In general, screws can be classified into ordinary type, gradual transition type, sudden transition type, mixing type, separation type, distribution type, barrier type, pin type, DIS type, vent type, etc. the screws can be single-head or variant-head.

Plastics with relatively special properties, special plastic products or particles shall apply special screws.

For a certain kind of plastic, an all-purpose screw is less efficient in plasticization and consumption than a special screw. The screws made for special products and demands are introduced below:

Special screw for PC

PC is a high-viscosity non-crystalline plastic with poor fluidity. It is easily degraded when it has been heated for a long time, and requires large torque when it is fed into the hopper. Since PC is uneasy to melt but easy to clog the feeder, it shall be injected with high temperature and high pressure, thus its temperature adjustment range is limited to some degree. In conclusion, PC is inferior to PMMA in processing property.

The screw is processed from the imported fully-hardened high alloy steel, effectively resistant to large torque, high pressure impact, abrasion, surface electroplating, and corrosion. Its smooth and gap-free surface helps reduce surface adhesion and degradation. It consists of well-processed details, effectively preventing the deposit of plastics; and adopts low shearing design for low heat release rate. The screw is generally of small or medium diameter, helping produce PCABS+PC, PP-R and flame-retardant ABS with perfect shapes. It can also be used to produce ordinary plastics and PMMA products, but providing poor color mixing effect. For those plastics produced with toners, reinforced screws for color mixing shall be customized.

To facilitate the molding of ultra-thin and precision plastic products in a way required by the industry, we also provide services such as reducing the screw diameter, shortening the screw length, increasing the injection pressure to over 3000Kgf/cm, and moving the injection base forward (the base can be restored to its original place after the movement)

Special screw for PMMA

PMMA is a high-viscosity non-crystalline plastic with low pliability, requiring large torque, and is uneasy to melt but easy to clog the feeder. It is also sensitive to the shearing and degradable, with relatively poor fluidity. All the above attribute to its injection with high temperature and high pressure. However, it has wide temperature adjustment range and relatively good processing property. The screw is compressed to a great degree, and processed from the imported fully-hardened high alloy steel, effectively resistant to large torque and high pressure impact. The surface electroplated with hard chromium helps reduce surface adhesion and degradation. PMMA provides uniform plasticizing effect, good color mixing effect, low heat release rate by adopting low shearing design, and low degradation rate. The screw is generally of medium diameter, used to produce PMMA, PP-R, PC and flame-retardant ABS with perfect shapes once toner is added.

Screw components for acid CP, CA

For strong corrosive acid plastics such as CP and CA, special design has been adopted to the structure and surface treatment of the screws, barrels filled with plastic melt and other parts used for plasticization. The screw components have good corrosion resistance. For better corrosion resistance, we recommend using the double alloy charging barrel.

Special screw for PVC

There are granular and powder PVC plastics. PVC is sensitive to the temperature and easy to decompose, with high viscosity and strong corrosiveness. The design of screws for such use have two features: the surface shall be chrome-plated; free of separation ring (or non-return valve) and washers. The screw is resistant to sour corrosion, ensuring the products to be well plasticized and have low heat release rate during shearing. Due to lack of the non-return valves, such screws cannot be used to process low-viscosity plastic melts and those with accurate gradations of injection speed and pressure.

Such screws have two designs: the screw and stirring head are integrated; or the two are separated. The products with high quality requirements and made from one kind of plastic (PVC) shall apply the integrated type; otherwise the separated type. Note: the structure with the screw and stirring head separated is prone to the deposit of materials. No matter which type of screws do you choose, we suggest that a special thermometer be installed for ease of precision temperature control. Given the heat dissipation for temperature reduction, when producing PVC products (excluding toys made from soft PVC), forced air cooling measures shall be adopted to the barrels filled with plastic melt and screws. 

Special screw for PU, TPU 

Such screws are specially designed according to the features of PU including sensitivity to shearing, easy to be over-burnt and strong adhesive force. The screw is electroplated with hard chromium surface to ensure that the raw materials are fed smoothly, helping improve the plasticizing efficiency and slow down the temperature rise.

