Brief introduction of 2D simulation thermoforming

2022-06-23
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Introduction to two-dimensional simulation thermoforming analysis

the outer surface of high-quality automotive interior parts usually has a layer of polyurethane semi-rigid surface or film attached to the injection molding substrate (carrier). This provides designers with a degree of design freedom that other machining methods do not have. The designer can design almost conceivable complex geometric shapes without designing the flow lines visible in the notch. The outer surface layer or film of the part can cover the micro cracks of the clamping line formed on the part surface during foaming molding, so the part surface will not leave flow lines

the surface layer or film on the part surface can be formed in a variety of ways: the surface layer sprayed, coated or cast is made by the so-called primary forming method, that is, the liquid or molten material is put into an integral mold. The mold surface is the desired rough (textured) surface made of nickel. At the same time, thermoformed films have been less and less used in the past decade, because stretching the film at some positions may cause the particles on the rough surface in some areas to become smaller or the texture to deform. Recently, however, this processing method has begun to attract people's attention. Thanks to the emergence of a new generation of TPO film (tpo2), the stretching of the film can be reduced and the quality of the thermoformed film can be improved

simulation replaces prototype

in various fields, engineers are facing pressure to shorten the development cycle, including the development of automotive interior parts. Due to the significant improvement of simulation technology, the number of physical prototypes required for product development has been reduced. For example, simulation software for repeated three-dimensional thermoforming based on thin film material characteristics and all boundary conditions has recently been launched. However, the software needs specially trained technicians to complete the calculation, and it takes a certain amount of time to prepare and carry out the subsequent calculation. For example, the three-dimensional geometry of the whole part, the viscoelastic properties of the film and the required machining parameters must be known. However, such data is usually unknown in the early stage of product development. At this stage, the processing methods of products are obtained through assumptions

when developing parts and testing the placement of part geometry in the hot forming tool, a simple two-dimensional calculation program helps developers quickly obtain the approximate value of part stretch

■ in order to make the calculation as simple as possible, the model described in this paper is based on the following assumptions:

■ all geometric aspects of the calculation are carried out through one or several sections of the hot forming tool

the contact point between the film and the thermoforming tool is fixed relative to the thermoforming tool during the thermoforming process

■ the film is solid, that is, it is incompressible

■ the movement of the film is described by splines related to three points (starting point, middle point and end)

■ if the spline length changes, the film thickness also changes accordingly

■ as 3 Is there a problem with the software? If the spline contacts the thermoforming tool, it will split at the contact point. At the contact point, the spline is tangent to the surface of the thermoforming tool

■ the middle point of the spline moves towards the surface of the thermoforming tool until it contacts the thermoforming tool

at the beginning of the calculation, the film must be "blown" to the extent that it is not in contact with the thermoforming tool. Figure 1 shows the starting point of the simulation. The middle point moves downward in the direction indicated by the arrow until it comes into contact with the thermoforming tool for the first time. Divide the spline into two halves and repeat for each half (Figure 2). This interaction will continue until all splines are in contact with the hot formed surface. For each spline, the program automatically saves the thickness value D of each segment, which is determined by elongation

l front: segment length before thermoforming

l rear: segment length after thermoforming

assuming that the density of the film is constant, that is, the film is solid, and the cross-sectional area of the whole film remains constant throughout the interaction process. As a verification, it can be proved by the following formula at the end of the simulation:

in the scientific and technological literature, elongation is used as a measure of material stretching:

in the practical application of the automotive industry, elongation is often used to evaluate the stretching of materials. The definition of elongation is:

for example, if the length of a segment is increased from 10mm to 11mm, the corresponding elongation is 10%, and the thickness is also reduced from 100% to 90.9%

in this way, the elongation can also be calculated by the following formula through thickness change:

on a typical CAD workstation, the simulation takes about 10 to 15min. This can be run directly within the CAD system through additional applications. Figure 3 shows the simulation results

if the film thickness is μ The results of Figure 4 can be obtained by plotting. For comparison, we also list the measured results of thermoformed tpo2 film samples. The initial thickness of the film is 1.2mm, which is a strict geometric simulation. Neither the characteristic parameters of the material nor the process parameters are considered, so the simulation results are in good agreement with the actual values

only a few mouse movements are required for evaluation

in relatively thin positions, such as 1 and 2, the calculated results are very close to the measured values. Therefore, this result is very applicable to the feasibility study of hot formed product orientation and the study of optimizing product performance. On the one hand, the deviation comes from the fact that the stretching of the cross section direction relative to the study section is not considered. For planar instruments, the elongation of this type is generally small; On the other hand, the program used in the calculation does not consider any kind of plunger auxiliary part. The auxiliary part is arranged along a component that moves in the inflatable film and moves the film between different areas. Only in this way can we say that the forming process of thermoforming tools has been successful

if a very high thermoforming tool is used to process the film, the film must be highly inflated before the simulation, which leads to a high elongation of the film, even in the area that is in quick contact with the thermoforming tool. In fact, the main reason for the deviation of the results is that the film cannot be blown to this extent before the forming tool rises to the hot forming position. This means that the thermoforming tool contacts the film as it rises and stretches the film as it rises further. Therefore, the extent of stretching in the area where the thermoforming tool first contacts the film is smaller, while the other areas are gradually stretched. The program can also simulate this process. Fig. 5 shows the simulation results of highly inflated films, and Fig. 6 shows the simulation results of slightly lower inflated films at the same position. There is a great difference between the two figures at the red mark

in actual processing, it is common to further reduce the tensile degree of the visible area by using the plunger to assist the pressure, provided that higher tensile degree of the invisible area is accepted according to the size of the steel strand. The new generation of TPO film can maintain high tensile strength without tearing

the simplified two-dimensional simulation introduced here cannot replace the three-dimensional simulation of hot forming, but as an auxiliary tool, it can show the early development trend in the expected stretching process. With a few mouse clicks, the designer can analyze the key areas of the film in the CAD system within 10 to another 15 minutes, and verify the effectiveness of the possible correct measurement methods. Through this method, the physical test can focus on specific parts, and the operation is faster. (end)

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