YIXIN SCREW

The screw is an important part in the plasticizing components of an injection molding machine. From the perspective of the plasticizing process of plastic entering the melt cylinder, it goes through solid feeding and conveying, compaction and melting, venting, further plasticizing and homogenization, and metering processes. Therefore, the screw is usually designed into three sections: the feeding section, the compression section, and the homogenization section.

Introduction to screw dimensions

The screw diameter (D) is related to the required injection volume. The injection volume = 1/4π× d ² × (injection stroke) ×0.85

Generally speaking, D2 is inversely proportional to the maximum injection pressure. The larger D is, the greater the extrusion rate. Q = 1.29D2HmNr×60/1000(kg/Hr)

Feeding section

Be responsible for the transportation, pushing and preheating of plastics

It should be ensured that the material begins to melt at the end of the feeding section and is preheated to the melting point.

Solid specific heat ↑, melting point ↑, latent heat ↑. Heating to the melting point requires more heat. The feeding section should be long. Solid thermal conductivity coefficient ↓, heat transfer is slow, and the temperature rise at the center of the plastic is slow. The feeding section should be long for preheating ↑, and the feeding section can be short.

Crystalline materials are the longest (such as: POM, PA); Amorphous materials come second (such as PS, PU); The shortest heat sensitivity (e.g., PVC).

Compression section

It is responsible for the mixing, compression and pressurized exhaust of plastics. The raw materials passing through this section should have almost all melted, but they may not be uniformly mixed.

In this area, the plastic gradually melts, and the volume of the screw channel must decrease accordingly; otherwise, the material will not be compacted, heat transfer will be slow, and exhaust will be poor.

For amorphous plastics, the compression section should be longer; otherwise, if the volume of the screw channel decreases rapidly and the material volume does not decrease, blockage will occur.

Crystalline plastics are actually not all crystalline (for example, PE: 40-90% crystallinity, LDPE: 65% crystallinity), so there is currently a trend of increasing elongation in the compression section.

It generally accounts for 25% of the working length of the screw.

Nylon (crystalline material)2 to 3 turns, accounting for approximately 15% of the working length of the screw. High viscosity, fire resistance, low conductivity and high additives, accounting for 40% to 50% of the working length of the screw.

PVC can utilize 100% of the working length of the screw to avoid intense shearing heat.

Metering section

Theoretically, by the starting point of the metering section, the material should be completely melted, but at least 4 days of the metering section should be maintained to ensure uniform temperature and thorough mixing.

The longer the measurement section is, the better the mixing effect will be. If the metering section is too long, it is easy for the melt to stay for too long, resulting in thermal decomposition. If it is too short, it is easy to cause uneven temperature.

It generally accounts for 20 to 25% of the working length of the screw.

PVC is heat-sensitive and should not be kept for too long to avoid thermal decomposition (do not measure the section).

The main factors affecting plasticizing quality

Slender ratio, compression ratio, back pressure, screw speed, and electric heating temperature setting.

Slender ratio

The slender ratio = the working length of the screw/the diameter of the screw.

If the slender ratio is large, it is easier to eat the ingredients evenly, but it is prone to overcooking.

Plastics with better thermal stability can use longer screws to improve the mixing property without worrying about charring. Plastics with poor thermal stability can be treated with shorter screws or screws without threads at the tail end.

Considering the properties of plastics, they are generally slender as follows:

Plastic properties

Slender ratio

Thermosetting

14 ~ 16

Hard PVC, high-viscosity PU and other heat-sensitive materials

17-18

General plastic

18 ~ 22

High-temperature stability plastics such as PC and POM

22-24

Considering the color-mixing ability, the general slender ones are as follows:

Slender ratio

Color-mixing ability

12-16

It is advisable to form the colored rubber particles to avoid color differences.

16 ~ 18

The color masterbatch is mixed, dyed and formed in the material tube, ensuring uniform quality and with relatively small color difference defects.

20-24

The colorant is mixed and dyed in the material tube, and the dispersion is uniform, which has a better protective effect on the physical properties of the finished product.

Compression ratio

Compression ratio = feed tooth depth/metering tooth depth

Consider the compressibility of the material, the degree of filling, reflux, the compactness of the product, heat transfer and venting.

An appropriate compression ratio can increase the density of plastic, making the molecules bond more tightly with each other, which helps to reduce the intake of air and lower the temperature rise caused by pressure, thus affecting the difference in output. An inappropriate compression ratio, on the other hand, will damage the physical properties of plastic.

The higher the compression ratio is, the greater the temperature rise generated during the plasticizing process of the plastic in the material tube will be, and the better the uniformity of the plastic during gelation will be achieved. Correspondingly, the output will be greatly reduced.

High compression ratio is suitable for plastics that are not easy to melt, especially those with low melting viscosity and thermal stability.

Low compression ratio is suitable for fusible plastics, especially those with high melting viscosity and heat sensitivity.

Back pressure

Increasing the back pressure can increase the work done by the screw on the molten resin, eliminate unmelted plastic particles, increase the density and uniformity of the raw material in the barrel, and reduce problems such as injection pressure reduction and warping.

Back pressure is used to increase the temperature of the material pipe, and its effect is the most remarkable.

Excessive back pressure is prone to decomposition of heat-sensitive plastics. Low-viscosity plastics may cause a "runny nose" phenomenon.

If the back pressure is too low, the ejected finished product may have bubbles.

Screw rotational speed

The rotational speed of the screw directly affects the shear of the plastic in the helical groove.

The groove depth of the small screw is relatively shallow, and it absorbs heat sources quickly, which is sufficient to soften the plastic during the compression section. The frictional heat energy between the screw and the material pipe wall is relatively low, making it suitable for high-speed rotation and enhancing the plasticizing capacity.

Large screws should not rotate rapidly to avoid uneven plasticizing and excessive frictional heat.

For heat-sensitive plastics, if the rotational speed of the injection screw is too high, the plastic will be decomposed very easily.

Usually, screws of various sizes have a certain speed range. Generally, the speed is 100 to 150 rpm. If it is too low, the plastic cannot be melted. If it is too high, the plastic will be scorched.

At present, the maximum surface velocity is limited to 1m/sec. For shear-sensitive materials, it is less than 0.5m/ second.

Electric heating temperature setting

Melt the cold-hardened resin retained in the material pipe and the screw to facilitate the rotation of the screw and provide part of the heat required for the resin to be melted.

Set 5 to 10 degrees Celsius lower than the melting temperature (partly provided by frictional heat energy)

The adjustment of nozzle temperature can also be used to control problems such as runny nose, solidification (plug head), and wire drawing.

Crystalline plastics are generally controlled by temperature

The design concept of the injection screw of the screw to the plastic machine

Guided by theory and based on practical experience, we should combine theory with practice and have the courage to innovate. The standards for screw design are: whether the plasticizing is uniform, whether the measurement meets the requirements, and secondly, the requirements for plasticizing capacity. If the screw can be plasticized to produce quality products that meet the quality requirements and the molding standards, then the design of the screw is successful. When designing an injection screw for an injection molding machine, it is essential to first understand the performance of the object to be processed and the quality of the product to be achieved, as well as which parameters of the injection screw of the plastic injection molding machine are related to the performance of the material to be processed, such as the material's sensitivity to shearing and temperature. According to the different characteristics of engineering plastics, special screws and barrels should be designed and manufactured.