Vacuum degassing is one of the most process-critical steps in PVC compounding and is frequently the difference between a compound that meets surface finish and regulatory specifications and one that does not. Yet it is often the least discussed aspect of extruder selection and process optimisation.
This article covers what degassing is, why it is particularly important in PVC processing, how the vent system is designed and operates in planetary roller and twin-screw extruders, application-specific vacuum requirements, and how to diagnose and resolve the most common vent problems.
What Is Vacuum Degassing?
In extrusion compounding, polymer melt contains dissolved or physically trapped volatile components: water absorbed from atmospheric humidity, residual vinyl chloride monomer (VCM) from PVC manufacture, plasticiser vapours in soft PVC, HCl released by thermal degradation, pyrolysis gases from wood flour, or contaminant vapours in post-consumer recyclate. If these volatiles remain in the compound at the die exit, they cause:
- Surface blistering, pitting, or streaks in rigid PVC profiles
- Reduced clarity and increased haze in transparent compounds
- Internal porosity in pellets, reducing pellet bulk density and downstream processing consistency
- Melt pressure instability at the die, causing diameter variation in profile extrusion
- VCM carry-over, which has occupational health and regulatory implications
Vacuum degassing addresses these problems by applying partial vacuum at a vent port in the barrel. Where the polymer melt surface is exposed to the vent, volatile components diffuse to the surface under the partial pressure gradient created by the vacuum pump, and are extracted from the process stream before reaching the die.
Why Degassing Is Critical for PVC
PVC presents volatile management challenges that most other thermoplastics do not.
Hydrogen chloride (HCl). PVC undergoes thermal dehydrochlorination — loss of HCl along the polymer backbone — at temperatures above approximately 170°C, even with stabiliser present. Once HCl is released, it is a chain-reaction initiator: it catalyses further HCl release from neighbouring PVC repeat units. Effective vacuum degassing removes HCl as it forms, interrupting the autocatalytic cycle and substantially extending the useful thermal stability window of the formulation. This makes degassing not merely a quality measure but a process stability measure.
Residual VCM. Suspension-grade PVC contains residual VCM at levels typically below 1 ppm after stripping at the resin manufacturer, but compounding heat may release trace amounts. Where the compound is used for food-contact applications, VCM content must meet regulatory limits (EU regulation EC 10/2011 specifies a migration limit of 0.01 mg/kg food). Effective degassing during compounding provides an additional barrier.
Moisture. PVC dry blends exposed to atmospheric humidity absorb surface moisture rapidly. Moisture above approximately 0.05% by weight causes steam formation in the melt, leading to surface blistering on rigid PVC profiles and, at higher levels, visible foam voids in the compound pellets. Degassing removes this moisture before the compound reaches the die.
Plasticiser vapours (soft PVC). High-molecular-weight phthalate and non-phthalate plasticisers have low vapour pressure at PVC processing temperatures, but they do volatilise at measurable rates, particularly in the early part of the barrel where the compound is still warming. Vacuum degassing captures these vapours — relevant both for product quality and for compliance with workplace exposure limits for plasticiser aerosols.
Vacuum Degassing in Planetary Roller Extruders
In a planetary roller extruder, the melt film formed between the satellite rollers and the barrel wall is typically 0.1–1 mm thick. At the vent port position, this thin film is directly exposed to the partial vacuum. Volatile components need to diffuse only fractions of a millimetre to reach the melt surface — a distance that volatiles in a low-viscosity melt traverse in less than a second.
This high surface-area-to-volume ratio at the vent means that a single optimised vacuum vent positioned correctly in the planetary processing zone is sufficient for most PVC applications. There is no requirement for a dedicated decompression screw zone or a partially filled melt pool design, because the thin-film geometry inherently provides the exposed melt surface that twin-screw machines must engineer explicitly.
PLATEX by Takımsan machines are configured with one or two vacuum vent ports depending on the application. For demanding applications (WPC, PCR, highly plasticised soft PVC), a two-vent configuration provides two opportunities for volatile removal — the first vent removing bulk volatiles, the second removing residual traces. Vacuum levels are set independently for each vent.
The vent location on a planetary roller extruder is typically positioned at 60–80% of the processing length — after the material has fully melted and been homogenised, but before the final sealing zone that builds die pressure. This ensures that the melt at the vent is fully plasticised (no unmixed dry-blend powder that would block the vent) but not yet over-processed.
Vacuum Degassing in Twin-Screw Extruders (Reference)
For comparison: a twin-screw extruder requires a dedicated decompression zone — a section of reduced screw channel depth — to create a partially filled melt pool that exposes melt surface to the vent opening. Because the melt pool is much deeper than a planetary roller film (typically 3–10 mm compared to 0.1–1 mm), volatile diffusion distances are much longer, and a single vent stage is often insufficient for highly volatile compounds. Two or three vent stages, each with its own vacuum level, are common for WPC and PCR applications on twin-screw systems. The vacuum system complexity and the sensitivity of the vent design to operating conditions (screw speed, throughput, compound viscosity) are correspondingly higher. See Planetary Roller Extruder Technology, Explained for a broader comparison of the two technologies.
