HOW TO DESIGN GRAPHITE MOLD FOR 3D HOT BENDING GLASS?

The common graphite Mould for making 3D hot bending glass has two modules, three modules, multi-mode, multi-mode glass beads. Generally, the single curved glass is mostly designed with two or three modules. The design of multi arc or more complex curved glass adopts multi-mode or multi-mode glass beads.

Figure 1 (a) is an example of two Mold design. The graphite Mold is mainly divided into two parts: upper Mold and lower Mold. Generally, step design should be added to the edge of upper Mold and lower Mold for positioning, and positioning pins should be added to individual design schemes. This kind of design is simple in processing and loading, saving graphite Materials, but the forming chamber is relatively open, which has adverse effects on the Mold life, the control of foreign particles in the forming chamber, and the accurate positioning. It is suitable for the thermal bending of simple single arc curved glass.

Figure 1 (b) is an example of three die design. The graphite Mold is mainly divided into 3 parts: upper die, middle frame and lower die. Generally, it is necessary to design step or pin location among the three. Compared with the design of the two Mold, the three module design uses more graphite Materials, has a longer working time, and is not easy to touch pieces (glass pieces) and slide plates, and it is not easy to damage the Molding surface. The Mold life, the control of the external particles in the Molding chamber and the accurate positioning are better. It is generally used for the hot bending of single arc curved glass.

Figure 1 (c) is an example of multimode design. The graphite Mold is relatively complex. The main components generally include lower Mold, middle frame, upper Mold, locating pin, special Molding pressing plate, special auxiliary locating block, etc. Compared with the two and three die designs, graphite is used more frequently, the processing time is longer, and the assembly and disassembly of the Mould is more complicated. It mainly corresponds to the thermal bending of some complicated or high demand single curved or double arc curved glass, which generally improves the thermal bending defects which are difficult to solve for simple dies.

Figure 1 (d) is an example of multi-mode glass bead design. The graphite Mold is relatively complex. The main components generally include lower Mold, middle frame, upper Mold, positioning pin, glass bead, special auxiliary positioning block, etc. The biggest difference with multimode design is that the placement hole position of glass beads is added at the lower Mold position. A proper number of glass beads of appropriate size should be placed before each hot bending (generally 4-12 glass beads with a diameter of about 9mm should be placed in the mobile phone glass hot bending), and the deformed glass beads should be taken out after the hot bending. Generally speaking, the cost of thermal bending is higher than that of multimode design, which mainly corresponds to the thermal bending of double arc and other complex curved glass.

Design method:
1. Back shallow groove design


A proper number of shallow grooves on the back of graphite Mold (non forming surface) can effectively reduce the overall low pressure area and avoid adsorption. At present, the hot bending equipment on the market mostly uses heat-resistant alloy plate (as shown in Figure 1 and Figure 4) to contact the back of graphite Mold (as shown in Figure 2 and figure 3), to heat, pressurize and cool the graphite Mold. In order to control the heating, pressure uniformity and heating speed as much as possible, the contact surface between the alloy plate and the graphite Mold needs to control the flatness accurately, generally under 0.05mm. For the same reason, the back of the graphite Mold also needs to do the same control. In general, the contact surface between the die and the alloy plate fits very well in the process. After the previous preheating, heating and pressurization treatment, it needs to be annealed and cooled to normal temperature. Under the rapid cooling speed, the two large surfaces are easy to form more local low-pressure surfaces and produce adsorption effect, which affects the normal process flow of the die.

2. Positioning clearance design

When designing the positioning dimension for graphite Mold, the expansion amount shall be calculated according to the expansion coefficient and hot bending temperature of glass material and graphite material, and appropriate clearance shall be reserved in advance. First, excessive thermal stress and deformation caused by high temperature expansion shall be avoided; second, too large sliding of CNC plate glass in the Mold shall be avoided, which may cause forming deflection and scratch during the hot pressing process.

Calculation formula of expansion difference:
ΔL=（αl1-αl2）×L×（T-T0）（1）
In formula (1), Δ l expansion difference; α L1 is the average linear expansion coefficient of glass from normal temperature to hot bending temperature; α L2 is the average linear expansion coefficient of graphite from normal temperature to hot bending temperature; L is the normal temperature value of the size to be calculated; t is the temperature of hot bending process; t0 is the initial temperature.

When the shape is complex, the expansion mode is more complex. It is suggested to use software to do finite element analysis to understand the expansion situation of each position directly.

3. Determination of CNC size of flat glass

The thickness of common glass to be hot bent is uniform. During the design, the pressure test can be carried out according to the extension size of curved glass products, and the graphite Mold can be adjusted according to the size distribution of multiple hot bends and hot bending experience to obtain the best CNC size.

4. Setting of air vent

At present, the yield of 3D hot bending glass has always been a headache for most manufacturers. One of the important factors affecting the yield of 3D hot bending glass is the pockmarks. The causes of pockmarks are now generally recognized as three:

First, the graphite Mold material does not meet the requirements of hot bending, the particle size (generally 4 μ m and below) is too large, and there are obvious pores visible to the naked eye. After glass softening, it is easy to cause concave and convex points at large particles and pores;

The second is that the powder of graphite is dropped severely due to the raw material, processing, use environment and other reasons, and the powder particles are mixed in the Molding surface, resulting in concave and convex points of glass;

Third, when hot bending, there is gas escaping from glass and graphite at high temperature, and the discharging is not smooth, resulting in obvious concave and convex points at the position with large amount of escaping.

Three reasons need to be carefully controlled in the process of graphite material selection, processing, storage, use, etc. the third point can be improved in the Mold design, that is to add a specific exhaust hole or exhaust slot design. There are two principles in the design of the exhaust slot: to ensure the smooth flow of the forming chamber and the external gas, and to minimize the entry of large foreign particles into the forming chamber.