Basic Parameters of Screws

1. Screw diameter (D)

a. related to the required injection volume:

Injection volume per mold=1/4×π×D²×S(injection route)×density×0.85;

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

2. Conveying

a. In charge of mixing, compressing, pressuring the plastics and venting; after this process, the raw materials have almost been melted, but may not be uniformly mixed;

b. The crystalline plastics shall be comparatively long (e.g. LCP, PA), non-crystalline plastics come second (e.g. PS, AS, ABS), the heat-sensitive plastics shall be comparatively short (e.g. PVC).

3. Compression section

a. In charge of mixing, compressing, pressuring the plastics and venting; after this process, the raw materials have almost been melted, but may not be uniformly mixed;

b. In this area, the plastics are gradually melted, thus the volume of screw channel shall be correspondingly reduced in response to the overall reduction in plastic geometric volume; or the plastics may not be fully compressed, leading to slow heat transfer and poor venting;

c. Generally, the compression section accounts for 25%~30% of the working length of the screw, while the compression section of the screw for nylon (a kind of crystalline plastic)  accounts for approx. 15% of the working length of the screw; and the compression section of the screw for those plastics with high viscosity, resistance to fire, low conductivity and high adding proportion, accounts for 40%~50% of the working length of the screw.

4. Measuring section

a. Generally, it accounts for 20%~25% of the working length of the screw, ensuring that the plastics are fully melted with uniform temperature and mixing;

b. The longer the measuring section, the better the mixing effect. However, if it is too long, that may lead to thermal decomposition of the melt after the melt stays for a long period; if it is too short, that may lead to non-uniform temperature of the produced plastics.

c. Heat-sensitive plastics like PVC shall not stay for a long period to prevent thermal decomposition; under such case, the measuring section can be shortened or omitted.

5. Depth of screw channels of the charging section and measuring section

a. The deeper the channel in the charging section of the screw, the larger the conveying volume. But the screw strength counts. The shallower the channel in the measuring section of the screw, the higher the heat release rate of plasticization and mixing performance index. However, if the channel in the measuring section of the screw is too shallow, that may increase the heat release rate during shearing, lead to the increase in self-released heat; excess temperature may cause the discoloring or scorch of the plastics, or damage the heat-sensitive plastics in particular;

b. Depth of channel in the measuring section of the screw==KD(0.03-0.07) ★D; if D increases, K shall take small value.

II. Main factors influencing the plasticization quality

Main factors influencing the plasticization quality: length-diameter ratio, compression ratio, back pressure, screw speed, heating temperature of the barrel, etc.

1. Length-diameter ratio: the ratio between effective working length and diameter of a screw

a. Uniform material charging benefits from high length-diameter ratio;

b. Plastics with good heat stability shall adopt relatively long screws to improve the mixing property and avoid scorching; while plastics with poor heat stability shall adopt relatively short screws or those screws without thread on the end. Considering the characters of plastics, generally the length-diameter ratio is: 15-18 for thermoset plastics, 19-23 for ordinary plastics, and 22-24 for PC, POM and other plastics with high temperature stability.

2. Compression ratio: the ratio between the depth of the last channel in the charging section of the screw, and the depth of the first channel in the measuring section of the screw.

a. Considering the compressibility, filling degree, backflow and other properties of the plastics, the products shall be dense, heat transferable and well-ventilated;

b. Proper compression ratio can help increase the density of the plastics, making the combination between molecules closer, and contributing to reduce the absorption of air and temperature rise caused by the pressure; it may also make the outputs change from time to time; the improper ratio will damage the physical property of the plastics;

c. The higher the compression ratio, the higher the temperature rise generated by the plastics during plasticization in the charging tubes; which helps produce in-process plastics with good mixing uniformity, and reduce the injection volume correspondingly.

d. High compression ratio applies to plastics hard to melt, especially those with low melt viscosity and heat stability; low compression ratio applies to plastics easy to melt, especially those with high melt viscosity and heat sensitivity.

3. Back pressure

a. A rise in the back pressure can help increase the work done by the screw on the melt resin, eliminate the unmelted plastic particles, and increase the density and uniformity of raw materials in the tube;

b. Back pressure produces remarkable effect when it is used to increase the barrel temperature;

c. Under excess back pressure, plastics with high heat sensitivity may decompose, those with low viscosity may leak out; if the back pressure is too low, the products made by the injection molding machine may contain bubbles.

4. Screw speed

a. The rotate speed of the screw has direct impact on the shearing of plastics in the screw channel;

b. Small screw channel is shallow enough for ease of rapid heat absorption, allowing the plastics to be softened when staying in the compression section; the friction heat energy generated between the screw and barrel wall is low enough to allow the rapid rotation of the screw, thus increasing its plasticization capacity;

c. Large screw shall not rotate rapidly, to avoid non-uniform plasticization and excess friction heat;

d. Plastics with relatively high heat sensitivity are prone to decompose under excess screw speed;

e. In general, a screw of a certain diameter has a proper range of speed; it may fail to melt the plastics if rotates too slow; or scorch the plastics if rotates too fast.

5. Setting of electric heating temperature

a. The temperature shall be high enough to melt the cold and hardened plastics retained in the barrels and screws, to allow the screw to rotate, also provide enough heat energy needed to melt the plastics;

b. The temperature shall be 5-10℃ lower than the plastic melting temperature (the friction heat energy partly attributes to temperature rise);

c. The leakage, condensed plastics (clogged the nozzle), wire drawing and similar problems can be solved by adjusting the temperature of the injection nozzle;

d. The temperature of the plastic melt in the barrel is generally higher than the temperature set out of the barrel.

III. Principles for selection of plasticizing tube and design of components of plastics stirring head

(I) Main points: conveying section, compression section, measuring section, three-section ratio, compression ratio, depth of the channel in the measuring section of the screw, length-diameter ratio, screw compression times.

(II) Principles for selection

a. For ideal mixing effect, production of plastics such as PA, PE, PP and POM shall adopt tubes with such design factors: large length-diameter ratio, frequent screw compression, large compression ratio, narrow and shallow channel in the measuring section of the screw;

b. To prevent the overheating caused by shearing, production of plastics such as PC, PMMA, rigid PVC mixed with glass fiber or fire-proof materials shall adopt tube with such design factors: small length-diameter ratio, infrequent screw compression, small compression ratio, and deep channel in the measuring section of the screw.

c. For high plasticization rate, tube with small compression ratio, and deep channel in the measuring section of the screw shall be adopted.

(III) Principles for design of components of plastics stirring head

a. The components shall be flexibly opened and closed, to achieve rapid and reliable backstop effect;

b. Proper gap shall be reserved between the non-return valve and charging tube; the flow area shall be reasonably designed;

c. The material flows smoothly without any dead angle, which can prevent the deterioration of plastics caused by excess temperature due to local shearing;

d. Resistant to abrasion and corrosion;

e. Replaceable and applicable to production of various kinds of plastics