Utilization of Direct Metal Laser Sintering in Injection Mold Design

Transcription

1 Utilization of Direct Metal Laser Sintering in Injection Mold Design JAN NAVRATIL, MICHAL STANEK, STEPAN SANDA, MIROSLAV MANAS, DAVID MANAS, ALES MIZERA, MARTIN BEDNARIK Tomas Bata University in Zlin nam. T. G. Masaryka 5555, Zlin CZECH REPUBLIC Abstract: - The aim of this research paper is to design two variants of cooling systems (cavities) for an injection mold. Both systems are designed to be used on the same injection mold for producing the same product. The difference is in the manufacturing technology itself. The first variant was made by conventional methods of machining and the second variant was made by combination of conventional methods and unconventional rapid prototyping technology Direct Metal Laser Sintering (DMLS). It was dealt with design of the universal injection mold frame and both cooling systems for an existing product in the first part of this research paper. In the second part were both designs compared by mechanical analysis and their influence on final product by flow analysis. Last part is focused on economical evaluation of both designs. Key-Words: - Rapid Prototyping, Direct Metal Laser Sintering, Injection Molding, Analysis, Mold, Part 1 Introduction Rapid prototyping belongs among modern manufacturing technologies, where the resulting product is made by adding material layer-by-layer, unlike subtracting technologies (milling and drilling), where the resulting product is made by removing material [1-3, 13-19]. Fig.1 Manufacturing technologies There are several rapid prototyping technologies based on the adding material, the main differences are in a used material and product building technologies. Between this technologies belong, for example; stereolithography (SL), laminated object manufacturing (LOM), fused deposition modeling (FDM), selective laser sintering (SLS) and direct metal laser sintering (DMLS) [4-15]. All these technologies are based on creation of real product directly from 3D CAD data in a few hours; this results in speeding up process planning and tooling design, because of possibility to provide a real product at an earlier design stage [1, 4, 5, 21]. Laser sintering is one of the leading commercial processes for rapid fabrication of functional prototypes and tools. The process creates solid three-dimensional objects by bonding powdered materials using laser energy [6, 23, 24]. Direct metal laser sintering (DMLS) allows creating fully functional metal part without using any tools and without any shape restrictions. The parts produced by this technology have mechanical properties fully comparable with cast or machined parts. Benefits of this technology increases with shape complexity; that means the more complex part, the more economical the process becomes [9-11, 17-23]. Principle of this technology is based on melting very thin layers of metal powder. The process begins by applying first layer onto a steel platform and melting required contour. Then is another layer applied and process continuous until the whole part is made. Minimal thickness of each layer is 20 µm. It is also necessary to use supporting structure, which is applied simultaneously with base material. The supports are necessary because the powder itself is not sufficient enough to hold in place the liquid phase created by melting required contour [11, 16-22]. Once the part is created then it goes through some finishing operations including support removal, shot peening and polishing. Between advantages belong besides shape complexity and ISBN:

2 cost and time savings also possibility of recyclation of about 98% of not used powder. It also has some disadvantages such as limited working space, high acquisition costs and high porosity [11-12, 25]. 2 Design Child seat was chosen as a sample of the injected part. This product was chosen because it is complicated enough to use Direct Metal Laser Sintering technology. Its basic dimensions are 359*399*374 mm (height*width*depth). 2.2 Injection Mold Design of injection mold was made in consideration of interchangeability, this means that can be, for example; unconventionally manufactured cavity used together with conventionally manufactured core without any change in injection mold, however; comparison was carried out only with standard organization conventional versus unconventional cavities organization. The injection mold itself was specifically designed due to the shape of injected product, where could not be neither ejector nor gate traces visible on the exposed side. Therefore injection and ejection systems are on the same side in this case on the right. Hot injection system with three hot nozzles was chosen to carry out an injection of melted polymer. Ejection was carried out by eight standard ejection pins and control of this system was implemented by four external hydraulic pullers. Partially original parts and partially HASCO standards were used for designing whole mold. Fig.2 Injected part 2.1 Material Polypropylene was chosen as a product material due to its advantageous combination of processing properties, high stiffness, low price and availability. For analysis purposes was an actual polypropylene chosen, its trademark is Daplen BH 345 MO and the supplier is Borealis company. Its basic properties can be seen in table 1. Table 1 Material properties MATERIAL PROPERTY VALUE Elastic modulus E [MPa] 1340 Shear modulus G [MPa] Melt flow index MFI [g/10min] 45 Shrinkage [%] 1.34 Hardness [HRC R-scale] 89 Fig.3 Injection mold left side Fig.4 Injection mold right side ISBN:

