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  • 标题:Injection moulding of thermoplastics in hybrid tools.
  • 作者:Godec, Damir ; Sercer, Mladen ; Pilipovic, Ana
  • 期刊名称:Annals of DAAAM & Proceedings
  • 印刷版ISSN:1726-9679
  • 出版年度:2008
  • 期号:January
  • 语种:English
  • 出版社:DAAAM International Vienna
  • 摘要:Application field of rapid prototyping processes for manufacturing of mould elements is newer dated. Therefore the influence of materials of prototype (hybrid) moulds on moulding properties is still insufficiently explored. Many RT processes include use of non-metal or non-ferrous materials opposed to classic mould manufacturing processes. Mould material properties substantially determine final properties of manufactured moulding (Karpatis et al., 1998). Therefore it is necessary to understand the influence of metal hybrid moulds on moulding properties (Gebhardt, 2003). In this study RT mould inserts are produced by Indirect Metal Laser Sintering process. The material used for RT inserts was the LaserForm A6 steel, and for classic inserts 1.2312 steel. The injection moulding was performed with both inserts, and the properties of moulding were compared.
  • 关键词:Thermoplastics

Injection moulding of thermoplastics in hybrid tools.


Godec, Damir ; Sercer, Mladen ; Pilipovic, Ana 等


1. INTRODUCTION

Application field of rapid prototyping processes for manufacturing of mould elements is newer dated. Therefore the influence of materials of prototype (hybrid) moulds on moulding properties is still insufficiently explored. Many RT processes include use of non-metal or non-ferrous materials opposed to classic mould manufacturing processes. Mould material properties substantially determine final properties of manufactured moulding (Karpatis et al., 1998). Therefore it is necessary to understand the influence of metal hybrid moulds on moulding properties (Gebhardt, 2003). In this study RT mould inserts are produced by Indirect Metal Laser Sintering process. The material used for RT inserts was the LaserForm A6 steel, and for classic inserts 1.2312 steel. The injection moulding was performed with both inserts, and the properties of moulding were compared.

2. IMLS RAPID TOOLING PROCESS

The IMLS procedure uses powder which consists of metal component as the building material and the polymer component as the binder. In the first phase the insert was made by the usual SLS process which is the so-called green phase. The mechanical properties of such pre-form are not sufficient for the application in injection moulding. Therefore, the green insert needs solidification in the furnace. The process in the furnace can be divided into three steps: debinding, final sintering (brown phase), and bronze infiltration (Dalgarno & Stewart, 2001; King & Tansey, 2003). The IMLS procedure results in very good mechanical properties of the mould inserts (Fig. 1).

[FIGURE 1 OMITTED]

The final mould inserts have low porosity, and they can be used for the production of over 100 000 moulded parts. Compared to direct metal laser sintering (DMLS) procedure, the disadvantage is the subsequent treatment in the furnace, and also greater deviations in dimensions (Dalgarno & Stewart, 2001; King & Tansey; 2003).

3. CASE STUDY

Based on the analysis from the literature (Potsch & Michaeli, 1995; Gao et al., 2001), and performed screening design, it was concluded that there are four significant processing parameters of thin-walled injection moulding: injection time, packing pressure, packing pressure time and melt temperature. The packing pressure time, in case of the cold runner system, is reduced to the gate cooling time. An experiment was done to determine the maximum packing pressure time (in this study the value was 5,0 s). For further analysis, the remaining three significant parameters were used, maintaining maximum value of the packing pressure time.

The evaluation of the influence of prototype (hybrid) and classic mould, was based on the monitoring of several properties of the moulded parts. These included the moulded part surface properties (completeness of the moulded part and flush), mass, dimensional stability of the moulded part, deformation of the moulded part, and tensile impact strength.

3.1 Experimental results

For the analysis, the central composite design with three factors was used. The values of arithmetic means of the observed properties are given in Table 1. The resulting difference in the values of the mass of the moulded parts should be assigned o to the higher compressibility of the materials of the prototype mould inserts. Moreover, the flush on the moulded part produced in the hybrid mould occur at significantly lower packing pressures (Fig. 2.) This occurs also, to a lesser extent, in the results of the dimensional stability of the moulded part {Dimension_1 and Dimension_2).

Higher values of deformations of the moulded parts can be explained by the fact that in the injection moulding, as a rule, higher cavity wall temperature was achieved in the hybrid mould (Fig. 3). The moulded parts in case of hybrid mould left the mould cavity at higher temperatures which resulted in greater shrinkage of the moulded parts.

[FIGURE 2 OMITTED]

The reason for such difference lies in the lower heat penetration coefficient of the materials of prototype mould inserts. Lower heat penetration coefficient prevents fast conduction of heat into the interior of the mould, so that higher contact temperature of the cavity wall is achieved. Also, the heating of such wall during a constant injection moulding cycle is faster, so that maximal mould cavity temperature is achieved faster than in the case of classic mould. On the other hand, in Fig. 3 one may observe that the temperature gradient in case of hybrid mould is somewhat greater compared to the classic mould. This can be explained by a higher value of thermal conductivity of the material of hybrid mould inserts, which results in faster change of temperature on the mould cavity wall.

