With the continuous advancement of automobiles with plastics instead of steel, the proportion of auto parts designed and produced by engineering plastics in the entire vehicle is also increasing. In the car door lock system, the door handle has always been an important component that affects the car experience and highlights the appearance of the car. Compared with traditional metal alloy door handles, door handles made of engineering plastics have many advantages such as light weight, low cost, flexible design, and short production cycle.
In terms of design, door handles usually adopt design methods such as electroplating, spray body paint or no surface treatment. Since most OEMs have very stringent environmental resistance requirements for automotive exteriors, electroplating or painting on the door handles can help improve the parts' heat resistance, UV resistance, and salt spray resistance.
Because PC/ABS material has the advantages of high mechanical properties, dimensional stability, creep resistance, environmental stress cracking resistance, electroplating, spray painting, etc., this material is being used more and more in the design of automobile exterior door handles. Grip type is common. Gas-assisted molding of the grip-type door handle has the characteristics of high strength, light weight, and high dimensional accuracy. However, the requirements for molding accuracy and materials are also higher, and appearance defects or performance defects may occur in actual production. Research on common defects will help future design and production.
GIM (Gas-Assisted Injection Molding) refers to the injection of high-pressure inert gas when the plastic is properly filled into the cavity (90%~99%). The gas pushes the molten plastic to continue filling the cavity, and the pressure is maintained by the gas. A molding technology that replaces the plastic holding process.
In the production of car door handles, common defects are:
<1>Concavities on the surface of the material
Needle holes visible to the naked eye can be observed on the surface of the molded part, and ring-shaped pits can be seen after magnification using a microscope. Generally, shallower pinholes can be post-treated by polishing and other means, while larger ones cannot. This defect will cause pitting on the surface after subsequent electroplating or painting, thereby affecting the yield. This kind of needle hole is usually caused by the lack of close contact between the molten colloid and the cavity during the filling of the mold, and the gas is encapsulated between the two. By increasing the injection temperature and mold temperature during molding, it helps to reduce the frequency of pinholes. However, to fundamentally solve this problem, we still need to start with improvements in both materials and molds. First, it can improve the fluidity of the PC/ABS material, so that it can better replicate the mold; second, it can strengthen the exhaust of the mold, and regularly clean the exhaust groove during production.
<2>Bumps on the surface of the material
The convex point on the surface of the part is very close to the concave point when observed with the naked eye, but after observation with a magnifying glass, it can be found that the unmelted impurities with a diameter of 100 to 300um generally appear at the defect position. Once the bumps on the surface of the material are not found in time, serious appearance defects will occur after electroplating and painting. There are many possibilities to introduce impurities in production, so it is necessary to strengthen environmental management in all aspects of production.
<3>Bubble on the surface of the material
The unruptured bubbles on the surface of the part usually appear near the gate, and the diameter of the bubbles is more than 200um. 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 bubbles are more complicated. Air trapping and material degradation during molding may cause irregular bubbles on the surface of the part. Appropriately lowering the injection temperature and injection speed can alleviate the occurrence of bubbles. However, the lowering of the injection temperature may cause other defects, such as insufficient material and product blow-through. To solve the problem fundamentally, it is necessary to improve the fluidity of the material and adopt PC/ABS products with better thermal stability. Through TGA (thermal weight loss) analysis, it can be found that the gas-assisted molding PC/ABS material HAC8244GM of Shanghai Kumho Sunny has better thermal stability at high temperatures and is more suitable for gas-assisted molding.
<4> Air finger
Air finger defect means that during the blowing process, the bubbles pass through the thin-walled area outside the predetermined air passage of the product, forming finger-like branches. Severe gas fingering will reduce the strength of plastic products, cause the failure of gas-assisted molding technology, or fail to give full play to the advantages of gas-assisted molding technology. The blowing delay time is an important process condition that affects the gas finger defect. Due to the increase of the delay time, the plastic melt near the inner wall surface of the mold cavity can be cooled and solidified, and the thickness of the solid layer increases, so that the lateral filling resistance becomes larger, and the gas follows the resistance limit. The small principle extends longitudinally along the center of the airway, so that the length of the airway is deepened and the diameter is thinner, and the degree of air finger defects formed by bubbles passing through the thin-walled area outside the airway of the product is reduced. However, if the blowing delay is too long, it is easy to cause problems such as unsmooth blowing. In terms of design, keeping the injection direction consistent with the blowing direction can effectively alleviate the defects caused by excessive blowing delay.
<5> Flow marks at the end of injection molding
When the molten material fills the product into the marking part, the remaining part is filled with nitrogen assisted. Due to the lag of nitrogen, the temperature difference taking the marking line as the dividing line will inevitably be caused. In addition, the injection pressure and nitrogen pressure have different replication of the mold. When the difference between the two is large, a clear dividing line or flow mark is produced. Properly increasing the injection temperature and adjusting the blowing time can effectively reduce the flow mark at this position. However, the flow mark cannot be completely eliminated.
