Injection Screw
The basic function of either a injection molding or extrusion screw is to simply convey the material to the mold or die in an even mixture of resin and pressure to properly fill the mold cavities in order to generate a proper plastic part.
Benefits of Injection Screw
First, they help to move the plastic through the barrel. As the pellets are fed from the hopper into the barrel the screw is rotated, driving the material forward while more pellets are added.Secondly, the flights provide a continuous mixing action that distributes heat evenly throughout the mass.
The injection screw is responsible for providing the majority of heat to the thermoforming plastic. This is because the diameter of the screw increases as it approaches the tip. Plastic pellets being pulled along by the flights are compressed into a tighter space and sheared, or cut, by the action of the turning flights. This action creates friction that mixes the pellets to a uniform consistency and heats them to the proper temperature.
The final advantage of the screw comes in the form of a check ring that maintains consistent pressure at the tip of the screw where it enters the nozzle.
When enough molten plastic has filled the barrel in front of the tip, the injection screw is plunged forward to inject the resin into the mold.As the mold fills it will try to resist this compression, which forces the check ring back against the thrust ring. This effectively locks off any back pressure so the screw can now fully pack the mold.
1.PC special screw
For high viscosity plastics such as PC. Less shear heating, acid corrosion resistance, medium and small diameter, good effect of molding PC, PP-R, flame retardant ABS, etc., and can also mold general plastic and PMMA ordinary products. The effect of color mixing is poor. For example, adding color powder to the plastic needs to be customized to strengthen the color mixing screw. The screw material can be made of stainless steel or double-alloy nickel plating.
2.PA special screw
For PA low viscosity, difficult to color, fast melting speed, good self-lubrication and other characteristics. The screw has good color mixing effect, stable feed rate, good exhaust effect, and the middle diameter is good for molding PA, PP, LCP and other crystalline low-viscosity plastics, and it can also mold general plastics. It is not suitable for plastics with high viscosity and poor thermal stability such as PC and PMMA flame retardant ABS (high temperature and decomposition in the middle section).
3.PMMA special screw
For PMMA transparent products, the characteristics of good plasticizing effect and low decomposition rate are required. Good plasticization, low shear heat generation, good color mixing, good effect when adding toners such as PMMA, PP-R, PC, ABS, etc. in the middle diameter. If the plastic is added with flame retardant, the screw needs to be chrome plated.
4.PVC special screw
For UPVC high viscosity, easy decomposition, strong corrosion and PVC pipe joints require good plasticization and other characteristics. The screw has good plasticization, less shear heat and acid corrosion resistance. Because there is no rubber ring, it cannot be used for low-viscosity plastics and products with accurate injection speed pressure classification. In addition, due to the need for heat dissipation and cooling, when making UPVC products, the barrel (melting barrel) should adopt forced air cooling measures to be used in conjunction with the screw, and the screw should be chrome-plated.
5.PET special screw
For PET with low viscosity, large specific heat capacity, easy-to-stick materials and the characteristics of PET preforms requiring fast plasticization and uniform plasticization, the screw has good plasticization, high stability, non-stick material, fast melting speed, The finished product rate is high when the preform is blown. Large diameter, can also be molded general plastic.
6.PBT special screw
For the characteristics of PBT that is easy to decompose, sensitive to pressure and needs to add glass fiber, the screw generates stable pressure, and uses double alloy or fully hardened tungsten carbide screw to improve wear resistance.
7.Acidic screw components
In view of the strong corrosive properties of acidic plastics such as CP and CA, the screw, melt barrel and other plasticized parts have been specially designed in terms of structure and surface treatment. The screw assembly has good corrosion resistance.
8.Double alloy screw
For recycled materials with high impurity content, glass fiber reinforced plastics and inorganic mineral fillers (calcium powder, carbon powder, talc powder, etc.).
9.Fully-coated tungsten carbide screw
For highly corrosive materials (such as halogen-free materials), add glass fiber 40-50% reinforced plastic and some plastic products such as magnetic powder and ceramic powder.
Application of Injection Screw
Screw is an important component of injection molding machine. Its function is to transport, compact, melt, stir, and apply pressure to plastics. All of this is achieved through the rotation of the screw inside the barrel. When the screw rotates, plastic will generate friction and mutual movement on the inner wall of the barrel, the bottom surface of the screw groove, the advancing surface of the screw edge, and between plastic and plastic. The forward movement of plastic is the result of this combination of motion, and the heat generated by friction is also absorbed to increase the temperature of plastic and melt it. The structure of the screw will directly affect the degree of these effects.
Ordinary injection molding screw structures can also be designed with components such as release screws, barrier screws, or split screws to improve plasticization quality. The structure of the material barrel is actually a circular tube with a discharge port in the middle.
In the process of plastic plasticization, the driving force for its advancement and mixing comes from the relative rotation of the screw and the barrel. According to the different shapes of plastics in screw grooves, screws are generally divided into three sections: solid conveying section (also known as feeding section), melting section (also known as compression section), and homogenization section (also known as metering section).
