Rapid prototyping (RP) technologies have been widely used to reduce the lead-time and development cost of new products. Most of the developed thin-layered RP technologies have disadvantage due to their working principles: large building time, stair-stepped surface of a part and additional time consuming post-processing. In addition, it requires high cost to install, operate and maintain the RP apparatuses. The thick-layered RP technologies employ mostly very thick layers thicker than 10 mm, expensive cutting systems and fully manual or extremely large automatic manipulation systems. Hence, a new effective RP process is needed for RP technology innovation of RP and to meet the needs of the RP market.
In the present work, a new effective rapid prototyping process, Variable Lamination Manufacturing Process using expandable polystyrene foam (VLM-S), has been developed to improve building efficiency including a building time and cost, to extend capabilities of the current RP technology toward the fabrication of practical parts covering the volumetric range from small-and-medium size and to medium-and-large size, to minimize the post-processing, and its equipment cost. In addition, thermal characteristics of hotwire cutting for expandable polystyrene foam (EPS) is investigated to improve the quality of parts and to estimate the optimal cutting conditions.
VLM-S process has several advantageous features such as thick layers with a thickness of less than 4 mm, sloping surfaces with the first order approximation between top and bottom surfaces of each layer, the concept of unit shape layer (USL) and unit shape part (USP), and a building sequence in that it performs stacking and bonding after cutting. The thick layer and the sloping surfaces have reduced the building speed, while maintaining the dimensional accuracy in a reasonable range. Using the concept of USL and USP, the problems of cutting for a multiply connected domain have been overcome. The majority of excessive material has been removed by the building sequence with the cutting and stacking processes, so that the post-processing has been minimized.
In order to implement the concept of VLM-S process, two types of apparatuses have been developed. The two types of VLM-S apparatuses, VLM-ST and VLM-SP, apparatus have several common technical novelties such as the combination of a hotwire cutter and EPS foam, an automatic synchronized four-axis hotwire cutter.
In order to obtain the mechanical and thermal properties, the tensile tests and thermal tests have been carried out. From the results of the tensile tests, it has been found that the EPS foam sheet has anisotropic characteristics, resulting in a significant difference in mechanical properties of the sheet. The thermal properties, such as melting temperature and decomposition temperature, have been investigated by melting and decomposition tests using TGA (Thermal Gravimetric Analysis) and DSC (Differential Scanning Calorimetry). Bonding strength has been determined by the shear-by-tension-load-test as well as the shear-by-tension of laminated assembly test.
Various hotwire cutting tests and numerical analyses have been carried out to obtain the operating conditions of the hotwire cutter and to investigate the influence of operating parameters on dimensional accuracy and part qualities. The allowable working range of the hotwire cutter has been estimated by the normal cutting test in the material direction. From the results of the normal cutting test and numerical analysis, it has been shown that the material anisotropy of EPS foam results in a significant difference in maximum allowable cutting speed with respect to material direction. The influence of cutting speed and heat input on the kerfwidth and the cutting offset has been studied by cutting tests and numerical analyses in case of normal cutting. From the results of cutting tests, the relationship between the effective heat input and the kerfwidth or the cutting offset has been derived. Through the numerical analysis, the accurate temperature distribution in EPS foam sheet during hotwire cutting has been computed to estimate the kerfwidth and to predict the size of thermal front when the hotwire loses its stiffness
The influence of cutting angle on the kerfwidth and parts quality has been investigated by the inclined cutting tests and numerical analyses in case of single-sloped cutting. In the numerical analysis, the hotwire cutter is assumed as a volumetric heat flux with an elliptical cross-section. From the results of cutting tests, it is noted that cutting angle is a minor parameter to affect the kerfwidth and the cutting offset of a part as compared with heat input and cutting speed. In addition, the modified relationship between the kerfwidth or cutting offset and effective heat input has been derived from the results of single-sloped cutting. From the results of single-sloped cutting, it has been known that edge sharpness is strongly affected by cutting angle. Through the investigation of the cut surface, it has been known that surface roughness and quality are not appreciably influenced by the cutting which are determined by the effective heat input. Through the numerical analyses, the accurate temperature distribution in EPS foam sheet during hotwire cutting has been computed. From the results of the analysis, it has been known that cutting angle causes an asymmetrical thermal field in EPS foam and the asymmetry of thermal field is widely spread into the inside of EPS foam sheet when cutting angle is increased. The influence of cutting angle on the kerfwidth and the edge sharpness has been investigated by numerical analyses and cutting tests in case of generally sloped cutting described by two cutting angles. In the numerical analysis, the hotwire cutter is assumed to be a volumetric heat flux with an elliptical cross-section considering the influence of two cutting angles, as in normal cutting. The fully conformed mesh is introduced to obtain an accurate temperature distribution in case of generally sloped cutting. From the results of the analysis and the experiment for generally sloped cutting, it is shown that the rotational angle about the x-axis does not appreciably affect the kerfwidth and the sharpness of the edges.
