Main factors affecting plasticizing quality

                                                            Main factors affecting plasticizing quality

The main factors that affect plasticizing quality are: aspect ratio, compression ratio, back pressure, screw speed, barrel heating temperature, etc.

1. Aspect ratio: It is the ratio of the effective working length of the screw to the diameter of the screw.

a. If the aspect ratio is large, the food will be eaten evenly;

b. Plastics with better thermal stability can use longer screws to improve mixing properties without burning. Plastics with poor thermal stability can use shorter screws or no threads at the end of the screw. Considering the characteristics of plastics, the general flow length ratio is as follows: thermosetting is 14'16, heat-sensitive ones such as hard PVC and high-viscosity PU are 17'18, general plastics are 18'22, and high-temperature stable plastics such as PC and POM are 22'24 .

2. Compression ratio: It is the ratio of the depth of the last groove in the feeding section to the depth of the first groove in the metering section.

a. Considering the impact of material compressibility, filling degree, backflow, etc., the product must be dense, heat transfer and exhaust;

b. Appropriate compression ratio can increase the density of plastic, make the molecules more tightly bonded, help reduce the absorption of air, reduce the temperature rise caused by pressure, and affect the difference in output. Improper compression Than will destroy the physical properties of plastic;

c. The higher the compression ratio, the higher the temperature rise generated during the plasticization process of the plastic in the material tube, resulting in better mixing uniformity of the plastic during plasticization, and the relative discharge volume is greatly reduced.

d. High compression ratio is suitable for refractory plastics, especially plastics with low melt viscosity and thermal stability; low compression ratio is suitable for fusible plastics, especially plastics with high melt viscosity and heat sensitivity.

3. Back pressure

a. Increasing the back pressure can increase the work done by the screw on the molten resin, eliminate unmelted plastic particles, and increase the density and uniformity of the raw materials in the material tube;

b. Back pressure is used to increase the temperature of the barrel, and its effect is the most significant;

c. If the back pressure is too large, plastics with high heat sensitivity are easy to decompose, and plastics with low viscosity may cause drooling. If the back pressure is too small, the injection molded finished product may have bubbles.

4. Screw speed

a. The rotation speed of the screw directly affects the shear of the plastic in the spiral groove;

b. The small screw groove is shallow and absorbs the heat source quickly, which is enough to soften the plastic during the compression period. The friction heat energy between the screw and the barrel wall is low, which is suitable for high-speed rotation and increases the plasticizing capacity;

c. Large screws should not be rotated quickly to avoid uneven plasticization and excessive frictional heat;

d. For plastics with high heat sensitivity, if the screw speed is too high, the plastic will be easily decomposed;

e. Usually, each screw size has a certain speed range, the general speed is 100'150rpm; if it is too low, the plastic cannot be melted, and if it is too high, the plastic will be scorched.

5. Electric heating temperature setting

a. Melt the chilled plastic trapped in the barrel and screw to facilitate screw rotation and provide part of the heat required for the plastic to melt;

b. Set 5'10℃ lower than the melt temperature (partly provided by friction heat energy);

c. Adjustment of the nozzle temperature can also be used to control problems such as drooling, condensed material (nozzle plugging), and wire pulling;

d. General temperature control of crystalline plastics

Several important geometric parameters of the screw

1. Screw diameter (D)

a. Relevant to the required injection volume:.

Injection volume=1/4*π*D2*S (injection stroke)*0.85;

b. Generally speaking, the screw diameter D is inversely proportional to the maximum injection pressure and directly proportional to the plasticizing capacity.

2. Conveying section

a. Responsible for the transportation, pushing and preheating of plastics, and should ensure that they are preheated to the melting point;

b. Crystalline plastics should be longer (such as POM, PA), amorphous materials should be second (such as PS, PU, ABS), and heat-sensitive materials should be the shortest (such as PVC).

3. Compression section

a. Responsible for the mixing, compression and pressurized exhaust of plastics. The raw materials passing through this section have been almost completely melted, but they may not be evenly mixed;

b. In this area, the plastic gradually melts, and the volume of the screw channel must decrease accordingly to correspond to the decrease in the geometric volume of the plastic. Otherwise, the material pressure will not be solid, the heat transfer will be slow, and the exhaust will be poor;

c. Generally, it accounts for more than 25% of the screw working length, but the compression section of nylon (crystalline material) screw accounts for about 15% of the screw working length, and plastic screws with high viscosity, fire resistance, low conductivity, and high additives account for 40%. '50% screw working length, PVC screw can account for 100% screw working length to avoid intense shear heat.

4. Measuring section

a. Generally accounts for 20'25% of the working length of the screw to ensure that all plastics are melted, the temperature is uniform, and the mixing is uniform;

b. The longer the metering section is, the better the mixing effect will be; if it is too long, the melt will easily stay for too long and cause thermal decomposition; if it is too short, the temperature will be uneven;

c. Heat-sensitive 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 channel depth, metering screw channel depth

a. The deeper the depth of the feed screw groove, the greater the conveying capacity, but the screw strength needs to be considered. The shallower the depth of the metering screw groove, the higher the plasticizing heat and mixing performance index. However, if the depth of the metering screw groove is too shallow, shearing will occur. Heat increases, self-generated heat increases, and the temperature rise is too high, causing the plastic to discolor or burn, which is especially detrimental to heat-sensitive plastics;

b. Measuring screw groove depth = KD = (0.03'0.07) * D. As D increases, choose a smaller value for K.