Mixing device for varying composition of thermoplastic in additive manufacturing allowing gradient extrusion

Abstract

A thermoplastic mixing device for additive manufacturing comprises a pellet extruder that melts and delivers a molten thermoplastic feedstock to a downstream mixing chamber. One or more metering pumps

Description

Patent Title: Mixing Device for Varying Composition of Thermoplastic in Additive Manufacturing Allowing Gradient Extrusion Description: Field of the Invention: The present invention relates to the field of additive manufacturing (AM) and more specifically to a thermoplastic mixing device designed to alter the composition of a main thermoplastic feedstock during the 3d printing process, allowingfor gradient extrusion and enhanced control over the material properties in 3D printing applications. Background of the Invention: Additive manufacturing processes, such as fused granular fabrication (FGF), require precise control overthe thermoplasticfeedstockto achieve desired mechanical, thermal, and aesthetic properties in the final printed object. This often involves the addition of various materials, including pigments, additives, and fillers such as milled carbon fiber, to modify the base thermoplastic. Traditional methods of adding these materials during the printing process face challenges related to inconsistent composition, material waste during color or material changes, and difficulty in achieving gradual material property gradients within the printed structure. The invention addresses these challenges by introducing a thermoplastic mixing device that allows for precise control of material composition and enables gradient extrusion, providing better control over the final printed structure’s properties. This device also facilitates reducing material purging during color changes, ensuring consistent material flow and reducingwaste. Summary of the Invention: The invention provides a thermoplastic mixing device for additive manufacturing processes, comprising a main pellet extruder (which can be built-in or added as a separate component), a mixing chamber, one or more metering pumps mounted on the exterior of the mixing chamber, and a static mixer. The device allows forthe incorporation of pigments, additives, and fillers (e.g., milled carbon fiber) into the thermoplastic feedstock. The system enables gradient extrusion, providing controlled changes in material composition, and it is equipped with temperature and / or pressure monitoring and control systems. Additionally, positioning the device at the end of the pellet extruder helps reduce purging requirements during color or material composition changes, thus improving efficiency and minimizing waste. The thermoplastic mixing device comprises the following key components: Embodiment of all components comprising in this invention are not subject to any restriction in size orform. FIG 1,2 14. Main Pellet Extruder (Built-in or Added Component): The main pellet extruder is responsible for melting and extruding the base thermoplastic feedstock. The extruder can either be integrated into the device or added as a separate unit at the beginning of the material flow. It ensures a consistent flow of molten thermoplastic material, which is fed into the mixing chamber. This extruder is designed to accommodate a variety of thermoplastics and can be adjusted for different flow rates and processing conditions. 7. Mixing Chamber: The mixing chamber, located downstream of the pellet extruder, serves as the location where the composition of the thermoplastic feedstock is altered. It is equipped with one or more metering pumps mounted externally to the chamber. These pumps are responsible for introducing pigments, additives (e.g., stabilizers, modifiers), or fillers such as milled carbon fiber into the molten thermoplastic feedstock. The mixing chamber allows for thorough integration of these materials into the base feedstock. 4. Metering Pumps: Metering pumps have many design variants from geared, screw and diaphragm depending on material and required application. The metering pumps are designed to add precise quantities of additives, pigments, or fillers to the molten feedstock. They are located externally to the mixing chamber, ensuring accurate dosing of materials without interfering with the primary extrusion process. These pumps allowfor controlled addition of materials, facilitatingthe achievement of gradient extrusion where the composition of the feedstock can be varied over time or distance. This is especially usefulforapplications where gradual changes in material properties are required. 1. Static Mixer: Positioned in the lower section of the mixing chamber, the static mixer ensures uniform blending of the thermoplastic feedstock and the added materials. The static mixer comprises a series of fixed elements that create turbulence and shear as the material flows through them. This ensures that the feedstock and additives are thoroughly mixed without the need for moving parts, reducing wear and tear while improving reliability and efficiency. 