Introduction: When Recycling Meets Overkill - The Overlooked Trap Of Granulation
In March 2025, although the gross profit margin of the recycled PP industry has expanded, the downward pressure on granule prices has increased sharply, and downstream procurement has imposed more price reductions and limited acceptance on recycled materials. At the same time, the global recycled plastic granules market is growing at a compound annual growth rate of 5.8%, heading towards a scale of 329.92 billion yuan by 2031. This is an era of both opportunities and challenges: environmental regulations are becoming increasingly stringent, and international customers are no longer satisfied with "usable" products, but are pursuing recycled granules with high cost-effectiveness and performance close to that of new materials.
However, many customers who are new to PP recycling and granulation, especially those who deal with recycled materials from PP woven bags and used daily necessities, often fall into a misconception: they believe that "strong force equals high efficiency". They configure a screw combination with extremely strong shear on the co-rotating twin screw extruder, but this results in material degradation, broken strands, and yellowing. Not only do they fail to produce good granules, but they also waste valuable recycled resources.
Today, through a real-life case study from Nanjing Kelongwell Chemical Machinery Co., Ltd., we will explore how to transform "excessive shear" into "moderate plasticization" by adjusting the screw combination, thereby truly realizing the value of plastic extruders.
Core argument: The "subtraction" of screw combinations is equivalent to the "addition" of the value of recycled particles
In a co-rotating twin screw extruder, which is a type of high-efficiency extruder, the screw configuration serves as the "heart" of the equipment. For PP recycled materials (especially those that have undergone a thermal history and are fragmented), excessive shear force is akin to "using a bull's knife to kill a chicken," potentially leading to melt fracture and thermal degradation. Only through scientific rearrangement of screw elements to reduce unnecessary shear peaks can the plasticization quality of the material and the mechanical properties of the product be ensured.
I. Phenomenon Analysis: Why can't "strong shearing" produce good PP recycled materials?
1.1 Internal impurities and carbonization: the invisible "culprit of broken strands"
Many operators have found that when using strong shear thread elements (such as large misalignment angle engagement blocks) to process PP recycled materials, the pressure fluctuation in the plastic extruder head is significant, leading to frequent strip breaks.
According to industry technical analysis, excessive shear strength in the screw can lead to increased local carbonization of the material. These carbonized substances are carried out into the material strip, and under the influence of traction, they directly cause strip breakage. PP recycled materials often contain a small amount of impurities or residual catalysts that cannot be fully sorted. When subjected to a shear rate exceeding 30/1, local overheating accelerates their decomposition.
1.2 Out-of-control melt flow rate (MFR)
One major characteristic of PP recycled material is its unstable MFR. As the number of recycling increases, the molecular chains of PP break down, leading to an increase in MFR.
· Consequence of strong shearing: Further increasing MFR leads to a sudden decrease in material viscosity, making drawing difficult and causing particles to exhibit "tailing".
· Data support: Research indicates that at high screw speeds, excessively high shear stress can significantly reduce the quality of extrusion molding.
2. Approach to Problem Solving: Cologne's "Flexible Granulation" Screw Philosophy
When dealing with the processing of PP woven bag recycled materials with significant MFR fluctuations for an international client, Nanjing Kelongwell Chemical Machinery Co., Ltd. did not recommend an aggressive strong shear solution. Instead, they conducted a "reverse optimization" of the screw combination of the twin screw extruder.
2.1 From "three heads" to "two heads": the wisdom of decentralized mixing
Before the transformation: The original process of the client utilized a three-head meshing block with an eccentric structure, aiming to enhance mixing. However, the result was a sharp decrease in tensile strength from 79.9 MPa to 52.2 MPa.
· COLOWE solution: In a co-rotating twin screw extruder, replace the three-head meshing block in the core mixing section with a double-head meshing block.
· Effect: The double-headed meshing block provides a gentler shear, but with more space, allowing for better distribution and mixing of the melt. This effectively avoids agglomerates formed by mechanical extrusion of wollastonite-like impurities (or fillers), ensuring the mechanical properties of the final recycled material.
2.2 Golden section and devolatilization of aspect ratio (L/D)
Considering the characteristics of PP recycled materials, which contain a high amount of moisture and low molecular weight substances, Kelongwell has adopted a design with an aspect ratio (L/D) of 40.
