Vacuum Compression Molding of Thermoset Sheets
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Solution Overview
Problem
Compression molding techniques face challenges in controlling porosity and pitting in thermoset articles, leading to surface defects and high scrap rates, especially in complex three-dimensional parts with varying thickness, due to inadequate degassing during the molding process.
Innovation Solution
A controlled process of vacuum compression molding that involves a programmed sequence of die movement and vacuum pressure, where the dies are closed at specific rates, and the mold cavity is evacuated to a reduced pressure to facilitate kinetic degassing, ensuring improved surface quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional compression molding is used, then production efficiency is maintained, but surface porosity and pitting occur leading to paint defects and high scrap rates
Solution Approach 1:
The vacuum is applied before the dies are fully closed, creating a reduced pressure environment in advance to facilitate degassing of the charge. This preliminary vacuum application prevents air entrapment and surface defects before compression begins, resolving the contradiction between maintaining productivity and improving surface quality.
Solution Approach 2:
The patent employs a dynamic, multi-stage closing sequence where the dies move through intermediate positions rather than closing directly to full compression. This dynamic approach allows the vacuum to act during specific phases of closure, optimizing degassing while maintaining production efficiency and reducing surface defects.
2Manufacturing precision
If vacuum is applied during die closure, then surface quality improves, but control over porosity and pitting remains inadequate
Solution Approach 1:
The die closing process is segmented into distinct stages: initial rapid closure to an intermediate position, holding phase with vacuum application, and final closure to full compression. This segmentation allows precise control over when vacuum is applied and how long it acts on the charge, improving porosity control while maintaining manageable process complexity through structured phases.
Solution Approach 2:
The patent controls porosity by changing key parameters: the timing of vacuum application (before full closure), the degree of die closure (intermediate position), and the duration of vacuum exposure. These parameter changes optimize degassing effectiveness while keeping the process control system practical and manageable.
3Manufacturing precision
If in-mold coating is used to fix surface defects, then surface quality improves, but throughput decreases and environmental weathering concerns arise
Solution Approach 1:
Instead of applying coating after curing to fix surface defects, the patent applies vacuum before and during die closure to prevent surface defects in the first place. This preliminary preventive action eliminates the need for corrective coating steps, maintaining high throughput while achieving excellent surface finish quality.
4Manufacturing precision
If conventional vacuum compression molding is used, then some surface quality improvement occurs, but pitting and porosity remain problematic for complex three-dimensional parts
Solution Approach 1:
The vacuum is applied in advance before full die closure to create a reduced pressure environment that facilitates thorough degassing of complex charge geometries. This preliminary action allows gases to escape from difficult-to-reach areas in three-dimensional parts before compression locks the material in place, significantly reducing pitting and porosity.
Solution Approach 2:
The patent uses a dynamic closing sequence with intermediate positions that allow the vacuum to act during specific phases. This dynamic approach ensures continuous degassing action throughout the closure process, effectively removing trapped gases from complex geometries and preventing surface pitting and porosity.
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 process significantly reduces surface porosity and defects in molded articles, enabling the production of high-quality parts with minimal imperfections, as demonstrated by reduced defect rates in vehicle hoods and fenders compared to conventional methods.
Implementation Method 1
The mold cavity is then evacuated within a shroud volume for at least 3 seconds to a reduced pressure of less than 0.16 atmospheres (16.2 kPa)
Implementation Method 2
After the thermoset resin cures, the molds are opened and the finished part removed
Data Source
AI summary
A process of compression molding an article from a curable resin is provided that includes opening a mold to an open position by displacing an upper die having an upper mold surface from a lower die having a lower mold surface. Resin is placed a mold surface and the dies are closed to an intermediate start position at a rate of from 1 to 10 cm/sec. The dies are then closed to a start position at a rate of between 25 and 60% of the rate of closure to the intermediate position. A shroud volume is evacuated for at least 3 seconds to a reduced pressure of less than 0.16 atmospheres while continuing to press toward a mold cavity volume at a rate of between 12 and 30% of the rate of closure to the intermediate start. The reduced pressure is removed as the full compression is applied.