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Experience sharing of plastic mold structure design

2021-07-08

Experience sharing of plastic mold structure design

I have been engaged in the design of injection molds for many years, and have designed molds for home appliances, automobiles, and electronic products. The design level is not necessarily very high, but the work done is more complicated. Today I was just idle, and I would like to discuss with you the structure design of the injection mold.


1. First of all, after we get a product, don't rush to split the mold. The most important thing is to check the product structure first, including mold drafting, thickness and other molding problems. Of course, these may be more difficult for a person who has just been engaged in mold structure design. Because they may not know what is a more suitable product for mold design, these are okay, they are just a process of their daily accumulation. After you analyze the product's draft, wall thickness, and undercuts in the ejection direction, you basically know the direction of the mold parting surface and the location of the gate. Of course, you still have to talk to the customer in the end. comfirmed.

2. Some people say, after I analyze the product structure, I can start designing the mold, the answer is of course NO. If you want to avoid detours when designing, some items that affect the mold structure must be confirmed. The specific content is as follows:

(1) The tonnage and model type of the injection molding machine used by the customer. If this confirmation is not good, you will not be able to confirm the entrance diameter of the sprue sleeve of your mold and the diameter of the positioning ring, the size and position of the ejector hole, and There are the depth that the injection molding machine can reach into the mold, even the size of the mold base, the closing height, and so on. You worked so hard to design a mold structure that uses a cylinder to pull the core. You also have a sense of accomplishment, but the mold cannot be produced at the customer's place, because the customer only has an electric injection molding machine and no additional neutrons are added. Sometimes you will feel like crying without tears.

(2) The coding mode of the customer's injection molding machine, generally used are platen coding, screw coding, hydraulic coding, magnetic coding and so on. After confirming this, you will know whether you need to design the pattern screw through hole or the pattern groove when you design the mold.

(3) The problem points of the product we analyzed just now, as well as the product clamping line, product material and shrinkage rate. Don't take it for granted that the shrinkage rate of PP plastic must be 1.5%. This must be confirmed with the customer. You must know the grade of the material they ultimately use for production, whether there are any modified materials added, and so on.

(4) When possible, it is best to be familiar with the assembly relationship of the product and the purpose of the product, etc. This information is very helpful for the future mold structure design. Because of this, you will know which is the appearance, which is the non-appearance, where the draft angle can be increased arbitrarily, and which cannot be changed. It even includes the structure of some products. If you understand the actual assembly relationship and purpose of the product, you will know which undercut structure can be cancelled or changed to another simple form. It must be kept in mind that the process of making a mold is a process of simplifying complex issues. I often see some people proud of what a complex structure they have made. I think that is very ignorant. Because many product engineers may design some unreasonable structures due to their own experience problems. If they are not able to correct them as a downstream process, they may always think that the design is no problem. Then the progress of our product engineers will be very slow.


3. The external parameters of the mold water circuit, the external parameters of the oil circuit, the external parameters of the circuit, and the external parameters of the gas circuit. Only after you understand the customer’s requirements before designing can you predictably design the water, oil, and gas circuits. Don’t work hard to design the mold. Later, you find that the customer needs to connect the oil circuit in the mold. The changes are estimated to be exhausting, because your waterways, jacks, screws, etc. are finally arranged in place. The design sequence of the four-way design is generally to ensure the oil circuit first, because the oil circuit needs to be distributed and balanced, especially the mold structure that requires the cylinder to be ejected. If the oil circuit is unbalanced, the ejection of the cylinder will have a sequence, which is easy to eject. Out of imbalance. Of course, a gear oil separator can also be used, but that is more complicated. The second is the waterway, because the waterway must ensure the cooling effect, and uneven distribution will affect the product quality and mold life. The last is the gas circuit and the electric circuit. The order of placement on the mold is, the circuit is the closest to the TOP direction, then the water circuit, the air circuit, and the oil circuit connector is the lowest.

4. Other unfinished matters. After mastering this information, we can proceed to design the mold. The first is parting. This process is probably a process that everyone prefers. Because there is a sense of accomplishment after being separated from the model. The principle of pulling the parting surface is simple is good, and the ones that can be stretched are determined not to scan, or use other advanced commands. Secondly, when pulling the parting surface, it is necessary to have a big picture, try to simplify the parting surface, don't do sloppy ones, if it is not a precision mold, those 0.1-0.5 insertion positions that can be avoided should be avoided. In addition, the parting surface should follow a principle, which is to follow the product trend as much as possible. The parting surface made like that will be beautiful. To add a digression, when learning 3D software, you must remember to understand the principle of each command, then you will know when you can use that command. In many cases, the important thing is not that you don't know how to use software, but that you don't learn to use software. The same command can be used by experienced people in many flexible ways. This is most obvious in UG. In a word, the important thing is the idea, not the tool itself. When making a parting surface, you only need to know that the angle of the penetrating surface can be larger, the area of the contact surface can be larger, and the pillow position can be wider. You must fully consider whether the parting surface you are currently pulling will be made out of the actual mold in the future, whether the master in charge of the flying mold in the workshop will scold you. If you feel that you will not be scolded, then OK and continue. When you are really scolded in the future, these are your experiences.

