Tips for Perfect Combination of Twin-Screw Extruder And Thermoplastic Elastomer

1. High-speed energy twin-screw extruder

Advances in high speed energy input (HSEI) twin screw extruder (TSE) design are opening new opportunities for thermoplastic elastomer/thermoplastic polyolefin/thermoplastic polyurethane/thermoplastic vulcanizate (TPE/TPO/TPU/TPV) formulations. This family typically includes plastics and rubbers mixed with fillers, fibers, additives, liquids and reactants. Raw materials come in a variety of forms, including pellets, powders, chunks, strips, fibers and liquids – and they must always be metered as they are fed into the extruder process section. How and where the raw materials are metered is determined by a variety of factors and the overall recipe.

Achieving a well-mixed and/or reactive melt stream is always the goal. However, even this is only half the battle, as the melt stream must be cooled down and then formed into the end product, often pellets for injection molding machines or single-screw extruders. Direct extrusion of sheets or profiles bypasses this step. In this way, inline molding becomes possible.

High-speed energy input twin-screw extruders often suffer from underfeeding, and the extruder output rate is determined by the feeder. The speed of the twin-screw extruder screws is controlled individually. Single-screw and twin-screw augers are used for granules and powders. When feeding multiple raw material streams, loss-in-weight feeders are usually specified. Liquid raw material streams (i.e. oil or supercritical reaction liquid) use plunger pumps or gear pumps depending on the viscosity of the liquid. Liquid systems can be heated or kept at room temperature. Feeding rubber materials is often quite challenging. The size of the raw material must be smaller than the screw flight depth.

For example, a twin screw extruder with a screw diameter of 77.5 mm will have a flight depth of approximately 15 mm. Obviously, it is not possible to feed a bale of rubber into an extruder in one piece and it is necessary to modify its form; the feeding device should accurately meter it as it enters the extruder process section. Various methods are used, including crushing and grinding the rubber into chunks that the auger can measure. The rubber strips entering the twin screw extruder process section can also be metered. Some rubber is allowed to melt before being pumped into the extruder. It is common practice to use a dedicated single screw extruder equipped with a gear pump to regulate and meter the rubber entering the twin screw extruder.

A few examples of processes/applications related to this are briefly described below.

2. TPU reactive extrusion system with water-immersed pelletizer

The flow of liquid feedstock into the extruder is metered by precision metering pumps using flow meters or loss-in-weight monitoring/control.

The L/D ratio of a twin screw extruder is usually between 40/1-52/1 to allow the feed stream to maintain sufficient residence time in the twin screw extruder process section. The design focuses more on distributive mixing than dispersive mixing, with the choice of screw elements to manage low peak shear intimate mixing. The use of "combs" or other similar distributive mixing elements is usually specified, and the feed rate and the speed of the twin screw extruder screws can control the residence time required to promote the reaction and the distribution of residence time.

A twin screw extruder is matched with an underwater pelletizer to pump the TPU melt through a circular die and cut into pellets by rotating blades immersed in water. The aqueous pellet slurry is then conveyed to a rotary dryer for dewatering. The system can produce spherical pellets cut in an underwater environment.

3. TPE/TPO direct extrusion sheet

Combining compounding/devolatilization of materials with direct sheet extrusion using a high-speed energy input twin screw extruder presents a technical challenge. Such a system requires a combination of intimate mixing and continuous pumping of the materials. In actual production, twin screw extruders often compound polyethylene/polypropylene pellets with pre-mixed rubber formulations. Plasticizers are injected into the processing section of the extruder, and fillers (i.e. talc or fibers) are added to the downstream barrel section by side filling devices. This can produce useful end products, including TPO roofing membranes and/or high oil content sheet products.

Direct extrusion systems combine material handling/feeding equipment with an underfed twin-screw extruder and a gear pump that matches the die. Melt temperature plays an important role and the system is more complex than the pelletizing system. Specifically, a programmable logic controller (PLC) executes a pressure control algorithm that analyzes inputs at key points in the system, performs numerical calculations, and modifies screw speed, gear pump, and feed rate to keep the die inlet pressure constant. The die/downstream system is like a single screw extruder processing premixed pellets.

4.TPV and rubber strip feeding mechanism

Before adding the rubber, the polypropylene pellets are fed into the twin-screw extruder feed port, metered and melted. The rubber strands can be metered by a Planetruder internal gear pump extrusion system, which combines a single screw extruder (such as a pin barrel extruder) with an integrated gear pump. The polypropylene and rubber are intimately mixed and then the crosslinker is injected. Vacuum exhaust helps to remove volatiles from the material before pressurizing the pelletizing die or gear pump for direct extrusion.

5. TPO and long fiber feeding and downstream sheet/profile

For long fiber TPO products, the first 24L/D of the twin screw extruder process section is set to feed/mix rubber, polymer, minerals and liquids. The fiber roving is added to the melt stream, and distributive mixing elements are used to protect the fiber length to optimize the physical properties of the final product.

In addition to fiber length, it is also important to "wet" the fiber, making the twin-screw extruder ideal for this balancing act. If the fiber bundle is not fully opened, the fiber will be very long, but its physical properties will be very poor because the fiber is not wetted. Short single-screw pumps with deep threads and non-compression designs are often used to provide stable die inlet pressure at low speeds. Using a similar twin-screw extruder system configuration as part of an inline molding system is typical for automotive parts.

Advances in twin screw extruders offer opportunities for processing that is shear sensitive or limited by torque and/or temperature. Regardless of the formulation and/or end product, it is important for processors to understand and employ the latest technology to achieve success.

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