Evolution of Surface Texture and Cracks During Injection Molding of Fiber Reinforced, Additively Manufactured, Injection Molding Inserts
This paper investigates the lifetime and surface deterioration of additively manufactured, injection moulding inserts. The inserts were produced using digital light processing and were reinforced with oriented short carbon fibers. The inserts were used during injection molding of low density polyethylene until their failure. The molded products were used to analyse the development of the surface roughness and wear. By enhancing the lifetime of injection molding inserts, this work contributes to the establishment of additively manufactured inserts in pilot production. INTRODUCTION Prior experiments with digital light processing (DLP) or similar technologies and fiber reinforced photopolymer have been performed showing the possibility of manufacturing parts using this process [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] The possibilities of additive manufacturing (AM) for injection molding (IM) were already pointed out earlier in [12, 13, 14, 15] However, the effect of fibers for the lifetime and surface deterioration of the IM inserts was not yet extensively investigated, although significant advantages at the cost, environmental and efficiency levels can evolve. The fiber reinforced polymer (FRP) showed a directional placement of the fibers within the manufactured layers allowing the reinforcement of the insert in two directions [9] The lifetime of additively manufactured inserts made from photopolymer using DLP is located below those of inserts made from brass or steel as supported by [16, 17] and, therefore, lifetime in terms of surface quality was the subject of this investigation. This research contributes to the development of new technologies for injection molding inserts reducing production costs as well as the environmental impact of prototyping and proof ofconcept manufacturing. It was pointed out by [18] that composite materials for IM inserts made from polymer and copper particles improved the heat conductivity of the inserts by increasing the lifetime of the inserts. This investigation was developed further using short carbon fibers. METHODS The IM inserts were produced using DLP from a photopolymer resin with 5%wt short carbon fiber content equipped with an average diameter of 7.2mm and an average length of 100mm extending the research performed by [19] without fiber reinforcement. The layers were placed perpendicular to the expected pressure tensor from the polymer melt from the injection molding process resulting in a fiber placement in the manufacturing layers. The back and sides of the inserts were milled to reduce warpage of the inserts and increase the accuracy of the mold assembly. The inserts were used during manufacturing with an IM machine injecting low density polyethylene (LD PE) at 210 bar maximum injection pressure during a 3 s filling time followed by a 10 s packing time and 10 s cooling phase for the insert in order for the insert to cool down to a temperature of 36 *C.The insert dimensions were 20 x 20 x 2.7 mm. The total cycle time of the molding was 23 s. The additively manufactured insert was built into a multi functional frame in the IM machine as shown in Figure 1. The LD PE was injected from FIGURE 1. Single insert in the IM machine before the first shot. the reverse side of the insert and through channels guided to the mold. The final part before ejection can be seen in Figure 2. Tests of the surface structure in terms of roughness were performed using a focus variation 3D microscope system with a vertical resolution of 500 nm and a lateral resolution of 3mm. The data of the scanned surface was thereafter aligned using global levelling for the inspected areas. The mean roughness was thereafter determined. Moreover, a scanning electron microscope (SEM) JEOL JSM 5900 was used showing the deterioration of the surface during the molding process. A Dino Lite Pro AM 4000 digital microscope was used to inspect the surface of the inserts and the parts after the molding process. RESULTS During this investigation, the observations in [9] concerning outstanding carbon fibers at the border of the object were confirmed as the first manufactured parts showed residual carbon fiber material in the upper layer of the part. The outstanding fibers broke off after the first shot and stuck to the produced part. No residuals were found at the second shot and later. The outstanding fibers in the original IM insert are shown in Figure 3. It shall be noted that no fibers are standing perpendicular to the top surface layer. Surface investigations of produced parts showed cracks of the insert in the mm regime after about 300 shots leading to a change in roughness of the surface of the manufactured part. Compared to other tests on photopolymer inserts without fiberFIGURE 2. PE LD parts arranged in the IM machine. FIGURE 3. Fibers standing out of the produced layers in a SEM investigation confirming the findings in [9] The image was taken before the first shot. The raster pattern on the surface results from the DLP process and the projector resolution. 0 0.05 0.1 0.15 0.2 0.25 0.3 200 400 600 80