8. Barrel coil heater: Heater is mounted to the barrel containing static mixerto ensure consistent heat is maintained duringthe extrusion process. Type of electrical heater commonly used in industrial applications, particularly in extrusion processes such as thermoplastic and plastic molding, to provide controlled and uniform heating of barrels, cylinders, or similar components. These heaters are essential for maintaining the optimal temperature ofthe material inside the barrel, such as plastic feedstock, which is necessary for proper processing, flow, and molding. 9. Extruder Nozzle: Standard component made from metal-based materials, normally tool steel for large extrusion machines. I.D (internal dimension) can vary depending bead size required. 10,11. Temperature and Pressure Monitoring and Control: The device is equipped with integrated temperature and pressure sensors that monitorthe processing conditions of the thermoplastic feedstock throughout the mixing and extrusion process. Temperature control ensures that the feedstock remains in the optimal range for extrusion, while pressure control maintains consistent flow and prevents clogging or inconsistent extrusion. The control system uses feedback from these sensors to make real-time adjustments to the extruder, metering pumps, and mixing chamber, optimizing the overall process. FIG2: Showing a section view of the main mixing head and all parts there in previously described. FIG3: 1. Static mixer Showing a basic static mixer design. Design and Structure of Static Mixers: a. Fixed Mixing Elements: Static mixers consist of a series of fixed, internal mixing elements that are strategically positioned within the mixing chamber. These elements are typically made of materials like stainless steel, plastic, or other corrosion-resistant alloys, depending on the application. b. Geometry of the Mixer: The internal elements are often designed in spiral, helical, or zigzag patterns. These geometries create a series of turbulent flow patterns that disrupt the material as it moves through the chamber, promoting thorough mixing. The material is continuously divided and recombined, which enhances the mixing process without requiring mechanical energy. c. Flow Path: The material flows through a cylindrical tube or pipe where the mixing elements are arranged. As the material moves through the mixer, the elements force the flow to split, twist, and recombine, resultingin homogeneous mixing. Function: d. Blending of Materials: The primary function of a static mixer is to blend two or more materials effectively. The mixing elements within the device create a shearing action that disrupts the flow and induces turbulence. This turbulent flow breaks the fluid into smaller layers, promoting the rapid and uniform blending of materials. e. No Moving Parts: Unlike dynamic mixers (e.g., paddle mixers or agitators), static mixers have no moving parts. The mixing process is entirely driven by theflow dynamics of the materials and the internal design of the mixing elements. f. Low Energy Consumption: Since static mixers do not require motors or agitators, they are highly energy-efficient. The energy needed to mixthe materials comes from the pressure drop caused by the flowthrough the mixer. This makes them a cost-effective solution for continuous mixing processes. Applications: g. Polymer and Additive Manufacturing: In extrusion and injection molding applications, static mixers are used to mix thermoplastic polymers with additives (such as colorants, fillers, or stabilizers) to create a homogeneous material feedstock. This is especially important in processes like gradient extrusion, where varying material properties need to be achieved across the part. Benefits: h. Energy Efficiency: Static mixers are energy-efficient since they require no external energy source or mechanical components to create the mixing action. The pressure of the material flow provides the necessary energy for mixing. i. No Moving Parts: The absence of moving parts makes static mixers highly reliable, with lower maintenance requirements and longer operational lifespans. j. Consistent and Uniform Mixing: The design of static mixers ensures that materials are mixed to a consistent and homogeneous level, reducingthe risk of product variability and improving process control. k. Compact Design: Static mixers are typically compact and can be integrated easily into existing piping systems without requiring large, complex equipment. This makes them a space-saving solution for continuous mixing processes. I. Low Operating Costs: Since no external power is needed to operate the mixer, static mixers have low operating costs compared to dynamic mixers, makingthem cost-effective for long-term use in industrial applications. Customization: m. Material Choice: Static mixers can be made from a wide range of materials (e.g., stainless steel, plastics, ceramics) depending on the type of materials being mixed and the environment in which the mixer is used. For example, food-grade plastics are often used for food and beverage applications, while high-temperature materials may be required in chemical processing. n. Mixer Length and Diameter: Static mixers can be customized in terms of length and diameter to accommodate different flow rates and types of