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How to control the size of injection molds and injection products

2021-08-26
How to control the size of injection molds and injection products
With the rapid development of the mold industry in recent years, the application scope of new technologies and new processes continues to expand and progress, and there have been qualitative changes from the accumulation of traditional experience to the application of software development. The wide range of applications of CAD, CAM and CAE has opened up room for improvement in the geometric size control technology of our molds and mold products. Due to the large differences in market demand for molds and mold products, there are many types, in terms of shape, size, material, structure, etc. The large changes and high requirements in various aspects have caused us to encounter many problems and difficulties in the production of molds and products. Among them, how to effectively control the geometric dimensions of molds and products is very intuitively placed in front of us. There are different control technologies and methods for different types of molds and products. Today I am here to talk about the control of injection molding product forming dimensions. When it comes to injection molding products, I will definitely talk about injection molds. In general, I will start from the following aspects.


1. Control of mold design
1. First, we must fully understand the mold structure, material, hardness, precision and other aspects of the user's technical requirements, including whether the shrinkage rate of the formed plastic material is correct, whether the product 3D size modeling is complete, and reasonable processing analysis.
2. Full consideration should be given to all areas that affect the appearance of injection products, such as shrinkage holes, flow marks, draft angles, weld lines, and cracks.
3. Simplify the processing method of the mold as much as possible without hindering the product function and pattern shaping of the injection molded part.
4. Whether the selection of the parting surface is appropriate, the mold processing, the appearance of the forming and the deburring of the formed parts must be carefully selected.
5. Whether the ejection method is appropriate, whether the push rod, discharge plate, ejection sleeve, etc. are used or other methods, and whether the position of the push rod and the discharge plate is appropriate.
6. Whether the side core pulling mechanism is suitable, flexible and reliable in action, and there should be no jamming.
7. Which method of temperature control is more suitable for plastic products, what kind of structure circulation system is used for temperature control oil, temperature control water, coolant, etc., and whether the size, number, and location of the coolant holes are appropriate.
8. The gate type, the size of the material channel and the feed port, and whether the gate position and size are appropriate.
9. The influence of heat treatment deformation of various modules and mold cores and whether the selection of standard parts is appropriate.
10. Whether the injection volume of the injection molding machine, the injection pressure and the clamping force are sufficient, whether the nozzle R, the aperture of the sprue sleeve, etc. match properly.


2. Control of process manufacturing
Although comprehensive considerations and arrangements have been made in the design stage, there will still be many problems and difficulties in actual production. We must meet the original intention of the design as much as possible in the production, and find out that the actual processing is more effective and economical. The craftsmanship means.
1. Choose economically suitable machine tools and make 2D and 3D processing schemes.
2. Appropriate tooling and fixtures can also be considered for auxiliary preparations in production. The reasonable use of tools can prevent product deformations, prevent product shrinkage fluctuations, prevent product demolding deformation, improve mold manufacturing accuracy, and reduce Small errors, prevent changes in mold accuracy, etc., a series of production process requirements and solutions.
3. Here are the reasons for the size error of the formed parts of the British Plastics Association (BPF) and the distribution of the proportions:
A: The mold manufacturing error is about 1/3;
B: Error 1/6 caused by mold wear;
C: The error caused by the unbalanced shrinkage rate of the molded parts is about 1/3;
D: The error caused by the inconsistency between the predetermined shrinkage rate and the actual shrinkage rate is about 1/6.
The total error=A+B+C+D, so it can be seen that the mold manufacturing tolerance should be less than 1/3 of the dimensional tolerance of the formed part, otherwise it is difficult for the mold to guarantee the geometric size of the formed part.


3. General production control
The fluctuation of the geometric dimensions of plastic parts after forming is a common problem, and it is a common phenomenon:
1. Control of material temperature and mold temperature
Different grades of plastics must have different temperature requirements. Plastic materials with poor fluidity and the use of two or more mixtures will have different situations. Plastic materials should be controlled within the optimal flow value range. These are usually easy to achieve. However, the control of the mold temperature is more complicated. Different shapes, sizes, and wall thickness ratios of the formed parts have certain requirements on the cooling system. The mold temperature controls the cooling time to a large extent, so try to keep the mold as much as possible. In the allowable low temperature state, in order to shorten the injection cycle and improve the production efficiency, the mold temperature changes, so the shrinkage rate will also change, the mold temperature remains stable, and the dimensional accuracy is also stable, thereby preventing the deformation of the molded part and poor gloss. , Cooling spots and other defects, make the physical properties of the plastic in the best state, of course, there is still a debugging process, especially the multi-cavity mold forming part is more complicated.
2. Adjustment and control of pressure and exhaust
The injection pressure is appropriate and the matching of the clamping force should be determined when debugging the mold. The air in the gap formed by the mold cavity and core and the gas generated by the plastic must be discharged from the exhaust groove outside the mold, such as exhaust Insufficient air will cause insufficient filling, resulting in weld marks or burns. These three forming defects sometimes appear in the same part occasionally. When there are thick walls around the thin wall of the molded part, the mold temperature will appear when the mold temperature is too low. Insufficient filling and too high mold temperature will cause burns. Usually, weld marks will appear at the burned parts at the same time. The exhaust grooves are often ignored and are generally in a small state. Therefore, as long as there is no burr, the discharge The depth of the air shoulder should be as deep as possible. There is a larger vent groove at the back of the shoulder so that the gas after the shoulder can be quickly discharged out of the mold. If there is a special need, it is reasonable to open an exhaust groove on the ejector pin. It's the same. One is that there is no waste, and the other is that the vent is quick and the effect is good.
3. Supplementary shaping control of the size of injection molded parts
For some plastic parts due to different shapes and sizes, after demolding, with temperature and loss of pressure, different deformations and warpages will occur. At this time, some auxiliary tooling and fixtures can be used to adjust, and the molded parts can be quickly and quickly after being ejected from the mold. Remedial measures are taken, and a better correction and adjustment effect can be obtained after it is naturally cooled and shaped. If strict management is ensured in the entire injection molding process, the size of injection molded parts will be ideally controlled