Warpage Controller of Thinn Walled Sheets 2015
Warpage Controller of Thinn Walled Sheets 2015
Warpage Controller of Thinn Walled Sheets 2015
a r t i c l e i n f o a b s t r a c t
Available online 29 December 2014 Currently, 3C products are required to be lightweight, portable, and convenient. Injection molding is among the
most used techniques for mass production in plastic processing industries; however, producing thinner parts
Keywords: that do not warp is challenging. Although plastic components warp for numerous complicated reasons, warpage
Injection molding primarily is caused by variations in shrinkage during the injection process of plastic part manufacturing. Material
Neutral axis theory properties, part design, mold design, and processing conditions are factors influencing variations in the part shrink-
Thin-walled molding
age. For example, inconsistent thickness in component geometry, poor sprue–runner–gate or cooling design in the
Warpage
injection mold, and improper molding condition settings may cause plastic parts to warp excessively. Warpage
causes unpredictable component shapes, which may cause poor assembly quality. Although mold cooling achieved
by adjusting mold temperatures improves warpage, the conventional single mold temperature setting for each
male or female mold plate limits the cooling capability. Therefore, this paper describes local mold temperature set-
tings for a cooling system that can prevent severe warpage in an asymmetric plastic cover for handheld communi-
cation devices. The neutral axis theory is introduced to analyze the temperature distribution in the cross section of a
part, and then predict the warping trend. Through simulation and experiments conducted in this study, the feasi-
bility of using an effective local mold temperature setting in a cooling system to reduce part warpage was verified.
© 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.12.008
0735-1933/© 2014 Elsevier Ltd. All rights reserved.
S.-C. Nian et al. / International Communications in Heat and Mass Transfer 61 (2015) 102–110 103
flow and minimal near the gate region. A decayed pressure profile gen-
erates an approximately uniform volumetric shrinkage by causing the
parts of plastic cooling near the gate to freeze as effectively as if they
were under the same pressure in regions further from the gate.
2. Literature review
Fig. 3. Sprue–runner–gate system in the injection mold. Fig. 5. Injection-molded portable cover: (a) front; (b) back.
S.-C. Nian et al. / International Communications in Heat and Mass Transfer 61 (2015) 102–110 105
Fig. 9. Cross-sectional temperature profile of initially designed cooling channels: (a) partial C-C section; (b) A-A section.
as an axis in the cross section of a beam or shaft along which there are
no longitudinal stresses or strains (Fig. 8). If the section is symmetric,
isotropic, and not curved before a bend occurs, then the neutral
axis is at the geometric centroid. If all polymers on one side of the
neutral axis are in a state of tension, those on the opposite side are
in compression. Thereby, there is a compressive strain at the top of
the beam, and a tensile strain at the bottom of the beam. From statics,
a moment consists of equal and opposite forces; therefore the total
amount of force across the cross section must be zero [21].
Basically, the approach used in this study analyzed the cross-
sectional temperature distribution along the part thickness immediate-
ly after the filling process (i.e., the molten polymer within cavities was
under zero pressure). Without pressure affecting the temperature
distribution, the effects temperature exerted on warpage were able to
be observed. Molten polymer at a high temperature shrinks substan-
tially after cooling to room temperature. The cross-sectional tempera-
ture distribution and the location of its neutral axis can indicate the
Fig. 10. Newly designed cooling channels with 8-mm diameter.
warping trend. For example, Fig. 9 illustrates the cross-sectional tem-
perature profiles associated with long and short edges. According to
Fig. 11. Warpage of the long edge w.r.t. with various mold temperature settings.
S.-C. Nian et al. / International Communications in Heat and Mass Transfer 61 (2015) 102–110 107
the temperature profile in the C-C section of the short edge (Fig. 2), the
maximal temperature was located above the neutral axis and caused
relatively substantial shrinkage during cooling, suggesting that the
short edge will warp, forming a concave shape after cooling. By contrast,
the maximal temperature is located below the neutral axis in the A-A
section of the long edge (Fig. 2), which will warp into a convex shape.
Therefore, the neutral axis theory can be considered to explain the
warpage behavior depicted in the simulation (Fig. 7). Moreover, if the
temperatures of female and male mold plates are adjusted to enable
the maximal temperature to be located exactly on the neutral axis, the
warpage caused by uneven shrinkage can be eliminated.