In order to demonstrate the capabilities and advantages of the VLM-S process for fabrication of three-dimensional shapes with geometrical complexity, various prototypes have been fabricated from the VLM-S system. From the results of fabrication for three-dimensional shapes, the prototypes of the VLM-S process show good geometrical conformity in the CAD model. In addition, all prototypes could be fabricated within forty minutes and the building cost was less than $8. The dimensional accuracy of each shape is mostly less than 1% in plane direction and 0.2% in the building direction. Based on the above results, it has been shown that the VLM-S has various advantages and prominence capabilities for general three-dimensional shapes. In addition, from comparison of several prototypes by the proposed process with those of the commercialized RP systems, it has been shown that the VLM-S process is an efficient rapid prototyping process.
오늘날 수요자의 다양한 기호에 따라 제품 모델이 다양화 되고, 국내외 시장에서 경쟁이 치열해지면서 제품의 개발 기간과 시작 기간 및 비용의 단축이 절실이 요구되고 있다. 최근 이러한 시대적 요구에 따라 제품 개발 기간과 비용을 최소화할 수 있는 동시공학적 개념 제품 개발 방식인 쾌속조형공정이 폭 넓게 사용되고 있다. 현재 개발된 쾌속조형공정은 수직벽을 가지는 얇은 층을 이용하는 방법과 경사면을 가지는 두꺼운 층을 사용하는 방법으로 나눌 수 있다. 대부분의 수직벽을 가지는 얇은 층을 이용하는 쾌속조형공정은 장시간의 조형 시간, 제품 표면의 단층 및 추가적인 후처리 공정이 소요되며, 쾌속조형장치의 경우 고가의 장비 구입비 및 운영비가 소요되는 단점이 있다. 또한, 경사면을 가지는 두꺼운 층을 사용하는 쾌속조형공정은 10 mm 이상의 두꺼운 층을 사용하므로써 비교 작은 형상에는 적합하지 못하며 고가의 레이저, 워터젯과 같은 고가의 절단 장치를 사용하며, 수동식 접착/적층을 방법을 사용하고 거대한 가공재료 운송 및 처리 장치가 소요되는 단점이 있다.
본 연구에서는 제품 조형 시간을 현저히 감소시키고 후처리 공정이 거의 소요되지 않으며, 제품 정밀도가 개선되고 장치의 도입 및 운영비가 최소화되는 새로운 개념의 효율적인 쾌속조형공정인 발포 폴리스티렌 폼을 이용한 가변 적층 쾌속 조형 공정 (VLM-S)을 제안하고자 한다.
VLM-S 공정 개발을 위하여 공정 설계 및 장치 개발을 수행하였다. VLM-S 공정은 측면 경사각을 가지는 한층 두께가 4 mm 이하의 재료의 연속적 적층에 의한 제품 제작 시간 단축, 제품 정밀도 개선과 제품 크기가 중소형부터 중대형까지 폭넓게 적용성 및 절단 후 적층에 의한 후처리 작업의 최소화되는 기술적 고유성이 있었다. VLM-S 장치는 EPS 폼을 열선 절단기로 절단함으로써 절삭 저항이 매우 적어 쾌속절단이 가능하며 재료와 열선 절단기의 장비 도입비 및 운영비가 매우 저렴하여 전체 VLM-S 시스템의 장비 도입비 및 운영비가 매우 저렴하며 빠른 시간내에 쾌속 조형 할 수 있는 특징이 있었다.
재료 특성 분석과 접착 강도 평가를 수행하여 EPS 폼 내부에 재료 이방성이 존재함을 확인하였고 EPS 폼의 기계적 물성 데이터와 용융 온도 및 열분해 온도 절단와 열 특성에 대한 데이터를 취득하였으며, EPS 폼과 접착제 사이에 층간 분리가 발생하지 않음을 확인하였다.
열선 절단 실험과 유한 요소 해석을 통하여 열선 절단기에 의한 EPS 폼 절단시 열선 절단 공정 변수에 의한 EPS 폼 내부의 열 특성 변화를 연구하였다. 실험 및 해석 결과 열선 절단기의 최대 절단 가능 조건 및 최적 절단 조건을 취득하였다. 또한, 열선 절단기 공정 변수가 제품 정밀도, 모서리 용융량 및 표면 조도등에 미치는 영향에 대하여 정량적으로 평가하였다. 본 실험 및 해석 결과 열선 절단기 공정 변수와 절단 폭 및 모서리 용융량에 대한 관계식을 얻을 수 있었다. 해석 결과와 실험 결과를 비교 분석함으로써 열선 절단기에 의한 EPS 폼의 해석 모델을 개발하였으며, 열선이 강성을 잃지 않고 절단할 수 잇는 임계 유효 열입력 값을 취득할 수 있었다.
최종적으로 3차원 형상을 제안된 VLM-S 으로 제작하여 본 공정의 적용성을 입증하였다.
VLM-S 공정으로 제작된 제품은 모두 40시간 이내 제작되었으며, 제품 제작비는 약 $8 이하로 소요되었고 제품 정밀도는 평면내에는 1 %, 조형 방향에서는 0.2%의 정밀도를 나타내었다.
본 공정으로 제작된 제품과 상용화 공정인 박판 조형 공정 (LOM) 및 용착 조형 공정 (FDM) 으로 제작된 제품을 제작 시간, 제작 단가 및 제품 정밀도 측면에서 비교하여 본 공정의 효율성 입증하였다.
