How to Choose the Screw According To the Plastic Characteristics ?
Tips for choosing the Screw according to the plastic characteristics
The screw of the injection molding machine has the functions of conveying, melting, mixing, compression, metering and exhaust. Besides, they are the key factors affecting the quality of plasticization. Here are some key points for selecting injection molding machine’s screws according to the characteristics of the plastic when injection molding.
The characteristics of PC:
- Non-crystalline plastic
- No obvious melting temperature point
- Glass transition temperature of 140~150℃
- Melting temperature of 215~225℃
- Molding temperature of 250~320℃
- High viscosity
- More sensitive to temperature. It has good thermal stability in the normal processing temperature range. It will not decompose for a long time at 300℃, and will start to decompose when it exceeds 340℃. The viscosity is less affected by the screw shear rate.
- Strong water absorption.
Pay attention to the following points for the screw of injection molding PC:
(1) In view of its good thermal stability and high viscosity, in order to improve the plasticizing effect, choose a large screw L/R ratio as much as possible, generally 26. Because of its wide melting temperature range and longer compression section, a gradual screw is used. L1=30%L, L2=46%L.
(2) Compression ratio ε: The degree of gradient A needs to be compatible with the melting rate. But the current melting rate cannot be accurately calculated. According to the processability of PC from 225°C to 320°C, the degree of gradient A can be relatively higher than the middle value. In the case of large L2, the ordinary gradual screw ε=2~3, and A generally takes 2.6.
(3) Because of its high viscosity and strong water absorption, a mixing structure is added to the screw before the homogenization section and after the compression section to strengthen the disintegration of the solid bed. At the same time, the entrained moisture can be turned into gas and escape.
(4) Other parameters such as e, s, a and the gap with the barrel can be the same as other ordinary screws.
The characteristics of PA are as follows:
- Types diversity.There are many types, different types with different melting points, and a narrow melting point range. Generally, the melting point of PA66 is 260~265°C.
- Low viscosityand good fluidity
- Relatively obvious melting point, and poor thermal stability
- The water absorption is average.
Pay attention to the following points for the screw of injection molding PA:
(1) Select a mutant screw with a length-to-diameter ratio of 18-20.
(2) The compression ratio is generally 3~3.5, in order to prevent overheating decomposition h3 = (0.07 ~ 0.08) D (diameter of the screw).
(3) Because of its low viscosity, the gap between the check ring and the barrel should be as small as possible, about 0.05. Besides, the gap between the screw and barrel is about 0.08. If necessary, depending on the material, the nozzle should be self-locking.
(4) Other parameters can be designed according to general-purpose screw.
Polymethyl methacrylate (PMMA)
The characteristics of PMMA are as follows:
- The glass transition temperature is 105°C,
- The melting temperature is greater than 160°C
- The decomposition temperature is 270°C
- The molding temperature range is wide
- The viscosity is large
- The fluidity is poor
- The thermal stability is good
- Strong water absorption
The following points should be noted for the screw of injection PMMA:
(1) Choose a gradual screw with a length-to-diameter ratio of 20-22, and generally L1=40%L, L2=40%L according to the accuracy requirements of its product molding.
(2) The compression ratio ε is generally selected from 2.3 to 2.6.
(3) Because it has a certain degree of hydrophilicity, a mixing ring structure is adopted at the front end of the screw.
(4) Other parameters can generally be designed according to general-purpose screw. In addition, the gap between the barrels should not be too small.
The characteristics of PET are as follows:
The melting point is 250~260℃, and the molding temperature of blow molding grade PET is a bit wider, 255~290℃.
Pay attention to the following points for the screw of injection PET:
(1) L/D is generally taken as 20, three-stage distribution L1= (50%~55%) L, L2=20%L.
(2) Use a screw with low shear and low compression ratio. The compression ratio ε is generally 1.8~2. At the same time, to prevent discoloration or opacity caused by shearing overheating, set hs=0.09D.
(3) There is no mixing zone at the front end of the screw to prevent overheating and material storage.
(4) Because this kind of material is more sensitive to temperature, and general manufacturers tend to use recycled materials. In order to increase production, they generally use low-shear screw. So that the motor speed can be appropriately increased to achieve the goal. At the same time, in the use of recycled materials (most of them are flakes), according to the actual situation, in order to increase the conveying capacity of the feeding section, methods like increasing the diameter of the blanking and slotting in the barrel have been achieved, and relatively good results have been achieved.
Polyvinyl Chloride (PVC)
PVC is a heat-sensitive plastic, generally divided into hard and soft. The difference lies in the amount of plasticizer added to the raw material, less than 10% is hard, more than 30% is soft.
The characteristics of PVC:
- No obvious melting point, soft at 60℃. Its viscoelastic state is at 100~150℃, melting at 140℃, and decomposing at the same time.Decompose rapidly at 170℃. The softening point is close to the decomposition point, and the decomposition is released in HCl gas.
- The disadvantage is poor thermal stability and poor fluidity.
The following points should be paid attention to for the screw of injection molding PVC:
(1) The screw design should be as low shear as possible to prevent overheating.
(2) The screw and barrel should be corrosion-proof.
(3) Generally speaking, the screw parameters are L/D=16~20, h3=0.07D, e=1.6~2, L1=40%L, L2=40%L.
(4) In order to prevent material accumulation, there is no check ring. And the head taper is 20°~30°, which is more suitable for soft plastic. If the product requirements are higher, a separate screw without metering section can be used, which is more suitable for the High-quality PVC.
And further, in order to cooperate with temperature control, cooling water or oil holes are added to the screw in the feeding section. While the barrel is added with a water cooling or oil cooling system. The temperature control accuracy is about ±2°C.