Compared with the initial design cooling system (Fig. 6) that is lim-
ited when improving the warpage of thin-walled parts, a new design
cooling system that enables establishing local mold temperatures for
the four edges is proposed in this paper (Fig. 10). Fig. 11 shows the
comparison of z-directional warpage in the long edge with respect
to a uniform mold temperature setting (70 °C mold temperature)
and a local mold temperature setting (80 °C and 30 °C mold tempera-
tures for female and male mold plates, respectively). The female plate
was deformed in the range of −0.88–1.0 mm, whereas the male plate
was less deformed in the range of − 0.63–0.7 mm. Fig. 12 depicts the
cross-sectional temperature profiles of the long edges according to
various mold temperature setting. The simulation determined that a
local mold temperature setting at the long edge enabled the maximal
temperature to be near the neutral axis. Consequently, the simulation Fig. 13. Sensing positions for the z-directional position of the portable cover (back side).
verified the feasibility of using local mold temperature settings to elim-
inate the warping of thin-walled molding based on the neutral axis
theory. balanced. The height of each sample was measured and then compared
To further verify the effectiveness of using local mold temperature to with its standard value. Fig. 13 depicts the sensing positions for the
reduce the warpage of long edges, relevant experiments were conduct- z-directional height on the back side of the portable cover. Fig. 14
ed in this study. shows curvature profiles of the four cover edges that correspond to var-
ious mold temperatures, and Table 1 lists the warpage amount according
5.1. Single mold temperature setting to various mold temperature settings, including range, straightness,
and flatness analyses. Because the measurement was conducted by
In the single mold temperature experiment, the mold temperature probing the back face of the part, the long convex edges on the outer
was set at 50, 60, and 70 °C, and some samples were acquired after 40 surface became concave in shape on the inner surface. Similarly, the
cycles of injection molding ensured that data collected would be short edges that were concave on the outer surface were convex on
Fig. 12. Cross-sectional temperature profiles of the long edge w.r.t. with various mold temperature setting.
108 S.-C. Nian et al. / International Communications in Heat and Mass Transfer 61 (2015) 102–110
4.60
a respectively, and the warpage was minimal among these four levels of
4.50
single mold temperature settings.
4.40
4.10 atures in the female mold plate, and the corresponding B2 and B4 chan-
4.00
nels were given high temperatures in the male mold plate. The local
mold temperature setting was constrained by the capacity of available
3.90
mold heating devices in this experiment. Table 2 shows the actual
3.80
local mold temperatures; A1 and A3 (49 °C–52 °C) were hotter than
3.70 B1 and B3, and B2 and B4 (40 °C–42 °C) were hotter than A2 and A4.
3.60 To ensure the mold temperature control in the eight mold regions was
3.50 stable, eight temperature sensors were installed in each region
P2 P4 P6 P8 P10 P12 P14 P16 P18 P20 P22 P24 P26 P28 (Fig. 16). Fig. 17 shows the record of cycle-by-cycled mold tempera-
Position tures, which were controlled steadily. Fig. 18 shows the deformation
profile for the thin-walled injection molding that fabricated the portable
4.60
4.50
c device cover using local mold temperature setting. The greatest defor-
mation was located at P12 of the short edge (3.85 mm). Table 3 lists
4.40 the straightness of the four edges: 0.11 mm and 0.06 mm on the right
4.30 and left edges, respectively, and 0.19 mm and 0.23 mm on the top
4.20 and bottom edges, respectively. The flatness and range were merely
Z (mm)
Fig. 15. Layout of local mold cooling channels: (a) female mold plate; (b) male mold plate.
Fig. 16. Mold temperature sensing positions: (a) female mold plate; (b) male mold plate.
settings can be improved substantially compared with when using direction and a warpage profile according to various mold
conventional single mold temperature settings. The following con- temperature settings.
clusions were made: (2) A computer simulation and experiment was performed to
predict the warpage of the injection-molded portable cover,
(1) The study employed Moldex3D to predict the thermal effect fabricated using mold temperature settings of 50, 60, and 70 °C.
cooling channels exerted on the part warpage. The model The results indicated that the left and right thin edges warped se-
yielded a temperature profile along the part thickness verely upward, whereas the top and bottom thick edges warped
downward, results that were consistent with the actual molding.
A mold temperature of 50 °C achieved the minimal warpage with
a flatness of 0.65 mm.
100
90 4.60
4.50
80 4.40
Temperature (oC)
4.30
70 4.20
Z (mm)
4.10
60
4.00
3.90
50
3.80
40 3.70
3.60
30 3.50
P2 P4 P6 P8 P10 P12 P14 P16 P18 P20 P22 P24 P26 P28
0 50 100 150 200 250 300 350 400 450 500 550
Position
Time (sec)
Fig. 18. Z-directional deformation profile of the molded part fabricated with local mold
Fig. 17. Cycle-by-cycle local mold temperature profiles. temperature settings.
110 S.-C. Nian et al. / International Communications in Heat and Mass Transfer 61 (2015) 102–110
Table 3 References
Warpage of local mold temperature settings.
[1] J. Fasset, Thin wall molding different in processing over standard injection molding,
Straightness (mm) Range (mm) Flatness (mm) SPE-ANTEC11995. 430–433.
[2] P. Staeblin, Thin-wall technology increases component utility, Kunststoffe Plast. Eur.
Right edge Left edge Top edge Bottom edge
87 (1997) 21–22.