· Technical analysis: A longer aspect ratio means a more reasonable distribution of material residence time within the high-efficiency extruder. We have installed strong vacuum exhaust systems in zones 5 and 8 of the co-rotating twin screw extruder, coupled with a 15KW dual-stage water ring vacuum pump. This system immediately extracts the volatile components after the material has been fully plasticized, effectively solving the issues of "bubbles" and "yellowing" in the particles.
2.3 Reorganization of "misalignment angle" of kneading blocks
Reduce the proportion of kneading blocks with a 90° misalignment angle and increase the combination of 45° and 60° misalignment angles.
· Purpose: To reduce peak shear stress, increase heat transfer efficiency, and ensure that PP recycled material is "smoothly" plasticized in the plastic extruder, rather than being "forcibly" sheared.
3. Practical Verification: Success Behind the Data
In the laboratory of Nanjing Kelongwell Chemical Machinery Co., Ltd. and at customer sites, this adjustment plan has achieved significant data breakthroughs:
3.1 Balance between production capacity and energy consumption
· Production capacity: In response to the high production capacity (~1500kg/h) required by an Indian customer, we did not rely solely on shearing. Instead, we optimized the screw combination of a co-rotating twin screw extruder, coupled with a 500Kw main motor, to achieve stable high-output production.
· Energy saving: Due to the reduction in shear force, the load on the main engine decreases, resulting in a reduction of approximately 12% in unit energy consumption.
3.2 Leap in quality
· Particle shape: The original "tail-like" irregular particles have changed to cylindrical or spherical particles, significantly improving fluidity.
Mechanical properties: The retention rate of tensile strength and flexural strength of recycled PP has increased from 60% to over 85%.
· Filtration effect: The screen changer with a filtration area of 3000cm², combined with the efficient extruder head configuration, effectively intercepts the inertial impurities in the recycled material, ensuring stable die pressure and continuous granulation.
4. Key Q&A: The confusion you may be facing
To assist international customers who are either new to this or seeking alternatives to better understand, we have compiled the following key questions and answers:
Q1: Why do I always encounter "continuous granules" or "tailing" in my PP recycled granulation process?
A: This is usually due to excessive shear force from the screw combination of the plastic extruder, leading to excessively high melt temperature and excessive decrease in viscosity. When the material enters the pelletizing system, if the water temperature is not low enough or the cutter speed does not match, melt adhesion may occur. You can try reducing the screw speed, or as suggested by COLOWE, replacing the strong shear components with milder conveying components to reduce the melt temperature.
Q2: I want to add a large amount of calcium carbonate or additives to the recycled material. How can I prevent screw wear and poor dispersion?
A: The key lies in the screw combination design of the co-rotating twin screw extruder. Pure strong shear can lead to agglomerates (such as wollastonite agglomerates), which in turn wears down the equipment. The correct approach is to set up a distributed mixing element that disengages from the screw thread after the feeding section, allowing for pre-dispersion before gradually applying shear. COLOWE typically uses a combination of toothed discs and small misaligned angle kneading blocks in the mixing section of such high-efficiency extruders, ensuring distribution first and then dispersion, protecting the threaded elements while ensuring the plasticizing effect.
Q3: Facing the new environmental regulations of the European Union, how can I prove that my recycled particles are of high value?
A: High-value recycled particles must have a stable and traceable plasticization process. According to the data from the "2025-2031 Global Recycled Plastic Particle Industry Report", the market demands particles with low odor and high purity. Through the precise screw combination control of the Colongweil co-rotating twin screw extruder, the material is ensured to stay in the barrel for an appropriate duration, undergoing gentle plasticization without dead zones. This is the technical cornerstone for producing high-value recycled materials.
Conclusion: COLOWE, helping you accurately shape the future amidst the green wave
In this era where the global plastics industry is transitioning towards a circular economy, simple recycling can no longer meet the market's demands. Nanjing Kelongwell Chemical Machinery Co., Ltd. specializes in the core technology of twin screw extruders-screw combinations. We do not believe in "one-size-fits-all screws", but rather in customized engineering tailored to material characteristics, especially for variable PP recycled materials.
Whether it's the PP woven bags, waste films, or high-filled modified recycled materials in your hands, COLOWE can help you turn shear force from "resistance" to "motivation" by adjusting those seemingly simple but technologically advanced screws, successfully producing "green particles" that meet international standards.
Because true professionalism involves knowing when to "let go". Choose COLOWE, and choose a successful granulation experience.
