Theoretical knowledge of injection molding machine screw

Theoretical knowledge of injection molding machine screw:

The screw is a key component of the plasticized part, and it is in direct contact with the plastic. The effective length of the plastic passing through the screw groove passes through a long thermal history and undergoes a three-state (glass state, viscoelastic state, and viscous fluid state). The screw The length, geometry, and geometric parameters of each functional segment will directly affect the delivery efficiency and plasticization quality of the plastic, and will ultimately affect the injection molding cycle and product quality.

According to its adaptability to plastic, injection screw can be divided into general screw and special screw. General-purpose screw, also known as conventional screw, can process most thermoplastic plastics with low and medium viscosity, crystalline and non-crystalline civilian plastics and engineering plastics. It is the most basic form of screw.

The screw structure of the general-purpose injection molding machine generally adopts the standard middle diameter screw with strong versatility. When special conditions require it, a conventional rod suitable for high pressure or low pressure needs to be equipped.

This kind of screw configuration in general injection molding machine can basically meet the injection molding work of commonly used resins. According to the different properties of injection products, a reasonable screw structure can be selected. The structural conditions of several screws for injection molding products are presented below, and reference selection is made when supplying them.

1. Screws for rigid PVC injection products

Due to the poor thermal stability of rigid polyvinyl chloride resin and the high viscosity of the melt, the screw for injection molding rigid polyvinyl chloride products should be selected with a smaller compression ratio (usually 1.4~2), a smaller length-to-diameter ratio, and a smaller screw. The homogenization section (metering section) and the feeding section (feeding section) are shorter, and the thread groove of the homogenization section is deeper. The head of the screw should be tapered, and the injection nozzle should be a straight-through type.

2. Screws for non-crystalline plastic injection products such as ABS

Screw structure for injection molding of amorphous plastics such as ABS can be used without a homogenization section (metering section) but only a feeding section and a transition section. This can reduce the overheating of the melt and improve the plasticizing ability and mixing effect of the screw. For the injection of amorphous high-viscosity melts (ਜਿਵੇ ਕੀ: ਪੀ.ਐਸ, PPO, ਪੀ.ਸੀ, ਪੀ.ਐੱਮ.ਐੱਮ.ਏ, AS and POM materials), straight-through nozzles should be used.

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3. Screws for crystalline plastic injection products such as polyethylene and polypropylene

The screw for injection molding of crystalline plastics such as polyethylene and polypropylene should generally be selected with a screw with a larger compression ratio (3~3.5), and a non-return ring device should also be used. This is to ensure the working efficiency of the injection molding machine and prevent There is a large reflux when the melt is injected. Due to the low melt viscosity of this resin, a salivation prevention nozzle should be used.

injection molding machine screw use method

1. Do not start the machine when the barrel has not reached the preset temperature. New electric heating generally requires that the temperature reach the set value for 30 minutes before operating the injection molding machine screw

2. If the machine is stopped for more than half an hour each time, it is best to close the blanking port and clean the material in the barrel, and set up heat preservation

3. 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 should be added to prevent iron filings from entering the hopper.

4. During using anti-salvation, make sure that the plastic in the barrel is completely melted, so as not to damage the transmission system parts when the screw retreats.

5. Avoid the phenomenon of screw idling and slipping.

6. While 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.

7. Avoid mixing POM and PVC into the barrel at the same time, which will react at the melting temperature and cause serious industrial accidents.

8. When the temperature of the molten plastic is normal but black spots or discoloration of the molten plastic are constantly found, check whether the injection molding machine screw non-return ring (apron, meson) is damaged.

The slippage of the injection molding machine screw of the injection molding machine will cause the degradation of the material, which will affect the quality of the product. What are the reasons for the slipping of the screw? How to “catch” the slipping screw? Let me explain to you below.

As the screw slips, the material may gather at the feeding port and cannot be conveyed to the end of the injection machine normally. When the screw rotates and retreats in the barrel to convey the material and prepare for the next injection, the screw slippage will occur in the plasticizing section. ਇਸ ਸਮੇਂ ਤੇ, the rotation of the screw is still continuing, but the axial movement of the screw will stop, ਜੋ ਕਿ ਹੈ, slipping occurs. The slippage of the screw often leads to degradation of the material before injection, the quality of the product will be reduced (such as lack of material), and the molding cycle will be prolonged.

