1、 Classification of screw material of injection molding machine
1. HPT all hardening powder alloy screw
The tungsten cobalt powder alloy tool steel is used, and it is formed by special process and the internal and external hardness of the screw is the same, reaching about HRC65 °. It is suitable for processing various transparent products, precision products and LCP, PPS, PES, PPA and other special high temperature and high-corrosion plastics with fiber.
2. Twin alloy screw
Sacm645 and 38CrMoAlA were used as the base material, and then the tungsten carbide nickel base alloy was sprayed by the top of the tooth or the groove ion. The surface hardness of the screw was about HRC60 °. It is suitable for processing various transparent products and PA6, PA66 and other engineering materials with fiber added.
3. Electroplated screw
The surface of the product was electroplated with hard chromium, nickel, titanium and other elements. The hardness is about HRC60 °. Suitable for processing PC, PMMA, pet transparent material and corrosive plastic U-PVC material.
4. Stainless steel screw
The material is 9cr18mov special stainless tool steel, which is treated by integral quenching + overall hardening, and the overall hardness is about HRC55 °.
5. SKD61 screw
Hitachi SKD61 material is used for forming by special process, and the product has good toughness and strong torsion resistance.
6. Ordinary nitriding screw
Sacm645 and 38CrMoAlA were used as the base material, and the nitriding layer was formed by special process. The nitriding layer depth was 0.5-0.8mm, and the overall hardness was hv980-1080 °. Suitable for processing ordinary plastics, such as PP, ABS, PS, as, PVC and other ordinary plastics.
2、 Injection molding machine screw segment description
The screw of injection molding machine can be divided into feeding section, compression section and homogenization section (also known as metering section). (Note: the ratio of different screw sections is different, the depth of screw groove is different, and the transition form of screw bottom diameter is different)
1. Screw feeding section of injection molding machine
The depth of the groove is fixed, and its function is to be responsible for preheating and plastic solid transportation and extrusion. It must be ensured that the plastic begins to melt at the end of the feed section.
2. Screw compression section of injection molding machine
The section is the depth of spiral groove of the gradually shrinking screw, and its function is melting, mixing, shearing compression and pressure exhaust of plastic materials. Plastic will dissolve completely in this section, the volume will be reduced, and the design of compression ratio is very important.
3. Screw homogenization section of injection molding machine
This section is a screw screw groove fixed groove depth, its main function is mixing, glue conveying, measurement, and must also provide enough pressure to maintain uniform temperature of the melt glue and stabilize the flow of molten plastic.
3、 Description of screw parameters of injection molding machine
1. D - screw diameter (mostly expressed by Φ)
The diameter of screw directly affects the plasticizing capacity and the theoretical injection volume.
2. L / D - ratio of length to diameter of screw
L is the effective length of the screw thread. Under the premise of a certain screw diameter, the larger l/d, the longer the thread length, it directly affects the heat history of the material in the screw, and also affects the ability to absorb energy; if l/d is too small, it will directly affect the melting effect and melt quality of the material; if l/d is too large, the torque transfer will be increased and the energy consumption will increase.
3. L 1 - length of feeding section
The length of L1 should ensure sufficient conveying space for materials. Because too short L1 will lead to premature melting of materials, thus it is difficult to guarantee the stable pressure transportation conditions, and it is difficult to guarantee the plasticizing quality and plasticizing capacity of each section of screw in the future.
4. H1 - depth of screw groove in feeding section
If H1 is deep, it will hold more materials, which improves the feeding capacity and plasticizing capacity, but it will affect the plasticizing effect of materials and the shear strength of screw root. Generally H1 ≈ (0.12-0.16) d.
5. L3 - length of melting section
L3 length is helpful to the fluctuation of melt in the screw groove, and it has the function of stabilizing pressure, so that the material is discharged from the screw head with uniform material quantity. Generally l3= (4-5) d.
6. H 3 - depth of measuring section screw groove
H3 is small and the screw groove is shallow, which improves the plasticizing effect of plastic melt, which is beneficial to melt homogenization. However, too small H3 will lead to high shear rate and excessive shear heat, which will cause degradation of molecular chain and affect the melt quality; if H3 is too large, the plasticizing ability will be reduced due to the increase of the reflux effect caused by screw back pressure during pre molding.
