By Steve Gerbig
In plastic materials published data, polyamide moisture absorption is almost always expressed in terms of percent weight gain. While this information is important for comparison purposes, it doesn’t truly relate to the design engineers’ application and use of these materials. This study will quantify and compare the relative dimensional changes which occur in parts as they are exposed to a humid environment and move from the dry-as-molded state toward saturation using Nylon types 6, 66 and 46.
Being crystalline materials, Nylon 6, 66 and 46 molding materials exhibit similar behavior. That is to say, they produce high shrinkage as unfilled products and relatively low shrinkage when reinforced with glass fibers. They also absorb moisture at levels substantially higher than most other engineering plastics.
It is fairly common practice today for a new project to be introduced into the marketplace using a high-performance polymer such as Nylon 46. Over time, the product may be cost-reduced by making minor geometric changes, lowering performance requirements and substituting lower performance/lower priced materials such as Nylon 66 or Nylon 6.
One important consideration when making material substitutions is the cost of tool modifications required to accommodate the shrinkage, warpage and moisture uptake characteristics of the materials involved.
It is this propensity to absorb moisture that is the subject of this study. Increasing the moisture content of a Nylon part increases its weight and overall toughness while reducing strength and stiffness. Additionally, polyamide moisture absorption also affects change in part dimensions. This latter characteristic is the source of much concern to design engineers when contemplating material selection for new products or cost reduction programs. Quantification of these dimensional changes is the subject of this study.
The materials used for this study included Nylon 6, 66 and 46 in neat, 15% glass fiber reinforced and 30% glass fiber reinforced grades. Specimen were molded using a standard shrinkage plaque (per ISO 294-4) measuring 60mm x 60mm x 2mm thickness.
Injection Molded Test Specimen (Fig. 1)
After molding, parts were stored at all times in moisture proof packages. Parts were measured in the dry-as-molded (DAM) state then conditioned in 80° C water to speed up the conditioning process. Moisture uptake was then determined by measuring weight gain.
Dimensions were taken from the specimen using a purpose built measuring fixture.
Test Fixture (Fig. 2)
This fixture uses calibration standards based on room temperature cavity dimensions, thus the thermal expansion of the mold due to elevated processing temperature is disregarded for purposes of this discussion.
Moisture conditioning continued well past normal room temperature, 50% relative humidity equilibrium. In each case parts reached a level of moisture uptake near the theoretical maximum, however no effort was made to ensure that such a level had been reached.
Results and Discussion
The dimensional effect of polyamide moisture absorption on the various Nylon products tested is illustrated in Figures 3 through 11.
Note that the graphs are laid out so that the tool steel or base dimension is at the top. Dry-as-molded condition is shown as zero moisture content. This is the value typically reported by material manufacturers in published data.
Unfilled Nylon materials – Figures 3 through 5. Both shrinkage and moisture absorption are very isotropic regardless of the type of Nylon. It’s interesting to note that while type 6 Nylon absorbed more total moisture during the conditioning process, the dimensional change between the dry-as-molded state and equilibrium at 50% RH was less than that illustrated by either 66 or 46 Nylons.
Glass reinforced Nylon materials – Figures 6 through 11 clearly illustrate the anisotropy of these materials. Shrinkage in the dry-as-molded state ranges from about 50% to 200% more transverse to flow than in the direction of flow. This is not surprising as this information is often reported in product data and is the primary cause for the warpage commonly experienced when molding these products.
There are two interesting characteristics evident in these graphs. First, there is very little dimensional change in the linear direction as parts move from dry-as-molded to equilibrium in 50% RH air. Second, the dimensional change is much greater in all materials and all levels of moisture absorption in the transverse direction as opposed to linear.
The demands of the end-use product functionality and the conditions under which a part will be used determine what allowances need to be made for both moisture absorption and material substitution. Dimensionally, Nylon 46 is much more similar to Nylon 66 in both shrinkage and moisture absorption, than it is to Nylon 6. Based on this data, Nylon 66 may be a viable cost effective substitute for Nylon 46 provided other factors such as lower thermal and mechanical properties are not a factor. Similar assumptions may be made concerning substituting Nylon 6 for Nylon 66. Moving from Nylon 46 directly to Nylon 6, however, may require consideration of the dimensional differences which may occur if the same tooling is used without modification.
While Nylon 6 absorbs more total moisture than either Nylon 66 or 46, the dimensional change illustrated between dry-as-molded and equilibrium at 50% RH is substantially less for Nylon 6. This would imply that in the majority of applications, Nylon 6 would be more dimensionally stable when moisture absorption is the primary concern. This conclusion does not take into consideration other factors such as temperature variation, etc.
Finally, these moisture absorption characteristics need to be considered when designing and building molds to produce parts using any Nylon material.
Nylon 6, Unfilled (Fig. 3)
Nylon 66, Unfilled (Fig. 4)
Nylon 46, Unfilled (Fig. 5)
Nylon 6 + 15% Glass Filler (Fig. 6)
Nylon 66 + 15% Glass Filler (Fig. 7)
Nylon 46 + 15% Glass Filler (Fig. 8)
Nylon 6 + 30% Glass Filler (Fig. 9)
Nylon 66 + 30% Glass Filler (Fig. 10)
Nylon 46 + 30% Glass Filler (Fig. 11)
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