There are many reasons for screw slippage, which may be related to high back pressure, overheating or overcooling of the barrel end, wear of the barrel or screw, too shallow threads in the feeding section, unreasonable hopper design, and blocked hopper, wet resin, and excessive resin Factors such as lubrication, material too fine or unreasonable cutting of resin and recycled materials are related.

Process settings

Too cold at the end of the barrel is one of the main causes of screw slippage. The barrel of the injection machine is divided into 3 sections. At the end, the feeding section, the pellets will form a layer of melt film and stick to the screw during the heating and compression process. Without this film, the pellets are not easily transported to the front end.The material in the feeding section must be heated to a critical temperature to form that critical melt film.

ਹਾਲਾਂਕਿ, usually the residence time of the material in the feeding section is very short and cannot reach the required temperature. This situation generally occurs on small injection machines. If the residence time is too short, the melting and mixing process of the polymer will be incomplete, which will cause the screw to slip or stall.

How to choose a injection molding machine screw

injection molding machine screw can be said to be the heart of the injection molding machine. The quality of the screw determines the quality of the product. The plasticizing screw of the injection molding machine has the functions of conveying, melting, mixing, compression, metering and exhausting. It plays an important role in the quality of plasticization and is a key factor affecting the quality of plasticization.

The screw has several crucial parameters that affect the quality of plasticization. ਆਮ ਤੌਰ 'ਤੇ, the screw of the injection molding machine is selected according to the following principles:

1. ਪੇਚ ਵਿਆਸ (ਟੈਂਕ ਸਮੇਂ ਸਿਰ ਪਿਘਲਣ ਵਿੱਚ ਹਵਾ ਅਤੇ ਗੈਸਾਂ ਨੂੰ ਬਾਹਰ ਕੱਢਣ ਲਈ ਕਾਫੀ ਹੋਣਾ ਚਾਹੀਦਾ ਹੈ)
2. Related to the required injection volume: injection volume=1/4*π*D2*S (injection stroke)*0.85;
3. ਜੇ ਆਮ ਗੱਲ ਕਰੀਏ, the screw diameter D is inversely proportional to the maximum injection pressure and directly proportional to the plasticizing capacity.
4. Conveying section
5. Responsible for the transportation, pushing and preheating of the plastic, and ensure that the preheating reaches the melting point;
6. The crystalline plastic should be long, the amorphous material is the second, and the heat sensitivity is the shortest.
7. Compression section
8. Responsible for the mixing, compression and pressurization of plastics. The raw materials passing through this section have almost all melted, but they may not be uniformly mixed;
9. In this area, the plastic gradually melts, and the volume of the screw groove must be correspondingly decreased to correspond to the decrease of the geometric volume of the plastic, otherwise the material is not compacted, the heat transfer is slow, and the exhaust is poor;
10. ਆਮ ਤੌਰ 'ਤੇ, it accounts for more than 25% of the working length of the screw, but the compression section of the nylon (crystalline material) screw accounts for about 15% of the working length of the screw, and the plastic screw with high viscosity, fire resistance, low conductivity, and high additives accounts for 40% \50% screw working length, PVC screw can account for 100% screw working length to avoid intense shearing heat.
11. Measuring section
12. Generally occupies 20\25% of the working length of the screw to ensure that all the plastics are melted and the temperature is uniform, and the mixing is uniform;
13. If the metering section is long, the mixing effect will be good. If it is too long, the melt will stay too long and cause thermal decomposition. If it is too short, the temperature will be uneven;
14. Heat-sensitive plastics such as PVC should not stay for too long to avoid thermal decomposition. A shorter metering section or no metering section can be used.
15. Feeding screw groove depth, measuring screw groove depth
16. The deeper the feed groove depth, the greater the conveying capacity, but the strength of the screw needs to be considered. The shallower the metering groove depth, the higher the plasticizing heat and the higher the mixing performance index, but the shallow metering groove depth means shearing Heat increases, self-generated heat increases, and the temperature rise is too high, causing plastic discoloration or scorching, especially not conducive to heat-sensitive plastics;
17. Measuring groove depth=KD=(0.03\0.07)*ਟੈਂਕ ਸਮੇਂ ਸਿਰ ਪਿਘਲਣ ਵਿੱਚ ਹਵਾ ਅਤੇ ਗੈਸਾਂ ਨੂੰ ਬਾਹਰ ਕੱਢਣ ਲਈ ਕਾਫੀ ਹੋਣਾ ਚਾਹੀਦਾ ਹੈ, if D increases, select a small value for K.

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