Ultimate guide for Extruder screw

The function of the extruder screw is to make the rubber material gradually change into linear motion with the rotation of the screw, push it toward the head, and cooperate with the body to compress, generate heat, soften and stir, and mix the rubber material.

1 Basic knowledge

The screw is composed of threads and cylinders. The screw has a long hole along the center line, which can pass cooling water. The tail of the screw is installed in the thrust bearing to avoid the reaction force generated when squeezing the rubber to push the screw out.

The diameter of the screw is slightly smaller than the inner diameter of the steel sleeve of the body, that is, there should be a gap between the diameter of the screw and the inner surface of the steel sleeve, and the gap is generally controlled at 0.002~0.004 times the diameter of the screw. If the gap is too small, the screw will "sweep the bore", cause wear, and even cause the screw to get stuck; if the gap is too large, the rubber will flow back and affect the extrusion volume, low production efficiency, and product quality.

2 Thread characteristics of screw

Thread depth is directly related to the production capacity of the equipment. The greater the thread depth, the more rubber can be extruded under a certain pressure. However, the plasticization of the rubber is difficult and the screw strength is poor. The thread depth of the screw is generally controlled at 0.18~0.25 times the screw diameter. The propulsion surface of the thread should be perpendicular to the axis of the screw, and the opposite surface of the propulsion surface should have a certain slope. The axial distance between adjacent threads is called the pitch. The screw of the rubber extruder is generally a double-threaded screw with equal distance and unequal depth. The volume between pitches is calculated as follows:

tgФ=L/πD

F=h(πD tgФ-e)Where:Ф——the slope of the opposite surface of the screw push surface

L——pitch

D——screw diameter

e——thread crest width

F——volume between pitches

Thread crest width is generally 0.07~0.1 times the screw diameter, among which the screw of small-sized rubber extruder can take a larger value, while the screw of large-sized rubber extruder can take a smaller value. The thread crest width cannot be too small. If it is too small, the strength at the crest is too small; if it is too large, the thread volume will be reduced. Affecting the output, and causing the rubber to burn due to frictional heat. The distance of the thread is generally equal to or slightly greater than the screw diameter.

There are three shapes of screw heads: flat, semicircular and conical. Conical screws are commonly used.

3 Screw length-diameter ratio

The screw length-diameter ratio is the ratio of the screw length L to the screw diameter D. The larger the screw length-diameter ratio, that is, the longer the screw working part, the better the plasticization of the rubber, the more uniform the mixing, the greater the pressure on the rubber, and the better the product quality. However, the longer the screw, the easier it is to cause the rubber to burn, the more difficult the screw processing is, and the higher the extrusion power is. The screw used for hot-feed rubber extruders generally has a length-diameter ratio of 4 to 6 times, and the screw used for cold-feed rubber extruders generally has a length-diameter ratio of 8 to 12 times.

Length-to-diameter ratio L/D

Plastic extruders use a large variety of plastics for extrusion molding, and one screw cannot mold all plastics. The screw should be designed according to the characteristics of the raw materials and the commonalities of various raw materials should be considered as much as possible, so that one screw can extrude several plastics at the same time, which is economically meaningful in industrial production. The reverse thread at the rear end of the screw prevents leakage.

Screw length-to-diameter ratio L/D, screw diameter D refers to the outer diameter of the screw thread. The effective length L of the screw refers to the length of the working part of the screw, as shown in Figure 3-14. The effective length is different from the total length of the screw. The length-to-diameter ratio is the ratio of the effective length of the screw to the diameter. The early extruder screw had a relatively small length-to-diameter ratio of only 12-16. With the development of the plastic molding processing industry, the length-to-diameter ratio of the extruder screw has gradually increased. Currently, the commonly used ones are 15, 20, and 25, and the maximum can reach 43.

Increasing the aspect ratio has the following benefits:

①. The screw is fully pressurized, and the physical and mechanical properties of the product can be improved.

②. The material is well plasticized, and the product appearance quality is good.

③. The extrusion volume is increased by 20-40%. At the same time, the characteristic curve of the screw with a large aspect ratio has a small slope, is relatively flat, and the extrusion volume is stable.

④. It is conducive to powder molding, such as polyvinyl chloride powder extrusion tube.

However, increasing the aspect ratio makes the manufacture of the screw and the assembly of the screw and the barrel difficult. Therefore, the aspect ratio cannot be increased indefinitely.

