What Are the Common Specification Units of Extruder Screw?

Ⅰ. Metric and inch threads of screw elements

1. The outer diameter of the metric thread of the screw element is in millimeters, such as 6, 8, 10, 12, 18, 20 mm, etc., and the pitch is also in millimeters, such as 0.5, 0.75, 1, 1.5, 2, 3, etc.

2. The outer diameter of the inch thread of the screw element is in inches, (each inch is equal to 25.4 mm) such as 3/16, 5/8, 1/4, 1/2, etc. Therefore, when measuring the outer diameter with a metric caliper, the reading often has irregular decimals.

Imperial pitch is expressed in cusps per inch. Set the caliper at 25.4 mm, align one caliper tip with the thread cusp, and the other caliper tip should be inch thread if it is aligned with the thread cusp. If the thread cusp is not aligned, it should be a metric thread.

When measuring the screw pitch of the extruder screw, it is best to print the tip of the thread on the white chalk. The print on the chalk is clearer and easier to measure. To measure the metric pitch, you should measure a length, such as 10, 15, 20 mm, etc. Count how many cusps are included and calculate the pitch.

The thread specification specified in inches is the inch thread, such as: G1. The thread specification specified in metric unit millimeters is metric thread, such as: M30.

The imperial system is determined by the amount of cusps in an inch (2.54 cm), which is usually a 55 degree angle. The metric system is the pitch determined by the distance between the two cusps, which is usually a 60 degree angle.

Ⅱ. The extruder screw is divided into three sections: feeding section, compression section, homogenization section

1. Feeding area - the bottom warp is small and the main function is to transport raw materials to the rear section, so it is mainly a problem of transport capacity. Parameters (L1, h1), h1=(0.12-0.14)D.

2. Compression zone - basal warp changes and the main function is to compact and melt materials to establish pressure. Parameter compression ratio ε=h1/h3 and L2. Accurately, the gradient should be A=(h1-h3)/L2.

3. Metering area - extrude the melted material in the compression section to the front end of the extruder screw quantitatively and at a constant temperature. Parameters (L3, h3), h3=(0.05-0.07)D.

For the entire extruder screw, the parameter L/D - aspect ratio.

L/D advantages and disadvantages: L/D and speed n are important factors for the plasticizing ability and effect of the extruder screw. If the L/D is large, the material will stay in the barrel for a long time, which is conducive to plasticization. At the same time, the pressure flow and leakage flow are reduced, which improves the plasticization capacity and is beneficial to materials with high temperature distribution requirements. However, when the aspect ratio is large, it has a negative impact on the use of manufacturing and assembly. Generally, the L/D is (18 to 20), but there is a tendency to increase at present.

Other pitch S, helix angle φ=πDtgφ. Generally D=S, then φ=17°40′.

φ has an influence on the plasticizing ability. Generally speaking, the larger the φ, the faster the conveying speed. Therefore, the shape of the material is different and its φ changes. The powder material can be about φ=25°, the cylindrical material is about φ=17° and the square material is about φ=15°. However, the difference in φ is also difficult for processing, so generally φ is taken as 17°40'.

Edge width e. For materials with low viscosity, e should be as large as possible. If it is too small, it is easy to leak. If it is too large, it will increase power consumption and easily overheat. e=(0.08~0.12)D.

All in all, under the current circumstances, there is no complete design method for the design of the extruder screw. Most of them have to formulate parameters according to different material properties and experience to meet different needs. All factories are roughly the same.

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