Study of Moulding Defects in Automobile Relay Cover Dr. Shajan Kuriakose, Sunny K. George, Prisley Varghese Mathew

Study of Moulding Defects in Automobile Relay Cover Dr. Shajan Kuriakose, Sunny K. George, Prisley Varghese Mathew Abstract Automobile relay covers are used to protect various parts of the relay including coils and terminals. These are manufactured by injection moulding process. Defects during the moulding process leads to the rejection of the relay cover. Various defects identified in the relay cover are: short moulding, flash, black dots, ejector pin marks, burn marks, sink marks, flow marks etc. As a result these will have to undergo rework or rejection, which increases the total cost of production. It is found that moulding defects in relay cover can lead to serious quality problems. The main objective of this study is to find out the major causes of these problems and to find out remedies by which these defects can be reduced. Index Terms Automobile Relay Cover, Cause and Effect Diagram, Injection Moulding, Moulding Defects, Pareto Chart. I. INTRODUCTION Injection moulding is the most commonly used manufacturing process for the fabrication of plastic parts [1]. A wide variety of products are manufactured using injection moulding, varying in their sizes and applications. The injection moulding process requires the use of an injection moulding machine, raw plastic material and a mould. These machines consist of two basic parts, an injection unit and a clamping unit. The plastic is melted in the injection moulding machine and then injected into the mould, where it cools and solidifies into the final part. In this process, hot molten polymer is forced into a cold empty cavity of a desired shape and is then allowed to solidify under a high holding pressure [2], [3]. The entire injection moulding cycle can be divided into three stages: filling, post-filling and mould opening [4], [5]. Common quality problems or defects that come from an injection moulding process are short moulding, flash, black dots, ejector pin marks, burn marks, sink mark, flow marks, shrinkage etc.. The defects of injection moulding process usually arise from several sources, including the pre-processing treatment of the plastic resin before the injection moulding process, the selection of the injection moulding machine, lack of proper maintenance and the setting of the injection moulding process parameters etc. The effects of the above parameters on the physical and mechanical properties of thermoplastics have been studied by various researchers [4], [6]-[10]. A study on moulding defects were carried out in a leading electro-mechanical parts manufacturing company. Over the years, the company was involved in production of injection moulds of engineering plastics and precision parts. Automobile relay cover is one of the critical products manufactured in the company. It protects various components such as coils and terminals and hence the quality of automobile relay covers is very important. The objective of this study is to find out the major defects observed during the injection moulding of automobile relay covers and to find out remedies by which these defects can be minimised. II. EXPERIMENTAL DETAILS The material used to produce automobile relay cover was Nylon 66. The material was pre-conditioned at 45 c for three hours using a dehumidifying drier before moulding. Fanuc Roboshot S-2000 C injection moulding machine was used for manufacturing automobile relay cover. The data for the study were collected from actual production line of the plastic injection moulding department of the company for a period of two months and major defects in automobile relay covers were identified. Root causes for these defects were found out from cause and effect diagram and remedies were suggested. III. DATA COLLECTION AND ANALYSIS A. Data Collection The total production and rejection details of automobile relay cover during the months of June and July are given in Table 1. From Table 1 it is clear that the total percentage of rejection during the two months accounts to 15%. Table 2 shows the defects identified during the months of June and July 2012. Table 1: Production Data of Automobile Relay Cover for the Months of June & July 2012 Table 2: Type of Defects Identified In Automobile Relay Cover during the Months of June & July 2012 TYPE OF NUMBER OF (Approx.) JUNE JUNE % NUMBER OF (Approx.) JULY JULY TOTAL PRODUCTION 171,000 205,000 TOTAL NO. OF DEFECTIVE PARTS PERCENTAGE OF DEFECTIVE PARTS (%) 24,000 33,000 14 16 % 134