materials. Larger or more complex processes may require longer mixers with more internal elements, while smaller systems may use compact models. o. Mixing Efficiency: The number and design of the internal elements can be adjusted to meet the desired mixing efficiency. High-viscosity material may require more intensive mixing elements, while simpler blending may use fewer or less complex designs. Performance Considerations: p. Pressure Drop: The design of the static mixer affects the pressure drop across the device. Higher-pressure drops indicate higher mixing intensity but may also require more energy to pump the material through the system. q. Residence Time: The time that material spends in the static mixer is critical to the efficiency of the mixing process. Longer residence times may improve mixing but can also increase the size of the mixer and the amount of space required in the system. r. Flow Regimes: Static mixers work most efficiently under turbulent flow conditions, which can be influenced bythe viscosity of the materials, the flow rate, and the internal design of the mixer. Careful design is required to ensure the flow regime is optimal for the intended application. Types of Static Mixers: 1. Coil Static Mixers: These mixers consist of coiled tubes with internal mixing elements that create turbulence as fluid flows through. The coil design can be used for both liquid and gas applications. 2. Helical Static Mixers: A helical design uses spiral-shaped elements to create a uniform mixing action. This type is suitable for processes requiring high degrees of turbulence. 3. Basket Static Mixers: These mixers contain basket-like components that rotate around the flow axis to stir and mix the fluid efficiently. 4. Blade Static Mixers: These mixers contain blades or baffles that deflect the fluid into complexflow patterns for improved mixing. Astatic mixer is a highly effective and energy-efficient solution for blending materials in industrial applications. By usingfixed internal elements to create turbulence and shear forces within the medium, static mixers can achieve consistent and uniform mixing without the need for mechanical components. This makes them ideal for applications in industries such as chemicals, food and beverage, water treatment, and additive manufacturing, where precise mixing and low maintenance are essential. Gradient Extrusion Capability: The metering pumps and mixing chamber are designed to enable gradient extrusion. By gradually altering the flow of pigments, additives and / orfi Ilers, the device allows for gradual changes in the composition of the thermoplastic material. This capability is particularly useful for creating parts with varying material properties, such as color gradients, enhanced mechanical properties, or strength variations across different sections of the printed object. Reduced Purging Waste: By positioning the thermoplastic mixing device at the end of the pellet extruder, the device can utilize the same feedstockthroughoutthe entire extrusion process. This design significantly reduces the need for extensive purging when switching between different pigments (colours), additives or fiIler composition thereby minimizing waste and improving overall process efficiency. Control System: The control system manages all aspects of the thermoplastic mixing device, including the operation of the metering pumps, static mixer, temperature and pressure regulation, and extrusion flow. It ensures that the desired composition of the feedstock is mai ntained throughout the process and enables the creation of parts with highly specific material property profiles. Advantages of the Invention: Precision in Composition Control: The metering pumps allow for precise, accurate control of additives, pigments, and / or fillers, enabling the production of thermoplastic materials with specific desired properties. Gradient Extrusion: The ability to introduce gradual changes in material composition allows forthe creation of printed parts with graded material properties, such as varying stiffness, strength, or color. Reduced Material Waste: By positioning the mixing device downstream of the extruder, the system reduces the need for purging between different colors or material composition changes, thus decreasing waste and improving efficiency. Enhanced Mechanical Properties: Additives or Fillers such as milled carbon fiber can be added directly into the feedstock as and where requested, enhancingthe strength and stiffness of the final printed part. Customizable Design: The size and form of each component of the device (extruder, mixing chamber, metering pumps, static mixer) can be customized accordingto specific application needs, offering flexibility for different types of additive manufacturing processes. The thermoplastic mixing device described herein provides a flexible and efficient solution for controllingthe composition of the thermoplastic feedstock in additive manufacturing process. By enabling gradient extrusion during printing cycle and reducing purging waste, the device enhances the precision of material properties in 3D printed parts while offering greater control over the final output quality.