Design of two-material integrated rotating mould for automobile seals
Time：2021-06-21 11:38:13 / Popularity： / Source：
1 Introduction of PP+TPE automotive seals
Sealing of large auto parts on cars is usually based on installation of hard parts and then adding a sealing medium to achieve sealing effect, but often in actual production process, seal is unstable and fails. Two-component integrated injection mold is to inject two different materials in two stages in same mold. Through joint action of mold structure and injection molding equipment, plastic part integrates special effects of two materials. Through application of double-material integrated (such as PP+TPE) seals, not only production efficiency is improved, but also hidden quality problems caused by multiple installations are solved, and sealing effect of whole vehicle is improved [1~3].As shown in Figure 1, PP (polypropylene) + TPE (thermoplastic elastomer) two-material integrated plastic part has assembly rigidity of hard rubber and sealing performance of soft rubber, which can meet various needs of practical applications, but design scheme and process of plastic part molding need to be modified. According to characteristics of plastic part structure, hard rubber is required to achieve secondary injection molding without leaving mold cavity. This requires mold to adopt a rotary two-shot molding solution, which also requires mold to have high-precision two-stage clamping positioning accuracy, low-pressure injection soft rubber process guarantee, and effective control of multi-cavity single-side ejection balance [4~6].
2 Design of PP+PTE double-material integrated rotating injection mould
Key to successful research and development of two-component integrated precision mold lies in effective application of technologies in several major fields, such as repeated accurate positioning of the second mold clamping, CAE analysis, ejection balance of long and narrow plastic parts, and material selection.(1) Precise positioning of second mold clamping.
This project uses hierarchical mold opening and closing guidance, resetting technical solutions and scientific processing technology routes, supplemented by an independent large slide core-pulling guide mechanism on fixed mold side, MAKINO high-precision processing technology to stabilize processing technology of each part of second mold clamping. Matching relationship between parts meets requirements of producing double-material plastic parts, overcomes negative influence caused by cumulative error of movement of multiple moving parts in second mold clamping. For example, matching gap of one part corresponding to multiple parts needs to meet technological requirements of two-material injection molding, various related links are also relatively mature, with a one-time success rate of more than 80%.
(2) Controllable injection molding system.
This project uses MoldFlow to analyze injection simulation, develops a good dual-system sequential control pouring plan, which greatly reduces various risks caused by mold debugging. For example, related components are damaged due to excessive load caused by excessive pressure, surface phenomena during debugging mislead formulation of improvement plans. Through this analysis, debugging zone of injection process has been greatly widened, making process easier to adjust, and achieving expected goal.
(3) Ejection balance of long and narrow plastic parts.
In order to prevent plastic parts from being distorted and deformed due to unbalanced forces during ejection, elastic deformation characteristics of plastic parts are used, bottom of stiffener is selected as point of action of ejection force, and a balanced top is added on the other side of plastic part. At the same time, an effective hydraulic circulation circuit is designed, an oil distributor is added to system to make it easier to debug production. Ejection and reset actions of multiple cylinders are synchronized; ejection positioning device is added to elastic deformation of plastic part to reduce displacement caused by instantaneous contraction and elastic deformation of plastic part, effectively ensure ejection balance of plastic part.
2.1 Precise positioning of second mold clamping
Fixed mold side of two-material integrated mold is designed with a large slider. A special mechanism must be designed to realize synchronization of slider core pulling and mold opening. Because mold is relatively heavy, centrifugal tendency during rotation process will easily cause entire mold center to be misaligned; In addition, movable mold of large mold needs to be precisely matched with second cavity after rotating, a more accurate guiding and positioning system is required, otherwise it will cause scratches on the first molded PP plastic part. Therefore, large-scale two-material integrated mold must be designed with a more accurate guiding and positioning system.(1) Design of synchronous action between fixed mold slider and mold opening.
As shown in Figure 2, large sliding block is located in fixed mold and weighs 350kg. Since there is no power on fixed mold side, it brings some difficulties to design. Power problem should be solved first: two nitrogen gas springs are designed at the bottom of each slider. Nitrogen gas springs have characteristics of large elastic force, smooth elastic curve and long service life. Therefore, high elastic force of high-pressure nitrogen can successfully achieve synchronous ejection of slider; Secondly, in order to further ensure synchronous ejection of slider, a mechanical hook is designed on movable mold plate to hook slider when mold is closed. When mold is opened, slider is fully pulled out by opening force of injection molding machine, use of push and pull ensures that slide is safely performed at the same time as mold opening action.
(2) Solution to centrifugal tendency when mold is rotating.
As shown in Figure 3, movable mold of two-material integrated mold will rotate when mold is closed for second time. In order to prevent errors in cooperation between rotated movable mold and fixed mold, try not to design unnecessary moving parts on movable mold side, otherwise these parts will inevitably move unnecessarily during rotation process, which will affect accuracy of secondary clamping. Therefore, in mold, all parts that are prone to movement (such as sliders, guide posts) are designed on the side of fixed mold. Entire movable mold not only eliminates moving parts, but also reduces weight, thereby better solving problem of centrifugal tendency that occurs during rotation of movable mold.
(3) Progressive guiding positioning system.
