Regardless of whether you are doing injection molding or molds: 18 essential basics please keep in m

Time:2020-01-17 09:01:30 / Popularity: / Source:

injection molding production 

01. Nozzle

Melt usually flows from nozzle into gate, but for some molds, nozzle is part of mold because it extends to bottom of mold. There are two main types of nozzles: open-ended and closed-ended. In injection molding production, open-ended nozzles should be used because they are cheaper and less likely to stay.
If injection moulding machine is equipped with a pressure relief device, then even lower viscosity melts can use this nozzle. Sometimes it is necessary to use a closed-end nozzle, which acts as a check valve, blocking plastic in injection cylinder. Make sure that nozzle is properly inserted into nozzle sleeve, top hole is slightly smaller than nozzle sleeve, which makes it easier for nozzle to withdraw from mold. Nozzle sleeve hole is 1mm larger than shooting nozzle, that is, nozzle radius is 0.5mm thinner than nozzle sleeve radius.

02. Filter and combined nozzle

Plastic impurities can be removed by filter of extension nozzle, that is, melt and plastic flow through a channel, which is separated by 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 spray cylinder and nozzle to perform work of separating and remixing melt. Most of them are channels that pass melt through stainless steel.

03. Exhaust

Some plastics need to be vented in shot cylinder to let gas out during injection. In most cases these gases are only air, but it may be moisture or single molecular gas released by melting. If these gases cannot be released, gas will be compressed by melt and brought into mold, it will expand and form bubbles in product. To vent gas before it reaches nozzle or mold, reducing or reducing diameter of screw root can depressurize melt in shot cylinder.
Here, gas can be expelled from holes in shot tank. Then diameter of screw root is increased, and devolatized melt glue is directed to nozzle. An injection moulding machine equipped with this facility is called a vented injection moulding machine. Exhaust injection moulding machine should have a good smoke exhauster above catalytic burner to remove potentially harmful gases.

04. Role of increasing back pressure

In order to obtain high-quality melt glue, plastic must be uniformly heated or melted and thoroughly mixed. Correct screw is used for proper melting and mixing, and there is sufficient pressure (or back pressure) in shot cylinder to obtain mixing and thermal consistency.
Increasing resistance to return oil can generate back pressure in firing cylinder. However, screw takes longer to reset, so there is more wear and tear in drive system of injection moulding machine. Keep back pressure as far as possible, and keep it from air. Melt temperature and mixing degree should also be consistent.

05. Stop valve

No matter which type of screw is used, tip is usually equipped with a check valve. To prevent plastic from flowing out of nozzle, a decompression (inverting) device or a special shooting nozzle is also installed. If a stop-batch supply is used, it must be checked regularly as it is an important part of firing tank. At present, switch-type nozzles are not commonly used because plastics leak and decompose easily in nozzle equipment. Each plastic currently has a list of suitable nozzle types.

06. Screw backward (reverse rope)

Many injection molding machines are equipped with screw back or suction devices. When screw stops, it is withdrawn hydraulically to suck back plastic at the tip of nozzle. This device allows use of open nozzles, which may reduce number of suckbacks, because entering air can cause problems for some plastics.

07. Screw padding

In most injection cycles, rotation of screw village needs to be adjusted so that when screw injection is completed, most of plastic cushion will remain, so that screw can achieve an effective advance time and maintain a fixed firing pressure. Screw padding of small injection molding machines are about 3mm; large injection molding machines are 9mm. No matter how big value of screw padding is, it must be kept same. Size of screw padding can now be controlled within 0.11mm.

08. Rotating speed of screw

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

09. Shot volume

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

10. Plasticizing ability

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

11. Estimation of plasticizing ability

To determine whether production quality can be maintained throughout production process, a simple formula about output and plasticizing capacity can be combined, as shown below: t = (total injection shot g*3600) ÷ (plasticizing amount of injection moulding machine kg/h*1000),  t is minimum cycle time. If cycle time of mold is lower than t, injection moulding machine cannot fully plasticize plastic to achieve a uniform melt viscosity, so deviations often occur in injection molded parts. Especially when paying attention to quality of thin-walled injection molding or precision tolerances, amount of shot and plasticization must match each other.

12. Retention time of shooting cylinder

Rate at which plastics decompose depends 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, residence time of plastic in shooting cylinder is very important.
Actual residence time can be determined through experiments. Method is to measure time required for colored plastics to pass through shot cylinder, which can be roughly calculated by following formula: t = (rated shot cylinder g*cycle time S) ÷ (shot amount G*300). Please note that some plastics stay in shot tank longer than calculation time, because they can agglomerate in shot tank.

13. Calculate retention time and importance

As a general rule, residence time of a certain plastic on a particular injection moulding machine should be calculated. Especially when large injection molding machines use smaller shots, plastics tend to decompose, which is not detectable from observation. If residence time is short, plastic will not plasticize uniformly; if residence time is longer, plastic properties will be attenuated.
Therefore, retention time must be consistent. Method: Ensure plastic that is injected into injection moulding machine has a stable composition, consistent size and shape. Any abnormality or wear of parts of injection moulding machine must be reported to maintenance department.

14. Temperature environment of shooting cylinder

It should be noted that 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 feed opening.
Temperature in other areas gradually increases until it reaches nozzle. To prevent dripping, temperature at the tip of nozzle tends to be slightly lower. Mold is also heated and cooled. Due to dimensional relationship of many molds, molds are also divided, but unless stated, each zone should be set to 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 high temperatures of hot-melt plastics can burn and even corrode skin. In injection workshop, burns are accidental.
Therefore, gloves and face shields should be worn when handling hot plastic or in danger of splashing from hot melt plastic. To ensure safety, tip of heat-control needle should be pre-heated to temperature to be measured. Each plastic has a specific melt temperature. To reach this temperature, actual injection cylinder adjustment depends on rotation speed, back pressure, shot volume, and injection cycle of screw.

16. Mold temperature

Always check that injection moulding machine is set and running at temperature specified on record sheet, which is very important. Because temperature will affect surface finish and yield of injection molded parts. All measured values must be recorded and injection moulding machine checked at specified time.

17. Uniform cooling

Injection molded parts must be cooled uniformly, that is, different parts of mold must be cooled at different rates, so that entire product will be cooled uniformly. Injection molded parts must be cooled as quickly as possible, while ensuring that defects do not occur, such as uneven surfaces and changes in physical properties.
Cooling rate of each part of injection molded parts must be equal, but it refers to cooling mold by non-uniform methods, for example, cold water is input to inner core portion of mold, outside of mold is cooled by warmer water. Syringe plastic tolerance should be used when precision flat products or large products with long melt flow at spout.

18. Temperature and cooling checks

Always check that injection molding machine is set and running at temperature specified on record sheet, which is very important. Because temperature will affect surface finish and yield of injection molded parts. All measured values must be recorded and injection moulding machine checked at specified time.

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