Injection technicians must learn! Seventeen tricks to subdue injection molding machine

Time:2022-09-27 08:39:14 / Popularity: / Source:

1 Nozzle

Melt usually flows from nozzle into sprue, but in some molds, nozzle is part of mold because it extends to bottom of mold. There are also two main types of nozzles: open nozzles and closed nozzles. In injection molding production, open nozzles should be used more because they are less expensive and less likely to hold up. Even low viscosity melts can be used with this nozzle if injection molding machine is equipped with a depressurization device.
Sometimes a closed nozzle must be used, which acts as a stop valve to block plastic in injection cylinder. Make sure nozzle fits into sprue sleeve correctly, and top hole is slightly smaller than sprue sleeve to allow sprue to be withdrawn from mold more easily. Hole of nozzle sleeve is 1mm larger than that of shooting nozzle, that is, radius of nozzle is 0.5mm thinner than radius of nozzle sleeve.

2 Filters and Combination Nozzles

Plastic impurities can be removed by filter of extendable nozzle, i.e. molten and plastic flow through a channel, which is divided into narrow spaces by inserts. These narrows and gaps remove impurities and improve mixing of plastic.
So extended, a stationary mixer can be used for better mixing. These units can be installed between shot tank and nozzle to separate and remix melt, mostly through channels of stainless steel.

3 Exhaust

Some plastics need to be exhausted in injection cylinder to let gas out during injection molding. Most of time these gases are just air, but it may be moisture or monomolecular gases released from melting. If these gases cannot be released, gas will be compressed by melt and carried into mold, where it will expand and form bubbles in product. To vent gas before it reaches nozzle or die, reduce screw root diameter to depressurize melt in shot cylinder.
Here, gas can be expelled from holes or holes in shot cylinder. Then diameter of screw root is increased, and melted glue of volatiles is adapted to nozzle. Injection molding machines equipped with this facility are called vented injection molding machines. There should be a good smoke exhauster with catalytic burner above this exhaust injection molding machine to remove potentially harmful gases.

4 Effect of increasing back pressure

To get a high-quality melt, plastic is heated or melted consistently and mixed well. Proper melting and mixing requires correct screw and sufficient pressure (or back pressure) in shot cylinder for mixing and thermal consistency. By increasing resistance to oil return, back pressure can be created in firing cylinder. But screw takes longer to reset, so there is more wear and consumption in drive system of injection molding machine. Maintain back pressure as much as possible, isolate from air, also require consistent melt temperature and mixing degree.
injection molding production 

5 Check valve

No matter what kind of screw is used, tip is usually equipped with a stop valve, and in order to prevent plastic from flowing out of nozzle, a decompression (reverse cable) device or a special shooting nozzle is also installed. If a miscarriage stop is used, it must be checked regularly because it is an important part of shooting material tank. At present, switch-type nozzles are not commonly used, because nozzle equipment is prone to leakage and decomposition of plastic. Each type of plastic is currently listed with a suitable nozzle type.

6 Screw back (reverse cable)

Many injection molding machines are equipped with screw retraction or suction. When screw stops, it is hydraulically withdrawn to suck back plastic at the tip of nozzle. This unit allows use of an open nozzle. Amount of sucking back may be reduced, as incoming air can cause problems for some plastics.

7 Screw gasket

In most injection cycles, it is necessary to adjust rotation of screw, so that when screw is injected, there will be a small amount of soft cushion plastic left, which can ensure that screw reaches an effective advancing time and maintains a fixed shooting pressure. Pad material for small injection molding machines is about 3mm; for large injection molding machines, it is 9mm. Regardless of screw pad value used, it must remain same. Now size of screw spacer can be controlled within 0.11mm.

8 Rotation speed of screw

Rotational speed of screw significantly affects stability of injection molding process and amount of heat acting on plastic. The faster screw rotates, the higher temperature. When screw rotates at high speed, friction (shear) energy transmitted to plastic improves plasticizing efficiency, but also increases non-uniformity of melt temperature.
Due to importance of screw surface speed, screw rotation speed of large injection molding machine should be less than that of small injection molding machine, because shear heat energy generated by large screw is much higher than that of small screw at same rotation speed. Because of different plastics, speed at which screw rotates is also different.

9 Injection volume

Syringe presses are usually evaluated by amount of PS that can be injected in each shot, which may be measured in ounces or grams. Another ranking system is determined according to volume of melt injected by injection molding machine.
10 Plasticizing ability
An injection molding machine is usually evaluated on basis of amount of PS (in pounds kg) that can be uniformly melted in 1 hour, or heated to a uniform melting temperature, which is called plasticizing capacity.

11 Estimation of plasticizing capacity

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

12 Dwelling time of injection cylinder

Rate of decomposition of plastics depends on temperature and time. For example, plastics decompose after a period of time at high temperatures, but take longer to decompose when exposed to lower temperatures. Therefore, residence time of plastic in injection cylinder is very important.
Actual residence time can be determined through experiments by measuring time required for colored plastic to pass through injection cylinder, which can be roughly calculated by following formula: t=(rated material volume of the injection cylinder gX cycle time S)÷(Injection cylinder amount gX300).
Note that some plastics will stay in shot tank longer than calculated because they can agglomerate in shot tank.

13 Calculate residence time and importance

As a general practice, residence time of a plastic on a particular injection molding machine should be calculated. Especially when large injection molding machine uses a small shot size, plastic is easily decomposed, which is not detectable from observation. If residence time is short, plastic will not be plasticized uniformly; if residence time is long, plastic properties will be attenuated. Therefore, residence time must be kept consistent.
Method: Ensure that plastic entering injection molding machine has a stable composition, consistent size and shape. If there is any abnormality or loss of parts of injection molding machine, report it to maintenance department.

14 Injection cylinder temperature environment

It should be noted that melt temperature is important and any shot tank temperature used is only indicative. If you have no experience working with a particular plastic, start with the lowest setting. Usually first zone temperature is set to a minimum value to prevent premature melting and sticking of plastic in feed port.
Temperature in other zones is then gradually increased until it reaches nozzle, which tends to be slightly cooler at the tip of nozzle to prevent dripping. Molds are also heated and cooled, due to size of many molds, molds are also divided, but unless otherwise stated, zones should be set to same size

15 Melt temperature

It can be measured by nozzle or by air jet method. When measuring with the latter, care must be taken to ensure that there are no accidents when cleaning hot-melt plastics, as high temperature of hot-melt plastics can burn or even corrode skin. In injection molding workshop, burns are accidental.
Therefore, gloves and face shields should be worn when handling hot plastics or where there is a risk of splashing hot melted plastics. To ensure safety, tip of thermal needle should be preheated to temperature to be measured. Each plastic has a specific melt temperature. To achieve this temperature, actual injection cylinder adjustment value of injection molding depends on rotation speed of screw village, back pressure, injection volume and injection cycle.

16 Mold temperature

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

17 Even cooling

Injection molded parts must be cooled uniformly, that is, different parts of mold must be cooled at different rates, so that the entire product will be uniformly cooled. Injection molded parts must be cooled as quickly as possible without causing defects such as uneven surfaces, changes in physical properties, etc.
Cooling rate of each part of injection molded part must be equal, but it means to cool mold in a non-uniform way. For example, cold water is fed into inner core part of mold, and warmer water is used outside mold to cool. This technique should be used especially for precise straight products or large products with long sprue melt flow.

18 Temperature and Cooling Check

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

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