7. S - pitch
The size of the screw affects the screw angle, thus affecting the conveying efficiency of the screw groove. Generally, s ∧ D.
8. ε - compression ratio
ε = H1 / H3, that is, the ratio of the depth H1 of the feeding section to the depth of the molten segment H3. The larger ε, the shear effect will be enhanced, but the plasticizing ability will be weakened.
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4、 Evaluation standard of screw quality of injection molding machine
1. Plasticizing quality
A screw must first produce products that meet the quality requirements. The so-called quality requirements refer to that the products produced shall meet the following requirements:
(1) It has various performance that meets the requirements. Have the physical, chemical, mechanical and electrical properties in accordance with the regulations;
(2) Have the required apparent quality. If it can meet the user's requirements for bubble, crystal point, dyeing uniformity, etc.
(3) The plasticizing quality of screw meets the requirements:
Whether the melt temperature of screw extrusion is uniform, axial fluctuation and radial temperature difference are much larger.
Is there a minimum melt temperature to be formed.
Whether the extrusion melt has pressure fluctuation.
Whether the dyeing and other filling agents are evenly dispersed.
2. Output
The so-called output refers to the production or extrusion quantity of the given head under the premise of ensuring the plasticizing quality. As mentioned above, the output is generally expressed in kg / h or kg / R. A good screw should have a high plasticizing capacity (production capacity).
3. Unit consumption
The so-called unit consumption refers to the energy consumed by every kilogram of plastic (rubber) extruded, generally expressed in N or. Where n is power (kw), q is output (kg / h).
(1) The larger the value, the more energy it takes to plasticize the same weight of plastic, that is, the more heating power is consumed, the more mechanical work the motor does enters the material through the form of shear and friction heat. vice versa.
(2) A good screw should be as low as possible under the premise of ensuring the plasticizing quality.
4. Adaptability
(1) The adaptability of screw refers to the adaptability of screw to different plastics, matching different heads and products. Generally speaking, the stronger adaptability, the lower the plasticization efficiency.
(2) Therefore, we always hope that a good screw should be equipped with both adaptability and high plasticizing efficiency.
5. Manufacturing is difficult
A good screw must also be easy to process and manufacture, with low cost.
5、 How to choose the screw of injection molding machine
1. Screw diameter (d)
The injection volume is 1 / 4 × π × D ↑ 2 × s (ejection stroke) × 0.85. The injection pressure is inversely proportional to the maximum injection pressure and is proportional to the plasticizing ability;
2. Conveying section
Responsible for the transportation, extrusion and preheating of plastics, and ensure preheating to melting point. Crystalline plastics should be long (such as POM, PA); amorphous materials should be followed (such as PS, PU, ABS); and the shortest thermal sensitivity (such as PVC);
3. Compression section
Responsible for mixing, compression and pressure exhaust of plastics. Almost all the raw materials through this section are melted, but they are not necessarily uniformly mixed. In this area, the plastic gradually melts, and the volume of screw groove must be reduced accordingly to correspond to the decrease of plastic geometry volume. Otherwise, the material pressure is not solid, the heat transfer is slow and the exhaust gas is poor. The compression section generally accounts for more than 25% of the working length of the screw, but the nylon screw accounts for about 15%, the plastic screw with high viscosity, fire resistance, low conductivity and high additive accounts for about 40%, and PVC screw can account for 100% in order to avoid the occurrence of intense shear heat.
4. Metering section
Generally, it accounts for 20% of 25% of the working length of screw, ensuring that the plastic is melted and the temperature is uniform and the mixing is even. The long measuring section is good for mixing, but too long will make the melt stay too long and produce thermal decomposition, and too short will make the temperature uneven. PVC and other heat sensitive plastics should not stay for a long time to avoid thermal decomposition. Short measuring section or non measuring section can be used.
5. Depth of the groove
The deeper the depth of the feed screw groove, the larger the conveying capacity, but the screw strength of the injection molding machine should be considered. The shallower the measuring screw groove depth, the higher the plasticization heat and mixing performance index, but if the depth of the measuring screw groove is too shallow, the shear heat increases, the self generated heat increases and the temperature rise is too high, which causes the plastic to change color or burn, especially for the heat sensitive plastics.
