0.1mm with specialized micro injection molding techniques |
| Minimum Wall Thickness Between Holes | 0.8mm | - | 0.2mm minimum in advanced micro injection molding processes |
| Maximum Depth-to-Diameter Ratio | - | 8:1 (through holes) | Typically 5:1 in micro injection molding without special techniques |
| Maximum Depth-to-Diameter Ratio | - | 4:1 (blind holes) | 3:1 standard in micro injection molding applications |
| Hole Diameter Tolerance | ±0.05mm | ±0.1mm | ±0.01mm to ±0.03mm in precision micro injection molding |
3.3 Hole Spacing and Edge Distance
The spacing between holes and the distance from holes to part edges are critical factors in maintaining part integrity and moldability. Insufficient spacing can lead to weak areas, while excessive spacing may result in unnecessary material usage and increased part weight. In micro injection molding, these dimensions require even more precise calculation due to the smaller overall part size and the potential for increased stress concentrations.
Figure 2-16: Hole Spacing and Edge Distance
Diagram illustrating recommended distances between adjacent holes and from holes to part edges to ensure structural integrity, particularly important in micro injection molding designs.
Table 2-16: Recommended Hole Spacing and Edge Distance
| Hole Diameter (D) | Minimum Distance Between Holes | Minimum Distance from Hole to Edge | Micro Injection Molding Adjustments |
|---|---|---|---|
| D < 1mm | 1.5D | 1.0D | Often increased to 2.0D due to micro injection molding constraints |
| 1mm ≤ D ≤ 3mm | 1.25D | 0.75D | Slightly increased to 1.5D in micro injection molding |
| 3mm < D ≤ 10mm | 1.0D | 0.5D | Generally maintained in larger micro injection molding components |
| D > 10mm | 0.75D | 0.5D | Less common in micro injection molding applications |
Practical Considerations for Hole Dimensioning
When designing holes in injection molded parts, and particularly in micro injection molding, it's essential to balance functional requirements with manufacturing capabilities. The following practical guidelines should be considered:
- Hole diameters should be standardized to minimize tooling costs, especially beneficial in micro injection molding where tooling expenses are relatively higher
- Through holes are generally preferred over blind holes when possible, as they are easier to produce with consistent quality in micro injection molding
- When multiple holes are required, maintain consistent diameters where possible to simplify tooling in micro injection molding processes
- Consider draft angles on hole walls, even in micro injection molding, to facilitate easier ejection and reduce part stress
- For holes with critical functional requirements, specify appropriate tolerances based on micro injection molding capabilities
The advancement of micro injection molding technology has expanded the possibilities for producing small, complex holes with tight tolerances. However, even with these advancements, the basic principles of hole design remain relevant:
- Maintain appropriate depth-to-diameter ratios to ensure moldability in micro injection molding
- Provide adequate spacing between holes and from part edges to maintain structural integrity
- Consider the impact of hole placement on material flow, particularly important in micro injection molding where flow paths are more restricted
- Evaluate the need for additional support structures around holes in thin-walled sections
- Collaborate with micro injection molding specialists early in the design process to optimize hole features for manufacturability
4. Core Pin Design for Hole Formation
Core pins are critical components in the formation of holes in injection molded parts. These precision tools create the negative space that becomes the holes in the final part. In micro injection molding, core pin design takes on even greater importance due to the small sizes involved and the increased risk of deflection, breakage, and wear.
Material Selection
Core pins for standard injection molding are typically made from tool steels, while micro injection molding often requires materials with higher wear resistance and better polishability, such as carbide or stainless steel alloys.
Support Requirements
Proper support is critical, especially for longer core pins. In micro injection molding, even short core pins may require additional support due to the high pressures involved relative to the small cross-sectional area.
Ejection Considerations
Core pins must allow for proper ejection of the part. In micro injection molding, specialized ejection mechanisms may be required to prevent damage to small features while maintaining the precision of micro holes.
The design of core pins directly impacts the quality and consistency of molded holes. In micro injection molding, where hole diameters can be measured in hundredths of a millimeter, even minor variations in core pin dimensions or surface finish can render parts unusable.
Core pin deflection is a significant concern, particularly for longer pins and in micro injection molding applications where the small diameter pins are more susceptible to bending under injection pressure. This deflection can result in holes that are out of round, off-position, or with inconsistent wall thicknesses.
To address these challenges in micro injection molding, core pins may incorporate specialized designs such as stepped shafts, where a larger diameter section provides additional support for the smaller diameter working section that forms the hole. Additionally, advanced micro injection molding processes may utilize temperature-controlled core pins to manage material flow and cooling in critical hole features.
The formation of holes in injection molded parts, and particularly in micro injection molding, requires careful attention to design principles, material characteristics, and processing parameters. From blind holes to complex cross holes, each feature presents unique challenges that must be addressed through thoughtful design and appropriate tooling.
By following the guidelines presented for hole diameters, depths, spacing, and core pin design, manufacturers can produce high-quality parts with consistent hole features that meet functional requirements. As micro injection molding technology continues to advance, the possibilities for creating smaller, more complex hole geometries will expand, opening new opportunities for innovative product designs.
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