Liquid Nitrogen Pre-Cooling for Uniform Milling of Polyamides
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Solution Overview
Problem
Existing milling methods for fragmented materials, particularly soft, medium-hard, brittle, fibrous, or temperature-sensitive materials, face challenges such as uneven cooling leading to non-uniform particle distribution and thermal strain on the mill due to direct application of liquefied gas, which affects the quality and grindability of materials like polyamide at temperatures above -70°C.
Innovation Solution
A method involving pre-cooling fragmented materials to -150°C or lower by immersing them in a liquid nitrogen bath and using mechanical vibrations to transport them directly to a mill, avoiding direct application of liquid nitrogen to the mill, ensuring uniform cooling and homogeneous temperature distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If liquefied gas is applied directly to the mill during milling, then cooling effect is achieved, but uneven cooling and thermal strain on the mill occur
Solution Approach 1:
The fragmented material is pre-cooled in a liquid bath before entering the mill, rather than applying cooling directly during milling. This preliminary cooling action ensures uniform temperature distribution throughout the material before grinding begins, eliminating the uneven cooling problem while still achieving the desired low temperature for difficult-to-grind materials.
Solution Approach 2:
A liquid bath serves as an intermediary medium between the cooling source (liquid nitrogen) and the fragmented material. The material is immersed in the liquid bath where cooling occurs uniformly throughout, then the pre-cooled material is transported via chute to the mill. This intermediary approach avoids direct application of liquefied gas to the mill while still achieving effective cooling.
2Temperature
If liquefied gas is applied directly to the mill during milling, then cooling effect is achieved, but thermal strain on the mill increases
Solution Approach 1:
The cooling action is performed in advance in a separate liquid bath, allowing the material to reach the desired low temperature before entering the mill. This separates the cooling function from the grinding function, protecting the mill from thermal strain while still achieving effective cooling of the material.
Solution Approach 2:
The liquid bath acts as an intermediary that absorbs the thermal shock and cooling stress, transferring only the cooled material to the mill via a chute. This protects the mill from direct exposure to liquefied gas and the associated thermal strain, while still achieving the necessary cooling effect on the material.
3Ease of manufacture
If fragmented material is cooled to low temperatures, then grindability of difficult materials improves, but uniform cooling must be ensured
Solution Approach 1:
The liquid bath is designed with specific geometric features including inclination angle and depth variations that ensure all fragmented material is fully immersed and uniformly cooled. The bath geometry creates optimal local conditions throughout the entire volume, ensuring consistent cooling quality across all material particles regardless of their position in the bath.
Solution Approach 2:
Mechanical vibrations are applied to the liquid bath to enhance heat transfer and ensure uniform cooling throughout the fragmented material. The vibrations prevent stagnant zones and promote consistent thermal contact between the material and liquid nitrogen, achieving homogeneous cooling essential for uniform particle distribution after milling.
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
Enables uniform particle size distribution and continuous quality in milled materials, allowing grinding of difficult-to-grind materials like polyamide with high homogeneity, while reducing thermal stress on the mill and maintaining consistent product quality in additive manufacturing processes.
Implementation Method 1
providing an amount of the fragmented material at a material inlet to a liquid bath of liquid nitrogen
Implementation Method 2
transporting the fragmented material from the material inlet to a material outlet of the liquid bath by exciting mechanical vibrations in the liquid bath
Data Source
AI summary
According to the present invention fragmented material 2, e.g. made from a material having Polyamides, is passing through a liquid bath 6 filled with liquid nitrogen 7 to cool the fragmented material 2 before entering a mill 10 for grinding the fragmented material 2. The fragmented material 2 is moved through the liquid bath 6 by exciting mechanical vibrations in the liquid bath 6 e.g. by a vibrational motor 28 coupled to the liquid bath 6 and/or an ultrasonic resonator 26 attached to the liquid bath 6. The invention allows to grind even materials being difficult to grind by reaching a temperature of −150° C. and less before entering the mill 10 while avoiding a direct cooling e.g, by introducing liquid nitrogen directly into the mill 10.