Screw Combination: The Genetic Code Of Co Rotating Twin-screw Extruders - How To Determine The Success Or Failure Of Plastic Mixing?
In the microscopic world of plastic granulation, the screw combination beneath the metal casing of the equipment is the "gene" that truly defines the fate of the material. For international customers who pursue environmental protection, high cost-effectiveness, and try to produce new materials, screw combination is not simply mechanical stacking, but the underlying logic that determines production capacity, energy consumption, and physical properties.
Core viewpoint: The precise design of screw combinations directly determines the dispersibility, distribution, and thermomechanical process of co rotating twin screw extruders in plastic mixing. An optimized screw configuration can transform an ordinary formula into a high value-added product without increasing energy consumption.
1. Anatomy of "Genes": Composition and Physical Logic of Screw Combination
If the co rotating twin screw extruder is compared to the torso of the human body, then the screw combination is the blood vessels responsible for transporting nutrients and the gastrointestinal tract responsible for digestion and absorption. A complete set of screw combinations is usually composed of conveying elements, mixing elements, shearing elements, and kneading elements arranged in a specific order.
·Conveyor element (threaded element): This is the "skeleton" of the screw.
·Function: According to the lead size, it can be divided into forward delivery and reverse delivery. The large lead (such as 72/72) is responsible for fast feeding and establishing low filling levels; The small lead (such as 28/28) is responsible for establishing pressure and forming a melt seal.
·Professional perspective: In the development of new materials or the handling of high filling materials (such as calcium carbonate content ≥ 60%), the geometric shape of the conveying element determines the residence time distribution (RTD) of the material in the barrel, directly affecting the degradation risk.
·Mixing and shearing components (kneading block and toothed disc):
·Kneading block (KB): This is the core of plastic mixing.
·Neutral kneading block: Provides gentle dispersion and mixing, suitable for thermosensitive materials or PCR (post consumer recycling) plastics.
·Deflection kneading block: The shear strength is controlled by staggered angles (30 °, 45 °, 60 °, 90 °). The larger the misalignment angle, the stronger the axial reflux and the higher the shear heat.
·Tooth shaped element (TME/ZME): focuses on distributed mixing.
·Function: It can continuously divide and reorganize the material interface like a "mixer". When processing environmentally friendly biodegradable plastics such as PLA and PBAT, toothed components can achieve excellent phase control at low temperatures, avoiding high-temperature degradation.
Data support: According to the "White Paper on Extrusion Technology" released by the Institute of Plastic Processing (IKV) at RWTH Aachen University in Germany, when processing polypropylene (PP) systems containing more than 30% talc powder, using a combination of "single head thread+60 ° staggered angle kneading block" screws can improve the tensile strength uniformity (CV value) by about 22% compared to the ordinary "double head thread+90 ° staggered angle" combination.
2. Personalized expression of materials: screw combination strategies under different systems
The biggest misconception for international customers who are new to extrusion granulation is that 'one set of screws dominates the world'. In fact, the screw combination must be customized according to the physical state and chemical properties of the material.
2.1 High cost-effectiveness: The "fatigue war" of high filling masterbatch
In the pursuit of high cost performance calcium carbonate and talc powder high filling systems, the key lies in "wetting" and "dispersion".
·Problem: Powder is prone to "bridging" or "returning" at the feeding port.
·Solution: Use high lead conveying components (such as 72/72 or 64/64) in the feeding section to quickly push the powder towards the melting zone. Install a combination of toothed discs and reverse threaded components at the end of the melting zone.
·Refined analysis: This combination forces the material to withstand high shear stress in a fully filled state, breaking down powder agglomerates through "tensile rheology". Without this combination, even with expensive coupling agents, it is impossible to achieve a perfect combination of chemical bonding and physical adsorption.
2.2 Attempting to produce new materials: the "temperature game" of special engineering plastics
For customers attempting to produce high-temperature nylon (PPA), polyether ether ketone (PEEK), or liquid crystal polymer (LCP), screw combination is the key to thermal mechanical stability.
·Core pain point: Under high temperature, even small shear dead zones can cause material carbonization, forming "black spots", which is fatal for high-end thin films or fiber grade materials.
·Industry trend: With the increasing demand for temperature resistance of connector materials in electric vehicles (EVs) (requiring UL94 V-0 and CTI ≥ 600V), the screw combination of co rotating twin screw extruders is evolving towards "low shear, high dispersion".
·Authoritative quotation: According to the 2024 Global Advanced Plastic Processing Technology Trend Report, more than 75% of high-performance plastic modification accidents (such as black spots and gel) are due to improper design of shear peaks in screw combinations. Nanjing Kelongwell Chemical Machinery Co., Ltd. adopts a variable lead transition structure when customizing this type of screw, eliminating shear mutations and ensuring that the fluctuation of melt index (MFI) is controlled within ± 3%.
