J. Dobránsky, P. Baron, M. Kočiško, Faculty of Manufacturing Tech-nologies with a seat in Prešov, Technical University of Košice, Slovak RepublicL. Běhálek, Faculty of Manufacturing Technologies, Technical Univer-sity of Liberec, Czech RepublicE. Vojnová, 1.PN Moulds and Tools, Prešov, Slovak RepublicThis article deals with improvement of design properties of molded plastic parts. It can be achieved by modifying construction of metal injection mold and optimization of parameters in injection process. The subject of our ex-amination was depressions formed on molded plastic parts which are inacceptable in the process of approval. The problem which has arisen was solved in two phases. The first phase consisted in alteration of injection mold design – enlargement of injection molding gate. In the second phase, we have changed the location of injection molding gate. After performing constructional modifications, new molded plastic parts were manufactured and assessed.Key words: foundry, injection molding, construction, plastic partsINTRODUCTIONMold designing for products made of plastic places high demands on professional level. It is therefore nec-essary that an expert in this field has enough reliable information for implementing it correctly in manu-fac-turing of these products in practice. [1] Once such infor-mation is gathered, a series of questions related to a particular product to be designed is analyzed. [2]First of all, a detailed analysis of product’s function and related requirements on mechanical, thermal, elec-tric and other properties of plastic is carried out. [3,4] We also have to deal with economical requirements (e.g. product price limit, mold production costs), re-quirements for decreasing labor intensity, price and availability of material, visual questions (e.g. shape, color, surface pattern, shine, trademark), questions of packing and transport. [5,6]EXPERIMENTAL PROCEDUREThe subject of testing was molded thermoplastic part called Blende (Figure 1). It is used for headlights of Audi TT3 and serves as a bracket and a component of LED strips. It was made from polycarbonate (PC) with trade name Makrolon 1260. [7,8] This material has ex-cellent mechanical properties; it is hard, firm, solid and resilient. It is widely used in automotive industry in achromatic or colored variant.As it was already mentioned, the subject of our ex-amination is deep depressions on molded plastic parts.Since visual aspect of this molded plastic part is es-sential it must not be corrupted. The problem of deep depressions was noticeable after the first pressing. Thus, our aim was to improve visual aspect of a molded plas-tic part by modifying technology of pressing. By thor-ough examination it was discovered that parts were Figure 1   Molded plastic parts with depressions after 1st testing of a moldFigure 2   Quality percentage of a molded plastic part depending on the value of post-pressure poorly post-pressed, therefore, we decided to increase post-pressure gradually and then observe possible im-provement of quality of tested parts. At this stage, an-other problem has arisen. Higher pressure resulted in producing parts of higher quality, however, allowed threshold values of post-pressure caused deformation of parts during taking them out of a mold. Graphical repre-sentation of quality percentage of a part when increas-ing post-pressure up to deformation of a part caused by a high value of pressure is shown at the Figure 2.FIRST PHASE OF PROBLEM SOLVING – ENLARGEMENT OF INJECTION MOLDING GATEDue to persisting problem with the design of molded plastic parts it was necessary to modify the construc-tional design of injection mold. [9] Since modification of technology of pressing did not eliminate the problem in question we had to search for another way of improv-ing the design of molded plastic parts. Our suggested solution to depressions on parts was enlargement of in-jection molding gate (Figure 3) which is used for inject-ing melted material into the cavity of injection mold. The part is filled directly into the rib from where melted material gets into the whole cavity.The injection molding gate of the mold was enlarged by 0,3 mm in order to fill the cavity of the mold better and faster what decreases the post-pressure. After per-for-ming constructional modifications of the mold and adjusting injection parameters, further testing with in-jection machine took place. Effect of these changes on newly produced molded plastic parts was evident. En-largement of injection molding gate had a positive in-fluence on design of molded plastic parts which were again thoroughly visually examined. Since the design aspect of these parts is essential, our next aim was to improve their design even more, so that these parts were without any noticeable depressions at all (Figure 4).48117
SECOND PHASE OF PROBLEM SOLVING – ENLARGEMENT OF INJECTION MOLDING GATESince enlargement of injection molding gate and ad-justment of injection parameters did not contribute to 100 % improvement of the design of molded plastic parts further examination and modifications were car-ried out. The design was successfully improved by en-largement of injection molding gate, thus we tried to eliminate the problem completely in a similar way. However, further enlargement was not possible because a filling point was in the rib of the molded plastic point and further enlargement would make the injection molding gate larger than the width of the rib what is technically impossible.Thorough examination brought us to conclusion that achieving functionally and visually perfect molded plastic part requires different location of injection mold-ing gate. During construction of the mold it was re-quired to place the injection molding gate into the rib, thus we have introduced the problem and our solution. Our suggested change of the location of injection mold-ing gate was approved.Subsequently, a simulation of flowing melted mate-rial into the mold with new location of the injection molding gate (moved from the rib to the wall of the molded plastic part) was performed and an injection molding insert ensuring functionality of this change was created. This modification was then submitted for approval (Figure 5).Figure 3 A detail of enlargement of injection molding gate  RESULTS AND DISCUSSIONOur effort to improve quality of a molded plastic part by modifying injection parameters and an injection mold was not successful, thus we decided to change the location of the injection molding gate. After performing mold-flow analysis it was evident that changing the lo-cation of injection molding gate can eliminate depres-sions formed on molded plastic parts.Based on this analysis was changed the location of injection molding gate on the mold and on molded plas-tic part as well. Further testing of performed modifica-tions was carried out in order to confirm or refute cor-rectness of our solution to initial problem.New injection parameters resulting from changed location and design of injection molding gate were used during final functional testing. Table 1 shows adjust-ment of injection parameters during the first functional testing and parameters after suggested constructional modifications and recommendations.The final functional testing has proved that changed location of injection molding gate has improved visual aspect of a molded plastic part which was then fully ac-cepted. We have obtained molded plastic parts without any noticeable defects (Figure 6).thus the designer of the molded plastic part chose a spot where no collision with other parts was noticed. After explaining the problem and performing a simulation of injection process we have carried out suggested modifi-cation.The diameter of the original location of injection molding gate after enlargement was 1,65 mm. Since we were limited by width of the rib of the part changing the location of the gate provided larger diameter – 2,1 mm. This means that enlarged diameter (by 0,45 mm) was then used for filling the part. This modification enabled us to put more melted material into the cavity during the same time and the molded plastic part was not deformed by high post-pressure. Moreover, we succeeded in shortening whole injection cycle what has a positive in-fluence on financial aspect.AcknowledgementThis work has been supported by research grant VEGA 1/0013/11 (Innovation of methodology of risk identification and valuation process of undesirable events on technological workplaces).REFERENCES[1]  Ľ. Dulebová, B. Duleba, A. Krzyzak, Study of the mecha-nical properties re-processed of thermoplastic polymer composites, Advanced Materials Research 1001 (2014), 194-198.[2] M. Staněk, D. Maňas, M. Maňas, J. Navrátil, A. Škrobák, SW and HW optimization of injection molding process, International Journal of Mechanics 8 (2014) 1, 85-92.[3] J. Paško, Š. Gašpár, J. Ružbarský, 塑料成型生产过程中形成的凹陷英文文献和中文翻译:http://www.751com.cn/fanyi/lunwen_50468.html