As we all know, the "screw plasticizing component" of the injection molding machine is the main core unit of the injection molding machine, which directly reflects the performance of the injection molding machine. And the "heart" called the screw is a vital part, its performance determines the quality of processing products, efficiency and other indicators. With the rapid development of plastics industry, especially in our country after 20 years of development, injection molding equipment has been mature, as the injection molding equipment designers and users, need a clear understanding of what is suitable for processing screw which materials, especially in the complicated variety of materials on the market today, the screw type was different in various, but the actual use is more confusing, users often put a screw to physical properties not suitable for plastic processing. A more wrong concept is: "a screw can be omnipotent". In fact, this is a simple introduction to some commonly used screws with main structure, so as to provide some references for processing products.
(1) the characteristics of PC material (polycarbonate):
1. non crystalline plastic, no obvious melting point, vitrification temperature 140 -160 C, melting temperature of 215 C -225 C, molding temperature 230 degrees C
2. the viscosity is big, the processing temperature is high, the range is narrow, it is easy to decompose, and the phenomenon of avoiding overheating is avoided when using the screw.
3. is sensitive to water, and the resin is easy to hydrolyze at high temperature. The drying temperature is 120 -130 C, and the time is about 4 hours.
In order to improve the plasticizing effect, 1.L/D has a large length to diameter ratio in order to improve the plasticizing effect, because of its good thermal stability and high viscosity.
2. because the melting temperature range is wide, the feeding section is long, the homogenization section is short, the compression ratio is moderate, and the barrier type can be selected for the plasticizing effect.
3. such as adding PC flame retardant material or glass fiber reinforced, can choose to use high-quality stainless steel corrosion-resistant and wear-resistant tool steel material and three screw
(two) PMMA (organic glass) features:
1. the vitrification temperature is 105 C, the melting temperature is more than 160 C, the decomposition temperature is 270 degrees C, and the forming temperature range is 180-250 C.
2. the viscosity is big, the fluidity is poor, it is sensitive to the shear rate, and the temperature should be strictly controlled when processing.
The temperature of the 3. barrel is 240 -250 C, and the time lag of 260 degrees centigrade is not more than 8 minutes.
4. the water absorbability is stronger, the drying temperature is 80 -90 C, and the time is over 3 hours.
In 1.L/D, the ratio of length to diameter is 20-22, and the progressive type screw is generally L1=35-45% and L2=35-40%.
2. the compression ratio is generally 2.3-2.6;
3. in view of its hydrophilic property, the mixer head structure is adopted at the front of the screw.
(three) PA (polyamide), common PA6 (nylon 6), PA66 (nylon 66) properties:
1. kinds of crystalline plastics are different, their melting points are different, the range of melting point is narrow, and the melting point of PA66 is 260 centigrade at -270 C.
2. low viscosity, good fluidity, more obvious melting point and poor thermal stability.
3. the water absorption rate is high, the dryer temperature is 80 degrees centigrade, not more than 120 degrees centigrade, time 16 hours. If the raw material is exposed to the air for 8 hours, the vacuum oven should be used at about 105 degrees centigrade for 12 hours.
1.L/D selects a mutant screw with a length of 18-20.
The 2. compression ratio generally selects 2.5-3.5, which prevents superheat decomposition h3:0.067-0.089D;
3. because of its low viscosity, the gap between the rubber and the cylinder should be as small as 0.05-0.08, and the gap between the screw and the cylinder is 0.08-0.12.
The 4. ejection nozzle can be equipped with a spring self locking ejection nozzle.
(four) PET (polyester) chemical name: properties of Polyterephthalate two ester:
1. melting point 250 -260 C, blow molding PET molding temperature is more extensive, about 255 C -290 C, enhanced grade GF-PET can reach 290 degrees centigrade and so on.
2. high melting point, low viscosity, high adhesion to metal, easy to hydrolyze;
3. hydrophilicity, the sticky material is more sensitive to water at high temperature, the drying temperature is 150 -160 C, and the time is over 4 hours.
1.L/D generally takes 20-22, three segment distribution L1=50-60%, L2=20-30%;
2. the screw compression ratio of low shear and low compression ratio is =1.8-2.2, because the shear overheating causes discoloration or opaque h3=0.09-0.10D.
3. when the recycled material is used, the conveying capacity of the feeding section is increased, and the forced feeding and plasticizing system is adopted.
(five) PVC (polyvinyl chloride)
Heat sensitive material, generally divided into hard and soft, the difference is the number of plasticizer added raw materials, less than 10% to more than 30% for the soft hard.
1., there is no obvious melting point. It is soft at 60 C, 100 -150 C, and viscoelastic state. The melting temperature range of 140 C is narrow, the thermal stability is poor, it is easy to decompose, the decomposition temperature is 170 C, the softening point is near the decomposition point, and the decomposition is released to HC1 gas.
2. because the viscosity of UPVC melt is high, the thermal stability is poor, the temperature and time will cause decomposition, so it has the effect of corrosion to the general steel.
3. strict control of temperature, screw design as far as low shear to prevent overheating, the flow path must be smooth, no dead angle, to prevent the decomposition of material;
4. screw machine cylinder should prevent corrosion.
5. injection molding process should be strictly controlled.
1.L/D=18-21, h3=0.067-0.08D, epsilon -1.6-2, L1=40-45%, L2=35-45%, in order to prevent the material from not passing the rubber ring, the head is 20 C -30 C, using the hard chromium plating screw;
2. the choice of temperature control;
(1) the multi section air cooling device of the melt cylinder;
Second, low shear and high mixing UPVC special screw cylinder device.