5. When making the parting surface, the arrangement of the slider and the inclined top must be considered, because those also involve the change of the parting surface. The typical slider structure is a trigonometric function relationship. It’s fine to calculate it by yourself, but ensure that the angle of the oblique guide post is not too large, and try to be less than 30 degrees. The principle of the selection of the inclined guide post is to be as thick as possible, don't be too stingy, because the inclined guide post is subject to force. In addition, the sliding block is also divided into many variant structures, for example, uphill sliding block, downhill sliding block, internal sliding block, hydraulic cylinder pumping, front mold sliding block, sliding block with sliding block, sliding block with reverse top, and sliding block with Inclined top, etc., these special structures make full use of the trigonometric function relationship, the purpose is to achieve the demolding of the product undercut and the normal opening and closing of the mold. The calculation formula of the slider has been introduced in detail in various forums, so I won't repeat it here.

6. The second is the inclined roof. The inclined roof is more flexible, but the angle of the typical inclined roof should not be too large. Try not to exceed 15 degrees. Of course, you have to do 20 degrees, but the life span is difficult to guarantee, and the action is also It will be very difficult. For specific principles, refer to trigonometric functions and theoretical mechanics. The shape of the inclined roof has many evolution forms, for example, uphill inclined roof, downhill inclined roof, crooked neck inclined roof, large inclined roof inlaid with inclined mandrel, large inclined roof inlaid with round rods, and inlaid hanging platform. Small inclined roof, inclined roof under the top block, inclined roof on the slider, inclined roof on the inclined roof, reverse roof on the inclined roof, etc., all these structures are for one purpose, using triangles The function makes the inversion of the product. This shows how important it is to learn trigonometric functions well! So all kinds of special structures are thought of by people. You can give full play to your imagination. No matter what kind of structure, you can think about it. Okay, just go for a bold design, maybe you will have a new invention, but after designing, you must verify the relationship of the lower trigonometric function, which is to actually simulate the opening and closing movements of the mold, and consider the injection molding Will there be a problem and so on.

7. We are divided into models. The next step is the ranking of the mold structure. These contents must be clarified in advance. According to the actual situation of the product, different types of mold structures are selected, for example, two-plate mold, three-plate mold, hot runner, IMD, IML, Two-color, stacked molds, etc. All mold structure types are used to achieve mass production of products. When selecting the mold base, we must fully consider the four points of attention just mentioned.

8. After selecting the mold base, we have to consider the inlay of the mold. The principle of inlay is to simplify processing, save materials, and facilitate product molding, such as exhaust and so on. Do you think that there is a particularly thin part on the mold, which must be inlaid separately for easy replacement in the future. When inserting and assembling, we must fully consider the strength of the insert, the workability, and the design of the waterway in the future. After the inlay is finished, standard parts must be added. The principle of adding standard parts is to focus on the parts first, and try to arrange them as balanced and symmetrically. Generally, the ejector rod must be added first, and the arrangement of the waterway must be considered when adding the ejector rod. Under normal circumstances, priority should be given to the arrangement of the ejector rods, and then the waterway should be generally designed, and then the waterway ejector rods should be adjusted according to the actual situation to balance the two. The principle of adding ejection is to place where the holding force is large. In addition, it should be added to the place where the strength structure of the product is relatively good, such as the edge of the product, so as to avoid whitening or uneven ejection. The principle of drainage is the principle of adding ejection, because generally speaking, the place where the product has a strong holding force is the hot spot of the product, and the part that needs to be cooled. This contradiction needs to be adjusted. As to whether to use the ejector rod or the ejector block or push plate, specific issues need to be analyzed in detail. This should be analyzed in combination with the structural characteristics of the product. For example, the product is easy to stick to the front mold. It is necessary to consider whether to add an undercut on the back mold or an ejector on the front mold. These are all natural things, but many friends do not want to be comprehensive. What is the reason? It is because the product structure has not been fully analyzed, and the state of the product in the injection molding process has not been considered. This is the crux of the problem. After the ejector waterway has been added, what is left is the messy standard parts. When adding it, try to consider the symmetrical balance as much as possible.

9. After the complete mold design is completed, the following inspections must be done. The first is the draft analysis of the insert to see if there are any undercuts, and the second is the interference inspection of the mold parts, which is the most important. Today's 3D software has this function, which is very convenient. Then there is the simulation of the mold opening and closing action. If the simple mold is not too clear, it must be simulated in reality. The other is the machinability of each part of the mold and the assembly process of the mold. Don't work hard to design a mold. Ingenious structure, theoretical calculation is no problem, after processing, the result cannot be installed, or it is not easy to assemble.

10. At this point, congratulations, the general process is over. In fact, mold design is full of contradictions. If the design is perfect, the cost of the mold will be higher. If the design is simpler, the product may need to be changed or the strength of the mold and the service life will be affected. Therefore, mold design is not absolute. As long as you find the balance point, the mold you design is successful. Therefore, the structure designed by others may not be suitable for you. As long as the above mold design points are followed, it is estimated that everyone can design a more reasonable mold structure. Due to time constraints, some parts of the words are not expressing the meaning, and my level is limited, and some parts may be a bit biased, I hope everyone can discuss it. If there are friends who are interested in the mold structure, they may also discuss with me. We will learn from each other and make progress together! Let’s talk about it in general today, and we will further expand when we have time.