Screw is an important part of injection molding machine. Its role is to transport, compact, melt, stir and apply pressure to the plastic. All this is done by the rotation of the screw in the barrel. When the screw rotates, plastic will cause friction and mutual movement between the inner wall of the barrel, the bottom surface of the screw groove of the screw, the advance surface of the screw edge, and plastic and plastic. The forward movement of plastic is the result of this combination of movements, and the heat generated by friction is also absorbed to increase the temperature of the plastic and melt the plastic. The design structure of the screw will directly affect the extent of these effects.
Types and characteristics
Gradual screw features: long compression section, accounting for 50% of the total screw length, gentle energy conversion during plasticization, mostly used for plastics with poor thermal stability such as PVC.
Features of abrupt screw: The compression section is short, accounting for about 5% to 15% of the total screw length, and the energy conversion is severe during plasticization. It is mostly used for crystalline plastics such as polyolefin and PA.
Features of universal screw: The universal screw with strong adaptability can adapt to the processing of a variety of plastics.
Screw segment description
Generally speaking, the screw of injection molding machine can be divided into feeding section, compression section and homogenization section (also called metering section). (Note: The ratio of the three sections of different screws is different, the depth of the screw groove is different, and the transition form of the bottom diameter of the screw is different.)
(1) Description of feeding section: The depth of the spiral groove in this section is fixed, and its function is to be responsible for preheating and plastic solids conveying and pushing. It must be ensured that the plastic begins to melt at the end of the feed section.
(2) Description of the compression section: This section is a tapered screw sulcus tooth depth, and its functions are melting, mixing, shear compression and pressurization of plastic raw materials. The plastic will completely dissolve in this section, and the volume will shrink. The design of the compression ratio is important.
(3) Description of the homogenization section: This section is a fixed depth of the screw groove. Its main functions are mixing, melt transfer, and metering. It must also provide sufficient pressure to maintain the uniform temperature of the melt and stabilize the flow of the molten plastic.
Explanation of screw parameters
D-screw diameter (represented by Φ). The diameter of the screw directly affects the size of the plasticizing ability and the theoretical injection volume.
L / D-Screw length to diameter ratio, L is the effective length of the screw thread portion. Under the premise of a certain screw diameter, the larger the length-to-diameter ratio of the screw, the longer the thread length, which directly affects the thermal history of the material in the screw and the ability to absorb energy; if L / D is too small, it directly affects the Melting effect and melt quality; if L / D is too large, the transmission torque is increased and energy consumption is increased.
L 1-length of the feeding section. The length of L1 should ensure that the material has sufficient conveying space, because too short L1 will cause premature melting of the material, which will make it difficult to ensure stable pressure conveying conditions, and it will also be difficult to ensure the subsequent sections of the screw. Plasticizing quality and plasticizing ability.
h 1-Screw groove depth of feeding section. If h1 is deep, it will contain more materials, which will increase the feeding amount and plasticizing capacity, but it will affect the plasticizing effect of the material and the shear strength of the screw root. Generally h1≈ (0.12 ~ 0.16) D
L3-melt section length. The length of L3 contributes to the fluctuation of the melt in the screw groove, and has the effect of stabilizing the pressure, so that the material is discharged from the screw head with a uniform amount, generally L3 = (4 ~ 5) D.
h 3-meter section screw groove depth, h3 is small, and the screw groove is shallow, which improves the plasticizing effect of the plastic melt and is beneficial to the homogenization of the melt. However, if h3 is too small, it will lead to an excessively high shear rate and shear heat. If it is too large, it will cause the degradation of the molecular chain and affect the melt quality; if h3 is too large, the reflow effect due to the back pressure of the screw during pre-plasticization will increase, which will reduce the plasticizing ability.
S- pitch, its size affects the helix angle and thus the conveying efficiency of the spiral groove, generally S≈D
ε- compression ratio, ε = h1 / h3, that is, the ratio of the depth of screw groove h1 in the feeding section to the depth of screw groove h3 in the melting section. Large ε will increase the shear effect, but will weaken the plasticizing ability
3 screw material
The materials commonly used to produce screws are:
6.Tungsten carbide alloy, tungsten carbide nickel-based alloy
7, fully hardened powder alloy (all-alloy)
8.High temperature alloy
4 screw usage
1. When the barrel does not reach the preset temperature, do not start the machine. The new electric heating generally requires the temperature to reach the set value for 30 minutes before operating the screw
2. If the machine is stopped for more than half an hour, close the blanking port and clean the material in the barrel, and set the insulation
3. Avoid foreign matter falling into the barrel to damage the screw and barrel. Prevent metal fragments and debris from falling into the hopper. If processing recycled materials, add a magnetic hopper to prevent iron filings from entering the barrel.
