Seventeen moves to subdue the injection molding machine

Injection Nozzle

The molten glue usually flows from the shot nozzle into the injection port, but in some molds, the shot nozzle is part of the mold because it extends to the bottom of the mold. There are two main types of nozzles: open nozzles and closed nozzles. Open nozzles should be used more often in injection molding production because they are cheaper and have less potential for stagnation. If the injection molding machine is equipped with a depressurization device, then this type of nozzle can be used even for low viscosity melts.

Sometimes it is essential to use a closed nozzle, which acts as a stop valve to hold back the plastic yin in the injection cylinder. Make sure that the shot nozzle is properly inserted into the injection sleeve, with the top hole being slightly smaller than that of the injection sleeve, which allows the injection nozzle to be more easily withdrawn from the mold. The hole in the nozzle sleeve should be 1 mm larger than that of the shooting nozzle, i.e. the nozzle radius should be 0.5 mm thinner than the nozzle sleeve radius.

Filters and combination nozzles

Impurities in the plastic can be removed with the filter of the extension nozzle, i.e. the molten and plastic flows through a channel which is separated into narrow spaces by inserts. These narrows and gaps remove impurities and improve the mixing of the plastic.

Therefore, by extension, fixed mixers can be used to achieve better mixing results. These devices can be installed between the injection cylinder and the nozzle to separate and remix the melt, mostly by making the melt flow through a stainless steel channel. 

Venting

Some plastics need to be exhausted at the injection cylinder to allow the gases to escape during injection molding. In most cases, these gases are just air, but it may be moisture or single-molecule gases released by the melt. If these gases are not released, the gas will be compressed by the melt and carried to the mold where it will expand and form bubbles in the product. To drain the gas before it reaches the injection nozzle or the mold, lowering or reducing the screw root diameter will depressurize the melt in the injection cylinder.

Here, the gas can escape through the hole or holes in the injection cylinder. The screw root diameter is then increased and the melt, which has been de-volatilized, is directed to the injection nozzle. An injection molding machine equipped with this facility is called a venting type injection molding machine. This type of exhaust injection molding machine should have a catalytic burner well above the fume exhauster to remove the potentially harmful gases.

The role of increased back pressure

In order to get a high-quality melt, the plastic has to be heated or melted consistently and mixed well. Using the right screw is the only way to properly melt and mix, and to have enough pressure (or back pressure) in the injection cylinder to get consistent mixing and heat. Increasing the resistance of the return oil will create backpressure in the shot cylinder. However, it takes longer for the screw to reset, so there is more wear and tears in the injection molding machine drive system. Maintaining backpressure and isolation from the air as much as possible also requires a consistent melt temperature and mixing level.

Stop flow valve

Regardless of the type of screw used, the tip of the screw is usually equipped with a stop valve, or to prevent the plastic from flowing out of the injection nozzle, it is also equipped with a decompression (backpressure) device or a special shooting nozzle. If a stopcock is used, it must be checked periodically, as it is an important part of the shooting cylinder. At present, switching nozzles are not commonly used because of the tendency to leak plastic and disintegrate in the nozzle equipment. Currently, there is a nozzle type listed for each plastic.

Screw setback (reverse cable)

Many injection molding machines are equipped with a screw back-out or back-suction device. The screw is hydraulically withdrawn to suck back the plastic at the tip of the nozzle when it stops rotating. The number of Xi's feet that will be sucked back may be reduced, as the entry of air can cause problems for some plastics.

Screw bedding

Most of the injection cycles require the rotation of the screw village to be adjusted so that when the screw is finished injecting, there will be a small amount of soft cushioned plastic left, which will ensure that the screw achieves an effective advance time and maintains a constant shot pressure. This ensures that the screw achieves effective propulsion time and maintains a constant firing pressure. Nowadays, the size of the screw bedding can be controlled within 0.11mm.

