Discussion on the calculation of injection mold

Discussion on the calculation of injection mold

Introduction: The following is the information about the calculation of injection mold that I have carefully prepared for you. I hope I can help you!

1. Introduction

The purpose of industrial design is to make people use products more conveniently and make products play a better role through reasonable planning. On the basis of studying the product performance, industrial design also makes the product have a product form full of the spirit of the times, in line with the product performance and in harmony with the environment through reasonable modeling means, so that people can enjoy beauty. Industrial design emphasizes the combination of technology and art, so it is the product of the integration of modern science and technology and modern culture and art, which not only studies the aesthetic problems of products, but also studies the practical performance and environmental effects caused by products, so as to make them coordinate and play a better role. The law of the jungle means natural selection, survival of the fittest, survival of the fittest, and the law of the jungle. Fierce market competition makes plastic products use industrial design, but also have to introduce jungle law. It is industrial design and jungle law that make the appearance of plastic products more and more complex, and the development of computer technology, especially computer-aided design and manufacturing, makes it possible to realize these complex design shapes.

The molding of plastic products, the vast majority can not be separated from the mold. In recent years, the development of computer-aided design and manufacturing has brought earth-shaking changes to the design of plastic products and mold manufacturing. The technology of mold manufacturing has changed from locksmith's manual to CNC machine tool processing, and the design of plastic products has also changed from manual drawing to computer drawing. The expression of product drawings has also changed from 2D drawings to 3D data, and the shape of products has also changed from regular shapes such as squares, triangles and circles to complex spatial surface modeling. These changes make the appearance of products more and more complicated, and also bring great challenges to mold design and manufacturing. Therefore, our mold design must adapt to this challenge and keep pace with the times.

For the calculation of injection mold, there are many formulas and materials in mold teaching materials, technical materials, papers and design manuals. In the past decades, these formulas have been widely used in the mold industry. Now with the use of computer-aided design and manufacturing, the limitations of these formulas are also highlighted, so some traditional formulas for mold design and calculation have lost their use value in practice. Continuing to use some formulas may bring troubles to freshmen majoring in mold design. The purpose of this paper is to discuss what needs to be calculated and what doesn't need to be calculated in mold design practice, and how to choose the calculation formula.

2. Calculation of cavity size of injection mold

2. 1 Misunderstanding of cavity size calculation

Plastic will be injected into the cavity after melting, and its volume will be reduced after cooling. Therefore, the shrinkage factor will be considered when designing the mold, and the size of the large cavity will be increased, so that the plastic products can get the size we need after cooling. In the formula, the calculation methods can be summarized into two categories: average size method and limit size method. In fact, the above formula can't be used in actual mold design at all, and can only be applied to simple products with simple shapes as shown in Figure 1. Because many plastic products in reality are irregular three-dimensional models composed of multiple complex surfaces, it is difficult to find the dimensions and application formulas in these irregular products. On the other hand, after years of development, great changes have taken place in the accuracy of mold processing machine tools and tool materials, and numerical control technology has been widely used. The machining accuracy of modern CNC machine tools, wire cutting and precision EDM machine tools has reached micron level. In European mold design materials, similar calculation formulas have been cancelled for more than 20 years, so we must keep pace with the times. The wear of mold cavity is not considered in modern mold design.

If the cavity is excessively worn and the product size exceeds the tolerance requirements, replace the mold core or redesign the manufacturing mold. The background of using these formulas in the past is that the precision of machine tools is not high or the machining is not in place, and some machining needs skilled fitters to repair. At this time, it is very necessary to adopt conservative calculation formula.

2.2 Calculation method of shrinkage of plastic parts

As mentioned above, it is difficult to adapt to the mold design work by using the traditional calculation method. In developed countries such as Europe, America and Japan, Hongkong and Taiwan Province Province all adopt the following simplified calculation methods. After years of practice, the following calculation methods can meet the actual needs.

When there is no dimensional tolerance requirement (free tolerance) for plastic parts:

The dimensional tolerance of molding is still 1/2~ 1/3 of that of plastic parts.

2.3 Calculation of cavity size of fiber-containing plastic parts

The shrinkage of plastic parts with fibers is inconsistent in all directions, and the values vary greatly. Usually, the shrinkage along the plastic flow direction is small, and the shrinkage perpendicular to the plastic flow direction is large. At this time, it is more difficult to use the formulas in table 1 and table 2. In the popular three-dimensional design software PRO/E and UG, different shrinkage rates can be set in X, Y, Z, Y and Z directions, which solves this problem well.

2.4. Adjustment of high-precision dimensions

For high-precision dimensions, firstly, the dimensional tolerance is converted into the tolerance with equal upper and lower deviation values, and then according to the above calculation method, 0.05 steel material is reserved for high-precision parts according to the method of later steel reduction, which is convenient for trimming after die test.

