Syringe Plunger Thread Flank Angles for Engagement Strength
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Solution Overview
Problem
Small-capacity syringes face challenges in maintaining thread engagement strength between the piston and plunger rod without increasing sliding resistance, as existing designs struggle to balance thread engagement strength with piston expansion and sliding performance.
Innovation Solution
A plunger kit with a piston made of elastomer and a plunger rod made of harder material, featuring non-complementary basic groove and thread profiles, where the thread and groove flanks are designed to provide stronger engagement at specific points without substantial piston expansion, using angles between 90 to 150 degrees and varying contact points to enhance thread engagement strength without increasing sliding resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the external screw is designed larger to increase thread engagement strength, then the thread engagement strength between piston and plunger rod is improved, but the outer diameter of the piston increases causing increased sliding resistance
Solution Approach 1:
The patent applies local quality by creating non-uniform thread and groove profiles where the contact area varies along the engagement length. The thread flanks are designed with specific curvature radii (R1, R2) that create localized high-strength contact zones while maintaining a compact overall piston diameter. This allows strong engagement at critical points without uniformly increasing piston size throughout.
Solution Approach 2:
The invention uses asymmetric thread and groove profiles where the thread flank angles and curvature radii differ on opposite sides. The external screw has asymmetric flanks with different curvature radii (R1 ≠ R2), and the internal screw groove corresponds with asymmetric flanks. This asymmetry enables optimized stress distribution and engagement strength without requiring symmetric expansion of the piston diameter.
2Quantity of substance
If the syringe capacity is reduced for smaller dose sizes, then the burden on patients is reduced, but the moldability of threaded portions deteriorates making precise molding difficult
Solution Approach 1:
The patent changes the geometric parameters of the threaded portions by introducing specific curvature radii (R1, R2) and flank angles (α, β) that optimize both engagement strength and moldability. The thread profile parameters are carefully selected to maintain adequate engagement strength even in small-capacity syringes, while the rounded corners and optimized flank angles facilitate precise molding in small sizes.
3Reliability
If the threaded engagement is made stronger to prevent sudden disengagement, then the reliability of piston-plunger rod connection is improved, but the device complexity increases
Solution Approach 1:
The patent applies curvature by rounding the corners of the thread profiles with specific curvature radii (R1, R2). This curvature design eliminates stress concentration points that would lead to sudden failure, while the smooth rounded profiles actually simplify the molding process and reduce structural complexity compared to sharp-angled threads. The curved flanks provide gradual load distribution enhancing reliability without adding complexity.
Data Source
Figure 1A
Figure 1B
Figure 2A
AI summary
A leading flank (15G) of each thread (15A) of an external screw (15) and a groove flank (14G) of a corresponding groove (14A) of an internal screw (14) have mutually-different angles (γ=100°,δ=90°) with respect to a push direction and pull direction of a piston, respectively. When the piston is slid in the push direction in a syringe barrel, a hard contact part (15C) and soft contact part (15D) occur on the leading flank. Increased thread engagement strength can hence be provided between the external screw and the internal screw without substantially increasing screwing resistance. A following flank (15H) of the thread and an opposite groove flank (14H) of the groove are also constructed similarly.