Injection molding technicians must learn! Eighteen tricks to subdue injection molding machine

Time:2021-04-03 11:25:40 / Popularity: / Source:

01 Nozzle

Melt glue usually flows from nozzle into nozzle, but in some molds, nozzle is part of mold because it extends to bottom of mold. There are also two main types of nozzles: open nozzles and closed nozzles. In injection molding, open nozzles should be used more because they are cheaper and less likely to stay. If injection molding machine is equipped with a pressure-removing device, this nozzle can be used even for low-viscosity melts.
Sometimes a closed nozzle must be used. This nozzle acts as a stop valve to block plastic in injection cylinder. Make sure that nozzle is correctly connected to nozzle sleeve. Top hole is slightly smaller than nozzle sleeve. This makes nozzle easier to withdraw from mold. Hole of nozzle sleeve is 1mm larger than that of nozzle, that is, radius of nozzle is 0.5mm smaller than radius of nozzle sleeve.

02 Filter and combined nozzle

Impurities in plastic can be removed by filter of extensible nozzle, that is, melt and plastic flow through a channel, which is divided into narrow spaces by insert. These narrows and gaps can remove impurities and improve mixing of plastics.
Therefore, to extend it, a fixed mixer can be used to achieve a better mixing effect. These devices can be installed between injection cylinder and nozzle to separate and remix melt. Most of them make melt flow through stainless steel channel.

03 Exhaust

Some plastics need to be exhausted in injection cylinder during injection to allow gas to escape. In most cases, these gases are just air, but it may be molten moisture or single-molecule gases. If these gases cannot be released, gas will be compressed by melt and carried into mold, it will expand and form bubbles in the product. To get rid of gas before it reaches nozzle or mold, reduce diameter of screw root to depressurize melt in injection cylinder.
Here, gas can be discharged from holes or holes in shooting cylinder. Then diameter of screw root is increased, and melted material of devolatile matter is adapted to nozzle. Injection molding machines equipped with this facility are called exhaust injection molding machines. This kind of exhaust injection molding machine should have a good fume exhauster with a catalytic burner to remove potentially harmful gases.

04 Increase effect of back pressure

In order to obtain a high-quality melt, plastic must be heated or melted consistently and mixed thoroughly. Use correct screw to melt and mix properly, and there is enough pressure (or back pressure) in injection cylinder to achieve consistency of mixing and heat. Increasing resistance of oil return can produce back pressure in shooting cylinder. However, screw takes longer to reset, so there is more wear and consumption in drive system of injection molding machine. Keep back pressure as much as possible, isolate it from air, melt temperature and mixing degree must be consistent.

05 Check valve

No matter what kind of screw is used, its tip is usually equipped with a stop valve. To prevent plastic from flowing out of nozzle, a pressure reducing (reverse cable) device or a special shooting nozzle is also installed. If you use anti-abortion supply and marketing, you must check it regularly, because it is an important part of shooting tank. At present, switch-type nozzle is not commonly used because plastic inside nozzle equipment is easy to leak and decompose. At present, each type of plastic has a suitable nozzle type.

06 Screw back (reverse cable)

Many injection molding machines are equipped with screw retreat or suction devices. When screw stops, it is retracted by hydraulic pressure to suck back plastic at the tip of nozzle. This device allows use of an open nozzle. Amount of sucking back may be reduced because ingress of air can cause problems for some plastics.

07 Screw gasket

In most injection molding cycles, it is necessary to adjust amount of rotation of screw, so that after injection of screw, there will be a small amount of soft cushion plastic, which can ensure that screw reaches an effective advance time and maintains a fixed shooting pressure. Cushion material for small injection molding machines is about 3mm; for large injection molding machines, it is 9mm. No matter how large screw gasket value is used, it must remain unchanged. Now size of screw gasket can be controlled within 0.11mm.
injection molding machine 

08 Rotation speed of screw

Rotation speed of screw significantly affects stability of injection molding process and heat acting on plastic. The faster screw rotates, the higher temperature. When screw rotates at a high speed, friction (shear) energy transferred to plastic improves plasticization efficiency, but also increases unevenness of melt temperature.
Due to importance of screw surface speed, screw rotation speed of a large injection molding machine should be less than that of a smaller injection molding machine. Reason is that shear heat generated by large screw is much higher than that of small screw at same rotation speed. Due to different plastics, speed of screw rotation is also different.