4. When using anti-salin, make sure that the plastic in the barrel is completely melted, so as not to damage the transmission system parts when the screw is retracted.
5. Avoid screw idling and slipping.
6. When using new plastic, the rest of the barrel should be cleaned. Use POM, PVC, PA + GF and other materials to minimize the degradation of raw materials. After shutdown, flush with ABS and other materials in time.
7. Avoid mixing POM and PVC into the barrel at the same time. Reaction will occur at the melting temperature and cause serious industrial accidents.
8. When the temperature of the molten plastic is normal, but the black spots or discoloration of the molten plastic are constantly found, check the screw check ring (passing ring, meson) for damage.
5 ways to prevent slipping
The slippage of the screw of the injection molding machine will cause material degradation, which will affect product quality. What are the reasons for screw slippage? How to "grasp" the slippery screw rod? The following explains it for you.
When the screw slips, the material may collect at the feeding port and cannot be delivered to the end of the injection machine normally. When the screw rotates and retracts inside the barrel to convey the material and prepare for the next injection, screw slip will occur in the plasticizing section. At this time, the rotation of the screw is still continuing, but the axial movement of the screw will stop, that is, slippage will occur. Screw slippage often results in degradation of the material before injection, product quality degradation (such as lack of material), and prolonged molding cycles.
There are many reasons for the screw slippage, which may be caused by too high back pressure, overheated or cold end of the barrel, worn barrel or screw, too shallow thread in the feeding section, irrational hopper design, blocked hopper, wet resin, excessive resin Lubrication, material is too fine or improper cutting of resin and recycled materials.
Overcooling of the barrel end is one of the main causes of screw slippage. The barrel of the injection machine is divided into 3 sections. At the end, that is, the feeding section, the pellets will form a layer of melt film and stick to the screw during the heating and compression process. Without this film, the pellets cannot be easily transported to the front end.
The material in the feed section must be heated to a critical temperature to form that critical melt film. However, the residence time of the material in the feeding section is usually short and cannot reach the required temperature. This situation usually occurs on small injection machines. Too short residence time will cause incomplete melting and mixing of the polymer, which will cause the screw to slip or stall.
6 selection method
The screw can be said to be the heart of the injection molding machine. The quality of the screw determines the quality of the product. The plasticizing screw of injection molding machine has the functions of conveying, melting, mixing, compressing, metering and exhausting. It plays an important role in plasticizing quality and is a key factor affecting plasticizing quality.
There are several important parameters of the screw that affect the plasticization quality. Generally, the screw of the injection molding machine is selected according to the following principles:
1. Screw diameter (D)
a, related to the required injection volume: injection volume = 1/4 * π * D2 * S (injection stroke) * 0.85;
b. Generally speaking, screw diameter D is inversely proportional to high injection pressure and proportional to plasticizing capacity.
a. Responsible for conveying, pushing and preheating of plastic, and should ensure preheating to the melting point;
b. The crystalline plastic should be long, followed by the amorphous material, and the heat sensitivity is short.
a, responsible for plastic mixing, compression and pressurized exhaust, the raw materials through this section have almost all melted, but not necessarily uniformly mixed;
b. In this area, the plastic gradually melts, and the volume of the spiral groove must be reduced accordingly to correspond to the decrease of the geometric volume of the plastic, otherwise the material pressure is not solid, the heat transfer is slow, and the exhaust is poor;
c. Generally accounts for more than 25% of the working length of the screw, but the compression section of the nylon (crystalline material) screw accounts for about 15% of the working length of the screw. High viscosity, fire resistance, low conductivity, high additives and other plastic screws account for 40%. \ 50% screw working length, PVC screw can occupy 100% screw working length, so as not to generate intense shearing heat.
a. It generally accounts for 20 \ 25% of the working length of the screw to ensure that all the plastic is melted, the temperature is uniform, and the mixing is uniform;
b. If the metering section is long, the mixing effect is good. If it is too long, the melt will stay too long and thermal decomposition will occur. If it is too short, the temperature will be uneven.
c. Thermal-sensitive plastics such as PVC should not stay too long to avoid thermal decomposition. A shorter or no metering section can be used.
5, feeding screw groove depth, measuring screw groove depth
a. The deeper the depth of the feeding screw groove, the larger the conveying capacity, but the strength of the screw needs to be considered. The shallower the depth of the measuring screw groove, the higher the plasticizing heat and the mixing performance index. Increased heat, increased self-generated heat, too high temperature rise, causing plastic discoloration or scorching, which is especially bad for heat-sensitive plastics;
b. Measuring screw groove depth = KD = (0.03 \ 0.07) * D, if D increases, K will choose a smaller value.
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