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18 basic knowledge of injection mold

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18 basic knowledge of injection mold

Issue Time:2019-04-19
01 Shot Tsui

The melt 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 type nozzles and closed type nozzles. In injection molding production, 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-relief device, even nozzles with low viscosity can be used. Sometimes it is necessary to use a closed nozzle, which acts as a stop valve and blocks the plastic shade in the shot cylinder. Make sure the nozzle is correctly inserted into the nozzle sleeve. The top hole is slightly smaller than that of the nozzle sleeve. This allows the nozzle to be easily withdrawn from the mold. The hole of the nozzle sleeve is 1mm larger than the shooting nozzle, that is, the radius of the nozzle is 0.5mm thinner than the nozzle nozzle radius.

02 filter and combined nozzle

Plastic impurities can be removed by the filter of the extension nozzle, that is, the melt and plastic flow through a channel, which is divided into narrow spaces by inserts. These narrow and gaps can remove impurities and improve the mixing of plastics. Therefore, it can be extended to use a fixed mixer to achieve a better mixing effect. These devices can be installed between the injection cylinder and the nozzle to perform the work of separating and remixing the melted glue, most of which is to make the melt flow through the stainless steel channel.

03 exhaust

Some plastics need to be vented in the injection cylinder during injection molding to let the gas out. In most cases, these gases are just air, but it may be water or single-molecule gas released by melting. If these gases cannot be released, the gas will be compressed by the melt and brought into the mold, which will expand and form bubbles in the product. To remove the gas before it reaches the nozzle or mold, reduce or reduce the screw root diameter to decompress the melt in the shot cylinder.

Here, the gas can be discharged from the holes or holes in the shot cylinder. Then the diameter of the screw root is increased, and the melt of the devolatiles is directed toward the nozzle. The injection molding machine equipped with this facility is called exhaust type injection molding machine. Above this exhaust type injection molding machine, there should be a good smoke exhauster with a catalytic burner to remove potentially harmful gases.

04 The effect of increasing back pressure

In order to obtain a high-quality melt adhesive, the plastic must be heated or melted uniformly, and must be thoroughly mixed. Use the correct screw to properly melt and mix, and have enough pressure (or back pressure) in the shot cylinder to achieve uniform mixing and thermal power.

Increasing the resistance of the oil return can produce back pressure in the firing cylinder. However, the screw takes longer to reset, so there is more wear and consumption in the injection molding machine drive system. Keep the back pressure as far as possible, isolated from the air, and also need to have the same melt temperature and mixing degree

05 check valve

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

06 Screw back (reverse cable)

Many injection molding machines are equipped with screw retraction or suction devices. When the screw rotation stops, it is hydraulically withdrawn to suck back the plastic at the tip of the nozzle. This device allows the use of open nozzles. The rule of sucking back the amount may be reduced, because entering the air may cause problems for some plastics.

07 Screw padding

In most injection molding cycles, it is necessary to adjust the rotation amount of the screw village, so that when the screw is injected, most of the plastic cushion will be left in a small amount, which can ensure that the screw reaches an effective advancement time and maintains a fixed firing pressure. The padding of the small injection molding machine is about 3mm; the large injection molding machine is 9mm. No matter how much the value of screw padding is used, it must remain unchanged. Now the size of the screw pad can be controlled within 0.11mm.

08 Rotating 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 high speed, the friction (shear) energy transmitted to the plastic improves the plasticization efficiency, but at the same time also increases the unevenness of the melt temperature. Due to the importance of the surface speed of the screw, the screw rotation speed of the large injection molding machine should be less than that of the smaller injection molding machine, because the shear heat energy generated by the large screw is much higher than that of the small screw at the same rotation speed. Due to different plastics, the screw rotation speed is also different.

09 Injection volume

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

10 Plasticizing ability

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

11 Plasticizing capacity estimation

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

12 residence time of shot cylinder

The rate of decomposition of plastic is dependent on temperature and time. For example, plastic will decompose after being at a high temperature for a period of time; but when it is in a lower temperature environment, it will take longer to decompose. Therefore, the residence time of plastic in the shot cylinder is very important.

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

13 Calculate residence time and importance

In general, the residence time of a plastic on a specific injection molding machine should be calculated. Especially when large injection molding machines use less shots, the plastic is easily decomposed, which is not detectable from observation. If the residence time is short, the plastic will not be plasticized evenly; if the residence time becomes longer, the plastic properties will decay.

Therefore, it is necessary to keep the residence time consistent. Method: Ensure that the plastic input to the injection molding machine has a stable composition, consistent size and shape. If there is any abnormality or loss of the injection molding machine parts, they must be reported to the maintenance department.

14 shooting cylinder temperature environment

It should be noted that the melt temperature is very important, and the temperature of any shot cylinder used is only a guide. If you have no experience in processing a particular plastic, start with the lowest setting. Usually the temperature in the first zone is set to the lowest value to prevent the plastic from melting and sticking prematurely in the feed port.

The temperature in other areas gradually increases until it reaches the nozzle. To prevent dripping, the temperature at the tip of the nozzle tends to be slightly lower. The mold is also heated and cooled. Due to the dimensional relationship of many molds, the molds are also distinguished, but unless specified, each zone should be set to the same size

15 Melt temperature

The nozzle can be measured or measured by the air jet method. When using the latter to take measurements, 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 can burn or even corrode the skin. In the injection molding plant, burns are accidental.

Therefore, gloves and face shields should be worn when handling hot plastics or where there is a risk of hot melt plastic splashing. To ensure safety, the tip of the heat control needle should be preheated to the temperature to be measured. Each plastic has a specific melt temperature. To achieve this temperature, the actual injection cylinder adjustment value also depends on the rotation speed of the screw, back pressure, injection volume and injection cycle.

16 Mold temperature

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

17 even cooling

The injection-moulded parts must be cooled uniformly, that is to say, different parts of the mold must be cooled at different rates, so that the whole product will be cooled evenly. The injection molded parts must be cooled as quickly as possible, and at the same time, it must be ensured that no defects, such as uneven surfaces and changes in physical properties, are produced. The cooling rate of each part of the injection molded part must be equal, but it refers to cooling the mold by an uneven method, such as entering cold water into the inner core part of the mold, and the outside of the mold is cooled with warmer water. This technique should be used when the plastic tolerance of the syringe is precise for straight products or large products with a long gate melt flow.
18 Temperature and cooling check

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