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Dimensional study of thermoplastic parts made using sequential injection molding.

INTRODUCTION

Injection molding is the most popular process of plastic manufacturing for complex parts, because of its economical advantages for large series, different shapes capabilities, and high production rates. In the same way, narrow tolerances can be achieved if the mold and the process are properly designed. All these reasons make the injection molding process a necessary element for automotive industry, especially when all companies are interested in offering attractive parts and lighter components, reducing car fuel consumption. In addition, films or textile layers can be over-molded, getting a new appearance of aesthetic parts.

Basic rules for injection molding design are well developed in different research articles and literature since the 80s (1), (2), and the computer aided engineering (CAE) changed the traditional sequential development method to a concurrent one where all the actors involved are working together from the beginning of the project (3).

Injection molding has many variations (4) over the basic procedure to produce new parts improving their stiffness or reducing assembly operations, for example. These special injection methods, not only sequential injection but gas-assisted or injection-compression, are not always analyzed with care, giving to their users inadequate results. For example, gas-assisted technology offered shorter cycle times but could not be used easily, because it required specific part design, or gas extends in non uniform ways inside the thickness of plastic part, giving a non adequate aspect (5), (6).

In this way, there are no technical references for sequential injection molding far from its basic definition and its general advantages (7-11): this technology uses independent gate opening control on each injection point, operated by a computer system. The mold requires a hot runner system. Fig. 1 (mold used in this work), and a control device, adding new costs not required in a conventional system (mold investment plus operating costs). Figures 2 and 3 show a conventional filling sequence for a rectangular part using two injection points. Weld line will be placed between injection gates, and the aesthetical aspect and the mechanical strength will be poor. Figures 4 and 5 show sequential injection process for the same plastic sample: first, only one gate is open: when polymer flow reaches the second gate this will be opened, avoiding in this way weld lines caused by shock of both flow fronts. After this action, generally previous gates remain closed. At the beginning of the packing phase, all the gates will be opened again, to improve part quality. Figure 6 shows conventional filling of real parts, and Fig. 7 illustrates the sequential process. Authors describe in (12) that weld line elimination in sequential process, giving interesting results about mechanical behavior of different samples.

[FIGURE 1 OMITTED]

This injection molding technique is much used for complex parts which can not present visible weld lines as dashboards or car bumpers. Plastic painted parts are usually made by sequential injection too due to flaws caused by weld lines. The designer has to combine weld line position and general rules during mold design, and many items are manufactured successfully (as dashboard or car bumpers).

However, designers should be careful with common rules cited above, because under simple rules, the position of some gates may increase clamping force required due to "overpacking" effects (13-15).

This article is focused on results about shrinkage in injection molding comparing conventional and sequential samples made using a prototype mold. Other added results are the needed pressure to fill the cavity and the sample weight. Both are included to explain some significant differences between both technologies. All these data were obtained at T.I.I.P. research group. Unit Associated to C.S.I.C., (University of Zaragoza. Spain) with the support of Foundation a.i.T.I.I.P. (www.aitiip.com). which lent its facilities for the injection trials.

MATERIALS AND METHODS

For this study, a prototype mold was made. The mold was planned looking for a frontal weld line and it is also prepared, changing the cavity plate, for injecting an adjacent weld line in the future as the literature explains (16), (17). Figure 1 shows a scheme of the actual part modeled using C-Mold software (AC-Technology), showing the hot runner system and the layout of mold cooling lines. This computer program is used in this article to illustrate how the flow fills the mold in the types of injection described.

The specimen tested is a rectangle, 450 X 150 mm. 2.5 mm thick, with two injection points separated 150 mm. A Mold-Masters[TM]hot runner was used equipped with two Dura[TM] hydraulic valve gates. This disposition generates a weld line very well defined, when conventional process is used and the defect can be avoided with sequential injection. Indirect gate was added between the hot runner and the cavity to improve the thermal isolate from the manifold plate, eliminating "hot spots" and to look for symmetry in the thermal conditions for both faces of the plastic part. The reader can revise and compare again the melt front advancement simulated with both injection technologies at Figs. 2 and 3 (conventional); at Figs. 4 and 5 (sequential process); and. furthermore, the real samples injected in the Figs 6 and 7.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Mateu&Sole 340 tons injection machine was used, equipped with a hydraulic pressure sensor. This device was combined with three pressure cavity sensors in the mold. These transducers were mounted in significant positions: near to one gate, center of the part--where the weld line is formed, and at the end of filling close to the edge of one short side. Figure 8 indicates those locations using an image captured from C-Mold software, reproducing a sequential process (notice that two different flow directions can be selected, we will discuss later).