<6> Cracking after painting
Spray painting is a common post-processing method for automotive exterior parts. Loss of gloss, sagging, particles, pinholes, and cracks after painting are more common painting defects. Among them, the part that is closely related to the material is the cracking of the parts after painting. The body paint, primer and its thinner are a kind of strong corrosive material, which has a strong corrosive effect on the material. Once the paint formula is not suitable for PC/ABS materials, it may cause the plastic parts to crack at the stress concentration. It is manifested as a tortoise crack on the workpiece, and even in extreme cases, the paint may explode. To solve the problem of spray paint cracking, it is necessary to start at the same time 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 in corrosive solvents; at the same time, it is necessary to improve the fluidity of the material, reduce the residual stress of the material during molding, and leave the part to stand after molding. Relieve stress for more than 48 hours. On the other hand, the paint formula can be adjusted, the type and proportion of thinner in the formula can be changed, and the expected paint leveling can be achieved by adjusting the spray gun distance during production, instead of blindly increasing the proportion of thinner.
<7>Plating leakage
PC/ABS material has a lower butadiene content than ABS material, so it is more difficult to plate than ABS material. Due to the process characteristics of electroless plating, the surface of the plastic will be etched during the roughening process to form pits with uniform distribution and uniform size, so that the coating and the plastic material are combined more closely, and the coating fastness is higher [6]. The part that is etched away is the butadiene component in the material. The content of butadiene in PC/ABS is usually only about half that of ABS, which will inevitably reduce the yield of electroplating. The undesirable manifestations usually include: omission of plating, poor adhesion of the plating, and so on. In actual production, the solution of missing plating should be adjusted according to the situation. If the deposited coating is very bright, but the plastic part is not covered locally, it indicates that the surface is not roughened enough. The coarsening treatment should be further strengthened, and the coarsening temperature should be appropriately increased. If the plating layer is discontinuous or cannot be electroplated at all, it is necessary to adjust the formulations of the plating solution, activating solution and sensitizing solution, or even switch to materials that are more suitable for electroplating. Not all PC/ABS materials are suitable for electroplating processing. Under the same roughening process, the particle size and dispersion of butadiene will greatly affect the results of electroplating. At present, the common electroplating grade PC/ABS materials in the automotive industry include: Bayer's T45PG, SABIC (formerly GE) MC1300, and Kumho Sunny's HAC8244.
<8> Cracking after plating
The cause of PC/ABS material cracking after electroplating is similar to that of spray paint cracking. The chromic acid heated in the roughening process of electroplating will corrode the material. Once the corrosion is excessive, it may cause the cracking of the material after electroplating, and the crack extending to the surface of the coating will cause the defect of electroplating cracking. In the production, low-stress solution should be used to deposit micro-crack chromium or micro-porous chromium, etc., to shield the edges and corners of the door handle. But not all plating cracks are related to the material. In the standards for auto parts, major OEMs have experimental standards for cold and heat cycles and heat storage of electroplated parts. Taking General Motors as an example, all parts must pass the heat storage test at 90±3°C for 6 hours. After the test, some parts may have cracks in the coating. It can be seen from the microscope photos that the cracks only appear in the coating position, and do not extend into the plastic part. This situation indicates that the coating has poor resistance to environmental changes, and the electroplating process needs to be changed, especially to increase the deposition thickness of the copper layer to improve the ductility of the coating; at the same time, change the design of the fixture to make the coating thickness more uniform. These are all conducive to improving the heat resistance of the parts and the rate of results in the cold-heat cycle experiment. In the case of Figure 10, the cracking was caused by the thin copper layer, which did not meet the GM 40μm total plating thickness and 20μm copper thickness standard. The factory improved the uniformity of the coating by changing the position of the rack and adjusting the direction of the fixture; it also delayed the deposition time of the copper layer and increased the thickness of the coating, which met the requirements of the OEM standard.
In the actual production of automobile door handles, there may be problems such as failure of the tensile test and blow-through of the parts during gas-assisted molding. This is limited to the length of the introduction.
Among the defects of gas-assisted molding of car door handles, some are common problems that may occur in injection molding, and some problems such as air fingers and parts blow-through are unique problems of this molding method. In order to minimize the occurrence of these defects, in the early stage of design, CAE technology is used to analyze mold flow and optimize the design of molds and air passages, which can effectively reduce the rate of defective products in the production process. At present, the more mature commercial gas-assisted molding CAE software includes the following types: Moldflow Plastics Insight (MPI) from Moldflow, CAD/CAE software CADMOULD and HSC system from the German IKV Institute; before production, combined with the design of the host plant Standards, select appropriate raw materials for gas-assisted molding door handles to improve the yield of molding and electroplating, which can effectively reduce material costs; in the production process, analyze the causes of defective parts in time, and adjust the molding process and electroplating process. Changes in raw materials and other methods reduce the defect rate, increase production efficiency as much as possible, and achieve comprehensive cost selection.
