Useful Tips for Adjusting Injection Molding Speed and Pressure

The pressure speed is linearly proportional, but many people easily mistakenly think that they are independent and separate when adjusting the injection molding process. Let’s take a look at its control principle and make a metaphor:

  • Pressure can be compared to the limit of the number of people in a room, assuming that the room can only accommodate 10 people. Before this standard is reached, they will keep squeezing in until there are ten people. Adding one more person requires removing one of theprevious persons.
  • And the speed is how fast the peoplesqueeze in. When using high pressure but with a very low speed, in fact, the high pressure was not reached in the previous period of time. Because your room is empty. So how can we achieve the purpose of high pressure?

Especially when injecting glue, if filling is not achieved within a certain period of time, the material at the front end of the injecting glue will cool down immediately. And this is very easy to form defects such as short shots, flow marks, peeling, water lines, and dull surfaces. As for high speed and low pressure, they can’t form action at all. So we should pay attention to this linear relationship when adjusting the parameters.

For the issue of adjusting the injection molding process, professional craftsmen will analyze and understand it theoretically. Let’s take a look together:

1. Heat balance

Heat balance means that the heat generated and the heat discharged must be able to achieve a dynamic balance within a certain range.

As most technical staff know, the most important thing in the adjustment process is to maintain stability. That is to say, when adjusting the process, we should pay attention to solving all the problems at one time as much as possible, and ensure that the same problems will not occur for a long time later.

Heat balance formula:

Drying barrel + heating coil + screw shear + runner shear = cooling water + product + air

We can control mainly: the heating of heating coil (temperature), cooling water flow, screw speed, room temperature, product cycle.

2. Product cycle

In fact, the most important thing is the product cycle. Because profit is most related to production efficiency. All manufacturers want to shorten the molding cycle, but not everyone can make it to the most ideal state. You may wonder why others can produce it in 30% or even 80% less time than you, but with a much higher product yield rate and basically with no defects.   While you have done a lot of work, but still get a very low yield rate?

This is because the balance of the whole has been ignored. Maybe you have noticed some important parameters like pressure, velocity, temperature. But what about other details, such as back pressure, screw speed, nozzle hole shear?

Adjusting Injection Molding Speed and Pressure

3. The effect of multi-level speed and multi-level pressure

Many workers like to process products with multi-level pressure and speed when adjusting the process. Besides, for the current machines, the multi-level speed and pressure are also used to show the high grade of the machines. However, the simplest is actually the best. Because the more complex molding conditions, the higher the environmental and supporting facilities will be.

Of course, if you are producing high-end products, such as electronic connectors, etc., the requirements for your equipment and supporting environment must be very high. This is why many factories now use air conditioning equipment. And now many supporting equipment are becoming more and more advanced, because the requirements for various aspects of the product are getting higher and higher, such as size, surface, and internal stress.

While equipment are getting more advanced, simple is best. The main function of multi-stage speed and pressure is only exhaust. Because the structure of many molds is relatively complicated, I will talk about the mold structure and mold improvement later. Now by the way, the mold has sliders (row positions), inclined tops, core pulls, twisted teeth, etc. The structure of many products is very complicated, so the multi-stage speed pressure may need to be used for the molding conditions.

Before knowing why there is no need to use multi-stage speed pressure, it is first necessary to know what is holding pressure. Many people think that the holding pressure is the pressure that is marked with the word “holding pressure” on the machine. In fact, it is wrong. After the first section, it should be called holding pressure. It is customary that the first section is called mold filling pressure, also called injection pressure. When the cavity is about to be full, the main function of the subsequent pressure is not to fill the cavity, but to prevent the product from shrinking or generating air bubbles.

At the same time, we should know that no matter how good a computer is, it also has a response time. And the response time of a general computer is between 0.5 and 0.05. The response time of Fuji Computer is 0.02S. So when it reaches multiple segments, it may have wasted a few seconds. And within this time, the feed port has been closed.

But why use multi-stage injection? How to set the holding pressure? Multi-stage injection is mainly set for right angles, dead spaces, thin-wall transitions, etc. For flat or not too complex products, there is no need to use multi-stage speed and pressure in molding. Because maintaining a simple cost is completely different from the complex one. Then you may ask how to set the multi-stage speed and pressure?

When producing products, we should plan its injection curve before setting parameters. Of course, it is impossible for us to set a very specific number, because the pressure of each machine may be different. But we should be very clear about his curve requirements.

Generally speaking, it is necessary to pause for a while at right angles, dead spaces, etc., that is, to go slowly. Because this position is the easiest to accumulate gas. But now the mold basically does not use right angles. In addition, when the wall is thin or the aisle is small, it is necessary to divide the plastic type to adopt fast or slow speed.

  • For high-temperature materials, because of their high temperature, it is necessary to adopt high pressure to pass quickly.
  • But if it is a heat-sensitive plastic, it must be processed at a slow speed. Because the high speed is easy to generate shear heat, the plastic will decompose, discolor or even burn. This is also the reason why the needle tip glue port generally uses a slow speed.

Then how to set the holding pressure and holding pressure position?

Generally set the holding pressure according to the fluidity of the plastic:

(1) To manufacture PP, PE, PA and other plastic products with good flow properties, generally when the product has no thick wall and is filled to 85–90%, it should be turned to hold pressure. And its holding pressure should be generally within 60-80%.

(2) However, for products with average fluidity, it is generally switched to holding pressure at 90-95%, and the holding pressure setting is generally around 70-85%.

(3) For plastics with poor fluidity, such as PMMA, it’s necessary to switch to holding pressure when the cavity is about 95% full, and the holding pressure is generally higher.

If the wall thickness of the product is relatively thick, it can be squeezed with high pressure and low speed in a short period of time, and then turned to maintain pressure.

Multi-level speed pressure setting method:

Find out the location that needs to set the multi-stage pressure speed, and set the injection position of the first section at your estimated position. Then set all the subsequent pressure speeds to zero, and check whether it is in your estimated position after one injection. Then gradually set the breach pressure at the next position.

It is the critical pressure to break through the resistance. When adjusting the machine, we often find that there is a large burr on one side, and there is a shortage of material on the other side, or there are large burrs around the product. At this time, it is necessary to consider the breach pressure.

There will always be a certain gap in the mold, some are due to manufacturing process problems, some are due to exhaust, etc.. While due to the reason of the mold or the raw material, it is impossible to use less pressure to fill. Only when the maximum pressure that breaks through this position is found is the holding pressure, and a little bit in front of this position is the holding position.

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