[FIGURE 8 OMITTED]

An independent data recorder (Dataflow software, from Kistler) was used for register the information from machine and the three cavity pressure transducers.

Under each set-up conditions, eight shots were made to calculate average values. The sequential injection needs a control device for valve gate opening, using the controls of the injection machine or an independent computer desk. New software was developed by the a.i.T.I.I.P. team with this intention.

A polypropylene grade for automotive components. DSM Kelburon 95694, was selected for all experiences, (this material is used for bumpers, and it has an EPDM fraction to improve its behavior in the impact tests).

Setting up conditions were selected around generic producer recommendations: (a) temperature 215 and 230[degrees]C for plastic material and hot runner zones; (b) the filling of the mold cavity was made under ram speed control at constant flow rate until 95% of the cavity volume; (c) two values of filling time were used, 1 and 3 s, and finally, (d) during the packing phase two constant pressure levels were applied: 20 and 35 hydraulic bar. The result of cooling time plus the packing time remained constant at 45 s.

As we presented before, the sequential injection allows two directions for mold filling, depending on which valve gate was operated in first place. So, we are able to compare the experimental results for first sequential direction (see Fig. 8) and sequential reverse direction (see Fig. 9). Notice that the pressure transducers were not placed in a symmetrical way for both directions in mold filling, because they are fixed in their special locations.

[FIGURE 9 OMITTED]

RESULTS AND DISCUSSION

In the conventional process, all the samples show a visible weld line located in the middle of the part. For the sequential process, this defect is eliminated in all the experiences and the appearance of the part is improved. This is the main reason that forces the designers to use sequential injection molding. Authors describe in (12) that some results evaluating the mechanical behavior under tensile forces for specimens injected using conventional and sequential methods, which show that weld line reduce the tensile strength and make brittle fracture in normalized samples, according to Stevenson (9).

Other two basic results were obtained: first, experimental data from hydraulic pressure and cavity pressure to show differences between both technologies and to evaluate the integral of pressure; and, in the other hand, weight and main dimensions for injected samples, using a precision balance and a digital caliper. The complete results are not shown in order to make much easier the text; all the experiences had similar tendencies.

Table 1 shows the selected results of the maximum hydraulic pressure required depending on the process type. These experiences indicate that pressure in the sequential process was higher than in conventional filling. This is an expected result because the distance of flow when plastic reaches the second gate is longer in the sequential process. Notice that this maximum pressure does not always occur at the end of the filling phase, it is connected with the design of feeding system. This high internal pressure forces the raise of pressure level to complete the cavity during packing phase. For example, several samples were unfilled until an increase for pressure in the packing phase in order to force the material to reach the end of the cavity (sequential injection case with temperature set at 215[degrees]C and filling time at 1 s required an increase of packing pressure from 20 to 25 hydraulic bar).
TABLE 1. Compared pressure for conventional and sequential process
under several conditions.

                  Setting conditions for both
                processes. Screw rotation speed:
                 55 rpm; Screw diameter: 75 mm.
                      Total cycle time: 50 S

Conventional      Melt        Filling    Holding     Sequential
  process       temperature    time (s)   pressure     process
               ([degrees]C)

55 bar            215            1        35 bar    69 bar (+ 25%)
44 bar            230            3        35 bar    49 bar (+ 11%)
43 bar            230            3        20 bar    53 bar (+ 19%)
53 bar            230            1        35 bar    66 bar (+ 259%)

Maximum hydraulic pressure required at end of filling stage.


Both results indicate that sequential process should not be considered as "low pressure process" in the technical literature without further remarks. Only when the mold maker increases the number of injection points or combines the sequential process with additional technologies as injection--compression, this sentence will become true. Mold design and operative costs have to be considered as these restrictions before to plan the manufacture of new plastic parts using this technique.

Table 2 shows values of the pressure integral for both processes. This value is strongly connected with related to dimensional quality and mechanical properties of molded parts, as it is well established in the literature. The integral of hydraulic pressure shows that all the experiences, independently from process type, could be compared because this value is the same.
TABLE 2. The value of integral pressure under different setting
conditions (bar s).

            215[degrees]C 1 s 35 bar  230[degrees]C 3 s 20 bar

Conditions  Conventional  Sequential  Conventional  Sequential

Hydraulic       457           458         330           343
Gate           3354          3405        2048          2312
Weld line      2403          2522         784          1193
Cavity end      907          1118         110           332

            230[degrees]C 1 s 35 bar

Conditions  Conventional  Sequential

Hydraulic        452          456
Gate            3366         3457
Weld line       2608         2792
Cavity end      1149         1338

Hydraulic values show filling parameters accuracy.


