How to design the injection screw

With the deepening of people's understanding of polymer compounds, the injection screw has made great progress. Because the technical performance of the injection screw is the key to achieving excellent injection plasticization performance, it is particularly important for the reasonable design of the injection screw. This article analyzes the main technical parameters of the injection screw and the relationship between the main structure and technical performance, and puts forward the principle of determining the main technical parameters of the injection screw. Combined with the author's design practice, a more specific analysis and research have been made from both theoretical and practical aspects. 

1.Screw Length to Diameter Ratio

A.Screw length to diameter ratio and injection stroke

The injection stroke expresses the size of the injection volume and is an important technical parameter. The amount of material filled by the screw during the injection process is basically a very uncertain factor. During injection, the screw shaft moves forward, and the material flows into the screw groove, but it cannot fill the screw groove because the injection time is not enough to completely fill the screw groove. Because the material is sparsely filled, air is easy to be sucked in. If the air cannot be discharged in time, the quality of plasticization will be reduced. Generally, the size of the metering stroke is the main factor that determines whether air can enter the accumulator. Research on various materials has shown that if the metering stroke is greater than 3D, air will be trapped behind the non-return ring. At this time, if the screw L/D ratio is less than 18 (the screw L/D ratio that satisfies the 3D metering stroke), that is, the length at which the solids in the feeding section begins to melt is too short, which will cause the solids to remain in the melt when they transform to the melt in the compression section. The solids increase in a large amount, and in severe cases, it may even cause the transportation to stop. Therefore, in order to obtain a metering stroke exceeding 3D, the effective length of the screw must be increased, so that the solid material can have a sufficient melting path for melting during the feeding process, so as to reduce the solid content in the melt, and make the composition of the melt volume and the compression section The composition of the volume flow is matched, so that the melting volume required for metering is greater than the melting volume in the screw groove. Under normal circumstances, the screw length to diameter ratio reaches 20-25, which can meet the requirement that the metering stroke is greater than 3D. In addition, in order to be able to solve the problem of feeding air entrainment caused by the increase of the screw length-to-diameter ratio, the screw design must satisfy that the speed of the solid plug is greater than the speed of the solid bed during plasticization. Now, with the advancement of screw design and processing, the injection stroke of general injection molding machines has been increased from 3D to 4.5D~5D, and some even reached 6.5D, and the length-to-diameter ratio of the screw has also increased from 18 to 20~25, or even greater , Thereby improving the economics of the plasticizing mechanism. For example, in the Ergotech series of DEMAG, the injection stroke of the standard screw is about 4.5D, the effective length of the screw is 20D~24D, the length to diameter ratio of the special screw reaches 25, and the ratio of the pitch to the outer diameter of the screw is 1:1.

B. Screw length to diameter ratio and material melting characteristics

The specific enthalpy of semi-crystalline plastics is about 1.3 to 1.5 times higher than that of amorphous plastics. Therefore, amorphous plastics are easier to melt than semi-crystalline plastics. For example, ABS is melted through the 7D path, PS is melted through the 10D path, and PE is melted through the 14D path. Melting, and the melting process is not over after PP passes through the 23D path. It can be seen from the above phenomenon that different plastics have different requirements on the aspect ratio of the screw due to their different melting characteristics. For example, using an ordinary screw with a length-to-diameter ratio of 18 to plasticize PP, the quality of plasticization is definitely not ideal, so that there will be unmelted pellets in the molded product, but it can achieve good melting quality when used to plasticize PE. 

Now with the improvement of product performance, in order to overcome the shortcomings of general screw performance, special screws have emerged, such as: PET screw, PC screw, PVC screw, thermosetting plastic screw, EVA screw, PEEK screw and so on. In order to adapt to the plasticization of specific objects, the special screw has basically different length-to-diameter ratios, and the specific structure and technical parameters of the screw also have their own characteristics. For example, a special screw for injection of PET preforms, in order to adapt to the performance of the PET material and improve the quality of the preforms, the length-to-diameter ratio of the screw reaches 23D~26D.

C. Screw length to diameter ratio and product quality characteristics

The quality requirements of the products are different, and the requirements for the screw length to diameter ratio are also different. For example, for general PC products, the screw length-to-diameter ratio of 20 can meet the plasticization requirements, while for PC light guide plates, the ideal screw length-to-diameter ratio should be 23 to improve melting quality and measurement accuracy; precision injection screws require The ratio of length to diameter is larger than that of general purpose screws.

D. Screw length to diameter ratio and shear mixing elements

Although the screw length-to-diameter ratio and the shearing and mixing element have their own functions, they can make up for each other's shortcomings under certain circumstances. In the plasticization of plastics that are not sensitive to shear, such as PP, when the screw L/D ratio is insufficient, the melting rate can be increased by adding shear/mixing elements, such as the use of multi-threaded screws and barrier screws. Compared with the single-threaded screw, the multi-thread screw and the barrier type screw have the advantage that the melt layer formed between the barrel wall and the solid bed is not so thick, so the heat transfer from the barrel to the solid object is increased, thereby effectively using Energy. In addition, the surface of the barrier type screw can separate the solid from the melt, and the material undergoes a certain shear through the barrier thread, which makes the heat input more effective.