SHORT 1 Cause and Effect Diagram for Short Moulding 7680 32 11880 36 MOULDING FLASH 5520 23 8250 25 BLACK DOTS 4320 18 6600 20 EJECTOR PIN MARKS 2640 11 2970 9 BURN MARKS 1680 7 1650 5 SINK MARKS 1200 5 990 3 FLOW MARKS 960 4 660 2 B. Data Analysis Pareto chart was plotted based on the above data and are shown in figure 1 and figure 2. Both Pareto charts of June and July reveals that short moulding flash and black dots were the major defects. It is found that these defects were responsible for about 80% of the rejection and hence study was focused on these defects. Fig 3: Cause and Effect Diagram for Short Moulding Fig 1: Cumulative Rejection Percentage Based On the Type of Defect during the Month of June 2012 Machines are one of the factors that can contribute a lot to short shot moulding defect. Improper parameter setting like low injection pressure, low injection velocity and shot size are some of the major machine parameters which will result in short shot and was identified as the root cause as shown in figure 3. Aging machines and improper maintenance also can lead to defects. Experience of operator, condition of the mould tool, quality of material, contamination with foreign particles, presence of moisture, insufficient pre heating etc. may also causes defects. 2 Cause and Effect Diagram for Flash Fig 2: Cumulative Rejection Percentage Based On the Type of Defect during the Month of July 2012 C. Identifying Causes of Defects In order to identify the root cause of the main three defects viz, short moulding, flash and black dots, the cause and effect diagram was drawn. The major causes of all defects could be categorized in four categories: machines, materials, moulds, and man. The outcome was summarized in cause and effect diagram. A brainstorming session was conducted with the production engineer, quality engineer, and senior operator, to find out the root causes. Fig 4: Cause and Effect Diagram for Flash Improper proper parameter setting of machine, incorrect barrel temperature setting, insufficient press capacity, improper maintenance of moulding machine, unskilled operator etc. can lead to flash. Improper handling of mould, presence of impurities and other foreign particles inside the tool, the viscosity of the molten material and unbalanced material flow can also lead to flash. Mismatching of shut off faces of mould, damage of parting line of tool etc occurs due to improper maintenance and was identified as the root cause of flash (figure 4). 135

3 Cause and Effect Diagram for Black Dot 6. Increase the shot size Shot size is the amount of material required to completely fill the mould. The shot size must be set after trial runs. Less shot size may result in shot moulding. Therefore shot size must be increased to eliminate short shot. Fig 5: Cause and Effect Diagram for Black Dot As in previous case, improper proper parameter setting of machine, aging of machine, improper maintenance of moulding machine, improper training of the operator, incorrect procedure of moulding process etc. can lead to black dots in moulded parts. Presence of impurities and foreign particles inside the tool can also lead to black dot. Contaminated materials will affect the properties of the part and lead to defects. The root causes of black dot were identified as dirty machine and screw peeling (figure 5). A. Short Moulding IV. RESULT AND DISCUSSION After collecting and analyzing the data, cause and effect diagram was drawn to identify the causes of short moulding. The major cause of the defect was identified as improper machine parameter settings. The methods by which short moulding can be reduced are: 1. Increase the injection speed By increasing the injection speed, the rate at which the material entering into the mould cavity increases and thus reduces short moulding. 2. Increase the mould temperature If the mould is too cold, there might be chances of short moulding. It is seen that the mould will be initially cold during machine start up. Therefore, the mould temperature must be increased before the production. 3. Increase the injection pressure By increasing the injection pressure, the material can be forced to enter to every part of the mould cavity. 4. Increase the injection time Increasing the time of injection is the time taken by the melt plastic to completely fill the mould. Increasing the injection time can surely decrease the rate of rejection by short moulding. 5. Increase the material temperature If the material temperature is too low, there might be chances of plastic granules stuck at the nozzle and make a hindrance to the flow of molten plastic resin. Therefore, the material temperature must be increased by increasing the barrel temperature through which the material flow. It was noted that the rejection rate due to short shot is high during machine start up (when the mould is too cold) and at situations where frequent power failures occurs. Therefore, in order to decrease the percentage of rejection during start up the machine operator must ensure continuous power supply and proper machine parameter settings. A new technique known as Gas Assisted Injection Moulding (GAIM) can be employed. GAIM is the process of injecting an inert gas into the fluid resin mass at the final injection stage of the injection process, to ensure that the resin fills all the unfilled portions of the mould cavity. V. FLASH Improper moulding tool maintenance was identified as root cause for flash. This can be reduced by timely and effective maintenance of the tool. Preventive maintenance is the best strategy to reduce the