PC, PA, PET, and PBT are all engineering plastics, but their melt flow, moisture behavior, and crystallization are very different. Because of this, the same injection molding screws will not give the best plasticizing effect for all four materials. Matching screw geometry (length‑to‑diameter L/D ratio, compression ratio, and mixing or barrier features) to each resin can improve melt uniformity, reduce degradation, and lower scrap rate.

PC has high viscosity and good thermal stability, so it needs enough shear and residence time to melt completely, but must avoid excessive shear burn on sensitive grades or color concentrates. Guides for PC molding often recommend a screw L/D of at least 20:1, and many practical applications use 20–26:1 to achieve better plasticizing.

For injection molding screw selection for PC:

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In practice, if the PC part shows unmelted particles or shot‑to‑shot variation, a longer L/D or a barrier/mixing section can help improve melting and homogenization.

PA (such as PA6 and PA66) has strong moisture absorption, and its mechanical properties and molding behavior change with water content. Moisture leads to splay, silver streaks, and internal porosity that cannot be fully fixed by changing only temperature or speed.

For the injection molding machine screw design for PA:

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If your main issue with PA is bubbles and cosmetic defects from moisture, consider combining this screw design with better venting and strict moisture control instead of only increasing back pressure or screw speed.

PET for injection (for example, preforms) has high crystallinity potential and is very sensitive to moisture and degradation; many PET screws are barrier designs. In barrier screws, a second flight separates solid and melt so pellets can fully melt while keeping shear heating under control.

For injection molding screw selection for PET:

If you see acetaldehyde issues, yellowing, or loss of IV (intrinsic viscosity) in PET parts, it is usually better to optimize barrier screw design, drying, and residence time rather than only reducing temperature.

PBT is another polyester engineering plastic with faster crystallization than PET and good flow, but it is also sensitive to moisture and thermal degradation. Its processing window is generally lower than that of PET, but it still benefits from controlled shear and uniform melting.

For injection molding, the screw choice for PBT:

When PBT shows burn marks or brittleness, you should check screw shear level (compression ratio, speed, and back pressure) together with drying and residence time instead of adjusting only cavity venting.

The durability of an injection molding screw is determined by the base material and the surface treatments applied to it. In industrial environments, minimizing downtime due to screw wear is a priority.

Nitriding is a heat-treatment process that diffuses nitrogen into the surface of the steel (typically SACM645). This creates a hard, wear-resistant layer. Nitrided screws are suitable for processing non-abrasive plastics like standard PP or PS. However, they may not hold up well against glass-fiber-reinforced PA or PBT.

Bimetallic screws involve casting a wear-resistant alloy layer (usually a nickel-based or tungsten carbide alloy) onto the screw flights. This provides superior resistance to both abrasion and corrosion. For processing glass-filled resins (common in PA and PBT applications), bimetallic screws are the industry standard, as they offer significantly longer lifespans compared to nitrided versions.

For materials like PC and PET, which are prone to sticking and degradation, chrome plating is often applied. The plating provides a very smooth surface finish, reducing the coefficient of friction and preventing the resin from adhering to the screw, which reduces the occurrence of "black spots" or carbonization.

PC, PA, PET, and PBT each place different demands on injection molding screw design because of their unique viscosity, moisture sensitivity, and crystallization behavior. By focusing on L/D ratio, compression ratio, and whether you need barrier, mixing, or vented features, and by integrating proper drying and process control, you can choose an injection molding screw that supports stable plasticizing, fewer defects, and better part performance for each of these engineering plastics.