3 2.3 Cavities Cavities were divided into several smaller parts for easier manufacturing and possible repairs. These parts were manufactured by using only conventional methods of machining in case of conventionally manufactured cavities and in case of unconventionally manufactured cavities were their parts manufactured partially by conventional methods and partially by using Direct Metal Laser Sintering technology (marked parts), due to its very expensive manufacturing process. This can be seen in figures 5 and 6. All parts were assembled with screws and centered with pins. Fig.5 Core Fig.7 Core cooling channels Fig.6 Cavity 2.4 Cooling Systems Cores and cavities are the same on the first sight; the difference is in the cooling channels inside them. Input and output channels are same for both variants because of keeping interchangeability. Diameter of cooling channels varies from 6 to 12 mm. Water was chosen as a coolant liquid. Its temperature was 30 C and its pressure was 3.5 bar. In both cases was effort to design as good cooling system as possible to determine whether investment in to the direct metal laser sintering technology will even pay off at this product. Fig.8 Cavity cooling channels ISBN:

4 As can be seen from figures 7 and 8 most of the channels are completely the same for both variants and only at the most problematic areas was the direct metal laser sintering technology used for creation more complex cooling channels. This should also result in significant cost savings. 3 Analysis Flow and mechanical analysis was done to compare both variants. 3.1 Flow Analysis This analysis was done in Autodesk Moldflow Insight 2011 under same process parameters, which can be seen in table 2. Table 2 Process parameters PROCESS PARAMETERS VALUE Mold surface temperature [ C] 35 Melt temperature [ C] 200 Mold-open time [s] 5 Coolant temperature [ C] 30 Coolant pressure [bar] 3.5 Ejection temperature [ C] 115 There were only few different results among many obtained. The most significant of them was time to reach ejection temperature, where the difference between conventional and unconventional cooling system was 16 seconds. Ejection temperature was set to 115 C according to the material list; this temperature was achieved after s in case of conventional cooling and after s in case of unconventional one (figures 9 and 10). However this time was necessary only in places which does not restrict ejection of the part, therefore was ejection time set to 55 s in conventional cooling system and to 40 s in unconventional cooling system. Fig.10 Time to reach ejection temperature unconv. Filling time was another of the examined results and it was found out that both cavities are filled almost in the same time. Conventionally manufactured cavity was filled in s and unconventionally manufactured cavity was filled in s. The difference in both variants is minimal because there is very little influence of cooling system on this parameter. Fig.11 Filling time conv. Fig.9 Time to reach ejection temperature conv. Fig.9 Filling time unconv. ISBN:

5 3.2 Mechanical analysis Mechanical analysis was done in Catia V5R18 only at one cavity which was chosen core. Investigated injection pressure was approximately 47 MPa in both cases, but loading pressure was increased by safety coefficient to 50 MPa. For analysis purposes were both cavities clamped exactly the same like in the injection mold frame. Calculated von Misses stress was 502 MPa in case of conventionally manufactured core and 817 MPa in case of unconventionally manufactured core (figures 13 and 14). Despite higher stress in the second case the safe stress (1267 MPa) was not exceeded, therefore both variants comply. Table 3 Costs Figure 15 shows that despite higher investment costs to the direct metal laser sintering technology; it will be paid off after cycles due to a shorter injection molding cycle. This means that this variant will be better choice if the planned production is higher than cycles. Fig.15 Cost comparison Fig.13 Von misses stress conv. Fig.14 Von misses stress unconv. 4 Economical evaluation Economical evaluation was done according to the pricing of the manufacturing process of both variants, furthermore according to fixed costs and cycle length (table 3). 5 Conclusion In this research paper were designed injection mold and two types of cavities for one injected product child seat. Both cavities were compared from flow and mechanical points of view and economically evaluated at the end. Investigation of flow analysis results showed that both variants are equally suitable for chosen injected product, because the only significant difference was shorter injection molding cycle at unconventionally manufactured cavities. Mechanical analysis showed that both variants comply because neither of variants exceeded allowed von misses stress. Economical evaluation showed that using unconventional variant is eligible only for higher productions. Acknowledgement: This paper is supported by the internal grant of TBU in Zlin No. IGA/FT/2012/041 funded from the resources of specific university research and by the European Regional Development Fund under the project CEBIA-Tech No. CZ.1.05/2.1.00/ ISBN:

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