Rotation speed of the screw

The rotation speed of the screw significantly affects the stability of the injection molding process and the amount of heat applied to the plastic. The faster the screw rotates, the higher the temperature. When the screw rotates at high speed, the frictional (shear) energy transferred to the plastic increases the plasticizing efficiency but also 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 small injection molding machine, because the sheer heat energy generated by a large screw is much higher than that of a small screw at the same rotation speed. The speed of screw rotation varies depending on the plastic.

Injection volume

Syringe machines are usually evaluated by the amount of PS that can be injected in each shot, which may be measured in ounces or grams. Another scheduling system is based on the volume of melt that can be injected by the injector molder.

Plasticizing capacity

An injection molding machine is usually evaluated based on the amount of PS material it can uniformly melt, or the amount of PS heated to a uniform melt temperature in one hour (measured in pounds kilograms), which is called plasticizing capacity.

Plasticizing capacity estimation

To determine whether or not the quality of production can be maintained throughout the production process, a simple formula for yield and plasticizing capacity can be used, as follows: t = (total shot volume gX3600) ÷ (injection molding machine plasticizing volume 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 will not be able to fully plasticize the plastic to achieve a uniform melt viscosity, so the injection molded parts often deviate. Especially for thin-walled or precision tolerance injection molding, the injection volume and plasticizing volume must match each other.

Injection cylinder retention time

The rate of plastic decomposition is dependent on temperature and time. For example, the plastic will decompose after a period of time at a high temperature, but at a lower temperature, it will take a longer time to decompose. Therefore, the retention time of the plastic in the injection cylinder is very important.

The actual retention time can be determined experimentally by measuring the time required for the colored plastic to pass through the injection cylinder, which can be roughly calculated by the following formula: t = (injection cylinder rated material volume g X cycle time S) ÷ (injection volume g X 300)

Note that some plastics have a longer retention time in the injection cylinder than the calculated time because they can clump in the injection cylinder.

Calculating retention time and importance

It is common practice to calculate the residence time of a given plastic in a particular injection molding machine. Especially in large injection molding machines with low shot volumes, the plastic tends to break down, which is not detectable from observation. If the dwell time is short, the plastic will not be plasticized uniformly; if the dwell time is long, the plastic properties will be decayed. Therefore, it is important to keep the retention time consistent.

Method: Ensure that the plastic input to the injection molding machine has a stable composition, consistent size, and shape. Report any malfunctions or losses of the injection molding machine to the maintenance department.

Injection Cylinder Temperature Environment

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

The temperature in the other zones is then gradually increased until it reaches the injection nozzle, often at a slightly lower temperature at the tip of the nozzle to prevent dripping. The mold is also heated and cooled. Due to the size of many molds, the molds are also differentiated, but unless specified, each zone should be set to the same size.

Melt temperature

This can be measured by measuring the injection nozzle or by the air jet method. When using the latter method, care must be taken to ensure that no accidents occur when cleaning hot-melt plastic, as the high temperature of hot melt plastic can burn or even corrode the skin. Burns can be accidental in an injection molding plant.

Therefore, gloves and a face shield should be worn when handling hot plastic or when there is a danger of hot molten plastic splashing around. To ensure safety, the tip of the heat control needle should be preheated to the temperature to be measured. Each type of plastic has a specific melt temperature, and the actual injection cylinder adjustment to reach this temperature depends on the screw village rotation speed, back pressure, injection volume, and injection cycle.

Mold temperature

Always check that the injection molding machine is set and running at the temperature specified on the log sheet. This is very important. This is because temperature affects the surface finish and yield of the injection molded part. All measured values must be recorded and the injection molding machine checked at the specified time.

Uniform cooling

The finished injection molded part must be cooled uniformly, which means that different parts of the mold must be cooled at different rates so that the whole part will be cooled uniformly. The injection molded part must be cooled as fast as possible while ensuring that no defects, such as uneven surfaces or changes in physical properties, are created.

The cooling rate of each part of the injection mold must be equal, but it means that the mold is cooled in an uneven way, for example, by feeding cold water into the inner part of the mold, while the outside of the mold is cooled by warmer water. This technique should be used for flat products with precise tolerances or for large products with long melt flow at the watergate.