Do you know the 18 basic knowledge that injection molding technicians must know?

Date：[2020/3/20]

1 shot

Melt usually flows from the nozzle into the nozzle, but for some molds, the nozzle is part of the mold because it extends to the bottom of the mold. There are two main types of nozzles: open and closed. In injection molding production, more open nozzles should be used because they are cheaper and less likely to stay.

If the injection molding machine is equipped with a pressure relief device, even low viscosity melts can use this nozzle. Sometimes it is necessary to use a closed nozzle, which acts as a check valve and blocks the plastic shade in the injection cylinder. Make sure that the nozzle is correctly inserted into the nozzle sleeve, and the top hole is slightly smaller than the nozzle sleeve, which makes it easier for the nozzle to withdraw 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 that of the nozzle sleeve.

02 filter and combined nozzle

Plastic impurities can be removed by the filter of the extension nozzle, that is, the melt and the plastic flow through a channel, which is separated by the insert into a narrow space. These narrows and gaps remove impurities and improve plastic mixing. So extend it and use a fixed mixer for better mixing results. These devices can be installed between the shooting cylinder and the nozzle to perform the work of separating and remixing the glue, most of which are channels that pass the melt through the stainless steel.

03 exhaust

Some plastics need to be vented in the shot cylinder to let the gas out during injection. In most cases, these gases are only air, but they may be moisture or single molecular gases released by melting. If these gases cannot be released, the gas will be compressed by the glue and brought into the mold, and it will expand and form bubbles in the product. To vent the gas before it reaches the nozzle or mold, reducing or reducing the diameter of the screw root can depressurize the melt in the shot tank.

Here, gas can be expelled from holes or holes in the shot cylinder. Then the diameter of the screw root is increased, and the melted glue is directed to the nozzle. An injection molding machine equipped with this facility is called a vented injection molding machine. Above the exhaust injection molding machine should be a good smoke exhauster for catalytic burners to remove potentially harmful gases.

04 The role of increasing back pressure

In order to obtain high-quality melt glue, the plastic must be uniformly heated or melted and thoroughly mixed. The correct screw is used for proper melting and mixing, and there is sufficient pressure (or back pressure) in the shot cylinder to obtain mixing and thermal consistency.

Increasing the resistance to return oil can generate back pressure in the firing cylinder. However, the screw takes longer to reset, so there is more wear and tear in the drive system of the injection molding machine. Keep the back pressure as far as possible and keep it away from the air. The melt temperature and mixing degree should also be consistent.

05 check valve

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

06Screw backward (back cable)

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

07Screw gasket

In most injection cycles, the rotation of the screw village needs to be adjusted so that when the screw injection is completed, a small amount of cushioned plastic will be left, which can ensure that the screw reaches an effective advance time and maintains a fixed firing pressure. Smaller injection molding machines are about 3mm; large injection molding machines are 9mm. No matter how big the value of screw padding is, it must be kept the same. The size of screw padding can now be controlled within 0.11mm.

08 screw rotation speed

The rotation speed of the screw significantly affects the degree of 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 also increases the non-uniformity 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 smaller 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 speed of screw rotation is also different.

09 Shot volume

Syringe press evaluations are usually based on the amount of PS that can be injected per injection, and may be measured in ounces or grams. Another ranking system is based on the volume of melt that can be injected by the injection molding machine.

10 plasticizing capacity

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

11 plasticizing capacity estimation

To determine whether the production quality can be maintained throughout the production process, a simple formula about output and plasticizing capacity can be combined, as shown below: t = (total shot impact gX3600) ÷ (plasticizing amount of injection molding machine 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 deviations often occur in the injection molded parts. Especially when paying attention to the quality of thin-walled injection molding or precision tolerances, the amount of shot and plasticization must match each other.

12 shot cylinder residence time

The rate at which plastics decompose is dependent on temperature and time. For example, plastics decompose after being exposed to high temperatures for a period of time; however, it may take longer to decompose when exposed to lower temperatures. Therefore, the residence time of plastic in the shooting cylinder is very important.

The actual residence time can be determined experimentally. The method is to measure the time required for the colored plastic to pass through the shot tank, which can be roughly calculated by the following formula: t = (rated shot tank gx cycle time S) ÷ (shot GX300) Please note that some plastics stay in the shot tank longer than the calculation time, because they can agglomerate in the shot tank.

13 Calculating Residence Time and Importance

As a general rule, the residence time of a particular plastic on a particular injection molding machine should be calculated. Especially when large injection molding machines use smaller shots, plastics tend to decompose, which is not detectable from observation. If the residence time is short, the plastic will not plasticize uniformly; if the residence time is longer, the plastic properties will deteriorate.

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

14 Shot cylinder temperature environment

It should be noted that the melt temperature is important and that any shot tank temperature used is only a guide. If you have no experience processing a particular plastic, start with the lowest settings. The first zone temperature is usually set to the lowest value to prevent premature melting and blocking of plastic in the feed opening.

The temperature in the 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 stated, each zone should be set to the same size.

15 Melt temperature

Can be measured by nozzle or air jet method. When measuring with the latter, care must be taken to ensure that accidents do not occur when cleaning hot-melt plastics, as the high temperatures of hot-melt plastics can burn the skin and even corrode it. In the injection workshop, burns are accidental.

Therefore, gloves and face shields should be worn when handling hot plastic or in the danger of splashing hot melt plastic. 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 depends on the rotation speed, back pressure, shot volume, and injection cycle of Luocun.

16 mold temperature

Always check that 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 yield of injection molded parts. All measured values must be recorded and the injection molding machine checked at the specified time.

17 Uniform cooling

The 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 cooled uniformly. The injection molded part must be cooled as quickly as possible, while ensuring that no defects occur, such as uneven surfaces and changes in physical properties. The cooling rate of each part of the injection molded part must be equal, but it refers to the non-uniform method to cool the mold, for example, the cold water is input to the inner core portion of the mold, and the outside of the mold is cooled by warmer water. Syringe plastic tolerance should be used when precision flat products or large products with long melt flow at the spout.

18 temperature and cooling check