Due to the unique characteristics of the plastic parts of the complex parts of automobiles, the following factors should be fully considered in the design of its injection molding: the drying treatment of the material, the new requirements of the glass fiber reinforced material for the screw, the driving form and the mold clamping structure, etc.
When the commonly used resin materials such as automobile bumpers and instrument panels are modified resins such as modified PP and modified ABS, the resin materials have different hygroscopic properties. In order to meet the moisture content requirements during injection molding (generally required ≤0.2%), the resin raw materials must undergo hot air drying or dehumidification drying before entering the injection molding machine screw for pre-molding measurement.
Modified PP (PPEPDM) resin has low hygroscopicity. Generally, a hot air dryer can be used for 2-3 hours at 80-100°C. Modified ABS (PCABS) has strong hygroscopicity. Before automotive injection molding and drying, a dehumidifying dryer should be used for dehumidifying and drying. We have fully considered the functional application of the dehumidifying and drying system in the design of the injection molding system for complex plastic parts of automobiles.
At present, plastic parts for automobiles in China are basically plastic products without glass fiber reinforcement. Compared with the use of resin containing chopped glass fiber reinforcement, the material and structure of the screw of the automotive injection molding machine used for injection molding of glass fiber-reinforced plastic parts are quite different. When designing the injection molding machine, pay attention to the alloy material of the screw barrel and the special heat treatment process to ensure its corrosion resistance and strength.
Since auto parts are different from conventional products, their cavity surfaces are very complex, with unequal forces and uneven stress distribution. The machining capacity required for it is the key consideration in the design. The processing capacity of the injection molding machine is embodied in two aspects: clamping force and injection capacity (expressed by the maximum theoretical injection volume). When the injection molding machine is injecting molded products, the clamping force must be greater than the opening force generated by the mold cavity pressure, otherwise the mold parting surface will separate and cause flash.
The formula to determine the clamping force of the injection molding machine is as follows: P clamping ≥ P cavity × F/100. In the formula: P clamping is the clamping force (or clamping force) KN of the injection molding machine; P cavity is the average pressure of the mold cavity, related to the product structure shape, accuracy requirements and the number of cavities per model, etc. The value range is generally between 25-40MPa; F is the horizontal overlap projection area of the mold cavity, runner and inlet, CM3.
In order to ensure reliable clamping, the process clamping force during injection molding must be less than the rated clamping force of the injection molding machine. The maximum theoretical injection volume of the injection molding machine matches the clamping force tonnage of the injection molding machine. It is the nominal injection volume of the injection molding machine based on polystyrene (PS) resin before leaving the factory.
According to the characteristics of the complex plastic parts of automobiles, the design of the drive form and clamping mechanism of the injection molding machine is mostly used in China, which is hydraulic-mechanical or full-hydraulic hydraulic toggle, or injection molding machine with a direct pressure clamping mechanism at the center.
The advantage of this structure is that its mold opening stroke is much larger than that of the hydraulic toggle mold clamping structure, at least 2 times, which can meet the production requirements of large-scale automobile plastic parts (such as bumpers, dashboards). Secondly, the clamping adopts 4 composite oil cylinders to lock synchronously, and realize the stable clamping of large tonnage with small flow. The clamping force can be higher than the hydraulic toggle clamping structure of the same tonnage. And it fully guarantees the precision of the injection molding compound surface of the large and complex plastic parts of the automobile.
Because the cavity surface of auto parts is very complicated, the injection molding machine should consider its particularity when designing, and configure some special function programs: such as multi-group core pulling function, timing control function, injection molding machine supporting mold changing device function, supporting part picking manipulator device function. These special functions have obvious advantages in the production of injection molding automotive parts.
For example, in the bumper production process of this type of injection molding machine, in order to shorten the production cycle, the picking manipulator automatically takes out the large plastic parts after the mold is opened and ejected. Multiple sets of core pulls make the large plastic products stable and integral. It will not cause whitening or cracking. At present, we mostly use 3-axis pick-up robots in this injection molding system, and some use 6-axis pick-up robots.
1. The plastic can be processed and formed at one time, the processing time is short, and the accuracy is guaranteed.
2. The elastic deformation characteristics of plastic products absorb a large amount of collision energy, have a greater buffering effect on strong collisions, and protect vehicles and occupants. Plasticized dashboards and steering wheels are used in modern cars to enhance cushioning. The front and rear bumpers and the car body trim are made of plastic materials to reduce the impact of objects outside the car on the car body. Plastic also has the function of absorbing and attenuating vibration and noise, which can improve the comfort of riding.
3. The plastic has strong corrosion resistance and will not corrode if damaged locally.
4. By adding different fillers, plasticizers and hardeners to produce plastics with required properties to meet the requirements of different parts of the car. What's more convenient is that the color of the plastic can be adjusted to different colors through additives, avoiding the trouble of painting. Some plastic parts can also be electroplated.
On the other hand, the special appearance and size requirements of injection molded parts of complex automotive parts have raised the barriers and risks for the development of injection molded parts for automobiles.