As shown in Figure 3, in mold manufacturing process, there will always be machine tool processing errors, clamping errors, grinding errors and assembly errors, errors of large molds are more obvious. In order to ensure accuracy of mold, mold uses a high-precision machine tool MAKINO to precisely process parts, uses fast positioning fixture EROWA when clamping on different machine tools. At the same time, a progressive guiding positioning system is designed for large rotating mold. Progressive guiding positioning system adopts two-stage guiding positioning: first stage is coordinated by square guide column and square guide groove, which undertakes initial guiding and positioning of mold clamping, ensures initial accuracy of movable mold and fixed mold; second level of precise positioning composed of T-shaped convex blocks and T-shaped concave blocks ensures final precise positioning of movable mold and fixed mold.
2.2 Controllable dual injection molding system
After hard rubber injection blank is formed, mold is closed for second time, cavity and core together create closed mold cavity necessary for second injection molding; when soft rubber is injected for second time, it is necessary to reduce injection pressure to prevent damage to surface quality of molded hard rubber, to ensure that hard rubber molded parts are free from abnormalities such as bright spots and flashes under injection pressure. Therefore, in order to ensure quality of the plastic parts, low-pressure injection must be used, so multi-point hot runner pouring scheme shown in Figure 4 is adopted.Due to use of a hot runner system, it is possible to achieve controllable injection molding for dual pouring system (see Figure 5). Specific control plan: When mold is in the first clamping, hot runner system starts GA injection system to inject PP into cavity. Under control of hard plastic (PP) hot runner, gates are poured in accordance with sequential time, which ensures that PP main body plastic parts do not have defects such as flashing and welding marks.
After the first hard plastic (PP) injection, entire mold rotates 180°, hard plastic and movable mold rotate at the same time to cooperate with second cavity to form soft plastic (TPE) cavity space. At this time, hot runner system starts GB soft rubber (TPE) injection system to inject TPE melt into cavity. After injection is completed, cooling water circuit in mold cools plastic part, then mold is opened to start ejection system (various top blocks, inclined top, etc.) to eject plastic part, mold is closed for next cycle.
2.3 Adaptive ejector balance design
Because plastic part is long and narrow, in order not to deform plastic part during ejection, ejector pin is not used in ejection design of movable mold. Instead, more than 30 ejector blocks are used, which increases ejection area and ejection force. In addition, due to heat exchange of molten material in mold cavity, temperature of core part of movable mold will be significantly higher than that of ejector plate. During molding process, core will produce slight thermal deformation, which will directly affect fit between ejector hole inside core and top block in ejector plate, which may cause top block to be stuck when it is ejected. In order to solve this problem, top block is designed as an adaptive floating structure. As shown in Figure 6, unlike traditional rigid structure, top block of adaptive floating structure can be adjusted elastically as mold temperature changes. This solves problem of matching between ejector hole and center of ejector block due to thermal expansion.
Traditional ejection power of mold generally comes from rigid ejector pin of injection molding machine itself. However, since mold ejection mechanism is composed of many ejector blocks, thermal expansion coefficients of each ejector block are different. To realize stable ejection of each ejector block, hydraulic ejection must be used (see Figure 6). Therefore, four ejection cylinders are designed in mold, two cylinders provide a stable and balanced power source for ejection of each plastic part. In order to realize coordination of actions between oil cylinders, an oil circuit distributor is designed between oil cylinders (see Figure 6) to ensure that action error of each oil cylinder does not exceed 0.02mm. Due to above-mentioned design, entire ejection system achieves a balanced and orderly ejection action, no deformation or warpage occurs after plastic part is ejected, and effect is good.
2.4 Overall mold design
Large-scale rotary high-precision double-material integrated seal injection mold is composed of 7 parts: ①Hierarchical precision guidance and positioning system; ② Independently controllable dual pouring system; ③ Ejection balance automatic adjustment system for long and narrow plastic parts; ④ Synchronous core pulling system on fixed mold side; ⑤ Cooling system; ⑥ Lifting and handling system; ⑦ Forming system, etc.(1) Overall view of fixed mold part, as shown in Figure 7. Fixed mold part has a synchronous core-pulling mechanism, related cooling system, and a controllable dual pouring system distribution.
(2) Front view of movable mold part, as shown in Figure 8. Movable mold part has an automatic adjustment system for ejection balance, a hierarchical precise guiding and positioning system, main mechanical mechanism and distribution status of interface position of each cooling system.
3 Conclusion
(1) Progressive guide positioning system designed in this mold ensures smooth introduction and precise positioning of large molds in mold clamping, reduces amount of processing, and ensures accuracy of mold clamping, thereby ensuring molding accuracy of plastic parts, and effectively reducing mold clamping error to ±0.02mm. This design can be extended to positioning design of other large molds.(2) Designing a floating structure in ejection mechanism of large mold can reduce fit error between movable mold and ejection plate due to thermal expansion, reduce ejection jam, ensure ejection mechanism can eject plastic parts smoothly. Hydraulic ejector and oil circuit distributor are used to ensure synchronous action between oil cylinders and ensure consistency of entire ejection power source action, thereby achieving balance and stability of ejection action.
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