Multi-Tie-Rod Multi-Cylinder 6000t LFT-D Composite Material Hydraulic Press

Literature Overview and Technical Scope

This paper describes the design, fabrication, and performance of a 6000-tonne multi-tie-rod multi-cylinder hydraulic press using the LFT-D (Liquid Foamed Thermoplastic Direct) composite material forming technology. The press is designed for the production of large-scale composite components such as wind turbine blades, automotive body panels, and aerospace structural parts. From a pressure equipment and hydraulic system engineering perspective, this press represents a complex integration of high-pressure hydraulic power units, pressure vessel components, and precision mechanical systems that must operate reliably under demanding cyclic loading conditions.

Press Architecture and Key Technical Parameters

The 6000t LFT-D press employs a multi-tie-rod frame design with multiple hydraulic cylinders providing synchronized clamping force. The LFT-D process involves the injection of liquid thermoplastic compound into a mold cavity, where it foams and solidifies to form the final composite part. The press must provide uniform clamping pressure across the entire mold surface to ensure consistent part quality.

The hydraulic power unit (HPU) for this press must deliver a total flow rate of approximately 400 L/min at 25 MPa, representing a hydraulic power output of approximately 167 kW. The HPU is a critical pressure equipment component that must be designed and fabricated to meet the requirements of pressure vessel codes for all pressure-containing components.

Hydraulic System Design and Pressure Equipment Considerations

The hydraulic system of the 6000t press includes several pressure equipment components that require careful design and fabrication:

• Accumulators: Nitrogen-charged accumulators with water-seal or bladder-type designs, rated for 35 MPa, used for energy storage and peak flow supplementation
• High-pressure piping: Seamless steel piping with internal overlay cladding for erosion resistance, designed for 35 MPa with a safety factor of 1.5
• Control valves: Proportional and servo valves with hardfaced seats for wear resistance, rated for 35 MPa
• Manifolds: Cast or fabricated manifolds that distribute hydraulic fluid to multiple cylinders, requiring careful stress analysis at the multiple branch connections
• Reservoir: A large-capacity reservoir with baffles to promote fluid settling and air separation, designed for atmospheric pressure with a minimum freeboard height of 150 mm

The tie rods of the press frame are subjected to cyclic tensile loading during each press cycle. The stress amplitude in the tie rods must be evaluated for fatigue life using the S-N curve approach, with the maximum stress limited to below 50% of the yield strength to ensure a fatigue life exceeding 10^6 cycles. The tie rod material should be a high-strength alloy steel such as 42CrMo4 or 4340, with a minimum tensile strength of 1100 MPa and a minimum yield strength of 950 MPa.

LFT-D Process Integration and Control Requirements

The LFT-D forming process requires precise control of several parameters that are directly influenced by the hydraulic system:

1. Clamping force control: The holding pressure must be maintained within ±1% of the setpoint to ensure consistent part quality. This requires high-performance proportional valves with response times below 50 ms.
2. Temperature control: The mold temperature must be maintained within ±2°C of the setpoint. The hydraulic oil temperature must be controlled to prevent thermal expansion effects on the press geometry.
3. Cycle synchronization: The multiple hydraulic cylinders must close and open in perfect synchronization to prevent mold misalignment. This requires either a mechanical synchronization system or a sophisticated hydraulic synchronization circuit with pressure and position feedback.
4. Foaming pressure control: The LFT-D process involves the controlled foaming of the thermoplastic compound within the mold cavity. The hydraulic system must provide a controlled pressure profile that allows the foam to expand and fill the mold without premature solidification.

The hydraulic oil used in the press must meet stringent specifications to ensure reliable operation:

Fabrication Quality and Inspection Requirements

The fabrication of the 6000t press involves several critical manufacturing operations that require rigorous quality control:

• Frame fabrication: The frame consists of multiple cast or fabricated components that must be aligned with a total height variation of less than 0.05 mm per meter. This requires precision machining and assembly on a granite surface plate.
• Tie rod machining: The tie rods must be machined to a surface finish of Ra ≤ 0.8 μm on the threaded sections to minimize stress concentration and ensure fatigue resistance.
• Hydraulic cylinder fabrication: The cylinder barrels must be honed to a surface finish of Ra ≤ 0.4 μm to ensure seal longevity. The piston rods must be chrome plated with a minimum plating thickness of 0.05 mm.
• Welded components: All welded pressure-containing components must be inspected per NB/T 47014 or ASME IX qualification requirements, with UT or RT inspection of all welds and visual inspection of all accessible surfaces.

The hydraulic manifold is a particularly critical component that must be designed and fabricated with attention to stress concentrations at the multiple branch connections. Finite element analysis should be used to identify high-stress regions, and the design should be optimized to minimize stress concentrations through the use of generous fillet radii and smooth transitions.

Study Insights and Engineering Implications

This 6000t LFT-D composite material hydraulic press represents a state-of-the-art integration of hydraulic power technology, precision mechanical engineering, and composite materials processing. From a pressure equipment engineering perspective, the press demonstrates several important principles:

First, the reliability of the hydraulic system is directly linked to the quality of the pressure equipment components. Every valve, accumulator, pipe, and fitting must be designed, fabricated, and inspected to the highest standards to ensure reliable operation over the expected service life of 15–20 years.

Second, the synchronization requirements of the multi-cylinder system impose stringent demands on both the hydraulic circuit design and the control system implementation. Any variation in cylinder response time or pressure control accuracy can result in mold misalignment and part quality degradation.

Third, the cyclic nature of the press operation creates a fatigue environment for all pressure-containing components. The design must incorporate adequate fatigue margins, with particular attention to stress concentration areas such as welds, threaded connections, and sharp geometric transitions.

The successful implementation of this press technology requires close collaboration between hydraulic system engineers, pressure equipment designers, mechanical engineers, and process engineers. Each discipline must contribute its expertise to ensure that the final system meets all performance, safety, and reliability requirements. This case study reinforces the principle that complex industrial equipment is only as reliable as its weakest component, and that a systematic approach to design, fabrication, and inspection is essential for achieving long-term operational success.