Features:
① Non-crystalline plastic, no obvious melting point, glass transition temperature 140°～150℃, melting temperature 215℃～225℃, molding temperature 250℃～320℃.
② High viscosity, sensitive to temperature, good thermal stability within normal processing temperature range, basically no decomposition at 300℃ for a long time, decomposition begins at over 340℃, and viscosity is less affected by shear rate.
③ Strong water absorption
Parameter selection:
a. L/D: In view of its good thermal stability and high viscosity, a large aspect ratio is selected as much as possible to improve plasticizing effect, generally 26.
Since its melting temperature range is wide and compression can be longer, a gradient screw is used. L1=30% of the total length, L2=46% of the total length.
b. Compression ratio ε: Gradient A needs to be adapted to melting rate, but melting rate cannot be calculated at present. According to processability of PC from 225℃ to 320℃, its gradient A value can be relatively medium to high. When L2 is large, ordinary gradient screw ε=2~3, and 2.6 can be taken.
c. Because of its high viscosity and strong water absorption, a mixing structure is added to screw before homogenization section and after compression section to strengthen disintegration of solid bed. At the same time, entrained water can be converted into gas to escape.
d. Other parameters such as e, s, φ and gap with barrel can be same as other ordinary screws.

Features:
① Glass transition temperature 105℃, melting temperature greater than 160℃, decomposition temperature 270℃, and a wide molding temperature range.
② High viscosity, poor fluidity, and good thermal stability.
③ Strong water absorption.
Parameter selection
a. L/D: Selects a gradual screw with a length-to-diameter ratio of 20-22. Depending on precision requirements of product molding, L1=40%, L2=40%.
b. Compression ratio ε, generally selected from 2.3 to 2.6.
c. In view of its certain hydrophilicity, a mixing ring structure is used at the front end of screw.
d. Other parameters can generally be designed according to general screw, and gap with barrel cannot be too small.

Characteristics:
① Crystalline plastics, there are many types, different types, different melting points, and a narrow melting point range. Melting point of generally used PA66 is 260℃～265℃.
② Low viscosity, good fluidity, a relatively obvious melting point, and poor thermal stability.
Parameter selection
a. L/D: Selects a sudden change screw with a length-to-diameter ratio of 18-20.
b. Compression ratio, generally selected as 3~3.5, where h3=0.07~0.08D is used to prevent overheating and decomposition.
c. Due to its low viscosity, gap between check ring and barrel should be as small as possible, about 0.05, gap between screw and barrel should be about 0.08. If necessary, front end can be equipped with a check ring depending on material, and nozzle should be self-locking.
d. Other parameters can be designed according to general screw.
③ Water absorption is general.
④ Comparison between PA special screw and general screw

 

Characteristics:
① Melting point 250℃~260℃, blow molding grade PET has a wider molding temperature, about 255℃~290℃.
② Blow molding grade PET has a higher viscosity, temperature has a great influence on viscosity, and poor thermal stability.
Parameter selection
① L/D is generally taken as 20, three-stage distribution L1=50%-55%, L2=20%.
②Use a low shear, low compression ratio screw, compression ratio ε is generally 1.8~2, and shear overheating causes discoloration or opacity h3=0.09D.
③ No mixing ring is set at the front end of screw to prevent overheating and material storage.
④Because this material is sensitive to temperature, and most manufacturers use recycled materials, in order to increase production, our factory uses a low shear screw, so motor speed can be appropriately increased to achieve purpose. At the same time, in use of recycled materials (mostly flakes), our factory has also adopted methods such as increasing blanking diameter and grooving in barrel to increase conveying capacity of feeding section according to actual conditions, and achieved relatively good results.

