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 %