4 Screw compression ratio

The compression ratio required by various plastics is not fixed, but can have a range. Different raw materials require different compression ratios. For example, when extruding soft polyvinyl chloride plastic, if it is a granular material, the screw compression ratio is often 2.5-3, and if it is a powdered mixture, the compression ratio can be 4 to 5. Selection of screw compression ratio.

The ratio of the volume of the first pitch at the feed end of the screw to the volume of the last pitch at the discharge end is called the screw compression ratio. The compression ratio calculation formula is as follows:

(S1-e)(D-h1)h1

I = —————————

(S2-e)(D-h2)h2

Where: S1——The first pitch of the screw feeding end mm

S2——The last pitch of the screw discharge end mm

h1——The depth of the screw groove at the screw feeding end mm

h2——The depth of the screw groove at the screw discharge end mm

D——The diameter of the screw mm

e——The width of the thread peak mm

The screw compression ratio can be obtained by the following methods:

1. The pitch changes, and the depth of the screw groove remains unchanged;

2. The depth of the screw groove increases, and the pitch remains unchanged;

3. Both the pitch and the depth of the screw groove change.

Most wire and cable factories use equidistant and unequal depth screws. The compression ratio calculation formula is as follows:

I = h1/h2

The size of the compression ratio has a great influence on the quality of the product. The larger the compression ratio, the higher the density of the rubber and the smoother the surface. If the compression ratio is too large, the reaction of the rubber to the screw will be large, and the screw will be easily broken. The compression ratio of the rubber extruder screw is generally controlled at

1.3:1 to 1.6:1.

Another type of screw is a separation screw. This type of screw adds an additional thread to the middle section of the screw. The screw is divided into three sections: feeding section, melting section, and metering section. The middle section with additional threads is the melting section, the feeding section refers to the part from the feed point of the screw to the starting point of the additional thread, and the metering section refers to the part from the head of the screw to the end point of the additional thread.

The compression ratio can be obtained by the following methods:

(1) Pitch variation (equal depth and unequal pitch). The advantage of this structure is that it does not affect the strength of the screw when the compression ratio is large. The disadvantage is that the screw is difficult to process. When the helix angle is too small near the end of the screw, the material flow cannot be smooth, and it is easy to produce nests.

(2) Screw groove depth variation (equal pitch and unequal depth). Its advantages are easy processing and manufacturing, large contact area between the material and the barrel, and good heat transfer effect. The disadvantage is that the strength is greatly weakened, and special attention should be paid when using long screws and large compression ratios.

(3) Both the pitch and the depth of the screw groove vary (unequal pitch and unequal depth). If designed properly, this screw can achieve the greatest advantages and the least disadvantages. In actual production, equidistant and unequal depth screws are the most widely used, mainly for the convenience of processing and manufacturing.

5 Screw material

The screw must be heat-resistant, wear-resistant, and abrasion-resistant. Therefore, when processing the screw, heat treatment must be carried out, and the surface must be chrome-plated or nitrided. Commonly used materials include 45# steel or chrome-molybdenum aluminum alloy steel.

Segmentation

The movement of the material in the extruder screw is studied in three sections, so the design of the screw is often carried out in sections. Since each section is a continuous channel, in actual production, as long as the requirements can be met, it is not necessary to divide the screw into three sections. In fact, some screws have only two sections, and some are not segmented. For example, when extruding nylon, a material with good crystallinity, there are only feeding sections and homogenization sections. For general screws that extrude soft polyvinyl chloride plastics, all compression sections can be used without dividing them into feeding sections and homogenization sections.

The segmentation of the screw is obtained from experience and is mainly determined by the properties of the material. The length of the feeding section can be from 0 to 75% of the total length of the screw. Generally speaking, it is the longest when extruding crystalline polymers, followed by hard amorphous polymers, and the shortest for soft amorphous polymers. The length of the compression section usually accounts for 50% of the total length of the screw, of course, nylon and soft polyvinyl chloride plastics mentioned above are exceptions. When extruding polyethylene, the length of the homogenization section can be 20-25% of the total length. However, for some heat-sensitive materials (such as polyvinyl chloride), the material should not stay in this section for too long, and the homogenization section can be omitted. Some high-speed extruders have a homogenization section length of 50%.

Who are we?

We are Barrelize. We have been supplying extrusion screws to the plastic industry since 1990. We are currently supplying the industry with a total of 70 000 un its yearly