Minimizing Injection Molding Defects: Analyzing And Managing Quality Control In Manufacturing Processes

Tools and Techniques Used in Quality Control

Quality control has consistently been used in ideally all kind of business enterprises. It is a concept that not only focuses on the production areas but also on the service areas [1]

Its use has proved to be the core for those companies that intend to increase their competitive advantage. Improved quality of products and services will lead to greater success in the business, lower costs, greater customer loyalty as well as improved competitiveness amongst other related companies [2].

Worth noting is that in any given plant, continuous activities aimed at process improvement are needed in order to maintain better quality that meets a customer’s requirements and also assist the company to achieve the much required reputation in the market. Well controlled process play a major role in ensuring the quality costs are reduced, especially the internal, external and appraisal costs.

In order for an organization to achieve process improvement, it should build a proper team of professionals and experts in its field. The team will have a duty to recognize problems, analyze, put forward solutions and make implementations of the best solutions. This should help in solving problems or take care of the current situation [3]. Process improvement also purposes to reduce the costs and enable the schedule arrangement of the organization.

In a manufacturing shop floor, all the activities of process improvement do require the use assisting tools and techniques. The idea of lean manufacturing basically involves improving the process through waste elimination and cost reduction. Implementation of the KAIZEN concept will also help achieve process improvement. It represents the uninterrupted improvement of the processes in an organization.

For continued existence of a company, it is normal to experience changes and improvements. Whether or not the company will be able to survive in a very competitive business world relies on how the company manages and adapts to the ever changing environment demands [4]. The change in an organization depends on many resources, reduction in products prices, new products from competitors and the use of new technology in order to improve the product quality.

The expectations of a customer’s always keep changing as well and many companies therefore have also had to make improvements in their products or services in order to satisfy their customers’ requirements.

This is one of the most used fabrication tools used in the plastics industry. The process involves melting the plastic in an extruder [5]. This is done using the extruder screw to inject the plastic into a mold. At this point it is cooled. The vital tools of achieving a successful injection molding operation is speed and consistency. The whole process then recurs.

Analysis of Defects in Injection Molding Processes

Granular plastic materials are melted until soft enough for injection under pressure in order to fill a mold. The plastic molding cools sufficiently and hardens. The mold then opens thus releasing the part.

In order to achieve the research work objectives, historical data are used from the relevant departments to help identify and analyze concerns in injection moulding processes and also generate possible solutions and recommendations for the improvement plan [6].

Involves gathering reports from the (In Process Quality) IPQC and (Final Inspection) FI departments on the quality of the products as well as the effects of low quality products. Next is to identify a process map to help understand the order of activities [7]. Finally, define the future performance and state the goal of improvement.

With regards to the complaints from the customer as well as reports from the FI units and the IPQC, the problem is acknowledged as ‘IPQC keeps reporting defects occurrence during the process, FI keeps rejecting some parts that were released by the IPQC unit and OQC as well discover and reject some items that passed through the FI unit.

This is carried out by making reference to any scientific articles, papers and researches that have been performed earlier to understand more on the process of injection molding [8].

It is important to analyze and properly understand the injection molding processes to enable the evaluation of the present quality performance.

Data was collected through these processes; tracking rejection in the FI unit and the OQC and from IPQC department reports [9]. 21 injection moulding machines that are fully operational during the month presented data in the IPQC unit. The process focused on finding the proportion of defects while starting up the machines and comparing the total starts up.

The recommendations for improvements are to be developed grounded on the data collected.

The reliability and efficiency of the solutions put forward are checked and could then be executed permanently to the process.

Outgoing quality control

It is important to carry out half inspection within the OQC unit before shipping products to any customer. In case any defected parts are noticed from the final inspection unit, then it would also be realized in the OQC unit. OQC unit basically measures the performance of the FI unit. From the data collected from the 4 months, it was realized that there were some operators who kept releasing defected parts [10]. Such incidences would indicate wrong inspection processes or even lack of knowledge on how that defect occurs. The information gathered shows that some operators were require training in order to understand some of the defects they did not discover. Some of the rejection causes include silver streak, scratching and improper insertion process.

Causes of Defects and Recommendations for Solutions

Historical data for the data collection period is as shown below

Table 1customer complaints

A visualization of the above data is as shown below

Figure 1customer complaints

The types of defects reported by the customers are as shown in the table below from the most severe defect to the least severe defect. The data shows gas mark being the most severe defect with 17819 defected parts representing close to 50% of the total defect.

Table 2 types of defects reported by customer

A graphical presentation of the types of defects is as shown below.

Figure 2 presentation of the types of defects

Occurs when molten plastic is injected into a mold cavity and the air within the cavity requires a place to escape. Whenever the hot air can’t escape, a burn mark may be left on the part.

Marker highlight and manual spray are the most commonly used manual process to track gas mark [7]. The process of spraying parts surfaces in order to check if the parts are defected with any line or gas marks is called spray process.

Gas marks can be best controlled by strategically locating air vents of the proper depth within the mold.  In order to reduce burning, adjustments to process parameters including injection and screw speed could be altered [8].

This unit ensures smooth flow of the production process by continuously observing the process and looking for any failure that might occur.

Figure 3 Graphical presentation of the total defects

The total defects during the 4 months was observed at 165. The distribution of defects are as below;

• Wrong dimension- 91
• Flashing-31
• Sinkmark-16
• Short mould-8
• Silveratyreak-7
• Dented parts-7

The defects rate for the 4 months can be represented as follows.

Figure 4 defects rates

The total starts up for the months of October, November, December and January were 93, 78, 79 and 105 respectively and their corresponding rejected parts were 37, 42, 35, and 51 respectively.

Table 4percentage of defects

Conclusion

The research work has provided details of all the defects that occur in a company caused by plastic injection moulding. Data for the analysis was collected for a period of four months from the following departments; IPQC, OQC and customer complaints.

In IPQC, a total of 165 different defects were discovered during the said period. The distribution for those defects have been shown in the discussio9ns above. The most severe defects was wrong dimension, indicating 91 rejects. Next was flashing with 31 rejects, sink mark with 16 rejects, short mold with 8 rejects and silver streak and dented parts each contributed 7 defects. 3 rejection processes occurred due to high gating and other 2 due to string. Analysis have been carried out for the most severe defects and their main causes found. Their theoretical solutions have also been put forward.

In OQC, the analysis aimed at carrying out evaluation of the existing performance of the FI unit. Any defected part released by the FI unit would be realized by the OQC unit. During the same period, interviews were carried out on the operators that provided higher number of defected parts. Their reasons were put forward and possible solutions implemented.

References

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