18 plastic injection molding basics that injection molding process personnel must know, do you know them all?
Plastic injection molding basics 1. Injection nozzle
The molten rubber usually flows from the nozzle into the injection port, but in some injection molds, the nozzle is part of the injection mold because it extends to the bottom of the injection mold.
There are two main types of injection mold 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, this type of nozzle can be used even for low viscosity melts. Sometimes it is necessary to use a closed nozzle, which acts as a stop valve to hold back the plastic in the injection cylinder.
Make sure that the nozzle is properly inserted into the spout sleeve, with a slightly smaller hole at the top than in the spout sleeve, so that the spout can be withdrawn from the mold more easily. The hole of the nozzle sleeve should be 1mm larger than that of the shooting nozzle, i.e. the radius of the nozzle should be 0.5mm thinner than the radius of the nozzle sleeve.
Plastic injection molding basics 2. Filters and combination nozzles
Plastic impurities can be removed by the filter of the extension nozzle, that is, molten and plastic flow through a channel, which is divided into narrow spaces by the inserts.
These narrows and gaps remove impurities and improve the mixing of the plastic. Thus, 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, most of which is to make the melt flow through the stainless steel channel.
Plastic injection molding basics 3. Mold venting
Some plastics need to be exhausted in the injection cylinder during injection molding to allow the gas to escape. In most cases, these gases are just air, but they may be molten out of the moisture or single molecule gas.
If this gas is not released, it will be compressed by the melt and carried to the injection mold, where it will expand and form bubbles in the product.
To remove the gas before it reaches the injection nozzle or mold, the melt can be depressurized in the injection cylinder by reducing or decreasing the screw root diameter.
Here, the gas can escape through a 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. Injection molding machines equipped with this facility are called venting machines.
This type of vent injection molding machine should have a catalytic burner above the good fume exhauster to remove potentially harmful gases.
Plastic injection molding basics 4. Increase the role of back pressure
In order to obtain a high-quality melt, the plastic must be heated or melted consistently and mixed thoroughly. The right screw is used to melt and mix properly, and there is enough pressure (or back pressure) in the injection cylinder to obtain consistent mixing and heat.
Increasing the resistance of the return oil creates back pressure in the shot cylinder. However, it takes longer for the screw to reset, so there is more wear and tear in the injection molding machine drive system.
Maintaining back pressure and isolation from the air as much as possible also requires consistency in melt temperature and mixing level.
Plastic injection molding basics 5. Shut-off valve
Regardless of the type of screw used, the tip is usually equipped with a shut-off valve, and to prevent the plastic from flowing out of the injection nozzle, it is also equipped with a pressure-reducing (rewind) device or special shooting nozzle.
If a stopcock is used, it must be checked periodically, as it is an important part of the shooting cylinder. Currently, switching nozzles are not commonly used because of the tendency for plastic to leak and disintegrate in the nozzle equipment. At present, there is a type of nozzle specified for each plastic.
Plastic injection molding basics 6. Screw setback (reverse)
Many injection molding machines are equipped with a screw retraction or suction back device. The screw is hydraulically withdrawn to suck back the plastic at the tip of the nozzle when it stops rotating.
This device allows the use of open nozzles. The amount of suck-back may be reduced because the ingress of air can cause problems for some plastics.
Plastic injection molding basics 7. 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 over to ensure that the screw achieves an effective advance time and maintains a constant firing pressure.
The cushion material for small injection molding machines is about 3mm; for large injection molding machines, it is 9mm. no matter what screw cushion material value is used, it must be kept constant. Now the size of the screw bedding can be controlled by 0.11mm.
Plastic injection molding basics 8. The rotation speed of the screw
The rotation speed of the screw significantly affects the degree of stability of the injection molding process and the amount of heat acting on 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.
Because of the importance of the surface speed of the screw, the rotational speed of the screw in a large injection molding machine should be less than that of a small injection molding machine, because the sheer heat generated by a large screw is much higher than that of a small screw for the same rotational speed. Due to the different plastics, the speed of screw rotation is also different.
Plastic injection molding basics 9. Injection volume
Syringe molding 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 ranking system is based on the volume of melt that can be injected by the injector.
Plastic injection molding basics 10. Plasticizing capacity
An injection molding machine is usually evaluated on the amount of PS material that can be uniformly melted in one hour, or the amount of PS that can be heated to a uniform melt temperature (in pounds per kilogram), which is called plasticizing capacity.
Plastic injection molding basics 11. Plasticizing capacity estimation
To determine whether or not the mass 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 t, the injection molding machine cannot plasticize the plastic sufficiently to achieve uniform melt viscosity, so the injection molded parts often have deviations.
Especially when injection molding is thin-walled or precision tolerances, the injection volume, and the plasticizing volume must match each other.
Plastic injection molding basics 12. 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 = (nominal injection cylinder 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.
Plastic injection molding basics 13. 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 plasticize uniformly; if the dwell time is long, the plastic properties will decay. Therefore, it is important to keep the retention time consistent.
Method: Ensure that the plastic input to the injection molding machine has a consistent composition, size, and shape. Report any malfunctions or wear and tear of the injection molding machine to the maintenance department.
Plastic injection molding basics 14. 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 is set at the lowest temperature to prevent premature melting and sticking of the plastic in the inlet.
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, and due to the size of many molds, the molds are differentiated, but unless specified, the zones should be set to the same size.
Plastic injection molding basics 15. Melt temperature
The temperature of the melt can be measured by measuring the nozzle or by the air jet method. When using the latter method, care must be taken to ensure that no accidents occur when cleaning the hot melt plastic, as the high temperature of the hot melt plastic can burn or even corrode the skin. In an injection molding plant, burns can be accidental.
Therefore, gloves and a face shield should be worn when handling hot plastic or when there is a danger of hot plastic splashes. 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, and the actual injection cylinder adjustment to reach this temperature depends on the screw village rotation speed, back pressure, injection volume, and injection cycle.
Plastic injection molding basics 16. Mold temperature
Always check if the injection molding machine is set and running at the temperature specified on the record sheet. This is very important. This is very important 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.
Plastic injection molding basics 17. Uniform cooling
The finished injection molded part must be cooled uniformly, i.e., different parts of the mold must be cooled at different rates so that the entire part is 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, occur.
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 and using warmer water outside the mold.
This technique should be used in the case of precision flat products with syringe tolerances or large products with long melt streams at the watering ports.
Plastic injection molding basics 18. Temperature and cooling check
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 machine must be checked at the specified time.
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