Influence of the part shape on the polymer flowinside mould and overall strength.

Baltes, Liana ; Tierean, Mircea

1. INTRODUCTION

In industrial design is always a challenge to make the proper compromise between the most desirable part shape, tooling cost, weight part, how mouldable is the final part and how strong the part will be. As consequence, a nice shape increases the tool cost.

The same effect is obtained by the holes or other areas of the part that are not on the ejection direction, so the tool needs sliders with mechanically, hydraulically or pneumatically action, sliders that increase the tool price (Baltes et al., 2008).

A large thickness of the part can make the part stronger, but will increase the material consumption. Too thin thickness is cheaper, but can cause problems in casting or injection. Too large thickness is stronger as we already mentioned, up to a specific thickness helps the flowing of the materials, too big dimensions can cause problems also in casting process, so the general thickness is determinate on what technology the designer will decide. The values of the thickness, shapes, and ribs are different for each technology but there are some principles that always are the same.

The objective of this paper is to study the influence of the model reinforcement on the mouldability, stiffness of the piece and polymer consumption.

2. THEORETICAL ASPECTS

The commercial programs that simulate the mould filling start with 3D modelling of part, meshing, selecting the moulding process, selecting the material and setting of injection location. After analyse, the results will be: filling time, pressure, average velocity, core orientation, freezing time, bulk temperature, weld lines and air traps (Moldflow, 2002).

Using the Automatic Injection Time check box, the analysis finds the injection time which gives the lowest injection pressure (fig. 1). The variation of injection pressure against injection time has two influences:

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* as the injection time increases from zero, the pressure required to force the molten plastic through the part decreases;

* as the injection time increases, another factor affects the curve; as the flow rate of the molten plastic decreases, its temperature also decreases. This is because as the flow slows, the frictional heating decreases. The decrease in polymer temperature also increases the viscosity, again requiring more pressure to fill the part.

This study was done using ABS, with excellent surface appearance, strength, stiffness, toughness, and chemical resistance. This kind of polymer is used on automotive interior trim, computer housings, small appliances, and other consumer electronics.

The main goal in selecting polymer injection locations is to ensure that all flow paths in the model fill at the same time (balanced flow paths). The positioning of injection locations plays an important role in the effects of material orientation on part deformation. In some cases, changing the injection location position is the only way of controlling orientation effects to produce a satisfactory design. The essence of a good position is to avoid problems associated with overpacking, such as variation in shrinkage and product sticking in the cavity.

This result shows the flow path of the plastic through the part by plotting contours which join regions filling at the same time. These contours are displayed in a range of colors from red, to indicate the first region to fill, through to blue to indicate the last region to fill. A short shot is a part of the model that did not fill, and will be displayed as translucent. By plotting these contours in time sequence, the impression is given of plastic actually flowing into the mold (Tres, 2006; Bar-Meir, 2000).

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3. RESULTS ON MOULD FILLING AND STRENGTH SIMULATION

The study was done on the simple part, a luggage cover (480x380x40 mm, thickness 2mm), considered initially as flat shell, adding successively ribs in different directions. There are analysed the filling time (s) and von Mises stresses (MPa).

Figure 2 shows the flat luggage cover, without any filling problems (4.48s, fig. 3), but not strong enough at any pressure applied in the middle of the part. The material consumption is small, the mold is cheaper, easy to fill (software message: "your part can be easily filled using the current injection locations"), but no resistance at the 100N loading in the middle of the upper wall, resulting a big deformation.

Figure 4 shows luggage cover with 1.5 mm ribs on the flow direction (longitudinal). Filling is improved (4.16s), also the mechanical characteristics of the part ([sigma]=2.15 x [10.sup.5] Pa). As we calculated, the increasing of the material consumption compared with the design of figure 2 is 11.49%. The mold price increase, fewer than 5%.

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In figure 6 there are added some ribs on the radial direction in order to improve the stress behavior of the part (<t=1.88-105 Pa). The part is much stronger, but the tool cost is increasing, the injection cost is increasing and we start to have filling problems (software message: "your part may be difficult to fill"). All flow reports start to be negative. The material consumption increases by comparison with the precedent situation with 8.2%. The mold price increase with other few percents against the percent version.

In the last case studied there are added also transversal ribs. The part toughness increases ([sigma]=5.1 x [10.sup.4] Pa), but we have filling problems (software message: "your part may be difficult to fill"). The material consumption increases with 27.8%.

4. CONCLUSION

F.E.A. research emphasizes the relation between the thickness values, shapes, and ribs orientation. Appling different kind of ribs, starting with radial ribs, the mould could not be fully filled. The best compromise is the part with ribs oriented in the filling direction, which keep a good balance between the material cost, tooling cost, filling time and stress behavior.

5. REFERENCES

Baltes, L.; Tierean, M. & Eftimie, L. (2008). Advanced Design of High Pressure Die Casting Moulds, In: Annals of DAAAM for 2008 & Proceedings of the 19th International DAAAM Symposium, Katalinic, B. (Ed.), ISBN 978-3901509-68-1, ISSN 1726-9679, Vienna, Austria, pp. 0065-0066

Bar-Meir, G. (2000). Fundamentals of Die Casting Design, http://artikel-software.com/file/dieCasting.pdf, Accesed on: 2009-06-27

Tres, P.A. (2006). Designing Plastic Parts for Assembly, ISBN 1-56990-350-6, Carl Hanser Verlag Munich, Germany

*** Moldflow Corporation (2002). Moldflow Plastics Insight, Tutorial

*** SolidWorks Co. (2007). Solid Works documentation