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Common Defects of PC/ABS Gas-Assisted Forming Door Handles

With the development of replacing steel with plastics in automobiles, the proportion of automotive parts designed and manufactured by engineering plastics in automobiles is also increasing. In the automobile door lock system, the door handle has always been an important component which affects the automobile experience and highlights the appearance of the automobile. Compared with traditional metal alloy doorknobs, engineering plastic moulded doorknobs have many advantages, such as light weight, low cost, flexible design, short production cycle and so on.

In design, door handles are usually designed by plating, spraying body paint or without surface treatment. Because most of the main engine factories have very stringent environmental requirements for automotive exterior decoration, so plating or spraying paint on the door handle can help to improve the heat resistance, ultraviolet resistance, salt fog resistance and other properties of parts.

Because PC/ABS material has the advantages of high mechanical properties, dimensional stability, creep resistance, environmental stress cracking resistance, electroplating and painting, it is more and more used in the design of automotive door handles, among which the grip type is the most common one. Gas-assisted moulding grip door handles have the characteristics of high strength, light weight and high dimensional accuracy. However, the requirements for moulding accuracy and materials are also higher. In actual production, there will be appearance defects or performance defects. Through the study of common defects, it will be helpful for future design and production.

GIM(Gas-Assisted Injection Molding) is a kind of moulding technology that injects high-pressure inert gas when the plastic fills into the cavity properly (90%~99%). Gas drives melted plastic to continue filling the cavity and replaces the plastic holding process with gas holding.

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

< 1 > Material surface concave

Needle eyes visible to the naked eye can be observed on the surface of the moulded parts, and circular pits can be seen when magnified with a microscope. Usually shallow pinholes can be treated by polishing and other means, but larger ones can not. The defect will cause pitting on the surface after subsequent plating or spraying, thus affecting the yield. This kind of pinhole is usually due to the incompatibility of the melt colloid and the die cavity during filling, and the gas is coated between the two. By increasing the injection temperature and the mould temperature, it is helpful to reduce the frequency of needle eye. However, in order to solve this problem fundamentally, it is necessary to improve both the material and the die. Firstly, the fluidity of PC/ABS material can be improved so that it can replicate the mould better. Secondly, the exhaust of the mould can be strengthened and the exhaust groove can be cleaned regularly in production.

< 2 > Surface bump of material

The bump on the surface of the workpiece is very close to the concave point when observed by naked eyes, but the unmelted impurities with diameters of 100 to 300 um can be found at the defect position by magnifying glass. Once the existence of bumps on the surface of materials is not found in time, serious appearance defects will occur after electroplating and spraying. There are many possibilities of introducing impurities into production, so it is necessary to strengthen the environmental management in all aspects of production.

< 3 > Bubbles on Material Surface

The unbroken bubbles on the surface of the workpiece usually appear near the gate, and the diameter of the bubbles is more than 200 um. The main difference between bubbles and convex points is that the edges of bubbles are smoother, often showing a smooth arc. The causes of bubbles are complex, and irregular bubbles may appear on the surface of parts due to trapping gas and material degradation during forming. Proper reduction of injection temperature and injection speed can alleviate the occurrence of bubbles. However, the lower injection temperature may lead to other defects, such as material shortage, blow-through and so on. Fundamentally solving the problem requires improving the fluidity of materials and adopting PC/ABS products with better thermal stability. Through TGA analysis, it can be found that HAC8244GM, a special PC/ABS material for Rili gas-assisted moulding in Jinhu, Shanghai, has better thermal stability at high temperature and is more suitable for gas-assisted moulding.

<4> gas fingers

Gas finger defect refers to the finger branch formed by the bubble passing through the thin-walled area outside the predetermined airway of the product in the process of blowing. Serious gas finger will reduce the strength of plastic products, resulting in the failure of gas-assisted forming technology, or can not play the advantages of gas-assisted forming technology. The delay time of blowing is the most important technological condition affecting the defect of gas finger. With the increase of delay time, the plastic melt near the inner surface of the die cavity can be cooled and solidified better, and the thickness of the solid layer increases, so the transverse filling resistance increases. The gas extends longitudinally along the center of the air passage according to the principle of minimum resistance, which deepens the length and diameter of the air passage, and the air bubble passes through the outside of the product air passage. The degree of gas finger defects in thin-walled area decreases. However, too long blowing delay can easily lead to problems such as blowdown. In design, the defects caused by too long blowing delay can be effectively alleviated by keeping the injection direction in line with the blowing direction.

< 5 > Flow marks at the end of injection moulding

When the melt fills the product into the scribed part, the rest of the product is saturated with nitrogen. Because of the nitrogen hysteresis, the temperature difference with the scribed line is bound to be caused. In addition, the injection pressure and nitrogen pressure are different in the replication of the mould. When the difference between the two is large, the obvious boundary line, i.e. the flow mark, will be produced. Properly increasing injection temperature and adjusting blowing time can effectively reduce the flow marks at this position. But it is impossible to completely eliminate the flow marks.