0.11 0.06 0.19 0.23 0.31 0.31 [3] M.S. Despa, K.W. Kelly, J.R. Collier, Injection molding of polymeric LIGA HARNs,
Microsyst. Technol. 6 (1999) 60–66.
[4] J.W. Bozzelli, J. Cardinal, B. Fierens, Pressure loss in thin wall moldings, SPE-
ANTEC11997. 425–428.
[5] M.C. Huang, C.C. Tai, The effective factors in the warpage problem of an injection-
molded part with a thin shell features, J. Mater. Process. Technol. 110 (2001) 1–9.
[6] N.R. Subramanian, L. Tingyu, Y.A. Seng, Optimizing warpage analysis for an optical
housing, Mechatronics 15 (2005) 111–127.
[7] M.C. Song, Z. Liu, M.J. Wang, T.M. Yu, D.Y. Zhao, Research on effects injection process
parameters on the molding process for ultra-thin wall plastic parts, J. Mater. Process.
Technol. 187 (2007) 668–671.
[8] Y.J. Weng, S.Y. Yang, S.C. Nian, S.T. Huang, Y.J. Weng, Optimal process conditions of
shrinkage and warpage of thin-wall parts, Polym. Adv. Technol. 22 (2011) 891–902.
[9] S.C. Chen, Y. Chang, T.H. Chang, R.D. Chien, Influence of using pulsed cooling for
mold temperature control on microgroove duplication accuracy and warpage of
the Blu-ray Disc, Int. Commun. Heat Mass 35 (2008) 130–138.
[10] M.S. Huang, Y.L. Huang, Effect of multi-layered induction coils on efficiency and uni-
formity of surface heating, Int. J. Heat Mass Transf. 53 (2010) 2414–2423.
[11] R. Sánchez, J. Aisa, A. Martinez, D. Mercado, On the relationship between cooling
setup and warpage in injection molding, Measurement 45 (2012) 1051–1056.
[12] B. Ozcelik, I. Sonat, Warpage and structural analysis of thin shell plastic in the plastic
injection molding, Mater. Des. 30 (2009) 367–375.
[13] C.P. Chen, M.T. Chuang, Y.H. Hsiao, Y.K. Yang, C.H. Tsai, Simulation and experimental
Fig. 19. Comparison of z-directional deformation profiles for the cover fabricated with a study in determining injection molding process parameters for thin-shell plastic
single mold temperature of 50 °C (dashed line) and local mold temperatures (solid line). parts via design of experiments analysis, Expert Syst. Appl. 36 (2009) 10752–10759.
[14] H. Oktem, T. Erzurumlu, I. Uzman, Application of Taguchi optimization technique in
determining plastic injection molding process parameters for a thin-shell part,
Mater. Des. 28 (2007) 1271–1278.
[15] Y.M. Deng, Y. Zhang, Y.C. Lam, A hybrid of mode-pursuing sampling method and
genetic algorithm for minimization of injection molding warpage, Mater. Des. 31
(2010) 2118–2123.
(3) Applying the neutral axis theory can effectively indicate the [16] M.D. Azaman, S.M. Sapuan, S. Sulaiman, E.S. Zainudin, A. Khalina, Shrinkages and
warpage of each edge and suggested local mold temperature set- warpage in the processability of wood-filled polypropylene composite thin-walled
tings for the female and male mold plates at long and short parts formed by injection molding, Mater. Des. 52 (2013) 1018–1026.
[17] S.H. Tang, Y.J. Tan, S.M. Sapuan, S. Sulaiman, N. Ismail, R. Samin, The use of Taguchi
edges, respectively. In the local mold temperature experiment, method in the design of plastic injection mould for reducing warpage, J. Mater.
the mold temperature was set at approximately 40 °C–52 °C, as Process. Technol. 182 (2007) 418–426.
is listed in Table 2. The flatness (0.31 mm) of the cover fabricated [18] P. Larpsuriyak, H.G. Fritz, Warpage and Countermeasure for injection-molded in-
mold labeling parts, Polym. Eng. Sci. 51 (2011) 411–418.
using local mold temperature setting was superior to that fabri- [19] S.C. Chen, S.H. Tarng, C.Y. Tseng, Using pulsed cooling to reduce cycle time and im-
cated using a single mold temperature of 50 °C. Experimental prove part warpage, SPE-ANTEC22010. 867–871.
and simulation results both verified the feasibility of applying [20] H.L. Chen, S.C. Chen, W.H. Liao, R.D. Chien, Y.T. Lin, Effects of insert film on asymmet-
ric mold temperature and associated part warpage during in-mold decoration injec-
the neutral axis theory and local mold temperature settings to tion molding of PP parts, Int. Commun. Heat Mass 41 (2013) 34–40.
effectively control the warping of a thin-walled part. [21] http://en.wikipedia.org/wiki/Neutral_axis.