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04/30/2018 HYU News > Academics > 이달의연구자

Title

[Researcher of the Month] Applying Terahertz to Weld-Line Detection

Professor Kim Hak-sung (Mechanical Engineering)

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Contents
The weld-line among moulding injected plastic products have long presented a challenge in the production of plastic. Destruction of a specific plastic product has been considered necessary in the process of examining any existing weld-lines within the material. However, based on his newly released paper "Terahertz time-domain spectroscopy of weld-line defects formed during an injection moulding process," Kim Hak-sung (Mechanical Engineering) has coined the concept of applying terahertz radiation in this examination process. Based upon the terahertz time domain spectroscopy (THz-TDS), Kim has introduced a novel method of detecting weld-lines among moulding injected plastic products in a nondestructive manner.


The THz-TDS technique

In order to understand the THz-TDS system, one must become familiar with the actual concept of terahertz. Terahertz refers to a frequency unit of electromagnetic waves, counting up to one trillion cycles per second. Its long microwaves and wavelengths provide it a high permeability which allows terahertz to surpass materials other than metal. This high permeability leads to the THz-TDS, a spectroscopic technique in which the properties of matter are examined through different phases of terahertz radiation. In short, when shot at a specific target, the phases of terahertz radiation differ while surpassing different materials.
 
Professor Kim Hak-sung (Mechanical Engineering) explained the benefits of terahertz radiation
and how it can be applied to more practical fields.

Although THz-TDS was an already-existing technique, it was Kim who applied it to the more practical field of finding weld-lines among plastic products. Weld-lines are lines that occur around areas where two flow fronts meet, yet are unstably "welded" together in the moulding process. These weld-lines cause weak areas among the moulded part, which may lead to a breakage of the product when the part is under pressure. Until now, the inevitable destruction of the whole product has been regarded as the only method of scrutinizing the existence of weld-lines. However, Kim has introduced a new method, which allows the weld-lines to be detected without breakage.


Applying THz-TDS to weld-line detection     

As mentioned above, different phases of terahertz radiation occur when surpassing different materials. This variation of phases allows one to determine the specific material that the terahertz radiation is currently transcending through. Thus, when shooting terahertz radiation at moulding injected plastic products, the distinctions that occur among phases would be the areas where weld-lines, different layers than other parts of the product, are detected. This would eliminate the necessity of the current destructive weld-line determination process, as simply shooting terahertz radiation at the plastic products enables the investigation method to be possible without any force input.
 
The phases of terahertz radiation differ according to the existence of weld-lines,
which allows the detection of such weld-lines within plastic products.
(Photo courtesy of Kim)
 
Kim implemented a scanning method when conducting experiments to prove his theory. While attaching a mirror to the terahertz radiation, he moved the specimens according to their reflections. In order to make the reflections happen, the mirror was given a metalized-coating, considering the fact that terahertz surpasses all materials but metal. He managed to make a reflective-equipment that made the reflections occur on a much faster period, which allowed him to gather more results in a shorter time. According to Kim, the equipment is in its initial stage, yet developments are still being made towards totally eliminating minor errors.


Hardships and future plans

Despite achieving striking results, Kim also had hardships while conducting his research. Kim is a professor in the department of mechanical engineering, whereas terahertz research is related to the field of electronic engineering. Conducting research in a totally different field irrelevant to his major resulted in Kim having to look into two completely distinctive areas. However, he continued his research with only his students, without engaging in any form of joint research with others from departments in more closely-related fields.
 
Oh Gyung-hwan, one of Kim's student assistants and co-leaders of this research, and Kim (left and right) commented on the importance of one finding his or her own reasons for conducting research and remarked that they want to help students find such causes.

Such thought may result from Kim’s belief that research must be conducted in a positive manner. Kim mentioned, “I want my students to find their own reasons of pursuing specific research, while being proud of their achievements at the same time." He also added that this research was also conducted worldwide, and hence, his students should be proud of the significant results their global research. As for his future plans, Kim declaimed, “Despite my hardships in this study, I would like to do more research in a variety of other fields, while maintaining a firm stance within my major of mechanical engineering.”  
 



Choi Seo-yong
        tjdyd1@hanyang.ac.kr
Photos by Choi Min-ju

 
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