What are the core structures of an extruded feed pellet mill?

Nov 24, 2025

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While fish feed extruders have numerous components, only four truly determine feed quality: the extrusion chamber, the screw, the die, and the conditioner. These four components work together to complete the conditioning, extrusion, and shaping processes of the raw materials. Their design precision, material selection, and parameter settings directly affect key indicators such as the degree of extrusion, foam resistance, and nutrient retention rate of the feed.

The extrusion chamber is the "core location for raw material conversion," and its structural design directly determines the extrusion effect. A high-quality extrusion chamber adopts a split three-section design: a feeding section, a compression section, and a discharge section, each with different inner diameters and temperature controls. The feeding section has a relatively large inner diameter (typically 150-200mm), primarily serving to receive and preheat the raw materials, maintaining a temperature of 70-80℃. The compression section gradually decreases in diameter, creating a pressure gradient that compresses the raw materials and raises the temperature to 100-120℃, initiating starch gelatinization. The discharge section has the smallest inner diameter, where the pressure reaches its peak (5-10MPa), and the temperature rises to 120-140℃, where the raw materials undergo complete expansion. The inner wall of the expansion chamber is typically lined with wear-resistant alloy bushings with spiral grooves on the bushing surface. This enhances friction between the raw materials and the chamber wall, improving the expansion effect, while also reducing wear and extending service life. For aquatic feed production, the sealing of the expansion chamber is particularly important. High-quality equipment employs a labyrinth seal structure to prevent raw material leakage and pressure loss, ensuring pellet foam resistance.

The screw of the pet food extruder is crucial for power transmission and raw material extrusion; its structural parameters directly affect the degree of expansion and pellet density. Key screw parameters include pitch, screw head height, and length-to-diameter ratio, with different parameters corresponding to different feed requirements. When producing high-expansion pet food, a screw with a large pitch and low screw ridges is selected, with an aspect ratio controlled at 12:1. This increases the residence time of the raw material in the extrusion chamber, improving the extrusion effect. When producing low-expansion aquatic feed, a screw with a small pitch and high screw ridges is selected, with an aspect ratio of 10:1. This shortens the residence time and ensures compact and foam-resistant pellets. The screw material is typically 38CrMoAl alloy steel, which, after nitriding treatment, achieves a hardness of HRC60 or higher, capable of withstanding the abrasion of high-protein raw materials such as fishmeal and meatmeal. Some high-end equipment adopts a modular screw design, allowing for the replacement of screw sleeves with different pitches according to the characteristics of the raw materials, improving equipment versatility.

The die head is the "final stage of pellet forming," and its structure determines the shape, size, and stability of the pellets. The core component of the die head is the die hole, and the diameter, length, and arrangement of the die hole must be designed according to the type of feed. The die holes for aquatic feed are typically circular, with a diameter of 1.0-3.0 mm and a length-to-diameter ratio of 5:1, ensuring compact pellets. Pet food die holes can be designed as circular, square, or bone-shaped, with a diameter of 2.0-5.0 mm and a length-to-diameter ratio of 3:1, improving pellet palatability. The die head is made of high-speed steel or hard alloy, and the inner wall of the channel is polished (roughness Ra≤0.8μm) to reduce friction between the raw material and the channel, preventing burrs from forming in the pellets. The die head is also equipped with a temperature control system, which uses electric heating to maintain the die head temperature at 100-120℃, preventing the raw material from cooling and solidifying within the channel, thus preventing blockage.

The conditioner of a dog food pellet machine is crucial for nutrient retention; its structural design directly affects the conditioning effect and the degree of nutrient loss. A high-quality conditioner uses a dual-shaft paddle design, with two shafts rotating in opposite directions and the paddles arranged in an alternating pattern, ensuring thorough mixing of the raw material and steam. The conditioner is typically 1.2-1.8m long, with a paddle speed of 30-60 rpm. The speed can be adjusted according to the characteristics of the raw materials to achieve the best conditioning effect. To protect heat-sensitive nutrients, the conditioner uses low-pressure steam (0.2-0.3MPa) for heating, controlling the temperature at 85-95℃. It is also equipped with a moisture sensor to monitor the moisture content of the raw materials in real time, preventing excessive moisture from causing the pellets to become soft. For feeds with added probiotics and other active ingredients, some equipment is also equipped with a post-conditioning device for low-temperature conditioning after extrusion, ensuring that the active ingredients are not destroyed.