Many researches[1]~[6] indicate that both mold temperature and injection temperature are important factors making profound influence on injection molding parts. Therefore, in our case study, we focus on the effects of changing two parameters and the simulation is performed by Autodesk Simulation Moldflow Advisor 2015. To give a quick validation of the software, we compare the location of weld lines in realistic part and virtual results since weld lines is one of the most visualized and inevitable defects. Figure 1 indicates that the prediction in weld line position matches the real situation.

a.

b. c.

Figure 1- prediction of weld lines position in Moldflow® matches the real situation. (a) the result of simulation shows two local weld lines A and B. (b)Weld line A in real part. (c) Weld line B in the real part.

Effects of Melt Temperature on Defects Formation

Melt temperature is also a key process parameter. Parts quality, fill time of the polymer melt, and several defects formation are closely related to it (Yang, et al. 2004). Polymer melt temperature affects the injection pressure and filling time through fluid viscosity and thermodynamics, and therefore has strong influence on the formation of shrinkage and weld lines.

In our study, weld lines were quantified as weld line angle, which is the angle of the weld line gap. This gap angle indicates the depth of the gap; smaller the angle, deeper the gap. Thus, a

large gap angle is favored. Specifically in the case study of the fuel filter housing, a local weld line right beside a thread hole was looked into (shown in Figure 1 and Figure 2). Since it is the area that will bear the most amount of stress and torque, the existence of weld line is most likely to cause failure under operation. The minimum gap angle value of that weld line versus varying melt temperature is detailed later in the section.

Figure 2 – Local Weld Line being Studying

Figure 3 – Zoomed View of the Local Weld Line

Volumetric shrinkage was presented by the percentage of shrinkage relative to its original volume (Figure 3). Maximum shrinkage rate under each melt temperature was compared to derive their relation. Parts quality result is also presented in percentage, based on the mechanical properties and appearance of the part (Figure 4).

Figure 4 - Volumetric Shrinkage

Figure 5 - Part Quality Percentage

Data of local weld line angle, maximum shrinkage rate and quality percentage gained from simulation is shown below table. Diagrams of each feature’s trend versus melt temperature is also included. Melt temperature range and value selection was based on the injection molding guideline of Isoplast® 301 published by Lubrizol (Isoplast® Processing Guidelines - Lubrizol n.d.).

Table 1 - Data of Part's Features

Melt Temperature (° C)

Local Weld Line Angle (°)

Max Shrinkage Rate (%)

Max Quality Percentage (%)

230 34.4 5.308 0.80232.5 23 5.421 71.8235 27.4 5.541 71.8237.5 22 5.647 71.9240 26.3 5.764 71.9242.5 23.5 5.854 71.9245 35.4 5.904 72247.5 20.8 6.022 71.9250 34.4 6.112 71.9

Figure 7 - Weld Line Angle vs. Melt Temperature

The results show a clear trend of maximum shrinkage versus melt temperature – as melt temperature goes up, volumetric shrinkage increases accordingly. This result accorded well a previous study: as the temperature discrepancy between melt temperature and mold temperature goes up with higher melt temperature, the shrinkage during cooling is more furious due to entropy effect (Jansen, Van Dijk and M.H. 1998).

Figure 8 – Volumetric Shrinkage vs. Melt Temperature

Figure 9 – Quality Percentage vs. Melt Temperature

On the other hand, quality percentage of the part experienced a sudden rise when melt temperature went up from 230 ° C to 232 ° C, and remained at around 70% after that. It indicated that there existed a certain threshold of melt temperature, above which part quality percentage was stable around a value.

However, weld line angle didn’t show a clear relation with melt temperature. The angle value fluctuated around 28°, and the amplitude increased as melt temperature went up. This random trend accorded well with Zhai’s & Manjunatha’s research into weld line formation, both stating that weld line formation is heavily depended on gate position and runner size, while melt temperature and mold temperature is much less important (Zhai, Lam and Au 2006), (Manjunatha and Ramesh Babu 2014).

