Injection staff! Do you know all these 18 knowledge?

Injection staff! Do you know all these 18 knowledge?

1. Shooting nozzle

The melt usually flows from nozzle into gate, but in some molds nozzle is part of mold as it reaches bottom of mold. There are two main types of nozzles: open nozzles and closed nozzles. In injection molding, open nozzles should be used more often because they are cheaper and less likely to survive.

If injection molding machine is equipped with a pressure relief device, this nozzle can be used even for low viscosity melts. Sometimes it is necessary to use a closed nozzle that can be used as a check valve to block plastic in firing cylinder. Make sure nozzle is correctly inserted into sprue sleeve and top hole is slightly smaller than sprue sleeve to make it easier to remove sprue from mold. The hole of nozzle sleeve is 1 mm larger than that of firing nozzle, that is, radius of nozzle is 0.5 mm thinner than radius of nozzle sleeve.

2. Filter and combination nozzle

Plastic impurities can be removed by extension nozzle filter, that is, melt and plastic flow through a channel that is divided into narrow spaces by inserts. These narrow gaps remove contaminants and improve plastic mixing. Thus, stationary mixers can be used for better mixing. These devices can be installed between injection barrel and melt separation and remixing nozzle, most of which are channels to allow melt to flow through stainless steel.

Injection staff! Do you know all these 18 knowledge?

Injection workshop

3. Exhaust

During injection molding, injection molding cylinder needs to vent some plastics to release gas. In most cases, this gas is simply air, but it can be moisture or a monomolecular gas escaping from melt. If this gas cannot be released, gas will be compressed by melt and transferred to mold, where it will expand and form bubbles in product. To get rid of gas before it reaches nozzle or mold, reduce or decrease diameter of screw shank to release melt pressure in injection barrel.

At this point, gas may exit through hole or hole in firing cylinder. Then diameter of screw shank increases and directs non-volatile melt to nozzle. An injection molding machine equipped with this device is called a ventilation injection molding machine. At top of this exhaust injection molding machine, there should be a good fume extractor with a catalytic burner to remove potentially harmful gases.

4. The effect of increasing back pressure

To obtain a high quality hot melt adhesive, plastic must be heated or melted evenly and mixed thoroughly. Use correct screw to properly melt and mix plastic, and have enough pressure (or back pressure) in firing cylinder to mix and heat consistently.

The increase in oil return resistance creates back pressure in firing cylinder. But it takes more time to reset screw, so there is more wear and consumption in drive system of injection molding machine. Maintain highest possible backpressure, isolated from air, and require same melt temperature and mixing ratio.

5. Stop valve

Regardless of which screw is used, handpiece is usually equipped with a check valve. To prevent plastic from flowing out of nozzle, there will also be a decompression device (reverse cable) or a special shooting nozzle. installed. If an abortion arrest system is used, it must be checked regularly as it is an important part of injection tank. Switch nozzles are not commonly used these days because nozzle hardware easily leaks out of plastic and disintegrates. Each plastic type is now listed with appropriate nozzle type.

6. Scroll back

Many injection molding machines are equipped with a screw or suction device. When rotation of auger stops, it retracts hydraulically to suck out plastic at tip of nozzle. This device allows use of open nozzles. The amount of suction back can be reduced as air suction can be a potential problem with some plastics.

7. Threaded gasket

In most injection molding cycles, it is necessary to adjust rotation of auger so that when auger is injected, there is morepart of shock-absorbing plastic to ensure efficient auger advance time. and maintains a fixed pressure when firing. The padding for small injection molding machines is about 3mm, for large injection molding machines it is 9mm. Regardless of size of screw packing used, it must remain same. Now screw spacer size can be controlled within 0.11mm.

8. Screw speed

The rotation speed of screw significantly affects stability of injection molding process and thermal effect on plastic. The faster screw rotates, higher temperature. When screw rotates at a high speed, friction (shear) energy transferred to plastic improves plasticizing efficiency, but at same time increases melt temperature unevenness. Due to importance of surface speed of screw, rotation speed of screw of a large injection molding machine must be lower than that of a small injection molding machine, because thermal shear energy generated by large screw is much higher than that of large screw. small screw with same rotation speed. Due to different plastic, speed of rotation of screw is also different.

