Foamed Plastic Cup Wall Thickness Control
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Solution Overview
Problem
Existing methods for producing foamed plastic articles, such as cups or containers, face challenges in achieving a combination of thin walls, low mass, high thermal insulation, and precise geometrical shape with high wall strength, while also being recyclable and leak-proof.
Innovation Solution
A method involving a mould with precise cavity-forming surfaces, injection of a molten plastic composition with a physical blowing agent, controlled cooling to form solid skins, and exposure to reduced pressure to create a core layer of expanded cellular foam, allowing for uniform expansion and stretching of the solid skins to achieve the desired properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If an undulating surface is provided on the moulding surface to control expansion, then the expansion of molten plastic is controlled, but the outer surface exhibits residual undulation and is not highly smooth or precise
Solution Approach 1:
The invention segments the expansion control function from the surface formation function. The mould cavity is divided into a smooth outer surface portion and a separate expansion control mechanism (undulating surface) that acts on the molten plastic during expansion but does not transfer its undulation to the final outer surface. This allows independent optimization of both expansion control and surface quality.
Solution Approach 2:
The invention introduces an intermediary layer or mechanism between the undulating surface and the final article outer surface. The undulating surface controls expansion through this intermediary, preventing direct contact between the undulation and the outer surface that defines the final geometrical shape, thus achieving both controlled expansion and smooth surface.
2Temperature
If a thick wall is required to create thermal insulation, then thermal insulation is improved, but the mass of the article increases
Solution Approach 1:
The invention creates a foamed structure within the wall that introduces porosity (air cells) into the plastic matrix. This porous structure provides thermal insulation by trapping air, which is a poor thermal conductor, thereby achieving good thermal insulation properties with reduced material thickness and lower article mass.
Solution Approach 2:
The invention creates a composite structure combining solid plastic skin layers with a foamed cellular core. This composite architecture provides thermal insulation through the air-filled cells while maintaining structural integrity through the solid skin layers, achieving efficient thermal insulation with minimal mass.
3Temperature
If a blowing agent is added to create a foamed structure, then thermal insulation and reduced density are achieved, but the wall strength may be compromised
Solution Approach 1:
The invention creates a composite structure with dense solid skin layers on the outer surfaces and a foamed cellular structure in the core. The solid skins provide structural strength and integrity, while the foamed core provides thermal insulation and reduced density, achieving a balance between strength and insulation properties.
Solution Approach 2:
The invention applies different structural qualities to different regions of the wall: dense solid structure at the outer surfaces (skins) where strength is required, and foamed cellular structure in the core where thermal insulation is prioritized. This local differentiation optimizes both strength and insulation properties.
4Ease of operation
If paper cups with plastic lining are used, then disposable cup functionality is provided, but recycling becomes challenging due to difficulty in separating materials
Solution Approach 1:
The invention uses a single homogeneous plastic material (polypropylene) throughout the entire cup structure, including areas that would traditionally require different materials like paper or plastic linings. This homogeneity eliminates material separation challenges, making the cup fully recyclable while maintaining disposable functionality.
5Productivity
If conventional injection moulding is used, then production efficiency is maintained, but achieving uniform wall thickness with constant tolerance of +/−0.5% is difficult
Solution Approach 1:
The invention incorporates the undulating surface in the mould cavity before injection, which pre-configures the expansion pattern of the molten plastic. This preliminary action guides the expansion process to achieve uniform wall thickness and constant dimensional tolerances (+/−0.5%) throughout the article, eliminating the need for post-production adjustments.
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 method results in articles with high stiffness, excellent thermal insulation, and a smooth, precise surface, while being recyclable and free from leaks, with reduced material usage and uniform wall thickness, addressing the limitations of prior art.
Implementation Method 1
expose the molten plastic composition of the respective portion to an external pressure lower than the injection pressure thereby allowing the molten plastic composition between the first and second solid skins of the respective portion to expand by foaming
Implementation Method 2
the injected plastic composition in contact with the first and second cavity-forming surfaces is cooled to form first and second solid skins respectively adjacent to and in contact with the first and second cavity-forming surfaces
Data Source
AI summary
A method of forming an article by controlling the thickness of a portion of the plastic composition which comprises a layer of molten plastic composition between opposite first and second solid skins, so that the thickness is constant with a very low tolerance of +/−0.5%, preferably +/−0.2%, when the portion is subsequently exposed to a reduced pressure to cause expansion of the layer of molten plastic composition to form a core layer of expanded cellular foam between the first and second solid skins, and by controlling the stretch of the first solid skin to be within the range of 0.5 to 3%, the expansion of the plastic composition can be highly uniformly controlled in the expanded wall parts.