Effect of Mold Temperature

Likewise, mold temperature also plays an important role in injection molding process. To understand the influence of mold temperature, we fix the melt temperature at 230° C, maximum machine pressure at 95 MPa, and make the injection time auto-adjusted by Moldflow, and then we change the mold temperature form 66° C to 87 ° C. The results are shown in the Figure 10. In terms of injection pressure variation, increasing mold temperature causes less pressure variation, which is better for the injection process since large pressure difference in the same mold cause uneven shrinkage rate. Also, it also shows higher mold temperature requires less maximum pressure provided by machine, requiring less machine power. However, increasing mold temperature also causes negative effect on fill time. It ends up with needing more time in manufacture process. As to quality and weld lines angle, it shows the better mold temperature is below 72° C. It shows zero high-quality percentage and irreverent influence on minimum weld line angle above 72° C.

Most engineering problems are trade-offs between factors. Depending on which factor is critical to the problem, engineers tend to sacrifice less important factors. For example, in our case the production efficiency is more important than small pressure variation, we recommend setting mold temperature at lower point such as 70 ° C so that the fill time is reduced, and vice versa.

(a) 66 68 70 72 74 76 78 80 82 84 869.49.69.810

10.210.410.610.8

11injection pressure variation v.s. mold

temperature

(b) 66 68 70 72 74 76 78 80 82 84 861.4

1.45

1.5

1.55

1.6

1.65

1.7

1.75fill time v.s. mold temperature

(c) 66 68 70 72 74 76 78 80 82 84 860

0.2

0.4

0.6

0.8

1

quality percentage v.s. mold temprature

(d) 66 68 70 72 74 76 78 80 82 84 8605

10152025303540

weld line angle v.s. mold temperature

Figure 10 – influence of mold temperature on (a) fill time (b)quality (c)injection pressure (d) weld line angle

References[1] Akbarzadeh, Alireza, and Mohammad Sadeghi. "Parameter Study in Plastic Injection Molding Process Using Statistical Methods and IWO Algorithm." IJMO International Journal of Modeling and Optimization(2011): 141-45.

[2] Farshi, Behrooz, Siavash Gheshmi, and Elyar Miandoabchi. "Optimization of Injection Molding Process Parameters Using Sequential Simplex Algorithm." Materials & Design 32.1 (2011): 414-23. 

[3] Ahmad, A.h., Z. Leman, M.a. Azmir, K.f. Muhamad, W.s.w. Harun, A. Juliawati, and A.b.s. Alias. "Optimization of Warpage Defect in Injection Moulding Process Using ABS Material." 2009 Third Asia International Conference on Modelling & Simulation (2009)

[4] Lei Xie, “Study on relevant factors influencing the strength of weld line defect in micro injection molding process”, July 2010, doctor dissertation

[5] "What to Do About Weak Weld Lines : Plastics Technology." What to Do About Weak Weld Lines : Plastics Technology. N.p., n.d. Web. 01 Dec. 2015.

[6] "Coping with Weak Weld Lines : Plastics Technology." Coping with Weak Weld Lines : Plastics Technology. N.p., n.d. Web. 01 Dec. 2015.

n.d. Isoplast® Processing Guidelines - Lubrizol. Lubrizol. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwjas4yZn7rJAhUCoogKHXNFAnIQFggjMAE&url=https%3A%2F%2Fwww.lubrizol.com%2FLife-Science%2FDocuments%2FMedical-Polymers%2FProcessing-Guides%2FISOPLAST-Processing-Guidelines.pdf&usg=AFQj.

Jansen, K.M.B., D.J. Van Dijk, and Husselman M.H. 1998. "Effect of processing conditions on shrinkage in injection molding." Polymer Engineering & Science 385: 838-846. doi:10.1002/pen.10249.

Manjunatha, M., and K. Ramesh Babu. 2014. "The Influence of Gate Location in an Injection Moulded Basetta Tu Base Component." Int. J. Mech. Eng. & Rob. Res. (Int. J. Mech. Eng. & Rob. Res. 2014) 3 (3). http://www.ijmerr.com/v3n3/ijmerr_v3n3_58.pdf.

Yang, V., C-T. Huang, P-C Tsai, J. perdikoulias, and J. Vlcek. 2004. "Simulating the Melting Behavior and Melt Temperature Inhomogeneity in The Injection Molding Process." http://www.moldex3d.com/en/assets/2011/09/Simulating-the-Melting-Behavior-and-Melt-Temperature-Inhomogeneity-in-The-Injection-Molding-Processe.pdf.

Zhai, M., Y. C. Lam, and C. K. Au. 2006. "Runner sizing and weld line positioning for plastics injection moulding with multiple gates." Engineering with Computers 21 (3): 218-224. http://link.springer.com/article/10.1007/s00366-005-0006-6#/page-1.