[FIGURE 3 OMITTED]

Impact tensile strength (toughness) of the moulded parts produced in the classic mould is much higher than the toughness of moulded parts made in the prototype mould. The higher toughness of moulded parts made in the classic mould can be explained by the lower temperature gradient of the cavity. In general, the heating and cooling of these mould inserts is slower which is suitable to acquiring higher degree of cristallinity and higher values of toughness.

3.2 Optimizing the processing parameters

Within the research, the optimization (complex and partial) of the parameters with the classic and the hybrid mould was done with the aim of optimizing the observed properties of the moulded parts. As the result of such optimization, the obtaining of the corrected adjustable processing parameters in case of hybrid mould was expected. Based on the results of the optimization, it is possible to provide basic guidelines for the adjustment of the processing parameters in the hybrid mould (Table 2), in order to obtain comparable properties of the moulded parts to those produced in the classic mould.

4. CONCLUSION

By comparing the prototype hybrid and classic mould for the production of thin-wall PP moulded parts, it can be concluded that the biggest differences in the properties of moulded parts result from higher compressibility of the prototype inserts (higher mass of moulded parts, larger dimensions of moulded parts, occurrence of flush at lower values of packing pressure), and the difference in thermal properties of mould inserts materials. The material of prototype mould inserts has lower heat penetration coefficient, which results in achieving higher maximum cavity wall temperature, but also higher temperature gradient than in the mould with classic inserts, due to higher thermal conductivity. Such condition causes higher values of shrinkage of the moulded parts and more difficult maintenance of the dimensional stability of the moulded parts. Also, in the prototype mould, lower values of tensile impact strength are achieved. In order to achieve comparable properties of moulded parts made in both moulds, it is possible to define adequate corrections of the most significant parameters of the injection moulding in using the hybrid mould. The work analyses pure influence of the materials of both sets of mould inserts on the properties of the moulds. The possibility of "conformal cooling" in the case of hybrid mould has not been used. Future research will analyze the influence of different configurations of cooling channels on the mould properties. (Godec et al., 2008)

Acknowledgement

This work is part of the research included in the project Increasing Efficiency in Polymeric Product and Processing Development supported by the Ministry of Science Education and Sports of the Republic of Croatia. The authors would like to thank the Ministry for the financing of this project.

5. REFERENCES

Dalgarno, K. & Stewart, T. (2001). "Production tooling for polymer moulding using RapidSteel process", Rapid Prototyping Journal, Vol. 7, No. 3, pp. 173-179, ISSN 1355-2546

Gao, F., Koresawa, H., Narahara, H., Suzuki, H. (2001). Evaluation of thermal environment in plastic injection mold, Proceedings of ANTEC 2001, pp. 969-973, ISBN 158716-098-6, Dallas, May 2001, Society of Plastics Engineers, Brookfield

Gebhardt, A. (2003). Rapid Prototyping, Carl Hanser Verlag, ISBN 3-446-21259-0, Munchen

Godec, D. (2005). The influence of hybrid mould on thermoplastic moulded part properties, D. Sc. Thesis, Faculty of Mechanical Engineering and Naval Architecture, Zagreb

Godec, D., Sercer, M., Rujnic-Sokele M. (2008). Influence of hybrid mould on moulded parts properties, Rapid Prototyping Journal, Vol. 14, No. 2, pp. 95-101, ISSN 1355-2546

Karapatis, N.P., van Griethuysen, J.P.S., Glardon, R. (1998). Direct rapid tooling: a review of current research, Rapid Prototyping Journal, Vol. 4, No. 2, pp. 77-89, ISSN 1355-2546

King, D. & Tansey, T. (2003). Rapid tooling: selective laser sintering injection tooling, Journal of Materials Processing Technology, Vol. 132, pp. 42-48, ISSN 0924-0136

Potsch, G. & Michaeli, W. (1995), Injection Molding, Carl Hanser Verlag, ISBN 3-446-17196-7, MunchenEquations are centered and numbered consecutively, from 1 upwards.
Tab. 1. Values of the analysed properties (Godec, 2005)

 Arithmetic mean

Moulded part property / Hybrid Classic
Injection moulding parameter mould mould

Mass (g) 8,26 7,82
Dimension_1 (mm) 100,22 100,03
Dimension_2 (mm) 58,12 57,97
Deformation_1 (mm) 1,59 1,47
Deformation_2 (mm) 0,98 0,93
Tensile impact strength (kJ/[m.sup.2]) 96,6 123,48
Contact mould temperature ([degrees]C) 36,55 32,66
Temperature gradient ([degrees]C) 5,76 2,53

Tab. 2. Guidelines for adjusting parameters in hybrid mould
(Godec, 2005)

 Complex
Parameter optimization

Packing pressure +10 %
Melt temperature -5 %
Injection time -60 %
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