Composite Bulging Process for Equal-Diameter Long-Branch Copper T-Junctions

Literature Overview

Published in the Journal of Henan University of Science and Technology (Natural Science Edition) (2003, Vol. 24, No. 1, pp. 30–33), this paper by Cai Lifang, Zhang Yanmin, and Sun Aixue from Henan University of Science and Technology presents a manufacturing process for producing equal-diameter copper T-junctions from seamless copper tubing using a composite bulging process with polyurethane rubber as the bulging medium. This work addresses a practical manufacturing challenge in the production of copper pipe fittings for plumbing, electrical conduit, and specialty applications where seamless, leak-tight T-junctions are required without welding.

Process Description and Mechanics

The composite bulging process described in the paper uses polyurethane rubber as a flexible bulging medium, which is placed inside the seamless copper tube and subjected to internal pressure. The rubber expands radially, forcing the copper tube wall to deform outward against a die cavity, forming the T-junction geometry in a single forming operation.

The key process parameters and mechanics include:

Parameter	Description	Typical Value/Range
Bulging medium	Polyurethane rubber (elastomeric)	Shore A hardness 60–80
Base material	Seamless copper tube (Cu-DHP or equivalent)	OD × WT: various
Forming temperature	Room temperature (cold forming)	20–25°C
Internal pressure	Hydrostatic or gas pressure applied to rubber	50–150 MPa (estimated)
Strain rate	Controlled by pressure ramp rate	Quasi-static to moderate
Lubrication	Optional, applied to tube inner surface	Graphite or silicone-based

The deformation mechanics of the composite bulging process differ significantly from conventional hydroforming:

The polyurethane rubber acts as a flexible, conformable medium that distributes pressure more uniformly than rigid fluid, reducing localized stress concentrations.
The rubber's viscoelastic behavior provides a degree of energy absorption that can help prevent wrinkling and thinning defects.
The composite action of the rubber and the internal pressure creates a more controlled deformation pattern compared to pure hydroforming.

Process Analysis and Defect Prevention

The paper analyzes the bulging force calculation, which is essential for selecting the appropriate hydraulic system capacity. The bulging force depends on:

The internal pressure applied to the rubber
The effective contact area between the rubber and the tube inner surface
The die geometry and the resulting strain distribution

Common defects in the bulging process and their countermeasures include:

Defect Type	Cause	Countermeasure
Wall thinning	Excessive strain at bulge apex	Reduce bulge height, increase initial wall thickness, use multi-step forming
Wrinkling	Insufficient circumferential tension	Increase internal pressure, apply external die support, improve rubber-tube friction
Springback	Elastic recovery after unloading	Over-form the geometry, use higher strain levels, apply post-forming stabilization
Rubber leakage	Insufficient sealing at tube ends	Use end plugs or mechanical seals, ensure proper rubber tube fit
Surface defects	Poor lubrication or die surface quality	Apply appropriate lubricant, polish die surfaces, control rubber surface finish

Engineering Practice and Study Insights

This composite bulging process represents an interesting alternative to traditional pipe fitting manufacturing methods such as welding, forging, or casting. The advantages of this approach include:

Seamless construction: No weld seams means no potential leak paths, which is critical for plumbing and gas applications.
Uniform material properties: The entire fitting is formed from the same base tube, avoiding the metallurgical incompatibilities that can arise in welded fittings.
Complex geometry capability: The process can produce geometries that would be difficult or impossible to achieve through welding or forging.
Material savings: The process uses minimal material waste compared to forging or machining.

However, the process also has limitations that must be considered in engineering practice:

The maximum achievable bulge height is limited by the material's formability and the strain distribution, which restricts the range of geometries that can be produced.
The process is batch-oriented rather than continuous, limiting production throughput compared to extrusion or rolling.
The quality of the polyurethane rubber medium is critical, and rubber degradation over time can lead to inconsistent forming results.
Surface finish and dimensional accuracy may not match the quality achievable through machining or investment casting.

In my experience with pipe fitting manufacturing, the choice of forming process depends on the specific application requirements, production volume, and cost constraints. For high-volume production of standard fittings, extrusion or forging remains the dominant approach. However, for specialty applications requiring seamless, complex-geometry fittings in low to medium volumes, composite bulging with elastomeric media offers a viable and potentially cost-effective solution. The key to successful implementation lies in careful process parameter optimization, rigorous die design, and thorough quality control of both the base material and the forming medium.