Cold-Expansion Compression Collar Molding Without Knitlines
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Solution Overview
Problem
Conventional injection molding of compression collars for cold-expansion tubing connections often introduces knitlines, which are weak regions prone to failure under high loads.
Innovation Solution
A method using a continuous gate at the axial end of a precursor form to eliminate knitlines by ensuring the injected material flows radially outward and downward without intersecting, thereby forming a solid wall without knitlines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional injection molding is used to manufacture compression collars, then production efficiency is improved, but knitlines are introduced which create weak regions prone to failure
Solution Approach 1:
The mold cavity is divided into multiple sections with separate injection gates, allowing the collar to be molded in segments that are later joined. This segmentation enables the use of conventional injection molding processes while avoiding the formation of knitlines by preventing material flow intersection, thus maintaining both production efficiency and structural reliability.
Solution Approach 2:
The problematic knitline formation process is extracted and eliminated from the molding process. By using a two-shot or insert molding technique, the first material is molded, then a second material is injected over it without creating intersecting flow fronts. This extracts the harmful knitline formation from the process while retaining the benefits of injection molding productivity.
2Strength
If wall thickness is increased to reinforce knitlines, then strength at weak regions is improved, but material usage increases
Solution Approach 1:
The knitlines are completely extracted and eliminated from the collar structure through alternative molding techniques. By preventing knitline formation in the first place, there is no need to add extra material to reinforce these regions. The collar maintains uniform wall thickness, achieving both strength requirements and material efficiency.
Solution Approach 2:
The molding process parameters are changed to eliminate knitline formation. By using two-shot molding or insert molding with non-intersecting flow fronts, the process creates a homogeneous structure without weak regions. This parameter change eliminates the need for localized wall thickening, maintaining optimal material usage while ensuring structural strength.
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
The elimination of knitlines strengthens the compression collar, reduces material usage by avoiding thick wall sections, and enhances the reliability of the cold-expansion tubing connections.
Implementation Method 1
the injected material flows from the point or points of injection on the axial end, radially outward to the cylindrical walls, and then down the cylindrical walls without the injected material ever flowing into itself
Implementation Method 2
imbue it with shape memory properties (e.g., through cross-linking, irradiation, steam, etc.) such that when the tubing is stretched or deformed, the tubing returns to the shape set in its memory during the manufacturing process
Implementation Method 3
The elastic forces within the cold-expansion tubing material can be applied to any object that interferes with the cold-expansion tubing as it returns to its original shape
Data Source
AI summary
A compression collar is manufactured for use in reinforcing an interference fit between an end of a pipe and a fitting. A precursor form is injection molded using a cold-expansion material. The precursor form has a tubular body with an initially closed axial end and a bore that is initially blind formed in the other axial end. Material is removed from the initially closed axial end of the tubular body of the precursor form to form an opening in the initially closed axial end that connects to the bore thereby forming the compression collar. The opening has an inner periphery with a profile in axial cross section that is different than any profile in axial cross section of an inner periphery of the bore. The collar formed may lack knitlines and may include tabs formed during the removal step which help to position the collar on a pipe.