Gradual type screw - the compression section is long, accounting for 50% of the total length of the screw. The energy conversion is relaxed during plasticization, and it is mainly used in plastics with poor thermal stability such as PVC.
The mutant screw - the compression section is short, accounting for about 5% - 15% of the total length of the screw. The energy conversion is more intense during plasticization, and it is mostly used in crystalline plastics such as polyolefin and PA.
Universal screw - can be used for processing of many kinds of plastics.
6、 Screw use method of injection molding machine
Do not start the machine when the drum does not reach the pre-set temperature. The new electric heating generally requires that the temperature reaches the set value for 30 minutes before operating the screw.
If the shutdown is more than half an hour, it is better to close the material drop port and clean the material inside the barrel, and set up the heat preservation.
Avoid foreign matter falling into the material barrel to damage the screw and barrel. Prevent metal debris and debris from falling into the hopper. If the recycled materials are processed, magnetic hopper shall be added to prevent iron chips from entering the material barrel.
When using salivation prevention, make sure that the plastic in the barrel is completely melted to avoid damaging the transmission system parts when the screw is backward.
Avoid the phenomenon of screw idling and slipping.
When using new plastic, the surplus material of the material barrel shall be cleaned. When using POM, PVC, pa+gf and other materials, the degradation of raw materials shall be minimized, and the water inlet materials such as ABS shall be used to wash them out after shutdown.
It is necessary to avoid the mixing of POM and PVC into the tube at the same time, which will cause serious industrial accidents due to reaction at melting temperature.
When the temperature of the molten plastic is normal but the black spot or color change is found constantly, check whether the screw check ring (over rubber ring and meson) is damaged.
7、 Common problems and Solutions
1. Slip
In the pre molding stage, the screw rotates in the barrel to transport materials along the direction of the screw and back to accumulate the material to prepare for the next injection molding, the screw will also slip. If the screw starts to slide in the pre plastic stage, the axial movement of the screw will stop when the screw continues to rotate. The main reasons for the screw slip of injection molding machine are: the feeding port is difficult to press and the length of the barrel can not form enough adhesion, then the screw slip occurs.
There are two solutions to screw slip in injection molding machine:
The first method: add a small amount of materials to start cleaning the end of the cylinder, and check the melting temperature. If the residence time is short, the melting temperature will be lower than the set value of the barrel temperature.
The second method: observe the molding products. If there are black spots or light stripes or marble stripes, it indicates that the material has not been mixed well in the barrel.
2. No cutting
During injection molding, the screw rotation is often encountered but the material is not discharged, and the normal production cannot be completed. The common reasons and solutions for the failure of cutting the screw of injection molding machine are as follows:
(1) The material chute of the material cylinder is blocked and check whether there is any melted plastic block bond.
(2) The temperature control is inaccurate, the temperature at the back end of the barrel is too high, adjust the temperature setting, and check whether the cooling water path is blocked.
(3) The screw appears the phenomenon of glue - plastic wrapped screw rotates together.
(4) Too much oil is added to the plastic, causing the screw to slip.
(5) Too much return is added to the plastic.
(6) The wear of screw and barrel and rubber ring may lead to the screw not cutting, and the plastic leakage will not be transported to the front end of the barrel.
(7) The plastic particles are too large to bridge, only the plastic needs to be crushed again.
(8) If the new screw is not blanking, it may be that the design of the material barrel and screw blanking port is not appropriate. For the screw, the screw groove in the feeding section of the screw is too shallow, which causes the screw to not drive the plastic forward conveying when rotating, or it may be that the material quantity and the square surface of the barrel are the design of the blanking port.
Conclusion:
Speaking of this, we should have a good understanding of the screw of injection molding machine! As one of the four pillar materials after steel, wood and cement, plastics has been widely used in all fields of society. The development of screw industry of injection molding machine has been further improved. In the future, with the progress of technology, the grade, precision and technical content of screw will be constantly improved. The main trend of screw products will be to develop products that conform to the market development trend.
