18 basic knowledge of injection mold must know

As an injection mold craftsman, do you know all these 18 knowledge?


Melt glue usually flows from the nozzle into the nozzle, but in some molds, the nozzle is part of the mold because it extends to the bottom of the 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 the injection molding machine is equipped with a pressure-removing device, this nozzle can be used even for melts with lower viscosity.Sometimes a closed nozzle must be used. This nozzle acts as a stop valve to block the plastic in the injection cylinder.Make sure that the nozzle is correctly connected to the nozzle sleeve, and the top hole is slightly smaller than the nozzle sleeve, which makes the nozzle easier to withdraw from the mold.The hole of the nozzle sleeve is 1mm larger than the nozzle, that is, the radius of the nozzle is 0.5mm smaller than the radius of the nozzle sleeve.

02Filter and combined nozzle

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


Some plastics need to be vented in the injection cylinder during injection to allow the gas to escape.In most cases, these gases are just air, but it may be water or monomolecular gases released by melting.If these gases cannot be released, the gas will be compressed by the melt and carried into the mold, and it will expand and form bubbles in the product.To get rid of the gas before it reaches the nozzle or mold, reduce or reduce the diameter of the screw root to depressurize the melt in the injection cylinder.

Here, the gas can be discharged from the holes or holes in the shooting cylinder.Then the diameter of the screw root is increased, and the melt glue of the devolatile matter is adapted to the nozzle.An injection molding machine equipped with this facility is called an exhaust injection molding machine.Above the exhaust injection molding machine, there should be a good fume exhauster with a catalytic burner to remove potentially harmful gases.

04Increase the effect of back pressure

In order to obtain a high-quality melt, the plastic must be heated or melted consistently and mixed thoroughly.Use the correct screw to melt and mix properly, and there is enough pressure (or back pressure) in the injection cylinder to achieve the consistency of mixing and heat.

Increasing the resistance of the oil return can produce back pressure in the shooting cylinder.But the screw takes longer to reset, so there is more wear and consumption in the drive system of the injection molding machine.Keep back pressure as much as possible, isolate it from air, and also need the same melting temperature and mixing degree

05Check valve

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

06Screw back (reverse cable)

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

07Screw gasket

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

08Rotation speed of screw

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


09Injection volume

The evaluation of the syringe molding machine is usually based on the amount of PS that can be injected in each injection, and may be measured in ounces or grams.Another ranking system is based on the volume of melt that the injection molding machine can inject.

10Plasticizing ability

The evaluation of the injection molding machine is usually based on the amount of PS that can be uniformly melted or heated to a uniform melt temperature within 1 hour (in pounds and kilograms), which is called the plasticizing ability

11Plasticizing capacity estimation

To determine whether the quality of production can be maintained throughout the production process, a simple formula about output and plasticizing capacity can be combined, as shown below: t=(total injection shot weight gX3600)÷(plasticizing volume of injection molding machine kg/hX1000) t is the minimum cycle time. If the cycle time of the mold is lower than the value of t, the injection molding machine cannot fully plasticize the plastic to achieve a uniform melt viscosity, so the injection molded parts often have deviations.Especially when paying attention to the quality of injection molding with thin walls or close tolerances, the injection volume and the plasticization volume must match each other.

12Shooting cylinder residence time

The rate of decomposition of plastic is dependent on temperature and time.For example, plastic 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, the residence time of the plastic in the injection cylinder is very important.

The actual residence time can be determined through experiments. The method is to measure the time required for the colored plastic to pass through the injection cylinder, which can be roughly calculated by the following formula: t=(rated material volume of the injection cylinder gX cycle time S)÷(material injection Amount gX300) Please note that some plastics stay in the shooting tank longer than the calculated time, because they can agglomerate in the shooting tank.

13Calculate residence time and importance

In general, the residence time of a certain plastic on a specific injection molding machine should be calculated.Especially when the large injection molding machine uses a small amount of injection, the plastic is easy to decompose, and this is not detectable from observation.If the residence time is short, the plastic will not be plasticized uniformly; if the residence time is long, the properties of the plastic will be attenuated.

Therefore, the stay time must be consistent.Method: Ensure that the plastic input to the injection molding machine has a stable composition, consistent size and shape.Any abnormality or loss of the parts of the injection molding machine must be reported to the maintenance department.

14Shooting cylinder temperature environment

It should be noted that the 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 the temperature in the first zone is set to the lowest value to prevent premature melting and adhesion of the plastic in the feed port.

The temperature in other areas then gradually rises until it reaches the nozzle. In order to prevent dripping, the temperature at the tip of the nozzle is often slightly lower.The mold is also heated and cooled. Due to the size relationship of many molds, the molds are also distinguished, but unless otherwise specified, each area should be set to the same size.

15Melting temperature

The nozzle can be measured or measured by the air jet method.When using the latter for measurement, care must be taken to ensure that no accidents occur when cleaning the hot-melt plastic, because the high temperature of the hot-melt plastic will burn the skin and even corrode the 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 the danger of splashing hot-melt plastics.To ensure safety, the tip of the heat control needle should be pre-heated to the temperature to be measured.Each plastic has a specific melt temperature. To reach this temperature, the actual injection cylinder adjustment value for injection molding depends on the rotation speed, back pressure, injection volume, and injection cycle of Luocun.

16Mold temperature

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

17Uniform cooling

The finished injection molded parts must be cooled uniformly, that is, different parts of the mold must be cooled at different rates, so that the entire product will be evenly cooled.The injection molded parts must be cooled at the fastest speed while ensuring that no defects such as uneven surfaces and changes in physical properties will occur.The cooling rate of each part of the injection molded part must be equal, but it means that the mold is cooled in an uneven way, for example, cold water is introduced into the inner core of the mold, and warmer water is used to cool the outside of the mold.This technique should be used especially for flat products with precise tolerances of syringe plastics or large products with long melt flow at the spout.

18Temperature and cooling check

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

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