Injection staff! Do you know all these 18 knowledge?

Injection molding machine

9, injection volume

Syringe molding machines are usually priced in terms of amount of polystyrene that can be injected per shot, which can be measured in ounces or grams. Another ranking system is based on volume of melt injected by injection molding machine.

10. Plasticizing ability

Evaluation of an injection molding machine is usually based on amount of polystyrene that can be uniformly melted or heated to a uniform melt temperature for 1 hour (in pounds and kilograms), which is called plasticizing power

11. Evaluation of plasticizing ability

To determine whether quality of product can be maintained throughout production process, a simple formula for plasticizing capacity and capacity can be used, namely: hX1000) T is minimum cycle time. If mold cycle time is below T value, machine will injection molding cannot fully plasticize plastic to achieve uniform melt viscosity, so injection molded parts often have deviations. Especially when paying attention to quality of injection molding of thin-walled products or products with fine tolerances, injection volume and plasticizing volume must interact with each other.

12. Injection cylinder residence time

The rate of decomposition of plastic depends on temperature and time. For example, plastic decomposes when exposed to high temperatures over a period of time, but takes longer to decompose when exposed to lower temperatures. Therefore, residence time of plastic in injection cylinder is very important.

The actual residence time can be determined through experimentation. The method is to measure time required for colored plastic to pass through injection cylinder, which can be roughly calculated using following formula: T = (nominal injection volume cylinder gX cycle time S) ÷ (injection volume gX300) injection tank longer than expected because they can drain into injection tank.

13. Calculate stay time and importance

According to common practice, residence time of a particular plastic on a particular injection molding machine should be calculated. Plastic tends to degrade, especially in large injection molding machines using small portion sizes, which cannot be detected by inspection. If residence time is short, plastic is unevenly plasticized, if residence time is increased, properties of plastic deteriorate.

Therefore, make sure that residence time is constant. Make sure that plastic entering injection molding machine has a stable composition, same size and shape. If there is any deviation or wear of parts of injection molding machine, please do notmust be reported and repaired in time.

14. Injection cylinder ambient temperature

It should be noted that melt temperature is important and any crushing chamber temperature used is a guide only. If you don't have experience with a specific plastic, start with a minimal setting. Usually temperature of first zone is set to lowest value to prevent premature melting and sticking of plastic in feed throat.

The temperature in other areas gradually rises until it reaches nozzle. To prevent dripping, temperature at top of nozzle is often slightly lower. Molds are also heated and cooled, and due to size of many molds, molds are also zoned, but unless otherwise noted, zones should be set to same size.

Injection staff! Do you know all these 18 knowledge?

Injection tray

15. Melting point

Nozzle can be measured or temperature can be measured by air jet. When using latter for measurements, it is necessary to ensure that there are no accidents when cleaning hot melt adhesive, as high temperature of hot melt adhesive can burn or even corrode skin. Burns and scalds happen from time to time in injection molding plants.

Gloves and a face shield should therefore be worn when working with hot plastic or where there is a risk of splashing hot melted plastic. To ensure safety, thermostat tip must be preheated to a measurable temperature. Each plastic has a certain melt temperature. To achieve this temperature, actual injection cylinder adjustment value depends on screw speed, back pressure, injection volume and injection cycle.

16. Mold temperature

Always check that injection molding machine is set up and running at temperature specified in protocol. it is very important. Because temperature will affect surface finish and yield of injection molded parts. All measured values ​​must be recorded, and injection molding machine must be checked at a certain time.

17. Uniform cooling

Injection molded parts must be cooled evenly, that is, different parts of mold must be cooled at different rates so that entire product is cooled evenly. The injection molded parts should be cooled as quickly as possible without causing defects such as surface irregularities, changes in physical properties, etc. The cooling rate of each part of injection molded part should be same. Remember to cool mold unevenly, such as pouring cold water to inside of mold and using warmer water to cool outside of mold. This method should be used in particular when injecting straight products with small tolerances or large products with a long melt flow on sprue.

18. Checking temperature and cooling

Always check that injection molding machine is set up and running at temperature specified in protocol. Because temperature will affect surface finish, yield and quality of injection molded parts. All measured values ​​must be fully recorded and injection molding machine must be overhauled according to schedule.