Injection-molding screws: What exactly distinguishes A, B, and C? This article provides all the answers.

When visiting an injection-molding workshop or ordering a new machine, you will inevitably hear the same question from the manufacturer: "Which screw—A, B, or C—should you equip this machine with?" Many people tend to treat this like choosing clothing sizes S, M, and L; some even, for the sake of convenience, simply opt for the middle option, screw B. In actual production, however, choosing the wrong screw can lead to minor issues such as product shrinkage, uneven surface finish, black spots, or yellowing; in more severe cases, it can prolong the production cycle or even cause the raw material to burn and degrade directly inside the barrel. Today, let's discuss the fundamental differences among these three screw types and how to select the appropriate model.

I. First, get a clear understanding of the situation:

What exactly makes ABC screws inferior?

For the same tonnage (the same clamping force and injection-table base), when an injection-molding machine manufacturer produces these three types

of screws, they essentially adjust only two key physical parameters: diameter and length-to-diameter ratio (L/D). Type A screw: the smallest diameter and the highest length-to-diameter ratio; Type B screw: a medium diameter and a medium length-to-diameter ratio (the default "one-size-fits-all" configuration at the factory); Type C screw: the largest diameter and the smallest length-to-diameter ratio. Once the cross-sectional area changes, the two core process parameters are directly affected: injection volume (capacity) and injection pressure. The underlying physical principle is straightforward: under the same thrust, a smaller cross-section results in higher pressure, while a larger cross-section results in more distributed pressure. This creates a natural difference in performance characteristics:

· The screw: has a small capacity, but delivers extremely high extrusion pressure;

· C-type screw: Large capacity, but relatively low extrusion pressure;

· B-screw: balanced at both ends, conventional and well-balanced.

II. Apply These Principles to Your Own Situation:

What are they each best suited for?

The screw: a high-pressure performer dedicated to tackling the toughest challenges

Because of its fine design, the barrel volume is reduced, resulting in a slightly lower extrusion weight compared with B/C screws. Its advantage, however, lies in its extremely high extrusion pressure and high length-to-diameter ratio, which allows the material to be more thoroughly sheared and plastified inside the barrel and to achieve more uniform color mixing. ✅ Typical application scenarios:

i. High-viscosity engineering materials with poor fluidity—such as PC, PMMA, fiber-reinforced PA, PPS, and PEEK—cannot be fully filled into the mold cavity without sufficient high pressure.

ii. Ultra-precision and thin-walled components: internal mobile-phone components, ultra-thin lenses, and gears are highly sensitive to shrinkage, air bubbles, and dimensional tolerances.

iii. Prevent heat retention in the raw material: If low-density products are processed using a coarse screw, the rubber compound remains trapped in the barrel for an extended period before being discharged, which can easily cause discoloration and degradation. Using a fine screw allows for much faster material turnover, keeping the material "fresher."

B-Screw: A versatile go-to option that won't offend anyone

Unless otherwise specified at the time of factory shipment, almost all new machines come with B-type screws as the default configuration. ✅ Typical application scenarios:

i. It covers more than 70% of conventional general-purpose plastics, such as PP, PE, ABS, and HIPS.

ii. Appliance housings, daily-use plastic containers, and general-purpose toy accessories.

iii. Workshops well-suited for outsourced manufacturing: Today you source ABS, tomorrow switch to PP, and the day after that switch to a new mold. If you cannot predict which material your customers will request next month, choosing a B-type screw offers the highest margin of error tolerance.

C-type screw: the go-to choice for high-capacity meat processing

A thicker screw allows for the maximum amount of melt to be fed in a single cycle. However, this comes at the cost of diluted injection pressure; combined with its relatively low length-to-diameter ratio, its mixing and shearing performance is slightly inferior. ✅ Typical application scenarios:

i. Large-volume, thick-walled components: such as plastic turnover boxes, trash bins, storage boxes, and large-diameter bottle caps.

ii. Highly fluid resins: resins such as PP and PE with excellent fluidity can fill the mold cavity without requiring high pressure.

iii. Achieve the target critical weight: the total weight of a particular mold happens to sit exactly at the machine's upper limit; using a B-type screw leaves it just short of reaching the target weight, whereas switching to a C-type screw can increase the extrusion volume by 20%–30%, eliminating the need to force the mold onto a higher-tonnage machine just for a few dozen grams.

III. Three Golden Rules for Avoiding Common Pitfalls in Workshop Sizing

1. Keep in mind the "30%–70% optimal range"

❌ Overloading the machine: Using a 15 g injection for a single cycle while employing a large-diameter C-type screw; the material requires more than a dozen heating cycles in the barrel before injection is complete, by which time the heat-sensitive plastic has already developed black spots and silver streaks. ❌ Underloading the machine: The single-cycle injection volume exceeds 85% of the screw's maximum capacity, leaving the screw nearly at the end of its travel; this makes it easy for air to be entrained in the feeding section and for material feeding to become uneven, resulting in inconsistent product weight. ✅ The most rational configuration is to keep the single-cycle injection volume of the product (including the water outlet) between 30% and 70% of the screw's maximum capacity.

2. For transparent components, look for high-pressure fine screws.

When processing PC light guides and thick-walled acrylic bottles, even if the injection-molding volume is small, priority should be given to an A-type screw whenever possible. For transparent components, the key considerations are the prevention of air bubbles, stress concentrations, and black spots; a fine screw with a high length-to-diameter ratio and sufficient venting and shearing provides far greater stability than a large-diameter, bulkier screw.

3. When choosing fiber-reinforced wear-resistant materials, don't just look at ABC

When processing glass-fiber-reinforced materials, such as PA66 + 30% GF, using a single ABC-type screw is insufficient; more importantly, the alloy composition of the screw must be appropriate. In ordinary nitrided screws, the screw flanks are worn smooth before long processing has been completed; therefore, a double-alloy or fully hardened screw must be used.

IV. One-Minute Model Selection Guide

Choose Screw A: for high-pressure applications, material reprocessing, and the machining of engineering materials and thin-walled precision components; Choose Screw B: for versatility and contract manufacturing of a wide variety of miscellaneous materials; Choose Screw C: for high capacity, reduced equipment costs, and a focus on the distribution of large components. In conclusion, a screw is not about being expensive or inexpensive; when the material properties and product requirements are properly matched and the machine is adjusted on-site without excessive hassle, the product yield rate will naturally improve.

In conclusion, a screw is not about being expensive or inexpensive; when the material properties and product requirements are properly matched and the machine is adjusted on-site without excessive hassle, the product yield rate will naturally improve.

Proper screw selection is only part of the equation—reliable manufacturing and wear-resistant materials are equally essential to long-term performance.