Focus on injection molded parts: analysis of common defects in gas-assisted molded door handles

With the continuous advancement of automobiles and plastics, the proportion of auto parts designed and produced by engineering plastics in the whole vehicle has also increased. In the car door lock system, the door handle has always been an important component that affects the car experience and highlights the car's appearance. Compared with the traditional metal alloy door handles, engineering plastic molded door handles have many advantages such as light weight, low cost, flexible design and short production cycle.

In design, the door handles are usually designed with plating, body paint or no surface treatment. Since most of the OEMs have very strict environmental requirements for automotive exteriors, plating or painting on the door handles can improve the heat resistance, UV resistance and salt spray resistance of the components.

Because PC/aBS materials have the advantages of high mechanical properties, dimensional stability, creep resistance, environmental stress cracking, electroplating, painting, etc., this material is increasingly used in the design of automotive exterior door handles. The grip type is the most common. The gas-assisted grip type door handle has the characteristics of high strength, light weight, high dimensional accuracy, etc., but the molding precision and material requirements are also higher, and appearance defects or performance defects may occur in actual production. Research on common defects will help with future design and production.

Gas-assisted injection molding (GIM) refers to the injection of high-pressure inert gas when the plastic is filled into the cavity (90%~99%). The gas pushes the molten plastic to continue filling the full cavity, and replaces the plastic with gas pressure. A molding technique for the pressure holding process.

In the production of car door handles, common defects are:

<1> Material surface pit:

A pinhole visible to the naked eye can be observed on the surface of the formed part, and after the microscope is enlarged, an annular hole can be seen. Generally, shallower needles can be post-treated by polishing or the like, while larger ones do not. This defect can cause pitting on the surface after subsequent plating or painting, which affects the yield. Gas-assisted injection molding (GIM) refers to the injection of high-pressure inert gas when the plastic is filled into the cavity (90%~99%). The gas pushes the molten plastic to continue filling the full cavity, and replaces the plastic with gas pressure. A molding technique for the pressure holding process.

In the production of car door handles, common defects are:

<1> Material surface pit:

A pinhole visible to the naked eye can be observed on the surface of the formed part, and after the microscope is enlarged, an annular hole can be seen. Generally, shallower needles can be post-treated by polishing or the like, while larger ones do not. This defect can cause pitting on the surface after subsequent plating or painting, which affects the yield.

This type of eye is usually due to the fact that the molten colloid and the cavity are not tightly bonded during filling, and a gas is applied between the two. By increasing the injection temperature and mold temperature during molding, it helps to reduce the frequency of pinholes. However, in order to solve this problem fundamentally, it is still necessary to improve from both materials and molds. First, it is possible to improve the fluidity of the PC/aBS material so that it can replicate the mold better. Secondly, it can enhance the exhaust of the mold and periodically clean the exhaust groove during production.

<2> Material surface bumps:

The bumps on the surface of the workpiece are very close to the pits when viewed with the naked eye, but after observation with a magnifying glass, it is found that undecomposed impurities generally appear in the defect position of 100 to 300 um in diameter. Once the surface bumps of the material are not found in time, serious appearance defects will occur after electroplating. There are many possibilities for introducing impurities into production, so environmental management at all stages of production must be strengthened.

<3> Material surface bubbles:

Unbroken air bubbles on the surface of the part usually appear at the near gate, and the diameter of the bubble is above 200 um. The main difference between bubbles and bumps is that the edges of the bubbles are smoother and often appear as smooth arcs.

The causes of air bubbles are complicated, and the trapping and material degradation during molding may cause irregular bubbles on the surface of the workpiece. Properly reducing the injection temperature and injection speed can alleviate the occurrence of bubbles. However, the decrease in injection temperature may cause other defects, such as material shortage and product blow-through. Solving the problem fundamentally requires improving the fluidity of the material and using PC/aBS products with better thermal stability. Through the analysis of TGa (heavy weight loss), it can be found that Shanghai Jinhu Rili's special PC/aBS material HaC8244GM for gas-assisted molding has better thermal stability at high temperature and is more suitable for gas-assisted molding.

<4> Air refers to:

The gas finger defect means that during the blowing process, the air bubbles pass through a thin wall region outside the predetermined air passage of the product to form a finger branch. Severe gas refers to the reduction of the strength of plastic products, resulting in the failure of gas-assisted molding technology, or the advantages of gas-assisted molding technology.

The blowing delay time is the most important process condition affecting the gas finger defect. Due to the increase of the delay time, the plastic melt near the inner wall surface of the cavity can be cooled and solidified, the thickness of the solid layer is increased, and the lateral filling resistance is increased, and the gas follows the resistance. The minimum principle extends longitudinally along the center of the airway, making the length of the airway deeper and the diameter becoming thinner, and the extent to which the air bubbles pass through the thin-walled area outside the airway of the product to form a gas finger defect is reduced. However, if the blowing delay is too long, it may cause problems such as poor air blowing. In design, consistent injection direction and blowing direction can effectively alleviate the defects caused by excessive blowing delay.

<5> Injection end flow marks:

When the melt fills the product to the scribe line, the rest is filled with nitrogen, and the temperature difference caused by the scribe line is bound to be caused by the nitrogen lag, and the injection pressure and the nitrogen pressure are different for the mold. When the difference between the two is large, a clear dividing line, that is, a flow mark, is generated.