Special screw for flexible plastic melt and EVA foamed material

The screw adopts low shearing and high conveying efficiency, electroplated with hard chromium surface.

Special screw for PET 

PET plastic is currently used as the raw material of ordinary plastic bottles filled with drinks, with low viscosity, fine fluidity, large specific heat capacity, but is easy to adhere to materials, and requires high molding temperature, limited temperature adjustment range, reflecting poor processing property. When PET is being injected and molded, as a result of extremely high material temperature, huge shearing force or too fast rotate speed of the screw, it may produce acetaldehyde and acidize the products.

To meet the requirements to produce PET and PET bottle bases including quick and uniform plasticization, the screw is applied to achieve good plasticization effect, high stability, fast plastic melting, prevent the material from adhering to the screw, and increase the success rate of blowing and molding of the bottle bases. It can also be used to mold ordinary plastics. However, considering that manufacturers in the mainland usually inject materials into the water inlet when producing the PET bottle bases, under such case, the ordinary screw plated chromium is more applicable than the special screw for PET. A leakage preventing device shall be installed in the nozzle if necessary.

Special screw for PA6/66/46/6T

PA is a kind of low-viscosity crystalline plastic, melting fast under high temperature, with good self-lubrication, fluidity, but is easy to leak due to its water-based nature. Its end is prone to be cooled due to its sensitivity to temperature. PA also tends to clog the channel of screws in the material charging section due to high moisture absorption, and is hard to be colored.

The screw has large compression ratio and precision backstop design, ensuring good color mixing effect, stable feed volume, high plasticization efficiency and good venting effect. Such screw is generally of medium diameter, used to produce crystalline low-viscosity plastics such as PA, PP and LCP with good effect and shape. It can also be used to help mold ordinary plastics, but inapplicable to PC, PMMA, flame-retardant ABS and other plastics with high viscosity and poor heat stability (lead to the decomposition of middle segments of those materials as a result of high temperature). The screw shall be electroplated with hard chromium before being used to mold POM. The nozzle shall be heated under constant temperature, and be installed with a leakage preventing device if necessary.

Special screw for PPO

PPO is a non-crystalline material, hard to melt but easy to clog the feeder, sensitive to the shearing, and easily degraded.

We suggest that the screw be made from double alloy to improve its resistance to abrasion and corrosion, Grade C is enough. The screw will ease the charging of materials, and generate stable pressure.

Special screw for PBT+GF

PBT is a kind of saturate resin, semi-crystalline material, with high meltability, rapid crystallization rate, easy to decompose, sensitive to pressure and shall be blended with some glass fiber.

The screw shall be made from double alloy to improve its resistance to abrasion; according to the additional amount of braided alloy, Engineering Grade A, B, C are optional. The screw will keep the pressure stable, and can be used to produce crystalline low-viscosity ordinary engineering plastics, such as PBT+GF and PA+GF, with good effect.

Special screw/marbling screw

Two plans: A: only change those screws used to make marbling effect; B: change a whole set of marbling screws and charging tubes, modify the program in the control computer, or install the electric plug-ins.

Special screws and charging tubes for the bakelite and urea

The charging tubes are heated by hot oil in a loop. We suggest that the screws be made from Grade C double alloy; the nozzle of old injection molding machine can be transformed into bakelite mixing head, to enhance the color mixing effect; the mixing head is designed to be another stirring head, mainly used to enhance the color mixing effect, but has following limits: it may cause the back pressure to increase when plastic melts; increase the melting time; inapplicable to high viscosity plastics such as PMMA and PC. If the customers are unsatisfied with the color mixing effect, proper measures can be adopted to increase the back pressure for plastic melting, and reduce the rotate speed of the screws, which may help improve the effect. If the customers disagree to use a mixing head to enhance the color mixing effect, tooth-lock washer for plastic melt injection can be an option.