Polyethylene terephthalate (PET) has become one of the most widely used thermoplastics in the packaging industry due to its strength, clarity, and recyclability. However, processing PET requires careful attention to equipment configuration—especially the design of the extrusion or injection molding screw. Because PET is sensitive to thermal degradation and moisture, screw geometry and processing parameters must be engineered with exact precision to maintain material integrity and maximize product quality.
For any Industrial Polymers Company working with PET, understanding screw design fundamentals is critical to achieving consistent output and minimizing defects. The choice of screw affects every stage of processing, from melting and homogenization to devolatilization and pumping. This article explores key design considerations and how they influence PET resin processing efficiency and performance.
Thermal Sensitivity of PET and the Role of Screw Geometry
One of the defining characteristics of PET is its vulnerability to thermal and hydrolytic degradation. When exposed to excessive heat or moisture during processing, the polymer chains can break down, reducing molecular weight and compromising final product properties such as tensile strength, clarity, and barrier performance.
Screw design plays a central role in managing this thermal sensitivity. A well-designed screw provides sufficient shear to melt the resin quickly while minimizing residence time to avoid overheating. This balance is achieved through careful adjustment of screw length-to-diameter (L/D) ratio, compression ratio, and channel depth. PET processing typically benefits from longer L/D ratios—often in the range of 24:1 to 30:1—allowing for gradual melting and efficient venting zones.
An effective screw design ensures that heat is applied uniformly and gradually, preventing hot spots that could lead to localized degradation. This is particularly important in operations where recycled PET (rPET) is processed, as it often contains more contaminants or variable melt flow characteristics.
Importance of the Feed, Compression, and Metering Zones
The screw is divided into three functional zones: feed, compression (or transition), and metering. Each of these zones must be configured to suit the specific melting profile of PET resin.
In the feed zone, pellets are introduced and begin to heat up. The design must allow for controlled conveying while avoiding premature melting that can lead to bridging or inconsistent feeding. Groove feeding in the barrel wall is sometimes used to increase friction and stabilize flow in this zone.
The compression zone is where most of the melting occurs. This section of the screw reduces channel depth, compressing the material to generate shear and thermal energy. For PET, this compression must be carefully managed to achieve a complete melt without generating excessive shear heat, which could trigger degradation.
In the metering zone, the molten resin is homogenized to ensure a consistent melt temperature and viscosity before it is forwarded to the die or mold. Uniformity in the metering zone is essential to avoid flow inconsistencies, which can lead to part defects or poor mechanical properties.
A leading Industrial Polymers Company often customizes screw profiles in this section to match the thermal and rheological behavior of PET, balancing throughput with melt stability.
Devolatilization and Venting for Moisture Control
PET must be dried before processing, typically to a moisture content below 50 ppm, to prevent hydrolysis. However, even with adequate pre-drying, residual moisture or volatiles may remain. Devolatilization within the screw can serve as an additional safeguard.
To enable efficient venting, the screw may include a vacuum or atmospheric vent port in a devolatilization zone positioned after the melt has formed but before final homogenization. The screw channel in this section is often deep and open to allow trapped moisture, air, or volatile degradation byproducts to escape.
The effectiveness of venting depends not only on screw geometry but also on the integration of vacuum systems and barrel design. Preventing melt backflow into the vent port is crucial, which is why barrier flights or vent stubs are sometimes used in this region.
Advanced screw designs developed by experienced Industrial Polymers Company engineers incorporate these features, ensuring PET can be processed at high throughput rates without risking hydrolytic degradation.
Special Considerations for rPET and Additive-Loaded Blends
As sustainability initiatives expand, the use of recycled PET has grown rapidly. However, rPET introduces additional challenges due to its varying viscosity, presence of contaminants, and potential degradation from prior processing cycles. Screw designs that work well for virgin PET may not perform optimally with rPET.
To accommodate rPET, screw designers may increase the length of the compression zone to enhance melting or add mixing elements to improve homogenization. In some cases, twin-screw extruders are used instead of single-screw machines, especially for compounding PET with stabilizers, impact modifiers, or colorants.
The inclusion of additives such as colorants, UV blockers, or nucleating agents also influences screw performance. These materials must be dispersed uniformly without experiencing thermal damage or creating gel particles. Properly engineered screws maintain uniform shear and avoid dead zones where materials might stagnate.
An Industrial Polymers Company offering customized screw configurations will typically evaluate the full formulation and recommend changes to screw geometry to accommodate additive loading without compromising output quality.
Screw Surface Treatment and Material Selection
Because PET processing involves high temperatures and extended operation cycles, screw wear and corrosion are significant concerns. Screw materials must resist abrasion from glass-filled grades and chemical attack from potential degradation byproducts.
Screws are typically made from high-nitride steels or treated with hard-facing alloys such as Colmonoy to enhance durability. The surface finish must be smooth enough to reduce hang-up and encourage consistent melt flow but hard enough to withstand abrasive conditions.
In corrosive or demanding applications, bimetallic screws may be used, featuring a wear-resistant alloy bonded to a strong core. This hybrid design offers a combination of strength and durability, ideal for continuous PET production environments.
Innovation Through Precision Engineering
The effectiveness of PET processing ultimately rests on precise thermal and mechanical control. Screw design, while often overlooked outside of engineering circles, remains one of the most critical determinants of process efficiency, melt quality, and long-term machine performance.
From venting zones to metering geometry, every detail must be tailored to the unique thermal and flow requirements of PET. For manufacturers seeking to improve product consistency, reduce waste, and increase sustainability, collaborating with a knowledgeable Industrial Polymers Company for screw customization offers a distinct advantage.
Investments in advanced screw design not only enhance immediate processing results but also future-proof operations for emerging PET applications, whether in food-grade packaging, medical components, or high-performance industrial parts. In polymer engineering, precision begins at the screw.