+86 17727725162 jqj014950@topstarltd.com

Home / Injection Mold Machine Screw Selection Guide: Single Screw vs. Twin Screw Selection

Injection Mold Machine Screw Selection Guide: Single Screw vs. Twin Screw Selection

2025/12/01 By le zhan

Injection Mold Machine 1-1

The screw in an injection mold machine primarily functions to melt, mix, and precisely inject plastic. Choosing the wrong screw can lead to scrap, delays, and material waste. Selecting the right screw ensures consistent quality and lower costs. The choice between a single-screw and a twin-screw injection mold boils down to one question: what material are you molding, and how are you molding it? Most injection molding machines on the market (90%) use single-screws—including all Topstar models—because they perform exceptionally well with mainstream materials and applications (PP, ABS, PE, etc.). In contrast, twin-screws are specialized tools for processing niche, complex materials.

The Difference Between Single-Screw and Twin-Screw Injection Mold Machine

Before delving into the selection process, let us demystify these two designs—confusing them can lead to costly mistakes we see every day. Both types of screws are located in the injection unit of an injection molding machine, rotating to transport plastic pellets from the hopper to the mold. Their differences lie in their structures, mixing capacities, and applications.

A single-screw, as its name suggests, is a helical screw located within a cylindrical barrel. It utilizes three key zones to process plastics: the feed zone (which draws in granules), the compression zone (which melts the plastic through heating and friction), and the metering zone (which mixes the molten plastic and pushes it into the mold). Its design is simple, reliable, and optimized for stable flow—suitable for most materials.

Twin-screw injection mold machines consist of two meshing screws that rotate in the same or opposite directions. The overlapping threads (helical ridges) of the screws produce intense shearing and mixing, making them ideal for mixing additives (such as glass fibers) or processing materials containing impurities (such as recycled plastics). They are larger, more complex, and require specialized injection mold machines, which is why they are used in only 10% of industrial production.

injection molding machine screws

Single-screw injection mold machine—materials, applications, and industry standards

Single-screw injection mold machines dominate the industry for one simple reason: they can process 85% of the materials used in global manufacturing. Its simple and efficient design ensures stable melt quality, low maintenance costs, and short production cycles—which is why 90% of injection mold machines (including all Topstar injection molding machine series) use single-screw injection molding machines.

Suitable materials and applications for single-screw injection mold machines:

Common thermoplastics: PP (polypropylene), ABS, PE (polyethylene), PS (polystyrene), and PVC (rigid plastics). These materials have good flowability and do not require vigorous mixing.

Consumer goods: Toys, household appliances (e.g., plastic handles), and electronic product housings. Single-screw injection molding machines can provide the uniform wall thickness required for these parts.

Automotive interiors: Dashboard trim, door panels, and cup holders. Materials such as TPO (thermoplastic olefins) can be seamlessly processed with single screws.

Single-screw injection mold machine materials

Twin-screw—Advantages for Special Materials

Twin-screws are not “better” than single-screws, but somewhat different. They can solve specific problems that single screws cannot, but they also come with trade-offs that limit their applications.

Twin-screw injection mold machines excel at handling materials that require vigorous mixing, dispersion, or contamination resistance. Here are some of their key application examples:

Reinforced Composites: Glass fiber reinforced nylon, carbon fiber reinforced polyetheretherketone (PEEK), and mineral-filled polypropylene (PP). The interlocking screws of a twin-screw machine evenly distribute fibres, preventing fibre clumping and reducing part strength.

Recycled or Contaminated Materials: Post-consumer recycled (PCR) plastics containing small amounts of impurities. Twin-screw injection moulding machines break down impurities and evenly mix them, reducing defects.

Multi-Material Mixtures: Plastics mixed with additives such as flame retardants, colorants, or UV stabilizers. Twin-screw injection mold machines ensure even distribution, preventing “streaking” in colored parts.

However, twin-screw injection mould machines have significant drawbacks in most applications: they cost 2-3 times as much as single-screw machines, require specialised operators, and have cycle times 30% longer due to slower mixing speeds. Furthermore, twin-screw injection mold machines generate more heat, which can damage sensitive materials such as ABS or PC.

Injection Mold Machine Screw Selection: Five Key Factors

The choice between single-screw and twin-screw injection molding machines is not arbitrary; it is based on several specific factors. First, determine the type of material you are using. If you are using standard thermoplastics (PP, ABS, PE) or unreinforced materials, a single-screw is ideal. If you are using reinforced composites, PCR plastics, or multi-additive blends, a twin-screw may be necessary. Second, consider your tolerance requirements for the product parts. Parts with tight tolerances (±0.01 mm) or thin walls (less than 1 mm) require stable melt flow—a requirement that a single-screw can effectively meet. Inconsistent mixing ratios in twin-screw injection moulding machines can cause slight viscosity variations, leading to uneven filling.

Additionally, your high-volume production benefits from the faster cycle speeds of single-screw injection molding machines. The slower mixing speeds of twin-screw machines can cause a 20% to 30% decrease in output. Furthermore, the total cost of ownership for a single-screw injection molding machine is lower, with lower upfront and maintenance costs and a longer lifespan. Over a five-year operating cycle, the total cost of ownership for a single-screw injection molding machine is 45% lower than that of a comparable twin-screw injection molding machine.

Single-screw injection mold machine

Why do we design single-screws for 99% of applications?

All of Topstar’s injection molding machine series use single-screw machines. This is not an oversight, but a meticulous design based on 30 years of customer data and industry trends. We continuously improve our single-screws to outperform generic designs, even allowing for custom modifications to handle certain “twin-screw-only” materials.

We do not use “one-size-fits-all” screws. For low-viscosity PP materials, we design shallow threads to prevent backflow. For high-viscosity PC materials, we use deep threads for efficient material delivery. Additionally, the screws are electroplated with a hardness of HRC 60+. This makes them resistant to abrasive materials (such as 10% glass fiber reinforced polypropylene), extending their service life by 50% compared to uncoated screws.

Furthermore, the machining accuracy of each screw is controlled within ±0.005mm. This ensures a perfect fit between the screw and barrel, eliminating dead zones where plastics might scorch or degrade. Our single-screw design achieves ±2°F melt temperature stability—crucial for heat-sensitive materials like ABS. For customers requiring lightweight reinforcement (10-15% glass fiber), we add a mixing section (minor grooves on the screw threads) to the single screw to improve dispersion. This results in an average defect rate of only 1.2% for injection molding machines using single screws, significantly lower than the industry average of 3.5%.

Single screws are the most common and standard choice

Choosing the right screw is the most critical decision when purchasing an injection molding machine, and data clearly shows that for 90% of applications, a single screw is the correct choice. It is more economical, faster, more reliable, and optimized for most materials commonly used in the manufacturing industry (PP, ABS, PE, etc.). Twin screws also have their uses—for example, for reinforcing composites, recycled materials, and multi-additive blends—but they are specialized tools, not the default choice. At the same time, it is important to remember that when choosing a screw for an injection moulding machine, the material should guide your decision.



 

 

 

Prev: Guide to All-Electric Injection Molding Machine for Precision Plastic Parts: 90T-200T TEII Series

Next: Solving the problem of low cavity filling rate in high-speed packaging injection molding

TRENDING POSTS

HOT TOPIC

Get A Quick Quote