Types of Extruders: How They Work and Which One You Need

There are two ways to classify types of extruders: by the number of screws, and by the product they make. By screws, most plastic extruders are single-screw or twin-screw. By product, the six main types are blown film, cast film and sheet, pipe and profile, pelletizing and recycling, filament and lab extruders, and co-extrusion lines.

This guide is for buyers who need to know which extruder matches their product. We explain how an extruder works, how the six types differ and what to check before you ask for a quote.

A disclosure: at YONGBANG we build two of these six types, blown film extruders and recycling pelletizers. We describe the other four so you can tell them apart, but we do not sell them.

What is a plastic extruder?

A plastic extruder is a machine that melts a thermoplastic with heat and a rotating screw, then pushes it through a die to form a continuous product with a constant cross-section: film, sheet, pipe, profile, filament or new pellets. Unlike an injection molding machine, which fills a mold one part at a time, an extruder runs continuously as long as it is fed.

All extruders share the same basic parts. What changes is the screw size, the die and the equipment after the die.

1. Motor and gearbox: turn the screw at the set speed.
2. Hopper: holds the pellets or ground material.
3. Heater bands: heat the barrel in zones, each with its own temperature control.
4. Barrel and screw: the screw conveys, compresses and melts the plastic.
5. Screen changer: filters the melt before the die.
6. Die: gives the plastic its final shape.

How does an extruder work?

An extruder works in three zones along the screw. In the feed zone, the screw picks up pellets from the hopper and moves them forward. In the compression zone, the screw channel gets shallower, friction and the heater bands melt the plastic, and trapped air escapes backward.

In the metering zone, the molten plastic leaves at even pressure and temperature toward the screen changer and the die. What happens next depends on the type: blown film is inflated into a bubble, pipe is cooled in a vacuum sizing tank, and recycling strands are cut into pellets.

For a broader overview that also covers metals and food, see the Wikipedia article on plastic extrusion.

Types of extruders

Plastic extruders are grouped in two ways: by the number of screws and by the product they make. The first tells you how the machine mixes material. The second tells you whether it fits your business.

By number of screws

Single-screw extruders are the most common. They are used for film, sheet, pipe and most recycling pelletizers, because they melt and pump material steadily with a simple design.

Twin-screw extruders have two screws inside one barrel. Co-rotating twin screws, turning in the same direction, are used for compounding fillers, pigments and additives. Counter-rotating twin screws are mainly used for rigid PVC pipe and profile.

By the product they make

1. Blown film extruder: circular die and an air-inflated bubble. Makes polyethylene film for bags. See our blown film machines.
2. Cast film and sheet extruder: flat die and chilled rolls. Makes sheet for thermoforming and stretch film.
3. Pipe and profile extruder: die shaped like the pipe or profile, vacuum sizing tank, haul-off and cutter.
4. Pelletizing and recycling extruder: melts scrap or used plastic and cuts it into pellets. See our plastic pelletizing machines.
5. Filament and lab extruder: small machines for 3D printing filament, material trials or teaching.
6. Co-extrusion line: two or more extruders feed one die to form a multi-layer product, such as 3-layer ABA film.

General overview. Each manufacturer may use different configurations.

Why a film extruder and a recycling extruder use different screws

The screw design follows the job. A bag film can be 0.01 mm (about 0.4 mil) thick, so the plastic has to reach the die fully melted and mixed. Any unmelted gel shows up as a spot or a hole in the film.

That is why our blown film machines use 30:1 screws in SACM-645 / 38CrMoAl, a nitriding steel that resists wear. On the MF-B single-stage pelletizer the ratio is 20:1 to 22:1, and the screen changer and the cutter, which is synchronized with the extruder output, do the fine work.

Neither number is better than the other. The two machines have different goals, and a recycling extruder cannot replace a film extruder, or the other way around.

What about 3D printer and food extruders?

The word extruder also covers machines outside plastics processing. On a 3D printer, the extruder is the part that feeds filament into the hot end. The two common designs are direct drive, with the motor mounted on the print head, and Bowden, with the motor on the frame pushing filament through a tube.

In the food industry, single-screw and twin-screw extruders cook and shape products such as pasta, breakfast cereal, snacks and pet food. These machines follow the same screw principle but use food-grade construction. We build neither type.

Industrial vs. small or DIY plastic extruders

The difference between an industrial extruder and a small one is output per hour. Desktop lab extruders sold online are typically rated at a few kilograms per hour and suit filament or material trials. An industrial blown film extruder produces tens or hundreds of kilograms per hour.

Our smallest blown film machine, the mini YB-500 / YB-700, makes 40 to 50 kg/h. The largest, the MF-C series, reaches 220 kg/h. There is no useful middle ground between a DIY extruder and an industrial one if you want to sell bags: the DIY machine gives you neither the volume nor the even thickness.

Which extruder do you need to make plastic bags?

To make plastic bags you need a blown film extruder, which makes the film, and a bag making machine, which seals and cuts it. If you want to reuse your trim and rejected bags, add a recycling pelletizer.

These are the options, depending on your situation:

• You are starting to make your own film: the mini machine (40–50 kg/h) or the SJ-A (60–100 kg/h).
• You need wide film or high volume: the MF-C series, 100 to 220 kg/h.
• You want 3-layer film: the MF-ABA, with two extruders and a 3-layer die.
• You want to recycle your trim: the single-stage pelletizer (70–100 kg/h) or the two-stage pelletizer for film and filament (150–200 kg/h).

If you are setting up a factory from scratch, our plastic bag production line page shows which machines go together for each type of bag.

What to check before buying an extruder

Before you buy a plastic extruder, check these five points with the manufacturer:

1. Real output in kg/h with the material you will run, not only the catalog maximum.
2. Screw and barrel: diameter, L/D ratio and steel grade.
3. Voltage and frequency of your plant.
4. Options: on film lines, for example, corona treatment is required if you will print.
5. Warranty and support: what the warranty covers, how start-up works and which wear parts ship with the machine.

Frequently asked questions

Not sure which extruder you need? Write to us with the product you want to make and how many kilograms per day you need. We reply within 24 hours.