< 6 > Cracking after Painting

Spraying paint is a common post-processing method for automotive exterior parts. Loss of gloss, hanging, particles, pinholes and cracking after spraying are common poor spraying, among which, the cracking of parts after spraying is closely related to the material. Body paint, primer and its diluent are strong corrosive materials, which have strong corrosive effect on materials. Once the formulation of paint is not suitable for PC/ABS materials, plastic parts may crack at stress concentration. It is shown as tortoise cracks on the parts, and in extreme cases, even paint explosion may occur. To solve the problem of spray paint cracking, it is necessary to start with PC/ABS raw materials and paint at the same time. 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; on the other hand, it is necessary to improve the fluidity of materials, reduce the residual stress of materials during forming, and release the stress more than 48 hours after forming. On the other hand, we can adjust the formulation of paint, change the type and proportion of diluent in the formulation, and achieve the desired paint leveling in production by adjusting spray gun distance, rather than simply increasing the proportion of diluent.

< 7 > Leakage Plating

PC/ABS material is more difficult to electroplate than ABS material because of its lower butadiene content than ABS material. Because of the technological characteristics of electroless plating, etching will be carried out on the plastic surface in the coarsening process, forming uniform pits with uniform distribution and size, making the coating and plastic materials more closely combined, and the coating fastness is higher [6]. The etched part is the butadiene component in the material. The content of butadiene in PC/ABS is only about half of that in ABS, which will inevitably decrease the yield of electroplating. Usually, there are some disadvantages, such as leak plating, poor adhesion of coatings, and so on. In actual production, the solution of leakage plating should be adjusted according to the situation. If the deposited coating is very bright, but the part of the plastic part can not be covered, it indicates that the surface coarsening is not enough. Coarsening treatment should be further strengthened and coarsening temperature should be properly raised. If the plating layer is discontinuous or can not be electroplated at all, it is necessary to adjust the formulation of plating solution, activation solution and sensitizing solution, or even change the material more suitable for electroplating. Not all PC/ABS materials are suitable for electroplating. Under the same coarsening process, the particle size and dispersion of butadiene will greatly affect the final electroplating results. At present, the common plating grade PC/ABS materials in automobile industry are: T45PG of Bayer, MC1300 of SABIC (original GE) and HAC8244 of Rili Jinhu.

< 8 > Cracking after Electroplating

The cracking reason of PC/ABS material after electroplating is similar to that of spray paint. Chromic acid heated in the coarsening process of electroplating will corrode the material. Once the corrosion is excessive, it may lead to the cracking of the material after electroplating. When the crack extends to the coating surface, it will cause the bad cracking of electroplating. In production, low stress solution should be used to deposit MICRO-CRACKED chromium or micro-porous chromium, and to shield the edges and corners of door handles. But not all coating cracking is related to material. In the standards of automotive parts and components, major main engine factories have experimental standards for cold and hot cycle and hot storage of electroplated parts. Taking General Motors as an example, all parts have to pass the thermal storage test of 90 +3 C for 6 hours. After the test, some parts may crack. Microscopic photographs show that the cracking only occurs in the coating position and does not extend into the plastic parts. This situation shows that the coating has a poor ability to resist environmental changes, so 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 fixture is changed to make the coating thickness more uniform. All of these are helpful to improve the heat resistance of the parts and the experimental results of cold and hot cycles. In the case of Fig. 10, the cracking was caused by the thin copper layer, which did not meet the standard of GM's 40-micron total coating thickness and 20-micron copper layer thickness. The uniformity of the coating was improved by changing the position of the hanger and adjusting the direction of the fixture, and the deposition time of the copper coating was delayed and the thickness of the coating was increased, which finally met the requirements of the standard of the host plant.

In the actual production of automotive door handles, there may be tension test, but blow-through of parts in gas-assisted moulding, which is not introduced in detail.

Some of the defects of door handles in gas-assisted moulding are common problems that may occur in injection moulding, while others, such as gas finger and blow-through of parts, are special problems of this moulding method. In order to minimize the occurrence probability of these defects, in the early stage of design, using CAE technology to analyze the die flow and optimize the design of the die and airway can effectively reduce the rate of defective products in the production process. At present, there are several mature commercial gas-assisted forming CAE software: Moldflow Plastics Insight (MPI) of Moldflow Company, CAD/CAE software of IKV Institute in Germany, CADMOULD and HSC system, etc. Before production, according to the design standards of the main engine plant, selecting suitable raw materials for gas-assisted forming door handles can effectively reduce the cost of materials by improving the yield of forming and electroplating. In the production process, timely analysis of the causes of bad parts, through the shaping process, electroplating process adjustment, changes in raw materials and other ways to reduce the rate of bad, as far as possible to improve production efficiency, to achieve the optimal choice of comprehensive costs.

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