Plastic Granulator Yield Dropping & Black Spots? Check These Two Parts First
Anyone in the plastic recycling industry knows that when you buy a machine, it works well for the first few months. But after half a year, you’ll notice the yield gradually dropping, black spots appearing on the particle surface, and in severe cases, the extruded strands break easily when pulled. Many people’s first reaction is “the temperature is not adjusted properly” or “the filter needs to be replaced”.
In fact, 80% of late-stage quality issues stem from the “heart” of the plastic granulator — the screw and barrel (also called the cylinder in the industry).
plastic granulator
When these two parts work well together, the machine can last an extra 3 to 5 years; if they don’t match properly, or if the wrong material is chosen, no matter how large the motor power is, it will be useless. Today, we won’t talk about empty words. We’ll directly discuss how to choose, use, and judge when they need to be replaced from the perspective of actual production.
- First, the conclusion: What do the screw and barrel affect?
To put it simply, the job of a plastic granulator is to take in waste, melt it, and spit it out as particles. This entire process relies on the screw rotating inside the barrel.
The screw is responsible for pushing, pressing, and shearing. Waste enters from the feed port, is pushed forward by the screw threads, becomes tighter and tighter, generates heat through friction, and slowly turns into a paste (molten state).
The barrel is responsible for holding and heating. It is a fixed cavity wrapped with heating coils to stabilize the temperature.
You may ask: “Isn’t it just a shaft and a cylinder? How much difference can there be?”
The difference is significant. For a plastic granulator of the same power, just due to different screw designs and fit clearances, some manufacturers can achieve 1200kg/h, while others can only reach 800kg/h. The particle quality is even more obvious — good particles look like new materials, while poor ones have black spots and bubbles, which are directly rejected by downstream customers.
There’s only one core indicator: fit clearance. The normal clearance is between 0.1-0.3mm. When it exceeds 0.5mm, you’ll find material leaking back, the yield dropping by at least 20%, and the material being heated at high temperature in the clearance for a long time, carbonizing into black spots mixed into the particles.
- How to choose screw material? Don’t just focus on price
Many bosses only look at the motor power and yield of the plastic granulator when buying equipment, thinking “high power means it can work”. The screw material is often overlooked. After half a year, when they take it out and see the threads worn into blades, they realize they’ve lost out.
Let’s discuss two scenarios:
- Routine recycling: PE agricultural film, PP woven bags, ABS casings
This kind of waste contains some sand, dust, and label paper. 38CrMoAlA nitrided steel is sufficient. This material is characterized by a hard surface (above HRC60) after nitriding, but a tough core. Under normal use, the wear will not exceed 0.2mm in two years.
A stably operatingplastic granulator uses this type of screw — cheap, easy to repair, and sufficiently wear-resistant.
- High-fill, high-hardness waste: glass fiber reinforced plastic, engineering plastic containing metal powder
Ordinary screws can’t withstand this. A bimetallic screw — with tungsten carbide sprayed on the surface — is a must. It’s about twice as expensive, but its service life is 3-5 times that of ordinary screws. Do the math: two days of shutdown for screw replacement, disassembly labor, and lost production capacity are far more than the price difference.
How to judge when it needs to be replaced? A simple method: check the yield. If the yield is more than 15% lower than the normal value with the same raw materials, temperature, and speed, and the particles are obviously black, it’s basically due to screw wear or barrel scaling.
III. Europe vs. Southeast Asia: Why are screw designs different?
You may have noticed that plastic granulators used in different countries have similar-looking screws but significant detailed differences. These are not random changes, but based on local materials and market requirements.
plastic granulator
European and American markets (high-end models):
Requirements: high particle purity, few impurities, suitable for injection molding or wire drawing.
Screw length-diameter ratio: 28:1 to 30:1. Longer, so the material travels a longer path in the barrel, resulting in more sufficient melting.
The barrel is equipped with zoned temperature control, and each section can be adjusted independently. It won’t burn when processing heat-sensitive PET or high-fill materials.
Southeast Asian and emerging markets (economic models):
Requirements: high yield, durable, and easy to maintain.
Length-diameter ratio: 25:1 to 28:1. Shorter, so the material stays for a shorter time, not easy to carbonize, and the shorter screw has better rigidity and is not easy to bend.
Integrated barrel heating, simple. If it breaks, just replace the heating coil at low cost.
Advice: If you process mixed materials and often change varieties, choose a length-diameter ratio of about 28:1 for strong adaptability. If you only process a single material, 25:1-28:1 is sufficient for higher yield.
If you’re not sure about the wear of your screw and barrel, you can take photos and send them to us. We’ll help you judge the wear level for free and recommend the most suitable replacement material.
WhatsApp / WeChat: +86 15092868822 (Please note “Screw Consultation” when adding, priority processing)