Thermosensitive materials are generally divided into hard and soft. Difference lies in amount of plasticizer added to raw materials. Less than 10% is hard, and more than 30% is soft.
Features:
① No obvious melting point, softens at 60℃, viscoelastic at 100℃~150℃, melts at 140℃ and decomposes at the same time, decomposes rapidly at 170℃, softens at a point close to decomposition point, and decomposes to release HC1 gas.
② Poor thermal stability, temperature and time will lead to decomposition, poor fluidity.
Design principles
a. Strict temperature control, screw design should be as low shear as possible to prevent overheating.
b. Screw and barrel should be corrosion-resistant.
c. Injection molding process needs to be strictly controlled.
Generally speaking, screw parameters are L/D=16~20, h3=0.07D, ε =1.6~2, L1=40%, L2=40%.
To prevent material accumulation, there is no check ring, and the taper of the head is 20°~30°, which is more suitable for soft rubber. If product requirements are higher, a separate screw without a metering section can be used. This type of screw is more suitable for hard PVC. In addition, in order to cooperate with temperature control, cooling water or oil holes are added inside screw of feeding section, and cold water or oil tanks are added outside barrel. Temperature control accuracy is about ±2℃.
Note:
1. Screw diameter (D)
a. Related to required injection volume: injection volume = 1/4*π*D2*S (injection stroke)*0.85.
b. Generally speaking, screw diameter D is inversely proportional to maximum injection pressure and proportional to plasticizing capacity.
2. Conveying section
a. Responsible for conveying, pushing and preheating of plastics, and should ensure preheating to melting point.
b. Crystalline plastics should be long, followed by amorphous materials, and heat-sensitive materials are the shortest.
3. Compression section
a. Responsible for mixing, compression and pressurized exhaust of plastics. Raw materials passing through this section have almost all melted, but they may not be evenly mixed.
b. In this area, plastic gradually melts, and screw groove volume must decrease accordingly to correspond to decrease in geometric volume of plastic, otherwise material will not be pressed solid, heat transfer will be slow, and exhaust will be poor.
c. Generally, it occupies more than 25% of screw working length, but compression section of nylon (crystalline material) screw occupies about 15% of screw working length, and high viscosity, fire resistance, low conductivity, high additives and other plastic screws account for 40%-50% of screw working length. PVC screws can occupy 100% of screw working length to avoid intense shear heat.
4. Metering section
a. Generally, it occupies 20-25% of screw working length to ensure that all plastics are melted and temperature is uniform, and mixing is uniform.
b. The longer metering section, the better mixing effect. If it is too long, melt will stay too long and produce thermal decomposition. If it is too short, temperature will be uneven.
c. Thermosensitive plastics such as PVC should not stay too long to avoid thermal decomposition. A shorter metering section or no metering section can be used.
5. Feeding screw groove depth, metering screw groove depth
a. Deeper feeding screw groove depth, the greater conveying volume, but screw strength needs to be considered. Shallower metering screw groove depth, the higher plasticization heating and mixing performance index. However, if metering screw groove depth is too shallow, shear heat increases, self-generated heat increases, and temperature rise is too high, causing plastic to discolor or burn, which is especially unfavorable for thermosensitive plastics.
b. Metering screw groove depth = KD = (0.03-0.07) * D. If D increases, K should be a small value.

a. Good shot: "screw compression ratio" and "injection pressure" and other conditions are determined by plastic. Some engineering plastics require higher injection pressure and appropriate screw compression ratio design to achieve better molding effects. Therefore, in order to make product better, injection pressure requirements and compression ratio issues must also be considered when selecting screw. Generally speaking, a screw with a smaller diameter can provide a higher injection pressure.
b. Fast injection: that is, confirmation of "injection speed". Some finished products require high injection rate injection to form stably, such as ultra-thin finished products. In this case, it may be necessary to confirm whether injection rate and injection speed of machine are sufficient, and whether it needs to be equipped with accumulators, closed-loop control and other devices. Generally speaking, under same conditions, screws that can provide higher injection pressure usually have lower injection speeds, and conversely, screws that can provide lower injection pressure usually have higher injection speeds. Therefore, when selecting screw diameter, injection volume, injection pressure and injection rate (injection speed) need to be cross-considered and traded off.

 

D—screw diameter (mostly expressed as Φ). Size of screw diameter directly affects size of plasticizing capacity and size of theoretical injection volume.
L/D—screw aspect ratio, L is effective length of screw thread part. Under premise of a certain screw diameter, the larger screw length-to-diameter ratio, the longer thread length, which directly affects thermal history of material in screw and also affects ability to absorb energy; if L/D is too small, it directly affects melting effect and melt quality of material; if L/D is too large, transmission torque increases and energy consumption increases.
L 1—feeding section length,
Length of L1 should ensure that material has enough conveying space, because too short L1 will cause material to melt prematurely, making it difficult to ensure conveying conditions of stable pressure, it is also difficult to ensure plasticizing quality and plasticizing capacity of subsequent sections of screw.
h 1—feeding section screw groove depth. The deeper h1 is, the more material can be accommodated, which increases feeding amount and plasticizing capacity, but it will affect plasticizing effect of material and shear strength of screw root. Generally, h1≈(0.12～0.16)D
L3—melting section length.
Length of L3 helps fluctuation of melt in the screw groove, has effect of stabilizing pressure, and allows material to be discharged from screw head with a uniform amount of material. Generally, L3=(4～5)D.
h3—Depth of screw groove in metering section. When h3 is small, screw groove is shallow, which improves plasticizing effect of plastic melt and is conducive to homogenization of melt. However, if h3 is too small, shear rate will be too high and shear heat will be too large, causing degradation of molecular chain and affecting quality of melt. If h3 is too large, reflux effect generated by screw back pressure during pre-plasticization will be enhanced, which will reduce plasticizing ability.
S—Pitch, its size affects helix angle, thereby affecting conveying efficiency of screw groove. Generally, S≈D
ε—Compression ratio, ε=h1/h3, that is, ratio of screw groove depth h1 in feeding section to the screw groove depth h3 in melting section. If ε is large, shearing effect will be enhanced, but plasticizing ability will be weakened.