For the plasticization of high enthalpy plastics that are sensitive to shear, the shear/mixing element cannot replace the function of the screw length to diameter ratio. A screw with a large enough length to diameter ratio must be used to make the material have a longer residence time to absorb Enough heat, so as to obtain a sufficiently homogenized melt; plastics with longer fibers are often damaged by concentrated shearing during the plasticization process, so shear/mixing elements cannot be added. In order to achieve better dispersibility and kneading properties, a screw with a larger aspect ratio should be used.

2. Screw metering section

The main function of the metering section is homogenization and metering, so the metering section is also called the homogenization section. Since the inside of the melt output from the compression section has not yet reached the melting uniformity, there are still temperature differences between the various parts, and even unmelted solid materials, so that the plasticization amount and metering accuracy with high repeatability cannot be obtained, so the plastic melt The homogenization of the body needs to be completed in the metering section. From the function of the metering section and the practice in processing, the reasonable design of the metering section plays a very important role in the uniformity of plasticization and accurate measurement.

A. The homogenization of the metering section and the homogenization of the mixing shear section

Plastic melt homogenization has two meanings, one is the thermal uniformity expressed in the form of temperature difference, and the other is the optical and mechanical mixing uniformity that describes the distribution of pigments and fillers.

(1) Homogenization of the measurement section

The essence of the homogenization of the metering section is to make the temperature of each point inside the melt reach uniformity after the melt passing through the compression section passes through the path of the metering section, so as to obtain a melt with uniform density. This is the thermal uniformity expressed in the form of temperature difference.

(2) Homogenization of the mixing shear section at the metering front end

Because of its specific function, the injection screw has poor mixing and shearing ability, especially for color mixing and dispersibility. However, in order to reduce production costs, more domestic users seldom use color masterbatches with excellent color dispersion properties, but directly mix and stir pigments with plastic particles for plasticization. In order to meet the processing requirements of users, many domestic injection molding machine manufacturers set up a mixing shear section at the head of the screw to improve the uniformity of pigment mixing. The homogenization of the mixing and shearing section of this device is obviously different from the homogenization of the metering section, which is the optical and mechanical mixing uniformity expressed in the form of the distribution of pigments and fillers.

B. Design principles of the measurement section

(1) Length of measurement section

According to the purpose of the metering section, the groove depth of the metering section should be shallow rather than deep, and the length of the metering section is 25% to 30% of the length of the screw ridge portion. The depth and length of the groove of the metering section are two factors that affect the metering effect, and the length of the metering section plays a leading role. The reason why the increase in the screw length-to-diameter ratio can improve the metering accuracy and the uniformity of plasticization is because the length of the metering section is first lengthened and the compression ratio of the screw is reduced at the same time. For example, for a φ42mm screw, the screw length-to-diameter ratio is increased from 19 to 21.5, the length of the metering section is increased from 4.5D to 6D, the quality of plasticized PS varies between 241.3 and 241.8, and the quality repeatability reaches 0.2%, which is better than the original The 1% increase of 5 times, which shows that the length of the metering section plays a very important role in the metering accuracy.

(2) Compression ratio and groove depth of metering section

With the increase of the screw length-to-diameter ratio, the compression ratio can be appropriately reduced. When the screw L/D ratio was 18, the compression ratio was generally 2.8. Now, when the screw L/D ratio is increased, the compression ratio is generally 2.2. At the same time, the groove depth of the corresponding metering section is also reduced, so that a more uniform melt can be obtained and the plasticizing ability can be improved. Generally speaking, for the plasticization of plastics with low viscosity, good fluidity and good thermal stability, a large compression ratio and a deeper groove depth of the metering section are adopted; for plastics with high viscosity and easy thermal decomposition, a small Compression ratio and shallower groove depth of the metering section; for amorphous plastics, the groove depth of the metering section can be increased, and the groove depth is 0.06D～0.07D to reduce the flow resistance; for crystalline plastics, reduce the groove depth of the metering section. Deeply take 0.04D～0.05D to reduce the axial temperature difference of the melt.

(3) Clearance between screw and barrel

During high-speed injection, the gap between the screw and the barrel is a very prominent problem. When the screw is in high-speed injection, the shear force of the melt film and the molten pool acting on the solid bed in the direction of the compression section is greatly increased. If the gap between the screw and the barrel is too large, the melt will pass over the molten pool on the screw edge advancing surface The gap between the screw edge and the barrel enters the front groove, and the solid bed will be compressed by the melt, causing the solid bed to burst quickly, and even the solid bed at the end of the feeding section has begun to break. Therefore, in order to meet the requirements of high-speed injection, the gap between the screw and the barrel should be strictly controlled.

3. Non-returning valve

 There are three types of non-return valves: independent movement non-return valve, non-return valve that rotate synchronously with the screw, and double non-return valves that integrate the above two functions. The first two types of non-return valves are commonly used. Here we focus on double non-return valves. The double check ring is composed of an outer ring that rotates synchronously with the screw and an inner ring that moves independently. During injection, the end face of the outer ring and the end face of the screw are tightly sealed, and the end face of the inner ring and the inner end face of the outer ring are closely adhered to achieve a double sealing effect. Its characteristics It has long service life, good sealing effect, sensitive closing, long-term maintenance and stable high injection efficiency.