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Holes, slots, and recesses in plastic parts play critical roles in mechanical assembly, airflow, and even visual design. These features are typically formed using mold cores—protruding elements within the mold cavity. However, if a core is too long or too slender, it may deform or shift during injection due to the pressure of molten plastic, resulting in dimensional inaccuracies and quality issues. This guide outlines eight practical design strategies to optimize hole features in injection molded parts, helping you minimize defects and ensure structural reliability.
Dimensional Guidelines for Hole Types and Depths
Holes in molded parts generally fall into three categories: blind holes, through holes, and stepped holes. Through holes are often easier to form because cores can be supported from both ends, which helps prevent deflection. For deep hole requirements, stepped hole designs may improve moldability and reduce the risk of dimensional deviation.
Recommended dimensional rules include:
- For blind holes with D < 5 mm, hole depth (h) should not exceed 2D.
- For blind holes with D > 5 mm, hole depth should not exceed 3D.
- For through holes with D < 5 mm, hole depth should not exceed 4D.
- For through holes with D > 5 mm, hole depth should not exceed 6D.
Minimum Thickness Guidelines for Blind Hole Bottoms
For blind holes, the base must maintain a minimum thickness to avoid mechanical weakness and surface sink marks. The standard recommendation is to keep the blind hole bottom at a minimum of 0.2 times the hole diameter (h ≥ 0.2D). If the bottom is too thin, consider thickening this area to prevent failure or cosmetic defects.
Spacing Between Holes and Part Edges
To avoid part weakening or weld line defects, holes should be positioned at least 1.5 times the hole diameter (or 1.5 times wall thickness, whichever is greater) from other holes or the outer edge of the part. This buffer zone ensures structural integrity and reduces stress concentration.
Avoid Placing Holes in Load-Bearing Zones
Because hole features remove material from the part, they inherently reduce local strength. Avoid clustering multiple holes in structural load areas to prevent failure under stress. Additionally, hole concentration increases the chance of weld lines, which further weakens the part.
Use Bosses to Reinforce Hole Features
Adding a raised boss around a hole edge increases strength, improves mold filling, and prevents deformation. This is especially effective for long slots or holes requiring structural reinforcement. Typically, maintaining a boss thickness of 0.3–0.5 mm provides adequate strength without interfering with other design elements or coatings.
Avoid Side Holes Perpendicular to Mold Parting Direction
Designing side holes perpendicular to the parting plane complicates mold construction. These holes often require side actions, lifters, or sliding cores—features that increase tooling costs and cycle times. Whenever possible, align hole geometry with the mold’s parting direction to simplify the tooling.
Avoid Deep Holes That Disrupt Melt Flow
Long, narrow holes can obstruct the flow of molten plastic, especially when placed perpendicular to the material’s flow direction. This leads to incomplete filling and cosmetic flaws. Design elongated holes to follow the plastic’s flow path to ensure smoother molding and reduce the risk of short shots or air traps.
Designing Effective Vent Holes
Vent holes are often added for heat dissipation or airflow within a part. Cylindrical (round) vents are preferred due to their simple core shapes and low tooling cost. Hexagonal or complex vent holes are harder to mold and offer limited performance benefits. Additionally, too many vents may weaken the structure—so reinforcing them with ribs or surrounding bosses can maintain strength.