Stress and flow ratio of injection mold

Stress and flow ratio of injection mold

In the actual work of most mold practitioners, they always feel that plastics knowledge is used relatively little, but when reviewing new products, listen to leaders talk about “stress” “flow ratio” “flow direction” “viscosity”, etc. When it comes to nouns, it always feels like good and profound knowledge, like something unpredictable.

Mould practitioners secretly go to Baidu in private to check the information everywhere, and the information will always look at people more dizzy, and very little information can be written in a simple way. Today, let the editor of South Asia TEDA give you some popular science terms!

1. Stress

In life, we will see such a phenomenon. After pressing down the rubber and releasing it, it will slowly return to its original state. In fact, after a punch on the wooden table, the stress point of the wooden table will also be depressed. After the power is withdrawn, as long as it does not exceed its limit, it will return to its original state. It ’s just that it ’s too weak for us to feel.
What does this have to do with stress? Stress is actually when an object is subjected to an external force, the interior of the object will fight the external force to resist this external force, and try to return to the initial state. This resistance inside the object is what they call stress.

What’s the use of stressing this product? We often hear the words “stress concentration”, regardless of plastic products or steel, these words will appear, what does this mean? If the force is applied to the product in the direction of the arrow, what will be the result when this force is large enough? The circle will break at right angles.

The circle will break at right angles. Why?

It is because the stress is concentrated at a right angle, and when it exceeds its limit, it will crack from the stress concentration. So, how do we generally solve it?

Fillet is the most effective way to solve the stress concentration. It is not to say that it is all right when the corner is turned up. The corner is mainly to disperse its position so that it is not concentrated, so that it is not so easy to break.

Especially for the mold, the easiest cracks in the production process are at the right angles of the steel, including cracks in the heat treatment process, which are also prone to occur at these locations.

2. Stream-to-length ratio

The flow length ratio is the ratio of the distance the plastic flows to the wall thickness of the product. Does it feel particularly official? To put it simply, plastic flows through a slit. The larger the slit, the farther it can flow. This slit is the wall thickness of the product. The flow distance divided by the size of the slit is the flow length ratio.

Of course, this value is related to the material temperature, mold temperature, polishing degree, etc., but it is only a rough range value, and it is difficult to be precise because of different conditions.

What’s the use of flow ratio?

Do you have such doubts, looking at a certain product, what gate to use, you can probably judge one or two, as for how many gates? Maybe you will guess and feel it.

The feeling is always the feeling, it is not accurate, sometimes a gate can be done, you are worried about what to do in case? So I chose two, or made a preliminary plan. So, what are the criteria for judging? The stream-to-length ratio can actually roughly judge this thing.

For example:

PC material, product wall thickness 2mm, product filling distance is 200, flow ratio is 200/2 = 100. This value is basically the limit theoretical value of PC, which is the case without adding a flow channel. What if it counts as a runner? Therefore, for this product, it is very difficult for us to choose a gate for injection molding.

Why should the product gate be selected from thick places? It is because the thicker the area, the greater the flow.

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