09 Injection volume

Evaluation of syringe molding machines is usually based on amount of PS that can be injected in each injection, and may be measured in ounces or grams. Other sorting system is based on volume of melt that injection molding machine can inject.

10 Plasticizing ability

Evaluation of an injection molding machine is usually based on amount of PS that can be uniformly melted or heated to a uniform melting temperature within 1 hour (in pounds and kilograms), which is called plasticizing capacity.

11 Plasticizing capacity estimation

To determine whether quality of production can be maintained throughout production process, a simple formula about output and plasticizing capacity can be used together, as shown below: t=(total injection shot weight g*3600)÷(plasticizing volume of injection molding machine kg/hX1000)
t is minimum cycle time. If cycle time of mold is lower than value of t, injection molding machine cannot fully plasticize plastic to achieve a uniform melt viscosity, so injection molded parts often have deviations. Especially when paying attention to quality of injection molding with thin walls or close tolerances, injection volume and plasticization volume must match each other.

12 Shooting cylinder residence time

Rate of decomposition of plastic is dependent on temperature and time. For example, plastics will decompose after being exposed to a high temperature for a period of time; but in a relatively low temperature environment, it will take a long time to decompose. Therefore, residence time of plastic in injection cylinder is very important.
Actual residence time can be determined through experiments. Method is to measure time required for colored plastic to pass through injection cylinder. It can be roughly calculated by following formula: t=(rated material volume of injection cylinder g* cycle time S)÷(material injection G*300)
Please note that some plastics stay in shooting tank longer than calculated time, because they can agglomerate in shooting tank.

13 Calculation of residence time and importance

In general, residence time of a certain plastic on a specific injection molding machine should be calculated. Especially when a large injection molding machine uses a small amount of injection material, plastic is easily decomposed, which is not detectable from observation. If residence time is short, plastic will not be plasticized uniformly; if residence time is long, properties of plastic will be attenuated. Therefore, stay time must be consistent.
Method: Ensure that plastic input to injection molding machine has a stable composition, consistent size and shape. If there is any abnormality or loss in parts of injection molding machine, report to maintenance department.

14 Shooting cylinder temperature environment

It should be noted that melt temperature is very important, and any injection cylinder temperature used is only indicative. If you have no experience in processing a particular plastic, please start with the lowest setting. Usually temperature in the first zone is set to the lowest value to prevent premature melting and adhesion of plastic in feed inlet.
Temperature in other areas then gradually increases until it reaches nozzle. To prevent dripping, temperature at the tip of nozzle is often slightly lower. Mold is also heated and cooled. Due to size relationship of many molds, molds are also distinguished, but unless otherwise specified, each area should be set to same size.

15 Melting temperature

Nozzle can be measured or measured by air jet method. When using the latter for measurement, care must be taken to ensure that there will be no accidents when cleaning hot melt plastic, because high temperature of hot melt plastic will burn skin and even corrode skin. In the injection molding workshop, burns are accidental.
Therefore, you should wear gloves and face shields when dealing with hot plastics or when you encounter danger of hot melt plastic splashing. To ensure safety, tip of  heat control needle should be pre-heated to temperature to be measured. Each plastic has a specific melt temperature. To reach this temperature, actual injection cylinder adjustment value of injection molding depends on screw rotation speed, back pressure, injection volume, and injection cycle.

16 Mold temperature

Always check whether injection molding machine is set and running at temperature specified on record sheet. This is very important. Because temperature will affect surface finish and output of injection molded parts. All measured values must be recorded and injection molding machine checked at specified time.

17 Uniform cooling

Finished injection molded parts must be cooled uniformly, that is, different parts of mold must be cooled at different rates, so that entire product will be uniformly cooled. Injection molded parts must be cooled as fast as possible, while ensuring that no defects such as uneven surfaces and changes in physical properties will occur.
Cooling rate of each part of injection molded part must be equal, but it means that mold is cooled in an uneven way, for example, cold water is injected into inner core of mold, and warmer water is used to cool outside of mold. This technique should be used for precision flat products or large products with long sprue melt flow.

18 Temperature and cooling check

Always check whether injection molding machine is set and running at temperature specified on record sheet, this is very important. Because temperature will affect surface finish and output of injection molded parts. All measured values must be recorded and injection molding machine checked at specified time.

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