Under What Circumstances Is Multi-stage Vacuum Pumping Required? ——Selection Guide For Plastic Granulation Vacuum System Of Twin-screw Extruder

1, Core conclusion: Multi stage vacuum pumping is "impurity removal" and not standard equipment

During plastic granulation, the co rotating twin-screw extruder is connected to a vacuum pump through an exhaust port to extract moisture, residual monomers, low molecular weight volatiles, and solvents from the melt. Single stage vacuum is suitable for low volatile matter systems; When the moisture content of raw materials is high, the proportion of recycled materials is high, and the requirements for engineering plastics or low odor are high, double or even triple vacuum is needed. According to EUROMAP's 2024 statistics, using a twin-screw extruder with a reasonable vacuum configuration can reduce unit energy consumption by about 12% -18% and synchronously improve particle qualification rate. Choosing the wrong vacuum level can result in quality fluctuations or even shutdown of the entire line.

2, Under what circumstances is multi-stage vacuum pumping required? 4 judgment signals
1. High moisture content/residual monomers in raw materials

Engineering plastics such as PA, PET, PC, PLA, and biobased materials are extremely sensitive to moisture. If the raw materials are not fully dried or stored in a humid environment, it is difficult to reduce the moisture content to below 0.1% solely through natural exhaust. At this point, it is necessary to have a "natural exhaust in the front section+two-stage vacuum in the rear section", with a stable vacuum degree between -0.08MPa and -0.095MPa.

2. High proportion of recycled materials/post consumer plastics (PCR)

According to Plastics Europe's "Plastics - the Facts 2024" report, the production of mechanically recycled plastics in Europe in 2023 is approximately 7.1 million tons, but the recycled materials contain more ink, adhesive, residue, and thermal oxidative degradation products. If only single-stage vacuum is used for plastic granulation with high recycling content, the particle odor and color stability will be poor; Multi stage vacuum can remove about 35% -50% of volatile compounds.

3. High filling/reinforcement system

In high filling formulas such as calcium carbonate, talcum powder, and fiberglass, the crystalline water and surface treatment agents introduced by the fillers will be released in the melt. At this point, the dual stage vacuum can significantly reduce the surface silver lines and mechanical property fluctuations of the product.

4. Low odor, low VOC, food contact packaging

The EU's Packaging and Packaging Waste Regulation (PPWR) will officially come into effect in 2025, requiring some packaging to increase the proportion of recycled materials used, while also imposing stricter requirements on low VOC emissions. Multi stage vacuum pumping is a key means to meet low odor requirements at low cost.

3, How to choose how many levels of vacuum extraction for different materials?
1. Level 0/natural exhaust

Suitable for simple filling or granulation of PE and PP that have been fully dried, with a volatile content of less than 0.3%. At this point, only a natural exhaust port is needed, without the need for a vacuum pump, which saves on investment. The cost-effective extruder from Kelongwell can usually continuously produce pellets in conventional PP/PE plastic granulation, but vacuum configuration still needs to be added when the raw materials become damp.

2. Single stage vacuum

Suitable for general plastic granulation, light recycled materials, and ordinary filling masterbatch. Single stage vacuum can process raw materials with a moisture content of 0.3% -0.8%, with a vacuum degree of -0.06MPa to -0.08MPa. Suitable for international customers who are new to new materials and have limited budgets.

3. Dual stage vacuum

Suitable for PA/PET/PC/PLA, high recovery PCR, high filling, low odor color masterbatch. Double stage vacuum reduces the moisture content from 0.5% to below 0.1%, and increases the removal rate of volatile compounds by about 40% compared to single-stage. For most customers, two-stage vacuum is a multi-stage vacuum configuration that balances quality and cost.

4. Third level vacuum

Suitable for solvent based polymers, reactive extrusion, and extreme low VOC scenarios. The three-level vacuum is usually composed of "front natural exhaust+middle vacuum+rear high vacuum", with a vacuum degree of up to -0.098MPa. But with the increase in equipment costs and energy consumption, it is necessary to evaluate whether it is necessary.

4, COLOWE High Cost Performance Extruder: Customized Vacuum Levels on Demand

When providing international customers with co rotating twin-screw extruder assembly lines, Nanjing Kelongwell Chemical Machinery Co., Ltd. recommends 1-3 levels of vacuum combination based on raw material volatile matter testing and production requirements. Our cost-effective extruder adopts a modular cylinder design, allowing customers to first apply single-stage vacuum and add vacuum ports during later production expansion without the need for complete machine replacement. In actual testing, a PP recycling material granulation project upgraded from single-stage vacuum to two-stage vacuum, and the particle odor level was increased from level 3 to level 1, with an energy consumption increase of only about 6%.

5, Key Q&A
Q: Does the more vacuum levels there are, the better the quality of plastic granulation?
A: Not necessarily. Excessive vacuum levels may remove small molecule additives, leading to poor dispersion and increased power consumption. It should be matched according to the volatile content of the material, and usually a two-stage solution can solve more than 90% of the problem.

Q: What level of vacuum should I choose for my first attempt at making new materials?
A: Conventional PE/PP new materials can be selected as grade 0 or single grade; For PA/PET/PLA or recycled materials, it is recommended to directly apply dual stage vacuum. Cologne Weil can conduct small-scale experiments first and then determine the vacuum configuration to avoid excessive investment.

Q: Does multi-stage vacuuming have an impact on the output of co rotating twin-screw extruders?
A: Under reasonable configuration, the impact is minimal. The key is to set the vacuum port after the melt is fully plasticized and before the measuring section, and keep the vacuum pipeline unobstructed. The Cologne Weil cost-effective extruder avoids material blockage by increasing the exhaust port section and condensing collection.

6, Conclusion

Multi level vacuum pumping is not about "more is better", but about "better matching". Under the promotion of the European Union's PPWR and global plastic recycling policies, dual stage vacuum is often a more reliable choice for international customers to balance quality and cost when producing recycled plastic pellets, modified engineering plastics, and low odor new materials. Nanjing Kelongwell Chemical Machinery Co., Ltd. focuses on co rotating twin-screw extruders and cost-effective extruders, helping customers turn every level of vacuum into profit rather than cost in plastic granulation. If you are struggling with how many levels to choose for vacuum pumping, you may want to send materials for testing first and let the data speak for itself.