A specialized injection screw-barrel system that helps controllably remove moisture, air and volatile components from the melt during plasticizing. It can be considered for ABS, PA/Nylon, PC, recycled material and similar applications with moisture/volatile-related process problems.
Key message: this system is not gas-assisted injection molding. The goal here is not to inject gas into the mold, but to help remove gas, moisture and volatiles formed or carried within the melt before injection, through the degassing/vent zone on the barrel.
Let's Evaluate the Right Degassing Solution for Your ApplicationWhat Is a Degassing (Vented) Injection System?
In a standard injection plasticizing unit, the raw material is conveyed, compressed, melted and metered along the screw. In a degassing system, in addition to this flow, a degassing zone is created within the screw-barrel geometry where pressure is controllably reduced. In this zone the melt gains more free surface area, making it easier for moisture, air or volatile components to be vented through the opening.
After the degassing zone, the screw geometry recompresses the melt and creates the metering/pressure conditions required before injection.
Schematic illustration of the special degassing screw geometry and the vent zone on the barrel
Technical note: the location of the vent/degas zone, screw geometry and fill level must be designed together. If the melt overflows into the vent opening, gas venting can become inefficient and the process can become unstable.
Working Zones
- Supply / Feed Section — taking granule or regrind raw material into the screw channels and conveying it forward.
- Compression Section — compressing the raw material and initiating plasticizing through heat and shear.
- Pressing / Melting Section — supporting melt stability and homogeneity.
- Degas / Vent Section — controllably reducing pressure to allow venting of moisture, air and volatiles.
- Metering Section — restabilizing the melt after degassing and metering it for injection.
How Does It Work?
- The raw material is taken from the feed section and plasticizes in the first compression/melting zone.
- As the screw geometry approaches the degas zone, it controllably reduces melt pressure and channel fill.
- Moisture, air and volatile components in the melt can escape through the vent/degas opening.
- After degassing, the screw recompresses and homogenizes the melt and meters it for injection.
- Once metering is complete, the normal injection cycle continues.
Degas / Vent Zone on the Machine
Example placement of the gas venting / vent zone on the barrel of an injection machine
A degassing system is not simply a hole drilled into a standard barrel. In the zone where the vent opening is located, screw channel fill, melt pressure and material conveying must be controlled through special geometry. Depending on the system, atmospheric venting, a special vent element, or vacuum-assisted venting solutions can be considered.
- Location and geometry of the vent opening
- Screw channel volume / pressure drop at the vent zone
- Process settings that prevent melt overflow
- Safe venting of gas and volatiles
- Accessible structure for cleaning and maintenance
Maintenance note: the vent opening and gas venting elements can become contaminated over time with resin residue, dust or condensate. A clean, open venting path is important for stable system operation.
Which Problems Can It Be Considered For?
- Moisture-related surface marks / splay — can help remove water vapor and gas from the melt.
- Bubble / void formation — can help reduce trapped air and volatiles in the melt.
- Volatile components and odor — can support venting of certain volatiles generated during the process.
- Unstable plasticizing — can help reduce gas buildup and variable melt behavior.
- Recycled raw material — can provide additional process control for formulations with variable moisture and volatile content.
Effect on Drying Time and Pre-Drying Needs
Under suitable raw material and process conditions, a degassing/vented injection system can reduce the pre-injection drying load. Since moisture and volatiles in the melt are removed during the process, pre-drying time can be shortened in some applications; with sufficient vent capacity, suitable screw geometry and controlled raw material moisture, pre-drying can be eliminated entirely for some materials.
Caution: this advantage does not apply to every polymer and every moisture level. For very moist raw material, high cycle loads, or hydrolysis-sensitive grades, pre-drying may still be necessary. The final drying requirement must always be verified against the relevant raw material manufacturer's technical data sheet and process trials.
Applicable Raw Materials
- ABS — vented systems can be effective in reducing moisture-related splay and surface problems; with sufficient vent capacity and pellets that are not excessively moist, pre-drying can be eliminated for some grades.
