Isostatic graphite : the preferred material for 3D hot bending dies

With the development of the times, carbon materials are required to have more uniform fine particle structure and more stable performance. In this context, the pioneers of artificial graphite industry developed isotropic graphite. The graphite material is formed by cold isostatic pressing (CIP), then sintered and graphitized at high temperature. It has fine particles and isotropic structure and properties. Nowadays, isotropic graphite has been widely used in many industries, such as rapid quenching heat treatment industry, rapid development of semiconductor industry, environmentally friendly new energy industry, high precision mold industry, high reliability of the atomic energy industry. In the past two years, a new application of high-end isostatic pressing graphite has been added, which is used to make the mould of hot-bending 3D curved glass.
In 2016, 3D curved glass entered a rapid development stage, curved screen more in line with consumer habits, curved screen appearance and feel more soft, and can add more edge virtual shortcut buttons and reminders. At present, curved surface display is also used in wearing equipment. As a fashion, curved surface display is expected to be more widely used in the future.
Curved glass refers to the whole piece of glass is not in a plane, usually arc, J-shaped, V-shaped, hyperboloid curved, S-shaped, etc. At present, the shape of the curved glass used in mobile phones is relatively small. There are 2 main categories: the curved arc and the single arc. Glass composition is determined according to its performance requirements, usually there are three types: the first is alkaline aluminosilicate; the second is borosilicate; the third is soda-lime glass.
advantages of graphite mold
The materials used to make hot-bending glass molds in mainstream molding equipment now need good thermal stability, wear resistance, corrosion resistance, self-lubrication, thermal conductivity, and thermal expansion coefficient similar to that of hot-bending glass. Good thermal stability ensures that the die does not produce significant deformation during repeated high and low temperature applications. Wear resistance and corrosion resistance are beneficial to reduce the surface shedding, pits and other defects. Self lubrication makes the glass not adhere to the mold during hot bending and demoulding. Good thermal conductivity is conducive to the rapid and uniform heating of glass, thereby shortening the hot bending process time, improving production efficiency, uniform heating is conducive to avoid the occurrence of glass hot bending defects.
In addition, it is better to have good processability, economy and sustainable supply of the material, in order to obtain better cost performance, and to ensure that the mass production can be timely supply.
From the research and development stage of hot bending equipment up to now, high-end isostatic pressing graphite is considered to be the most suitable material for hot bending die. Compared with stainless steel, ceramics, tungsten steel and other materials, graphite has the best performance evaluation.
Isostatic pressure graphite, generally graphitized at about 3000℃, its thermal stability has long been verified in the application of high temperature vacuum furnace, such as heat treatment in the gas quenching furnace and other widely used, sometimes cooling rate reached 60 ~ 70℃/ s, in repeated use without obvious deformation, cracking, etc.
High end graphite has a hardness of over 75 and has good wear resistance. Graphite does not react with most acids and bases. It has excellent corrosion resistance and self lubrication.
Graphite has good thermal conductivity, and the coefficient of linear expansion is close to that of glass (the average coefficient of linear expansion of glass used for this purpose is (6.5-7.5)*10-6/K).
Graphite mold type
The common graphite mould for making 3D hot bending glass has two modules, three modules, multi-mode, multi-mode with glass beads. Generally, the single curved 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 with glass beads.
The graphite mold designed by the two modules is mainly divided into two parts: upper mold and lower mold. In general, step design should be added at the edge of the upper mold and lower mold to locate the graphite mold. The design and fabrication are simple and the graphite material is saved, but the molding chamber is relatively open, which has adverse effects on die life, external particle control and accurate positioning of the molding chamber. It is suitable for hot bending of simple curved curved glass.
The three mold design graphite mold mainly divides into the upper mold, the middle frame and the lower mold 3 parts, generally needs three steps to design the step or the pin localization. Compared with the two moulds, the graphite used for design is slightly more, the processing time is slightly longer, it is not easy to touch the sheets (glass sheets), slides, not easy to damage the mold surface, mold life, mold chamber external particle control, accurate positioning. It is generally used for hot bending of single arc curved glass.
The graphite mold with multi-die design is more complicated, and the main parts generally include lower mold, middle frame, upper mold, positioning pin, special forming and pressing sheet, special auxiliary positioning 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 curved curved glass, which generally improves the thermal bending defects which are difficult to solve for simple dies.
The graphite mold with glass beads is very complicated. The main parts include lower mold, middle frame, upper mold, positioning pin, glass beads, special auxiliary positioning blocks and so on. The biggest difference from the multi-mode design is that the place of glass beads is added in the lower mold position, and a proper number of glass beads (4-12 beads with a diameter of about 9 mm) should be placed before each hot bending, and the deformed glass beads should be removed after hot bending. Generally speaking, hot bending costs more than multi-mode design, mainly corresponding to double curved and other complex curved glass hot bending.
Design method
Back shallow groove design
A proper number of shallow grooves on the back of the graphite mold (non-forming surface) can effectively reduce the overall area of low pressure and avoid adsorption. At present, hot bending equipment on the market mostly uses heat-resistant alloy plate to contact the back of graphite mold, heating, pressurizing and cooling the graphite mold. In order to control the heating, pressure uniformity and heating speed as well as possible, the flatness of the contact surface between the alloy plate and the graphite mold should be controlled accurately, generally less than 0.05 mm. For the same reason, the back of graphite mould should be controlled by the same way. In general, the contact surface between the die and the alloy plate is very good in the process. After the first few preheating, heating and pressure treatment, it needs to be annealed and cooled to normal temperature. At a faster cooling rate, two large surfaces are easy to form more local low-pressure surface and produce adsorption effect, which affects the normal process flow of the die.
Positioning gap design
The expansion of graphite mold should be calculated according to the expansion coefficient of glass material, graphite material and hot bending temperature, and the suitable gap should be left in advance. First, the excessive thermal stress and deformation caused by high temperature expansion should be avoided; second, the CNC flat glass should be avoided from sliding too much in the mold and forming during hot pressing. Deviation and scratch.
Determination of CNC dimensions of flat glass
The thickness of the common hot-bending glass is uniform at all places. Generally, the pressure can be tested according to the extended size of curved glass products, and the graphite mold can be fine-tuned according to the size distribution of hot-bending products and the experience of hot-bending to obtain the best CNC size.
Setting of exhaust port
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 is pitting. There are three reasons for the pitting. First, the graphite mold material does not meet the demand of hot-bending, the granularity (generally 4 micron and below) is too large, and it has visible pores by obvious naked eye, glass softening is easy to cause large particles and pores in the concave and convex points. second, graphite due to raw materials, processing, use environment and other reasons, powder particles mixed in the molding surface caused concave and convex points of glass. third, hot bending glass and graphite at high temperature gas escape, discharged not smoothly, more obvious bump points in the location of escape of large amounts. Three reasons need to be carefully controlled in the process of graphite material selection, processing, storage, use and so on. The third point can be improved in the mold design, that is to add a specific vent hole or vent slot design. The design of the exhaust slot has two principles: to ensure the smooth flow of the molding chamber and the external gas, and to minimize the entry of large foreign particles into the molding chamber.
The industry predicts that graphite in mobile phones, automobiles, wearing equipment 3D curved glass applications in 2017 will enter an explosive period, at present, the industry chain is actively R & D and upgrading, graphite mold suppliers in the field of continuous exploration and innovation.
Author: Xiong Yun-hu, Huang Jian