Notice that, depending on flow direction, this value of the integral of pressure is different for the gate and cavity end position in all the examples (Table 3). The value of integral of pressure for the conventional process is placed between sequential ones depending on plastic flow direction. These differences indicate that local shrinkage will not be the same depending on selected process, or even more, depending on flow direction.
TABLE 3. The value of the integral pressure under different selling
conditions (bar s).

                      215[degrees]C 1 s 35 bar

Conditions  Conventional  Sequential  Sequential, reverse
                                         flow direction

Hydraulic       457           458             460
Gate           3354          3405            3239
Weld line      2403          2522            2494
Cavity end      907          1118             863

                     230[degrees]C 3 s 20 bar

Conditions  Conventional  Sequential  Sequential, reverse
                                        flow direction

Hydraulic       330          343              347
Gate           2048         2312             1935
Weld line       784         1193             1187
Cavity end      110          332              179

                    230[degrees]C 1 s 35 bar

Conditions  Conventional  Sequential  Sequential, reverse
                                        flow direction

Hydraulic       452          456             456
Gate           3366         3457            3298
Weld line      2608         2792            2768
Cavity end     1149         1338            1147

Flow direction is compared for the sequential process.


This asymmetric effect could be observed from other perspective regarding Figs. 10, 11, and 12, where we compared under identical setting conditions pressure curves from each transducer. In the Fig. 10, the sensor placed in the weld line shows a trace independent from tilling flow direction, whereas Figs. 11 and 12 pointed out the effect of flow direction "close to the gate transducer" and "far from the gate transducer". The conventional process has no differences because its filling is symmetric and easily anticipated.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

After these results, and according to theory of shrinkage and PvT diagrams (18), (19), local flaws as sink marks or voids can not be well predicted or controlled if pressure is not uniform in the mold during process.

In authors opinion, this asymmetric filling is the most significance characteristic of sequential process for this mold, and probably it has to be considered for many others parts as door panels, dashboards, or other aesthetical automotive parts to get a proper mold designed.

This conclusion allows suggest a basic rule for mold designers: a symmetric filling, should be regarded to avoid unexpected shrinkage behavior, or differences will be shown between both sides of the final component.

For the whole injected part, the measures (length and width) that we get when we use both processes are similar as Table 4 shows. The small differences are in the error limits of the measurement device. Linear shrinkage was encountered between the material limits (around 2-2.2%) for all samples, mold--makers can use data from material suppliers to machining molds.
TABLE 4. Experimental measurements for different samples produced
using conventional and sequential injection.

Conditions                215[degrees]C  1 s 35  bar  215[degrees]C
                                                       3 s 20 bar

Conventional

Longitudinal measurement            444.9                443.6
(mm)

First transversal                   147.8                147.3
measurement (mm)

Second transversal                  147.8                147.3
measurement (mm)

Sequential (first or      First                Rev        Rev
reverse direction)

Longitudinal measurement  444.8               444.8      443.8
(mm)

First transversal         147.8               147.8      147.6
measurement (mm)

Second transversal        147.9               147.7      147.4
measurement (mm)

Conditions                215[degrees]C 3 s 35 bar   230[degrees]C
                                                      1 s 35 bar

Conventional

Longitudinal measurement          444.6                  444.8
(mm)

First transversal                 147.8                  147.9
measurement (mm)

Second transversal                147.8                  147.9
measurement (mm)

Sequential (first or      First            Rev            Rev
reverse direction)

Longitudinal measurement  444.9           444.7          445.0
(mm)

First transversal         147.8           147.8          148.0
measurement (mm)

Second transversal        148.0           147.9          147.8
measurement (mm)


However, differences were found when measuring the both sides of the piece in the samples made with the sequential process. These changes were not found in conventional process. As we said before, results show an important divergence for sequential process depending on flow path, and this reason produces the alterations.

Finally, all the parts were weighed and heavier parts were made, in the most cases, using conventional injection. Table 5 shows these values. The conventional technique seems the best choice under the criterion of higher weight, although differences are small due to small weight of the part. As we can imagine, the aspect of big components could be affected when the sequential process was used, if sink marks could appear due to the presence of ribs under some aesthetical surfaces, or especially with asymmetrical fillings if there is one main flow direction.
TABLE 5. Experimental weight values for samples produced using
conventional and sequential techniques.