Appropriately increase the injection temperature and adjust the blowing time to effectively reduce the flow marks at this position. However, it is impossible to completely eliminate the flow marks.

<6> Cracking after painting

Painting is a common post-processing method for automotive exterior parts. Loss of light, sag, granules, pinholes and cracking after painting are common paint defects. Among them, the material is closely related to the cracking of the parts after painting. Body paint, primer and thinner are a strong corrosive material that has a strong erosive effect on the material. Once the paint formulation is not suitable for PC/aBS materials, it can cause the plastic parts to crack at stress concentrations. It is manifested as a crack in the turtle on the part, and even in the extreme case, there is a case of blasting.

To solve the problem of paint cracking, we must start from both PC/aBS raw materials and paint. On the one hand, it is necessary to improve the corrosion resistance of PC/aBS materials and reduce the risk of stress cracking of materials in corrosive solvents. At the same time, it is necessary to improve the fluidity of materials, reduce the stress residual of materials during molding, and let them stand after molding. Stress is released over 48 hours. On the other hand, it is possible to adjust the formulation of the paint, change the type and proportion of the diluent in the formulation, and achieve the desired leveling of the paint in the production by adjusting the distance of the spray gun, instead of simply increasing the proportion of the diluent.

<7> Electroplating leakage plating

PC/aBS materials are more difficult to plate than aBS materials because their content of butadiene is lower than that of aBS materials. Due to the characteristics of the electroless plating process, the plastic surface is etched in the roughening process to form a uniform distribution and uniform size pits, so that the plating layer and the plastic material are more tightly combined, and the plating adhesion is higher. The portion that is etched away is the butadiene component of the material. The component content of butadiene in PC/aBS is usually only about half of aBS, which will inevitably reduce the yield of electroplating. The defects that are usually exhibited are: leakage plating, poor adhesion of plating, and the like.

In actual production, the solution for leakage plating should be adjusted according to the situation. If the deposited coating is very bright, but the plastic part is not covered partially, it indicates that the surface is not rough enough. The roughening treatment should be further strengthened and the roughening temperature should be appropriately increased. If there is a discontinuity in the plating or no plating at all, it is necessary to adjust the formulation of the plating solution, the activation solution and the sensitizing solution, or even replace the material that is more suitable for electroplating.

Not all PC/aBS materials are suitable for electroplating. Under the same roughening process, the particle size and dispersion of butadiene will greatly affect the final plating results. At present, the electroplating grade PC/aBS materials commonly used in the automotive industry include: Bayer's T45PG, SaBIC (formerly GE) MC1300, and Kumho Riley's HaC8244.

<8> Cracking after plating

The reason for the cracking of the PC/aBS material after plating is similar to the reason for the cracking of the paint. The heated chromic acid in the roughening step of the plating will erode the material. Once the erosion is excessive, it may cause cracking of the material after plating. Cracks extending to the surface of the plating may cause poor plating cracking. Low stress solutions should be used in production to deposit microcracked chrome or microporous chrome, etc., to shield the edges and corners of the door handle parts.

But not all plating cracks are related to the material. In the standard of auto parts, all major OEMs have experimental standards for hot and cold cycle and heat storage of plated parts. In the case of General Motors, all components are subjected to a 90 ± 3 ° C, 6 hour heat storage test.

After the test, some parts may be cracked. It can be seen from the micrograph that cracking only occurs in the plating position and does not extend into the plastic part. This situation indicates that the plating has a poor ability to resist environmental changes. It is necessary to change the plating process, especially to increase the deposition thickness of the copper layer and improve the ductility of the coating. At the same time, the design of the fixture is changed to make the coating thickness more uniform. These are all beneficial to improve the heat-resistance and cold-heat cycle test results of the parts. In the case of Fig. 10, the cracking was caused by the excessively thin copper layer, and did not reach the standard of 40 μm total plating thickness and 20 μm copper layer thickness of General Motors. The factory improved the uniformity of the coating by changing the position of the pylon and adjusting the direction of the fixture; and delaying the deposition time of the copper layer, increasing the thickness of the coating, and finally meeting the requirements of the standard of the OEM.

In the actual production of the door handle of the car, there may be problems such as the tensile test, but the blowing of the parts during the gas-assisted molding, which is not limited to the space.

Among the defects of gas-assisted molding door handles, some are common problems that may occur in injection molding, and some problems such as air fingering and component blowing are the problems unique to this molding method. In order to minimize the probability of occurrence of these defects, in the early stage of design, the mold flow analysis by CaE technology to optimize the design of the mold and air passage can effectively reduce the defective product rate in the production process. At present, the more mature commercial gas-assisted molding CaE software has the following types: Moldflow's MoldflowPlasticsInsight (MPI), Germany's IKV Institute's CaD/CaE software CaDMOULD, and HSC system; before production, combined with the design standards of the host factory, Choosing the right raw materials for gas-assisted molding of door handles, improving the yield of molding and plating, can effectively reduce the cost of materials; in the production process, timely analysis of the causes of defective parts, through the adjustment of molding process, electroplating process