defect. The tool must be properly maintained after the moulding process or during tool idle time. Proper mould maintenance program is essential for good process result. Regular mould maintenance will help mould to last longer, run with less interruptions, and save time and money. Complex components like slides, lifters, moving cores, hydraulic and mechanical systems, runners, complex ejector systems or mechanisms with delicate components add the maintenance requirement. Excessive clamp pressures, high injection pressures, over-packing/flashing the part, jerking the mould during opening and closing, lack of lubrication, multiple ejection etc. also causes excessive wear and tear on the mould. Moulding tool maintenance is performed initially to reduce in-house tool abuse. Have a clean operation using well maintained machines and have the right tools. Do not use hard tools (screw drivers, hammers, punches on moulding surface, parting, or shutoff surface. Using soft tooling like rubber mallets, punches and pliers made from plastic, copper, or brass on hand to avoid damaging the mould. Use soft or treated water in cooling systems. Avoid excessive clamp pressures, high injection pressures, and over-packing/flashing the mould. Don't operate the press in such a way that the mould is rapidly jerked during opening and closing. Lubricate the components properly. Protect the work area and mould storage area from outside environment. The different levels of care/maintenance that has to be followed are: 136

Preventative maintenance: Simple Preventive Maintenance greatly improves the life of the mould. The following factors should be checked after each shift. The parting surfaces, core, and cavity should be gently cleaned with a mild, clean solvent and soft, clean towels to remove any buildup from vented gases, greases, and other resins that accumulate. Special attention must be given to the parting surfaces. Before the mould is removed from the machine, the mould should be returned to room temperature in order to avoid condensation to form on the mould and cause rust. All water lines should be drained and blown free of all residual water to avoid build up of rust due to standing water. It is critical that no water be trapped inside mould. The ejector system should be moved fully forward, and then sprays both the mould halves with light rust preventive lubricant (like WD-40). Retract ejector system and close the mould. Check whether parting line is damaged. Check and assure all bolts, plates, etc. are in place and tight. Before a production run, the following factors should be checked. Open the mould and clean the parting surfaces, core, and cavity with clean solvent and soft, clean towels. This removes the mould preservative and remaining dust or particles. Grease the guide pins, the ejection system, and any lifters or slides. Inspection: Inspection is carried out to identify the problems and to schedule for maintenance. This kind of maintenance should be performed by a lead operator or an experienced person in the tool room after about 20,000 cycles, after 10 production days, or at the end of a production run, whichever comes first. Inspect the tool and check for minor damage. The vents should be checked. Bent, worn or broken ejector pins should be checked. Maintenance: The periodic maintenance should be performed by a skilled mould maker. The following maintenance procedure is followed. This is generally carried out in every 1, 00,000 cycles (or every 10 production runs). All plates are separated and their faces cleaned. All components are checked for wear. Any excessive wear is noted repair, replace. Any cavity detail area with dings, dents or other signs of wear or abuse should be considered critical and should be carefully analyzed before any other replacements or repairs. All moving parts are to be lubricated. Use lubricant sparingly on all moving parts which make contact with plastic parts. Vents should be checked for depth, width and land. They should also be checked for corrosion and vent burns. All water lines are to be pressure tested for leaks and for flow capacity. The ejector system should be examined for proper alignment. Major Maintenance: Major maintenance is also performed by a skilled mould maker and it should be done after the mould fulfilling the required number of cycles for maintenance, or excessive wear or damage of the tool was noted. All components are thoroughly checked and repair or replace with original parts. This is generally carried out in every 2, 50,000 cycles (or half the anticipated life time). VI. BLACK DOT The major cause of this defect was due to screw peeling and dirty barrel. Two suggestions were recommended to reduce the defects. 