In fact, I didn't have idea to write this article at the beginning, but when I tried mold last night, I found that many people really didn’t understand it very clearly, and they even interfered with each other to debug many times. I will write down my personal understanding here, and I include a lot of inaccuracies. After all, it is my own understanding.

Let's talk about cavity balance first. Cavity balance should be best understood. This balance generally refers to products with multiple cavities in one mold. When injection molding such products, cavity of each product needs to be filled as much as possible at the same time, which means cavity balance.
For those molds that produce many products at one time, flow balance of cavity is very important. For example, bottle caps, golf balls, etc. at the beginning will be injected and weighed, and amount of feeding will be corrected with a file, so as to complete original intention of almost same feeding of each product cavity.

 

Because in production, especially production of golf balls, size requirements of this kind of precision mold are relatively high. A golf ball doesn't just need a size, it's a sphere that needs a roundness size on all four sides, which leads to a gap in filling of any of your cavities, which may end up with a gap in size of ball.
Therefore, in this multi-cavity product, priority is to ensure that product weight of each cavity is same, that is, to adjust balance of cavity. A problem that must be noted here is that cavity refers to cavity of a product, not a gate is a cavity. Again, I hope not to confuse concept. Because there will be flow balance in the back, it will be described relatively.
It is also mentioned here that it is a priority guarantee. Because of development of mold industry, there will be many multiple cavities in one mold, but products in each cavity are of different styles or sizes, so there are two ways of saying cavity balance. One is is weight balance, that is, weight and size of all cavity products are uniform or chiral symmetry, which can be done completely symmetrically, and gate can be adjusted to achieve an absolute balance of cavity. Second kind of balance is that two-chamber products are full at the same time, but weight will be different, which may involve another balance described below, flow balance.

Second, flow balance. Concept of flow balance is actually a word that I have always been very confused about. After consulting a lot of people, final conclusion is such a definition: simultaneous filling of multiple ends of a mold ends. This does not specifically refer to a single product or a multi-cavity mold. It actually feels like a balanced flow of individual products. However, in the case of multiple cavities, balance of cavity needs to be adjusted in a general direction, and balance of flow is to allow each product to be filled with products at the same time.

 

Having said so much, many people will feel that there is no difference, what nonsense are you talking about. This explanation may be able to make people more clear. Cavity balancing refers to filling of multiple cavities of product together, while flow balancing refers to filling of ends of a single product together.
Relationship between the two is indeed a rather entangled relationship. For a long time, I wondered whether there was a relationship between the two, but in the end I didn't understand relationship between them. Generally, we will try to debug product balance first, because now there are more molds with hot runners. After hot runner is controlled, it will be faster to perform a debugging of cavity balance again.
However, molds with cold runners generally have problems with machining accuracy, so balance of cavity is given priority. After ensuring balance of cavity, flow balance is leveled by some artificial grinding. Therefore, in many cases, quality of mold is not only manufacturing level of mold company, but also modification level of craftsman or technician.
A good set of molds always takes a long time to break in, not a brand new mold is a good mold. Slowly modifying problem will make mold more and more comfortable to use in a specific environment.

Third, pressure balance. Term stress balance is something I only recently started talking about. In fact, there is no pressure balance in CAE analysis, or you can't find point of pressure balance. Many people have misconception that pressure at the very end of filling is a balance. But it really is not a balanced pressure. Just like when I was before, I always felt that position pressure of final point should be exactly same.
It wasn't until I read final stress report that I realized what I was thinking was wrong. Final pressure may have a very large difference, because when product is multi-gated and glue is opened with a delay due to shape of product, pressure will be released instantaneously. In this case, it will actually lead to a big change in pressure. I imagine pressures are quite different.

 

As shown in picture above, shape of product is inconsistent at both ends, but gate position is uniform, and cavity balance can be satisfied. At the same time, flow balance can be achieved under condition of normal valve opening. Therefore, process is adjusted normally and product is debugged. However, an obvious hysteresis line was found during debugging process (some people call it a temperature difference line, and some call it a yin and yang surface).
It is difficult to push out through normal debugging. As shown in figure, pressure required by small head is extremely large. As long as large head is not fully filled, once large head is opened, a hysteresis mark will appear.

Later, after thinking about it, I don't care about balance of flow, big end is pushed back hard, and small end is completely full, then big end is opened, pressure needs to be smaller, material can be pushed better, hysteresis line will not appear on big end. Pressure has become more disparate, and flow balance in cavity is also abandoned.
After adjustment, analysis pressure chart is as follows:

 

Pressure of small end becomes a little larger, and pressure of big end becomes smaller. In this way, there is no hysteresis line when small end of product is filled first, but correspondingly, difference of flow balance becomes larger. It is necessary to adjust holding pressure of product to achieve the best possible pressure balance at both ends, so that a flat pressure is pushed out inside mold, and final product can also achieve expected idea.

 

Three balances that I understand are general meanings of above, pressure balance should be a pressure balance between the farthest end and its relative position. Not only this product, but all kinds of products should pay attention to order of three balances when adjusting. It is the best way to balance all three at the time of completion.
In mold and process debugging, please determine priority order by yourself, take final product as evaluation standard, do not use this as evaluation standard for a mold.