3. Calculation of thermal expansion

Molten resin flows into the gate, runner and fixed mold cavity, and the mold receives the heat of high-temperature resin, and the temperature is about 180 ~ 300℃. Generally, metal will expand when the temperature rises, so the parts of the injection mold will also expand. For the influence of thermal expansion on the performance of injection mold, many mold teaching materials and design manuals do not provide typical cases. Hot runner system mold, large mold, large slider multi-inclined top mold and high temperature forming mold have great influence on the above four kinds of molds. At high temperature, the normal mold movement gap will be reduced, so thermal expansion will lead to: affecting the matching of guide post and guide sleeve, affecting the matching of wiping position, the sliding of side core-pulling slider is not smooth, the inclined top and slider are easy to get stuck, and the core size expands.

For three-dimensional anisotropic materials, the difference between linear expansion coefficient and bulk expansion coefficient. For an object that can be approximately regarded as one-dimensional, the length is the decisive factor to measure its volume. The coefficient of thermal expansion at this time can be simply defined as the ratio of the length increase to the original length under unit temperature change, that is, the linear expansion coefficient. In mold design, the linear expansion coefficient is usually used to calculate the influence of thermal expansion.

4. Calculation of flow length ratio

The flow length ratio of plastic refers to the ratio of the length of plastic melt flow to the wall thickness. The ratio of flow length of plastic directly affects the number and distribution of pouring points of plastic products, and also affects the wall thickness of plastic.

We should keep in mind the flow length ratio of the plastic used, which is beneficial to our plastic mold quotation and mold design. The flow length ratio of LDPE is 270; The flow length ratio of HDPE is 230; The flow length ratio of PP is 250; The flow length ratio of PS is 210; The flow length ratio of ABS is190; The flow length ratio of PC is 90; The flow length ratio of PA is170; The flow length ratio of POM is150; The flow length ratio of PMMA is 150.

The flow length ratio of different plastics will be different, and the smaller the flow length ratio, the worse its fluidity. When designing small and medium-sized molds, it is generally not necessary to calculate the flow length ratio, but it cannot be ignored for large molds.

5. Calculation of cavity strength

Under the high pressure of plastic melt in the molding process, the cavity should have sufficient strength and stiffness. Practice shows that insufficient stiffness is the main contradiction of large-scale molds, especially deep cavity molds, and the cavity size should be based on meeting the stiffness conditions. For small and medium-sized molds, especially cavities with small depth, insufficient strength is the main contradiction, but the strength calculation of small cavities is generally omitted, and mold designers often use existing experience to design, so there will be no problems in practice. When the strength is insufficient, the mold cavity will rupture. Therefore, the condition of strength calculation is to meet the allowable stress in the stressed state. However, the lack of rigidity will lead to the expansion of the cavity size under stress, and as a result, the product will have burrs, the size is out of tolerance, and even it is difficult to demould. Please refer to the relevant mold design manual for cavity depth calculation of large cavity injection mold.

6. Calculation of clamping force

For small and medium-sized molds, the clamping force is generally estimated by experience. The clamping force of a large mold should be calculated according to the pressure in the cavity multiplied by the horizontal projection area, and the calculated clamping force should be far less than the rated clamping force of the injection molding machine. The same is true for the calculation of injection quantity.

7. Calculation of inclined roof and exhaust stroke

For simple dies, the stroke of inclined top can generally be calculated by tangent trigonometric function. For complex dies, after the stroke of inclined top is calculated by trigonometric function, it needs to be simulated and verified in 3D software. Many textbooks have listed formulas and examples for calculating line travel. In actual operation, the line drawing of a simple die may be clear at a glance, without calculation, such as the first example in Table 3.

Generally, a special case of core-pulling is listed, that is, semi-core-pulling with simple shape. In reality, it is difficult to calculate the core-pulling distance of many complex rows, but it can be easily determined by 3D software moving simulation. Therefore, excessive use of calculation formulas may mislead beginners and look up information everywhere to find calculation formulas.

8. Calculation of thread mould

For the calculation of demoulding die, it is necessary to calculate the transmission ratio and the number of revolutions of thread core. Please refer to the relevant manual.

9. Calculation of core-pulling force, demoulding force and jacking force

The calculation of core-pulling force, demoulding force and jacking force is quite complicated, and the existing formulas are also difficult to calculate accurately. We can only predict the demoulding force through the accumulation of experience and take corresponding countermeasures.

10, calculation of cooling system

Calculation of cooling system. There are many calculation formulas now, but few calculations are applied in reality. The manufacturing cycle of ordinary simple mold is very short, and the time of mold design is shorter, so the calculation of cooling system is mostly omitted and solved by empirical analogy. Although there is no need to calculate, Reynolds number calculation and laminar turbulence theory will provide thinking ideas for mold designers.

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The calculation of injection mold is essentially different between large mold and small mold. Small and medium-sized molds can be easily solved through experience, while large molds will cause huge losses once they are designed incorrectly. However, the calculation of mold design must be based on practice. Mold is a technology that practice is far greater than theory, and mold technology comes from practice.

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