Sample        215[degrees]C 1  215[degrees]C 3 s  215[degrees]C 3 s
conditions        s 35 bar         20 bar             35 bar

Conventional       136.0           123.2              133.9
weight (g)

Sequential         134.9           124.1              133.5
weight (g)

Sample        230[degrees]C 3 s  230[degrees]C 3  230[degrees]C 1 s
conditions        35 bar             s 20 bar         35 bar

Conventional      134.8               123.6           136.1
weight (g)

Sequential        133.7               123.7           135.9
weight (g)


CONCLUSIONS

Sequential injection is widely used in many molds nowadays because of high aesthetical requirements imposed to plastic parts. Weld lines are eliminated or moved to hidden locations using an adequate sequence of valve opening. However, mold makers and automotive suppliers are not always prepared to understand and assume all new commitments that this new technology implies.

In this article, a prototype mold was designed and injected using a 340 ton machine. Comparative values of shrinkage were obtained using conventional and sequential processes in plastic parts injected.

For mold shrinkage, similar values were encountered and data provided for material suppliers can be considered as correct for both techniques. In spite of this conclusion, manufacturers could discover unexpected defects when the cavity will be filled with an asymmetric way using sequential injection. These flaws will be much more significant when the size of the piece increases, because the gradient of pressure will be higher

Under the weight criterion, the conventional process is better if the injection conditions are equal. Even more, using some conditions, some short shots were encountered with sequential injection molding because higher filling pressure was needed.

Finally, the filling pressure and, logically, the clamping force, will be greater for sequential process because of longer flow path if the design of the tool is not considered in the mold layout. To reduce pressure and clamp force, new gates should be added.

New questions are opened now, for example, does this technology affect to the warpage phenomenon? Do the criteria of mold design have to change in sequential injection molding? We hope that the answers are affirmative, but values need further works.

REFERENCES

(1.) M.J. Gordon, Industrial Design of Plastic Products, Wiley, New Jersey (2002).

(2.) H. Belofsky, Plastics: Product Design and Process Engineering, Hanser/Gardner, Munich (1995).

(3.) J. Stevenson, "Lean Molding: faster=cheaper=better" in Innovation in Polymer Processing Molding, Chapter 10, Hanser, Munich (1996).

(4.) J. Rothe, Kunststoffe, 87, 338 (1997).

(5.) U. Resgren and A. Praeller, Kunststoffe, 12, 251 (2004).

(6.) J. Castany, I. Claveria, F. Serraller, and C. Javierre, J. Mater. Process. Technol., 143, 214 (2003).

(7.) W. Michaeli, G. Wisinger, S. Galuschka, and J. Zachert, Kunststoffe, 85, 1878 (1995).

(8.) W. Homes, Kunststoffe, 86, 1268 (1996).

(9.) J. Stevenson, "Controlled Low-Pressure Injection Molding," in Innovation in Polymer Processing Molding, Chapter 5, Hanser, Munich (1996).

(10.) D. Gao, K. Nguyen, P. Girad, and G. Salloum, ANTEC 1994 International Congress, 554.

(11.) D. Kazmer and P. Barkan, Polym. Eng. Sci., 37, 1880 (1997).

(12.) J. Aisa, J. Castany, and A. Fernandez, Revista de Plasticos Modernos, 87, 463 (2004).

(13.) J.P. Gazzonet, Caoutchoucs & Plastiques, 736, 44 (1994).

(14.) C. Javierre, A. Fernandez, J. Aisa, and I. Claveria, J. Mater. Process. Technol., 171, 373 (2006).

(15.) J. Aisa, J. Castany, and D, Mercado, Plast '21. 143. 29 (2005).

(16.) G. Mennig, Kunststoffe, 82, 235 (1992).

(17.) W. Brostow and R.D. Corneliussen, "Knit-Lines in Injection Molding and Mechanical Behaviour," in Failure of Plastics, Chapter 21, Hanser, Munich (1986).

(18.) G. Menges and P. Mohren, How to Make Injection Moulds, Hanser, Munich (1986).

(19.) F. Johannaber, Injection Molding Machines: A User's Guide, Oxford University Press, Munich (1983).

Jorge Aisa, Javier Castany

T.I.I.P-C.S.I.C. Associated Unit, Mech. Dpt of the University of Zaragoza, C/Maria de Luna 3, 50018, Zaragoza, Spain

Correspondence to: J. Aisa: e-mail: jorge.aisa@unizar.es

DOI 10.1002/pen.21310
COPYRIGHT 2009 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Aisa, Jorge; Castany, Javier
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Sep 1, 2009
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