- PA / Nylon 6 — technically feasible for moisture removal; depends on the grade, and many current PA6 manufacturers still require pre-drying.
- PC — can help reduce moisture- and gas-related surface/process issues; drying time can be reduced, but the resin manufacturer's moisture limit should be the basis.
- Recycled material — can help remove moisture, trapped air and certain volatiles during the process; pre-drying can be shortened or supported depending on the actual moisture and contamination level of the material.
- PBT / PET — these polyesters are highly sensitive to hydrolysis; they should generally not be marketed as a "dryer-free process," and strict pre-drying is required for most grades.
The reference application images show PC and PBT examples. In addition, a project-based degassing screw-barrel design can also be evaluated for ABS, the PA/Nylon family, PC, recycled raw materials and other formulations with moisture/gas/volatile-related problems. Final suitability must be verified according to the material grade and manufacturer process instructions.
Effect on Product Quality
Reference application image showing a visual comparison of PC and PBT samples with degassing injection versus general injection (this is an application example, not a guarantee of exact performance)
The purpose of a degassing system is not only to vent gas, but to reduce the negative effects of gas and moisture on melt behavior, creating more stable plasticizing. The result obtained depends on the raw material, drying, temperature, screw geometry, vent setting and mold conditions.
Expected Potential Benefits
- Helping reduce gas/moisture-related surface defects
- Contributing to lowering the risk of bubbles and internal voids
- Supporting melt homogeneity and repeatability
- Increasing process tolerance when using recycled or variable raw material
- Contributing to improved surface appearance and process stability of the molded part
Vent / Gas Venting Elements
The purpose of the elements used in the vent zone is to allow gas and volatiles to be vented while helping limit melt overflow and unnecessary material loss. Geometry, opening size, surface structure and mounting method can vary by machine and formulation.
Material and Surface Options
As with standard injection screw-barrels, the basic material and surface solution for degassing screw-barrels is selected according to raw material and wear conditions. Nitrided solutions can be used for general unfilled applications, while bimetal and high-wear-resistant options can be considered for glass fiber, mineral filler or high recycled-content formulations.
- Standard / low wear — nitrided screw-barrel based on 38CrMoAlA
- High wear — bimetal / hardfacing screw and wear-resistant barrel
- High corrosion — corrosion-resistant base material or a suitable special alloy
- Wear + corrosion — a special bimetal alloy system according to the formulation
Information Needed for Technical Evaluation
- Injection machine brand, model and injection unit
- Existing screw diameter, L/D ratio and total screw/barrel dimensions
- Raw material processed and additive/filler ratios
- Existing drying system and known information about raw material inlet moisture
- Cycle time, shot size and capacity expectations
- Existing surface problem: splay, bubbles, odor, gas marks, unstable plasticizing, etc.
- Technical drawing, existing screw-barrel sample or detailed photographs
- If there is an existing vent / vacuum connection on the barrel, its dimensions and location
Frequently Asked Questions
Is a degassing screw-barrel exactly the same as a standard screw-barrel?
No. In a degassing system, the screw geometry, vent zone and recompression/metering structure are specially designed.
Can production be done without a dryer?
In some applications, yes. In systems with sufficient vent capacity and when the raw material is not excessively moist, pre-drying can be eliminated entirely for some ABS grades. In other applications, drying time can be shortened. For materials more sensitive to hydrolysis, such as PA6/Nylon and PBT/PET, the resin manufacturer's moisture limit and drying instructions should be the basis.
Can an existing injection machine be converted to a degassing system?
This can be technically evaluated on some machines; however, the barrel, screw, vent placement, injection unit and control system must all be examined together.
Is a vacuum pump required?
Not in every system. Designs can use atmospheric venting or vacuum-assisted venting. The final solution is determined according to the material, gas load and machine structure.
Is a degassing system the same as gas-assisted injection?
No. A degassing system removes gas/moisture from the melt, whereas gas-assisted injection is a different molding method that injects pressurized gas into the melt inside the mold.
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