In plastic blow molding, the screw is often called the “heart” of the machinery. Its design directly affects production efficiency and product quality. Many people mistakenly believe that “a longer screw is always better” for plastic water tank blow molding machines.

Let’s break down this myth by examining three key factors: material processing, energy efficiency, and product quality.

1. How Screw Length Affects Plastic Melting

The length-to-diameter ratio (L/D) of a screw determines how well it melts plastic. While a longer screw can improve melting, there’s a limit. For example:

• For HDPE (common water tank material), increasing the L/D from 24:1 to 28:1 makes the melted plastic 15% more consistent. But beyond L/D 32:1, the improvement slows down.

• Longer screws keep plastic inside longer, which can overheat some materials. For heat-sensitive plastics like PVC, even a small increase in screw length (like 1D) raises temperatures by 3–5°C, risking damage. HDPE can handle 5–7 minutes of heating, but too much time in a very long screw can still break down the material.

Mixing efficiency matters more than length alone. A screw with advanced mixing parts (like “barrier” designs) can outperform a longer, simpler screw. Tests show these designs improve mixing by 40% without needing extra length.

2. Energy Costs vs. Performance

Longer screws guzzle more power. For example:

• A 65mm screw with L/D 32 uses 21.6% more energy than an L/D 28 screw but only boosts output by 8%.

• High-viscosity plastics (like LLDPE) face bigger pressure drops in longer screws. Shortening the screw from L/D 30 to 28 reduced one factory’s system pressure by 17%, lowered oil temperatures by 12°C, and cut machine breakdowns by 30%.

Maintenance costs also rise with length. Screws longer than L/D 32 wear out 20% faster and need 50% more repairs. One factory saved $12,000 yearly by switching to shorter screws (L/D 28).

3. Product Quality: Longer Isn’t Always Better

Wall thickness consistency peaks at L/D 26–30, with deviations under 1.5%. Beyond L/D 32, excessive heat and shear cause uneven walls (over 2% deviation). For 200L water tanks, an L/D 28 screw makes them 8% stronger than an L/D 32 screw.

Surface quality also suffers with overly long screws. At L/D 30, water tanks have a glossy finish (85GU rating), but at L/D 34, overheating creates defects like “silver streaks,” dropping the rating to 78GU.

Matching screws length to water tank size works best:

• 5–30L water tanks: L/D 26–28

• 50–100L water tanks: L/D 28–30

• 200L+ water tanks: L/D 30–32
  This approach boosts product quality by 4.7% compared to using one long screw for all sizes.

Longer screws aren’t automatically better. Modern screws designs use smarter features (like wave-shaped grooves or multi-edge mixing zones) to work better in shorter lengths. For plastic water tank production:

• Choose screws with L/D 26–32 based on your water tank size and material.

• Prioritize screws with advanced mixing parts.

• Balance energy use, maintenance costs, and quality.

The optimization design of the blow molding machine screw is essentially a system engineering. Contemporary screws technology has broken through the traditional mode of simply relying on length to improve performance. Through comprehensive means such as innovative mixing elements (such as corrugated screws, multi-faceted screws), optimized temperature control strategies, and intelligent adjustment systems, better processing effects can be achieved in a more compact size.

For the production of plastic buckets, it is recommended to select a screws with L/D=26-32 according to product specifications and raw material characteristics, and give priority to solutions that configure efficient mixing elements to achieve the best balance between energy consumption and efficiency while ensuring product quality.