Low Pressure Injection Molding for Thin-Walled Parts
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Solution Overview
Problem
Conventional injection molding processes face challenges in reducing wall thickness of molded parts without increasing capital expenses and in preventing premature hardening of thermoplastic material, which can lead to warped or weak parts with reduced mechanical properties.
Innovation Solution
A method of injection molding at low, substantially constant melt pressures, allowing for the filling of mold cavities without premature hardening and eliminating the need for heated mold cavities or pressurizing means, using a mold cavity pressure that is either at atmospheric pressure or slightly positive, and maintaining a constant melt pressure of less than 6000 psi during filling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of substance
If conventional injection molding processes are used to reduce wall thickness, then plastic content and cost are reduced, but capital expenses increase due to need for higher pressure equipment
Solution Approach 1:
The patent changes the pressure parameter from high to low (maintaining melt pressure below 6000 psi), and changes the temperature parameter by eliminating the need for heated mold cavities. These parameter changes allow thin-walled parts to be produced without requiring high-pressure equipment, thus reducing capital expenses while achieving reduced plastic content
Solution Approach 2:
The patent applies preliminary anti-action by preventing premature hardening through low pressure injection and eliminating heated mold cavities before the material can freeze. This prevents the boundary layer from closing off the flow path, allowing continuous flow into thin-walled sections without needing high pressure
2Productivity
If mold cavities are cooled to reduce cycle time, then productivity increases, but thermoplastic material hardens prematurely causing warped or weak parts
Solution Approach 1:
The patent changes the temperature parameter by eliminating heated mold cavities and using ambient or cooled molds, while simultaneously changing the pressure parameter to low pressure (below 6000 psi). This combination allows the material to flow into cooled molds without premature hardening, achieving both reduced cycle time and maintained part quality
Solution Approach 2:
The patent replaces the thermal field (heated mold cavities) with a mechanical field (low pressure injection). Instead of using heat to prevent hardening, the patent uses low pressure to allow continuous flow into cooled molds, substituting a thermal control system with a mechanical control system
3Manufacturing precision
If pressure is increased to force material into thin-walled sections, then flow continues before freezing, but molded-in stresses and warping increase
Solution Approach 1:
The patent inverts the conventional approach by using low pressure instead of high pressure to fill the mold. Conventionally, high pressure is used to force material into thin sections, but this patent uses low pressure (below 6000 psi) combined with eliminated heated cavities to achieve complete flow without creating molded-in stresses or warping
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 method enables the production of high-quality, thin-walled parts with reduced sensitivity to material variations, allowing for the use of recycled plastics and low-cost, high-thermal-conductivity mold materials, while minimizing molded-in stresses and warping, and reducing energy costs.
Implementation Method 1
filling substantially the entire mold cavity with the shot comprising the molten thermoplastic material while maintaining the melt pressure of the shot comprising the molten thermoplastic material at a substantially constant pressure of less than 6000 psi
Implementation Method 2
The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part
Data Source
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AI summary
Injection molding methods at low, substantially constant melt pressures in particular a method comprising: (a) filling with a shot comprising a molten thermoplastic material an unpressurized mold cavity of a molding apparatus, the shot comprising the molten material having a melt pressure that, upon injection into the mold cavity, exceeds a pre-injection pressure of the shot comprising the molten material; and, (b) while filling substantially the entire mold cavity with the shot comprising the molten material, maintaining the melt pressure substantially constant at less than 6000 psi, wherein: an internal portion of the mold cavity spaced at least 2 mm from a most immediate surface of the mold cavity contacting the shot comprising the molten material is maintained at a temperature of less than about 100[deg.]C, and the thermoplastic material has a melt flow index of 0.1 g/10 min to about 500 g/10 min.