1. Screw and barrel cleaning 2. Purging of screw and barrel A. Screw and Barrel Cleaning The injecting screw becomes carbonized as time proceeds. It is due to overheating of material in the barrel. The overheated material will stick on the screw and will be released slowly during injection process and caused for the black dot on the surface of the cover. Sand paper and chemical solvents can be used to clean the screw. Lot of scrap material will accumulate on the barrel and hydraulic unit area due to improper cleaning of the machine. This condition will lead to a situation where the foreign materials or scrap material will mix with original material and leads to black dot and other defects. Use proper cleaning agent to clean the barrel and the machine must be covered with a plastic sheet to make sure that no dirt or dust affects the machine. B. Purging Of Screw and Barrel The technique of purging the screw and barrel of the injection moulding machine is as follows: 1. Retract the injection unit. Run the barrel empty using maximum back pressure. Wipe the hopper and feed throat. 2. Feed the required amount of cleaning agent into the hopper. About one to two barrel capacities of cleaning agent is required for purging a typical injection moulding machine. 3. With completely forward the screw; increase the back pressure to the maximum level. 4. After cleaning agent begins to come from the nozzle, increase the screw speed to the maximum safe level. 5. Drop the back pressure once the nozzle is clean. 6. Retract the screw and perform short, high-velocity injection shots. 7. Repeat steps 1 through 6 if contaminants are still visible. 137

8. Purging is complete when cleaning agent coming from the machine is visibly free of contamination. VII. CONCLUSION AND FUTURE WORK The main objective of this study was to identify the moulding defects in automobile relay cover. It was found out that almost 15% of the total production was defective. The various defects identified in the automobile relay cover were short moulding, flash, black dot, ejector pin marks, burn marks, sink marks and flow marks. From the Pareto chart it was found out that 80% of the total rejection during the two months was due to short moulding, flash and black dot. The cause and effect diagram was drawn to identify the major causes of these three defects. It was found out that improper machine parameter was the major cause of short moulding. Flash in the relay cover was due to poor maintenance strategies and improper screw and barrel cleaning was the major cause of black dot. Short moulding is the major defect occurring during moulding and a new technique Gas Assisted Injection Moulding (GAIM) is recommended in order to reduce short moulding. GAIM ensures that the molten material is pushed to the unfilled extremities of the mould cavity and thereby reducing short moulding. Proper moulding tool maintenance is the only way to eliminate the defects caused due to flash. Preventive maintenance, inspection and major maintenance are some of the maintenance strategies that can be used. Black dots can be reduced by proper screw and barrel cleaning. Purging can also be employed to remove the presence of dirt s and other materials. By adopting the above techniques it is possible to reduce the defect percentage from 15% to 4% thus helping the company to increase its total profit. There is a further scope of reducing the various moulding defects found in the automobile relay cover. These defects can be minimized further by optimizing the machining parameters using ANOVA or any other optimization techniques. REFERENCES [1] S.A.Brent, (2000). Plastics materials and processing Jersey: Prentice Hall. [2] J.-F. Agassant, P. Avenas, J. Processing Principle and Modelling 1991 [3] T.A. Osswald. Polymer Processing Hanser Publishers, 1998. [4] W.C. Chen, P.H. Tai, M.W. Wang, W (2008) Neural network-based approach for a dynami injection molding process. Expert Systems with Applications 843 849. [5] C.M. Seaman, (1994). Multiobjective optimization of a plastic injection molding process. IEEE Transactions on Control Systems Technology 2(3), 157 168. [6] R.D. Chien, S.C. Chen, P.H. Lee (1999) Molding characteristics and mechanical properties of injection foaming polypropylene parts. Composites, 23(4), 429 444. [7] H. Ismail, & Suryadiansyah (2004). A comparative study of the effect of degradation on the properties of PP Plastics Technology and Engineering, [8] Y.H Lin, W.J. Deng, C.H. Huang (2000). Injection molding process for tensile and wear properties of polypropylene components via Taguchi and de method. Polymer Plastics Technology and Engineering [9] H. Oktem, T. Erzurumlu (2005). Optimization technique in determining plastic injection molding process parameters for a thin-shell part. [10] H.SadAbadi, & M. Ghasemi, (2007). Effects of some injection molding process parameters on fiber orientation tensor of short glass fiber polystyrene composites (SGF/PS). Composites, 26(17), 1729 1741. AUTHOR BIOGRAPHY Dr. Shajan Kuriakose, Professor, Dept. of Mechanical Engineering, Mar Athanasius College of Engineering, Kothamangalam, Kerala. Sunny K. George, Assistant Professor, Dept. of Mechanical Engineering, Mar Athanasius College of Engineering, Kothamangalam, Kerala. Prisley Varghese Mathew, Post Graduation Student, Dept. of Mechanical Engineering, Mar Athanasius College of Engineering, Kothamangalam, Kerala. 138