Defects and eliminating methods of Ultra-thick Rubber products
In the design of the mould, not only the size of the cavity is calculated according to the shrinkage ratio of the rubber material, but also the certain structural elements are given to design the mould, and the technological process and the flow mechanism of the vulcanization of the rubber molding are also fully understood. Because some defects of rubber products appear to be technological problems on the surface, not die problems, but in fact the process is more difficult to solve, but can be considered in the mold design to solve the problem. The following is a solution to the defects of super-thick rubber products.
Second, the defects and causes of the products
1. The location of defects in two typical thick Rubber products
Figure 1 is a typical thick rubber product, and the cross section of the die is usually as shown in figure 2. The surface of such products will often appear wrinkle and cracks, serious tongue layer. Fig. 3 shows the defect of the product at place A, that is, in the molding process, a wrinkle or tongue layer appears on the parting surface with relative motion, that is, the upper template 1 corresponds to the parting surface of the middle template 2 or the lower template 3.
2. Defect cause analysis
Based on the characteristics of the mould, the blank is filled directly into the cavity, and the heat is transferred to the inner layer at a certain rate of heat transfer. In this way, no matter how fast the transfer rate is and how long the vulcanization time is, the outer layer is the first to reach the positive curing point. Because of the large thermal expansion rate of the compound, it is customary to call it negative pressure, that is, the oil pump stops working after the vulcanization pressure indicator is expressed to a certain number; After the vulcanization process, the number of the oil pump indicator is several hundred Ns higher than the original figure, which is caused by the reaction force (negative pressure) on the template caused by the thermal expansion of the rubber compound. All heavy moulded products, regardless of vulcanization pressure This phenomenon will occur as much as possible. The change of the reaction force (negative pressure) in the mold is shown in figure 4. As a result of the negative pressure, there is a slot in A where the negative pressure is proportional. The positive pressure on the original rubber edge is close to zero, and the friction force on the rubber edge is also greatly weakened, approaching to zero. In this way, the adhesive edge benefits out of the cavity under the action of the expansion force of the cavity, resulting in the serious local pressure relief at the part A of the mould cavity. Because of the weakening or disappearing of the external pressure, the surface of the product which had already reached the positive curing point in the cavity in the part A is broken by the internal pressure and spills out with the rubber edge. After the product is taken out, the internal pressure disappears and after cooling, it overflows during vulcanization. Partially retract to the surface of the product. This is the reason for crease cracks or tongue clamps in part A.
Third, consider the method of solving the problem in die design.
Reaction force (negative pressure) is inevitable in die, the negative pressure is caused by rubber thermal expansion. In order to eliminate this negative pressure, the curing time can be extended to ensure that the inner and outer layers can reach the positive curing point together in the shortest possible time. In this way, in the process of mold pressing and heating, the rubber material will overflow gradually with the increase of internal pressure at the parting surface of relative movement of die template. The oil pump meter basically does not reflect the negative pressure. However, after the product is out of the mold, the product loses its original geometry because of the cooling shrinkage. In the section shape of the product shown in figure 5, the fine solid line is the standard shape outline of the product. The thick solid line is the shape of the product section after contraction. If the product by increasing the vulcanization pressure to offset this negative pressure is not. In a word, due to the negative pressure, the volume of the cavity is forced to expand because of the thermal expansion of the rubber.
The problem to be solved is to ensure that there is no local pressure relief in the mold cavity, although there is a gap between the templates. Through the experimental analysis and comparison, it is effective to add a resistance cavity near the cavity where the mould may have a gap. The distance from the resistance cavity to the cavity is less than 1 mm, preferably around 0.5mm, with a right triangle with a shape of 45 degrees. The length of the right side is 4-6mm. As shown in figure 6, region B is the cavity area. The resistance chamber has the function of residual glue groove or running glue groove, but it can not be equated with them, it has the function of preventing the pressure from unloading out. Due to resistance cavity Close to the cavity, after the cavity is full, then the resistance cavity is filled with the rubber with a certain internal pressure; When the negative pressure in the cavity and the gap between the mold templates appear, the internal pressure of the resistance cavity rubber also weakens and the volume increases slightly, which basically makes up for the gap between the mould templates. The most important is that a right side of the resistance cavity is perpendicular to the parting surface, so there will not be an overall outward overflow. In this way, the pressure unloading will not occur in the A part of the mold cavity, thus avoiding the occurrence of defects.
Four, several noteworthy points
(1) the distance from the resistance cavity to the cavity should be as small as possible. Otherwise, the thin flange from the resistance cavity to the cavity section will be wrinkled and the cavity will have pressure relief space, so the resistance cavity can not prevent the internal pressure from unloading or the effect is poor.
(2) the residual rubber strips in the resistance cavity should be cleaned every time. Otherwise, the flying edge of the next overflow is covered on the resistance tape, which is prone to delamination, and the resistance rubber strip can not prevent it.
(3) in order to avoid the opening of resistance cavity, the die of thick product should not be completely closed. The cavity is prone to deform because of negative pressure. If the wall thickness is increased, enough strength can be guaranteed. The punch will be tightly clamped by a strong pressure glue edge and the product cannot be removed from the die. This mold can be scrapped only a few times. Especially for multi-cavity die, because the weight of preformed rubber billet is different, and the heating rate of rubber material in each cavity is not the same, the heating rate of rubber material in each cavity is different. Each cavity volume expansion has a sequence, easy to cause die cover plate deformation clamping.
To sum up, good products need not only good moulds, but also good rubber machinery. The rubber machinery produced by Qingdao Running Machinery Co., Ltd can meet the needs of various products.