3. Environmental Protection and Sustainability: The Screw Logic of PCR Plastics and Biobased Materials
The global plastic industry is currently facing strict regulation from the European Union's Packaging and Packaging Waste Regulation (PPWR), which imposes mandatory requirements on the proportion of recycled materials (PCR) used. This directly poses a challenge to the plastic mixing process.
"Impurity tolerance" in plastic mixing:
PCR plastics often contain cross-linked gel, ink residue and mixed polyolefins with different melt fingers.
·Screw combination response: Introduce the arrangement of multi-stage kneading blocks and reverse threaded components.
·Mechanism of action: By establishing a circulation path of "melting filtering shearing remelting", the low melting finger "hard block" is uniformly dispersed using strong distribution mixing, rather than being cut off by strong shearing (to avoid the formation of burrs).
·Current news: In the third quarter of 2024, several modification giants in Europe announced that the quality of recycled materials did not meet the standard due to the inability of traditional single screw to handle PCR with high impurity content. The industry solution is shifting towards using a high free volume ratio co rotating twin screw extruder, which optimizes the axial force distribution in the screw combination and achieves direct modification of soft pack recycling materials containing less than 5% aluminum foil impurities.
4. Key Questions and Answers (Q&A)
In order to help you better choose equipment that suits your materials, we have extracted three core questions and answers to address common confusion among international customers:
Q1: Why do I use the same formula, but there are always color deviations in different batches of granulation?
A: This is usually due to a problem with the symmetry of the mixed element arrangement in the screw combination. If the spacing (L/D) between the kneading blocks is not an integer multiple, or if there is a lack of reverse conveying elements to establish a stable melt barrier, it will lead to a "tailing" phenomenon in the residence time distribution (RTD) of the melt inside the barrel. Some materials stay for too long and undergo thermal oxidation degradation, resulting in color difference. It is recommended to set precise melt sealing sections in the screw combination to ensure narrow distribution of RTDs.
Q2: Why does the smell of "burning" always appear when processing biodegradable plastics (such as PBAT/starch blends)?
A: Biodegradable plastics are extremely sensitive to thermal history and shear heat. If traditional screw combinations use kneading blocks with large column angles (60 ° or 90 °), they will generate extremely high local shear heat. The correct approach is to use multiple sets of small staggered angle kneading blocks (30 ° or 45 °) alternately arranged with toothed mixing discs to disperse the shear energy in a longer L/D (aspect ratio) range, achieving the effect of "slow temperature rise and uniform mixing". At the same time, the screw core shaft should adopt a high torque design, providing high torque at low speeds to avoid friction overheating.
Q3: When producing high filling (70% fiberglass or mineral powder) materials, the screw wear is extremely fast. How can I solve this problem through combination optimization?
A: Wear mainly occurs at the top of the kneading block in the melting zone and the compression surface of the reverse thread. The optimization strategy for screw combination for high filling materials is:
a. Segmented processing: Split the originally concentrated strong shear segments into three segments: "pre dispersion main dispersion fine dispersion".
b. Material upgrade: Kneading blocks made of powder metallurgy wear-resistant alloys (such as high vanadium high-speed steel) are used in the wear area.
c. Topology optimization: Reduce the use of 90 ° kneading blocks and switch to eccentric or spiral kneading blocks. These new components can disperse wear to a larger contact surface. Nanjing Kelongwell Chemical Machinery Co., Ltd. provides screw wear life prediction services based on finite element analysis (FEA) to help customers achieve the lowest maintenance cost per ton under high wear conditions.
5. Conclusion: The technological dividend from "universal" to "customized"
In the application world of co rotating twin screw extruders, screw combination is the only carrier that can directly interact with materials. For international customers who pursue environmental protection, high cost-effectiveness, and try to produce new materials, abandoning the fantasy of "universal screws" and embracing customized screw combinations based on material rheology is the key to enhancing competitiveness.
Data review: Through precise screw combination optimization, enterprises can usually achieve:
·Energy consumption reduction: 12% -18% more energy-efficient than traditional universal screw solutions (data source: International Rubber and Plastic Exhibition K Show 2022 Technical Forum).
·Production increase: Under the same torque, by optimizing the axial force distribution, the production capacity can be increased by up to 25%.
·Quality leap: For optical grade PC or LED diffuser special materials, the number of gel points has dropped from 50 per square meter to less than 5.
When you face a complex plastic mixing task next time, remember: every rotation of the screw combination in your hand, every misalignment of the kneading block, is writing the physical and chemical properties of your product. Choosing the right 'genetic code' is giving your product strong vitality.
