Hello and welcome to this plastics multipoint design data post in which we discuss the shrinkage of amorphous (ABS, PC) and semi-crystalline (PA 6) polymers as a function of wall thickness and fillers.
Impact of filler and reinforcing materials on shrinkage
Table 1 compares the shrinkage in longitudinal and transverse direction of a non-reinforced Polyamide 6 with filled and reinforced Polyamide 6. It is possible to influence the shrinkage values by using certain filler content and filler type. Using spherically-shaped fillers reduces the shrinkage in longitudinal and transverse directions. However, using glass fibers helps even more in the longitudinal direction since they impede thermal contraction in the glass fiber direction. In the transverse direction it is almost the same range as the spherically shaped fillers. Adding glass fibers is an effective way to influence shrinkage values of PA 6 (decrease between 50-80%) and the optimum level of glass fibers is at 30 wt%. From there on it levels off and adding more glass fibers will not result in lower shrinkage values. This is illustrated in Figure 1 where a PA 6.6 is loaded with different amounts of glass fibers, reaching a maximum load at 35 wt%.
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| Table 1: Comparison shrinkage in longitudinal and transverse direction of a non-reinforced Polyamide 6 with filled and reinforced Polyamide 6. |
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| Figure 1: Estimation of moulding shrinkage of a PA 6.6 loaded with different amounts of glass fibers, reaching a maximum load at 35 wt%. |
Shrinkage of semi-crystalline (PA 6) polymer as a function of wall thickness and fillers
Figure 2 shows the correlation between shrinkage and wall thickness of an unreinforced PA 6 and a 30 wt% glass fiber PA 6 (both dry as moulded - DAM). Influence of the wall thickness on the shrinkage can be in particular seen with unreinforced PA 6. Explanation for this is that an increased wall thickness leads to slower cooling and allows the crystalline regions to fully grow. As a consequence, higher crystallisation results in higher shrinkage values. If the shrinkage becomes too much, warpage will most likely be the consequence in your final part, especially when it is a flat shaped part.
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| Figure 2: Correlation between shrinkage and wall thickness of an unreinforced PA 6 and a 30 wt% glass fiber PA 6 (both dry as moulded - DAM). |
Shrinkage of amorphous (PC, ABS) polymer as a function of wall thickness and fillers
Figure 3 shows the correlation between shrinkage and wall thickness of an unreinforced Polycarbonate (PC) and glass fiber reinforced PC. Figure 4 shows it for Acrylonitrile Butadiene Styrene (ABS). Amorphous polymers have a lower shrinkage level due to its missing crystalline sections.
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| Figure 3: Correlation between shrinkage and wall thickness of an unreinforced Polycarbonate (PC) and glass fiber reinforced PC. |
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| Figure 4: Correlation between shrinkage and wall thickness of an unreinforced ABS. |
More multipoint data post can be found here:
Thanks for reading and #findoutaboutplastics
Greetings,
Herwig Juster
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Literature:
[1] Zöllner, O.: “Prozessgrößen beim Spritzgießen von Thermoplasten als Produktionskostenfaktor” paper presented at the SKZ Würzburg, Bayer AG, 1993
[2] Covestro: The fundamentals of shrinkage in thermoplastics, 2016: https://solutions.covestro.com/-/media/covestro/solution-center/whitepapers/the-fundamentals-of-shrinkage-in-thermoplastics.pdf
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