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Application and Practice of Injection Molding Process in the Field of Auto Plastic Parts

Date: Aug 27 2021 | By:LC Rapid
Tag:Injection Molding,

Due to the unique characteristics of the plastic parts of the complex parts of the automobile, the design of its injection molding needs to fully consider the following factors, those are 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 Wait.


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Ⅰ. Application of different plastic parts for automobile injection molding


1. 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 hygroscopicity, in order to meet the moisture content requirements during injection molding (general requirements ≤ 0.2%), the resin raw materials must be dried with hot air or dehumidified before entering the screw of the injection molding machine for pre-plastic 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 injection molding and drying, a dehumidifying dryer should be used for dehumidifying and drying. We have fully considered the function and application of the dehumidifying and drying system in the design of the automotive complex plastic parts molding system.


2. 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 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.


3. Since auto parts are different from conventional products, their cavity surfaces are very complicated, with unequal forces and uneven stress distribution, so the machining capacity required for them should be considered 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. Where: P mold clamping is the clamping force (or clamping force) of the injection molding machine KN; P cavity is the average pressure of the mold cavity, MPa, which is related to the shape of the product, the accuracy requirements and the number of cavities per model, etc., take the value The range is generally between 25-40MPa; F is the mold cavity, the horizontal overlap projection area of the runner and the feed port CM3.


In order to ensure reliable mold 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, which is the nominal injection volume of the injection molding machine based on polystyrene (PS) as the benchmark resin before leaving the factory.


4. As to the design of the drive form and mold clamping mechanism of the injection molding machine according to the characteristics of the complex plastic parts of automobiles, China currently uses hydraulic mechanical or full hydraulic hydraulic toggles, or direct pressure mold clamping at the center. Mechanism of injection molding machine.


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, to meet the production needs of large-scale automobile plastic parts (such as bumpers, dashboards, etc.). Secondly, the clamping adopts 4 composite oil cylinders to lock synchronously, realize the stable clamping of large tonnage with small flow, and the clamping force can be higher than the hydraulic toggle clamping structure of the same tonnage. This can fully guarantee the precision of the injection molding compound surface of the large and complex plastic parts of the automobile.


5. 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 multiple core pulling functions, timing control functions, injection molding machine supporting mold changing device functions, Supporting the functions of pickup manipulators, etc. These special functions have obvious advantages in the production of automotive plastic 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 produce 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.


Ⅱ. The advantages of injection molding complex plastic parts for automobiles


1. The plastic can be processed by injection molding 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 impacts, and protect vehicles and occupants. Plasticized dashboards and steering wheels are used in modern cars to enhance cushioning. The front and rear bumpers and body trim strips are made of plastic materials to reduce the impact of objects outside the car on the 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, saving 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 for complex automotive parts have raised the barriers and risks for the development of injection molded parts for automotive injection molding.

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