Stepped Die Shrinkage Fitting Ratio for Corner Crack Prevention
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Solution Overview
Problem
Existing stepped die designs for powder molding are prone to cracks at corner portions due to stress concentration, and methods to alleviate this often require additional parts or excessive stress, leading to inefficiencies and inaccuracies in the molding process.
Innovation Solution
A stepped die with an inner ring made of sintered hard alloy and an outer ring fitted by shrinkage fitting, where the shrinkage fitting ratio is set between 0.12% and 0.25%, ensuring appropriate compressive stress is applied to the corner portion to prevent cracking without increasing the number of parts or man-hours.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the stepped die is lowered together with the die plate to remove the molded part, then the part can be easily removed from the die, but a support cannot be disposed below the die since it becomes an obstacle, leading to stress concentration at the corner portion of the stepped portion
Solution Approach 1:
The patent applies preliminary anti-action by pre-generating compressive residual stress at the corner portion of the stepped portion through controlled shrinkage fitting of the outer ring onto the inner ring. This pre-applied compressive stress counteracts the bending stress that occurs during pressure molding, preventing crack formation before the harmful stress is applied. The shrinkage fitting process creates a pre-compression state that neutralizes the subsequent tensile bending stress at the vulnerable corner region.
Solution Approach 2:
The patent utilizes parameter changes by precisely controlling the shrinkage fitting ratio (0.05% to 0.30%) and the ratio of outer diameter to inner diameter of the inner ring (1.03 to 1.20). These parameter adjustments optimize the distribution of compressive residual stress, ensuring sufficient stress at the corner portion to prevent cracking while avoiding excessive stress that could cause deformation or damage to the die structure.
2Strength
If a ring is mounted on the outer periphery of the die by tight fitting to prevent crack occurrence, then the stress concentration at the corner portion is alleviated, but an additional part and an additional fitting step are required
Solution Approach 1:
The patent merges the protective function into the existing die structure by integrating the outer ring and inner ring as integral components of the stepped die assembly. The outer ring serves dual purposes: it provides structural support and generates the necessary compressive residual stress at the corner portion through shrinkage fitting. This eliminates the need for separate protective rings or additional strengthening components, reducing both part count and assembly complexity.
Solution Approach 2:
The patent applies self-service by enabling the outer ring to automatically generate the required compressive residual stress through its own shrinkage fitting process onto the inner ring. The shrinkage fitting operation simultaneously achieves two functions: securing the outer ring in position and creating the stress distribution pattern needed to prevent cracking. This self-generating stress mechanism eliminates the need for additional protective components or post-assembly stress application steps.
3Strength
If the shrinkage fitting ratio is increased to generate compressive residual stress, then crack prevention at the corner portion is improved, but excessive stress is generated in other portions of the stepped portion leading to crack occurrence
Solution Approach 1:
The patent utilizes parameter changes by precisely controlling the shrinkage fitting ratio (0.05% to 0.30%) and the ratio of outer diameter to inner diameter of the inner ring (1.03 to 1.20). These parameter adjustments optimize the distribution of compressive residual stress, ensuring sufficient stress at the corner portion to prevent cracking while avoiding excessive stress that could cause deformation or damage to the die structure.
Solution Approach 2:
The patent applies local quality by creating a non-uniform stress distribution through controlled shrinkage fitting. The compressive residual stress is concentrated at the corner portion of the stepped portion where it is most needed to prevent cracking, while the stress gradually decreases toward other portions of the inner ring. This localized stress distribution ensures protection at critical areas without subjecting the entire structure to excessive stress.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively prevents cracks at corner portions during pressure molding by optimizing the compressive stress distribution, maintaining the accuracy and integrity of the molded parts without additional components or increased complexity.
Implementation Method 1
an outer ring which is fitted on an outer periphery of the inner ring by shrinkage fitting
Data Source
AI summary
Provided is a stepped die which includes: an inner ring having a cylindrical shape, and an outer ring having a cylindrical shape which is fitted on an outer periphery of the inner ring by shrinkage fitting, in which a recessed portion for molding which has a stepped portion is formed on an inner side of the inner ring. A shrinkage fitting ratio of the outer ring to the inner ring is set to a value which falls within a range of from 0.12% to 0.25%.