In textbooks related to plastic plasticization, the solid conveying section of the screw is regarded as a solid bed where plastic particles do not move with each other. Then, the speed of plastic forward transportation is determined by calculating the ideal state of motion and friction between the solid bed and the barrel wall, the screw edge pushing surface, and the screw groove surface. There is a considerable gap between this and the actual situation, and it cannot be used as a basis to analyze the feeding situation of plastic particles with different shapes. If the plastic particles are not large, they will layer and roll when pulled forward by the inner wall of the material barrel, and gradually be compacted to form a solid plug. When the diameter of the material particles is similar to the thickness of the screw groove, their motion trajectory is basically a linear motion along the radial direction of the screw groove plus a linear motion at an angle. Due to the loose arrangement of plastic in the screw groove when the particles are large, their conveying speed is also slower. When the particles reach a certain size and enter the compression section with a diameter greater than the thickness of the screw groove, the plastic will get stuck between the screw and the barrel. If the forward pulling force is not enough to overcome the force required to flatten the plastic particles, the plastic will get stuck in the screw groove and not push forward.
How to Maintain Injection Screw
The screw is an important part of the injection molding machine. The screw of the injection molding machine works under high temperature, high pressure, high mechanical torque and high friction environment for a long time, and loss is inevitable. Even if the screw of the injection molding machine has undergone various surface treatments, from quenching to nitriding, boronizing, spraying, and double alloy spraying, it is still necessary to pay attention to daily maintenance.
1)When the injection molding machine is started, the injection molding must be fully preheated. After the barrel reaches the preset temperature, keep the temperature for 15 minutes below Φ60mm. After the above 30 minutes, start the motor and start the screw related actions.
2)Material and screw must match. Especially corrosive materials and hard materials, such as PVC, flame-retardant materials, and glass fiber-added materials should use professional screws instead of ordinary screws.
3)If the corrosive materials used for ordinary machines must be cleaned and replaced with stable materials such as HDPE, HIPS, etc. immediately after use.
4)If glass fiber material is occasionally used in ordinary machines, be sure to use high material temperature, low screw speed, and low back pressure.
5)Regardless of the material, try not to produce at the lower temperature limit. For example, PC material can be produced at 250 degrees, and ABS can barely be made at 185 degrees, but it is not good for the screw because it requires large torque and severe wear. At the same time, if you must use the lower limit problem, use a low screw speed.
6)If the machine stops for more than half an hour each time, close the blanking port, clean the material in the barrel, and set up heat preservation.
7)Avoid foreign matter falling into the barrel and damage the screw and barrel. Prevent metal fragments and debris from falling into the hopper. If processing recycled materials, a magnetic hopper must be added to prevent iron filings from entering the hopper.
8)When using anti-salivation, make sure that the plastic in the barrel is completely melted to avoid damage to the transmission system parts when the screw is retracted.
9)Avoid the phenomenon of screw idling and slipping.
10)When using new plastic, the remaining material in the barrel should be cleaned. When using POM, PVC, PA+GF and other materials, minimize the degradation of the raw materials, and rinse them with ABS and other nozzle materials in time after shutdown.
11)Avoid mixing POM and PVC into the barrel at the same time, which will react at the melting temperature and cause serious industrial accidents.
12)When the temperature of the molten plastic is normal but black spots or discoloration of the molten plastic are constantly found, check whether the rubber screw non-return ring seat (apron, meson) is damaged.
Considerations When Designing an Injection Screw
Screw size
The size and diameter of the screws should match the size and wall thickness of the injection parts. Choosing a smaller screw will result in poor strength, while choosing a larger screw may result in unnecessary cost increases.
Screw size is usually determined by thread diameter, pitch, and length. Thread diameter is a measurement of the outside diameter of the screw, pitch is the distance between two adjacent threads, and length is the total length of the screw.
Thread design
The thread design shall conform to the standard of the selected screws. If possible, avoid installing threads near thin walls or holes of injection molded parts, as this may cause cracking or deformation of injection molded parts.
Thread design refers to the shape and size of the thread, usually including the outside diameter of the thread, pitch, thread shape and thread length, etc. The thread design shall conform to the standard of the selected screw and take into account the specific requirements of the injection molded part.
Thread direction
When installing screws, the direction of the thread should be consistent with the direction of the injection part. This helps to ensure that the screws do not come loose or rotate during use.
Thread direction refers to the direction of rotation of the thread, usually divided into clockwise direction and counterclockwise direction. In injection parts, usually use right-handed thread, that is, clockwise rotation of the thread.
The advantage of right-handed thread is that when screws are fixed to injection parts, they are less likely to loosen or rotate by themselves due to vibration or force because of matching clockwise thread rotation. In addition, right-handed screw threads are widely adopted in international standards, making it easy to obtain matching screws.
Screw hole design
The screw holes on the injection parts should be designed with a suitable depth of hole bottom so that the screws can be tightly fixed to the injection parts. In addition, the screw holes should be large enough so that the screws can easily enter when installed.
Screw hole design refers to the process of designing and manufacturing holes for mounting screws in injection parts.
Screw position
As far as possible, install the screws on the thicker wall of the injection parts to ensure that the screws can be firmly fixed on the injection parts. Also, when selecting the screw location, make sure that the screws do not interfere with the rest of the injection molding.
The position of screws is a very important consideration in the design of injection parts. The correct positioning of the screws ensures that the injection molding will be securely fixed during use and will meet its specific functional requirements.
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