Vacuum System Design: Operating Parameters
Vacuum level. All vacuum levels below are absolute pressures:
| Application | Recommended vacuum (mbar abs.) | Rationale |
|---|---|---|
| Rigid PVC (profiles, pipe) | 100–200 | Moisture + HCl removal; low risk of melt foaming |
| Soft PVC (cable, hose) | 100–150 | Plasticiser vapour + moisture; avoid plasticiser loss |
| WPC (50–65% wood flour) | 50–100 | Wood pyrolysis gases + moisture require lower pressure |
| PCR (post-consumer recyclate) | 30–80 | Variable volatile content; adjust based on product quality |
| Colour masterbatch | 150–200 | Primarily moisture; higher vacuum not required |
Vacuum pump selection. A liquid ring vacuum pump is the most common choice for compounding applications, because it can handle wet gas streams (moisture-laden vapour) without pump damage. Dry-running vane pumps offer better energy efficiency but require a condensate separator upstream. Size the pump for the vent gas flow rate — typically 10–50 m³/h per vent port depending on throughput and volatile content — plus a margin of at least 50%.
Condensate collection. Moisture and volatile organics condensed from the vent stream must be collected and disposed of correctly. A glass or stainless-steel condensate trap between the vent and the vacuum pump prevents liquids from reaching the pump. For VCM-containing streams, closed condensate collection with activated carbon capture is required under EU industrial emissions regulations.
Vent valve. Install a manual isolation valve at each vent port. This allows the vent to be closed quickly if vent freeze occurs, and allows vent cleaning without shutting down the entire line.
Application-Specific Vacuum Requirements
Rigid PVC profiles and pipe: Set vacuum to 100–200 mbar absolute. Verify compound moisture by Karl Fischer before processing if storage conditions are uncertain. A well-functioning vent should draw clear vapour; brown or discoloured vapour indicates HCl generation and a thermal stability problem requiring formulation review.
WPC (wood-plastic composite): Two-vent configuration recommended. First vent at 80–120 mbar (bulk moisture removal), second vent at 50–80 mbar (pyrolysis gases). Verify wood flour moisture below 2% before compounding — moisture above 2% will overwhelm the vent system and cause foaming at the die. For process parameters, see WPC Compounding.
Post-consumer recyclate (PCR): Variable volatile content makes vacuum control critical. Start at 80–100 mbar and monitor compound surface quality at the die. If surface blistering occurs, reduce vacuum in 10 mbar steps until the problem resolves. PCR streams containing PVC need HCl monitoring at the vent exhaust. See Plastic Recycling Compounding for additional process guidance.
Flame-retardant cable compound (ATH/EVA): ATH begins to release water at approximately 200°C and decomposes fully by 220°C. Processing temperature must be held below 190°C — enforced by the planetary roller’s barrel-temperature-dominated thermal profile. Vacuum at 100–150 mbar removes surface moisture from ATH before it can cause compound foaming. The planetary roller’s thermal control advantage here is significant; see Filler Loading Limits in Compounding for ATH loading guidance.
Troubleshooting Vacuum Degassing Problems
Vent freeze (vent plugging): Symptom: Vacuum gauge shows rising pressure; visual inspection shows compound blockage in the vent port. Causes: Vent positioned in a zone where melt pressure is not fully relieved; throughput too high for the vent section design; vacuum level too aggressive. Corrective action: Close the vent valve. Remove the blockage with a vent screw or compressed air. Review vent position relative to the barrel fill profile — the vent must be in a section where the barrel is only partially filled. Reduce throughput by 10–15% and retest. Reduce vacuum level by 20 mbar and retest.
Insufficient degassing (poor surface finish, high moisture): Symptom: Compound pellets show surface blistering or internal voids; Karl Fischer moisture of pellets exceeds specification; HCl odour at die. Causes: Vacuum level too low; condensate trap blocked; vacuum pump undersized or degraded; vent partially blocked. Corrective action: Verify vacuum level at the vent port (not just at the pump). Check condensate trap — if full, volatile vapour is bypassing the trap and reducing pump efficiency. Inspect pump for signs of wear (reduced flow rate, elevated running temperature). Clean the vent port.
Melt foaming at vent: Symptom: Melt rises into the vent port, creating foam-like compound; vacuum gauge fluctuates. Causes: Vacuum too high (below approximately 30 mbar) causing dissolved gas to nucleate within the melt; water content too high causing steam generation at the vent. Corrective action: Increase vacuum set-point (raise absolute pressure) by 20–30 mbar. Verify incoming material moisture. For WPC applications, check wood flour drying.
Plasticiser condensate overload (soft PVC): Symptom: Excessive liquid accumulation in condensate trap; plasticiser odour in the processing area; vacuum pump running hot. Causes: Plasticiser loading too high relative to the compound viscosity at processing temperature; vent temperature too low (plasticiser condensing in the vent port rather than being drawn to the pump). Corrective action: Heat-trace the vent port and condensate line to keep above the plasticiser condensation temperature. Verify the plasticiser specification matches the formulation design. Increase condensate trap drain frequency.
For engineering support on vacuum system specification or PLATEX by Takımsan machine configuration for your specific compound, contact the Takımsan technical team through the enquiry form. For the broader technology context, see Planetary Roller Extruder Technology, Explained and Energy Efficiency in PVC Compounding.
