Hygroscopicity, the tendency of a solid substance to absorb moisture, is a great enemy of 3D printing, as it is a prominent characteristic of almost all thermoplastic materials. Even a small percentage of humidity can negatively affect most filaments and, therefore, the end result of the print job. This white paper explores the detrimental effects of humidity on polymers and compares traditional 3D printing filament drying methods to the effectiveness of the BCN3D Smart Cabinet.
Download the White Paper: Investigation of the Detrimental Effects of Humidity on Hygroscopic Polymers and Filament Drying MethodsDownload the White Paper: Investigation of the Detrimental Effects of Humidity on Hygroscopic Polymers and Filament Drying Methods
Water is one of the most important constituents of the atmosphere, depending on geographical factors and the weather, it can make up as much as 2% of the volume of the air that we breathe.
Most polymeric materials interact with water by absorbing it, regardless of its physical form. That is why raw thermoplastic materials are normally subjected to a so-called dehydrating phase, when they are processed at a high temperature before being used, to ensure that there is no water left in the form of bubbles. These water bubbles would stay trapped in the polymer matrix and would generate localized imperfections, which are detrimental to the aesthetical and mechanical properties of the plastic object made out of them.
Depending on the nature of the polymer and its behavior in a water-rich environment, it can be characterized as hygroscopic or non-hygroscopic. Non-hygroscopic materials tend to absorb water on their surface only, making it easier to remove by simple heating. On the other hand, hygroscopic materials are able to absorb big quantities of humidity from the air and store it deep within their matrix. Heating up a hygroscopic plastic helps to remove the water absorbed on the surface, but not the water stored deep in their matrix. That is why hygroscopic polymers need to be looked after and stored more carefully before being processed and to be kept in a dry and sealed environment.
Research conducted on wet 3D printing filaments by the BCN3D Engineering team
The BCN3D Engineering team has been conducting research on the effects of humidity on 3D printing filaments, with the aim of generating basic knowledge about how various materials absorb moisture, how that affects their performance, and the best methods to keep them dry. Firstly, they examined PA, PVA, and TPU filaments’ water absorption characteristics, and their ability to produce reliable end results after being subdued to certain levels of relative humidity as they are the most hygroscopic materials in our filament portfolio, therefore, the most sensitive to incorrect storage conditions.
To get to know how exactly the BCN3D Engineering team conducted these experiments, download the white paper: Investigation of the Detrimental Effects of Humidity on Hygroscopic Polymers and Filament Drying Methods
The results reported in Table 1, above, show the amount of water these 3D printing spools absorbed in relation to the environment’s water content. When the spools were kept in a dry atmosphere (Entry 1, Table 1) no relevant increase in weight was recorded, however, when they were exposed to higher humidity rates, the spools absorbed a proportionally higher amount of water.
As the table shows, each material followed a unique absorption profile: PVA is the most hygroscopic of the tested materials, and when kept at 70% RH for 4 days it absorbed the equivalent of 1.22% of its original weight in water. Meanwhile, PA and TPU seemed to behave in a similar way when exposed to low environmental humidity levels (Entries 1-4, Table 1, 10-40% RH), absorbing equal amounts of water. However, while TPU’s absorption capacity flattened out above 40% RH, PA showed a more hygroscopic behavior in a high humidity environment (Entry 5, Table 1).
Afterwards, the team started printing a few simple geometric shapes with these preconditioned filaments to verify if the exposure to humidity compromised the printability of the materials. The test prints consisted of a fine-walled cylinder and a cuboid shape, ideal for inspecting the presence of bubbles, voids, and stringing.
In the control experiment, humidity was kept below 10%, all spools produced perfect prints, as shown in the picture below, the printed samples show no imperfections.
At 12% humidity, while PA and PVA still printed as good as during the control experiment, TPU already showed significant stringing, meaning that the viscosity of the melt was reduced by the presence of water, acting as a plastifier.
PA started producing cloudy surfaces and showing stringing at 30% RH while PVA, even though it is the most hygroscopic of the three tested materials, withstood high levels of humidity, maintaining its printability at up to only 40% RH.
This experiment sheds light on important information about the behavior of PA, PVA, and TPU filaments when stored in an environment with different levels of relative humidity. By measuring the amount of water absorbed, the BCN3D Engineering team was able to identify PVA as the most hygroscopic material, followed by PA and finally TPU. However, it turned out that TPU, which is the least hygroscopic out of these three materials, is also the most sensitive to incorrect storage.
The second experiment that the team conducted was meant to take into account the effect of time on the rate of water absorption of different materials.
To read more about the second experiment that the BCN3D Engineering team conducted, download the white paper: Investigation of the Detrimental Effects of Humidity on Hygroscopic Polymers and Filament Drying Methods
The Engineering team at BCN3D could also calculate a theoretical shelf life at 60% RH, in a well-ventilated room, intersecting the data from the first and second experiments. From this calculation, the team could determine that PVA needs 12 hours to reach the critical water content of 0.47%, which leads to unsuccessful prints (Table 3). For PA, this time is reduced to 4 hours to reach the critical water content of 0.10%. According to this calculation, TPU only needs 1.5 hours at 60% RH and it already fails the print test (Table 3). These numbers are quite alarming and show how easily humidity and incorrect storage can affect the outcome and consistency of the FFF 3D printing process.
They also examined the materials’ behavior when inside the BCN3D Smart Cabinet, an environment designed to be protected against humidity, as explained further down this document. The following graph shows the environmental parameters of the Smart Cabinet in function, with the external relative humidity at 55%.
The BCN3D Smart Cabinet maintains the filaments in a low-humidity environment, greatly reducing the chances of print failure due to excessive water content. As shown in Graph 6, even with an external humidity of 55%, the Smart Cabinet ensures an internal humidity rate between 15-20%, to preserve filaments’ printability, extend their shelf life, and to reduce aesthetic defects on the end results.
The effectiveness of traditional filament drying methods compared to the BCN3D Smart Cabinet
There are a couple of filament drying methods that are well-known in the 3D printing industry. However, these methods have various disadvantages, which can end up even damaging the polymer.
Traditional filament drying methods
Oven baking filaments can mount up to high energy costs while decreasing tensile strength and even melting the filament, if too hot. It is also a very time-consuming process.
Air conditioner or dehumidifier
Using an air conditioner or a dehumidifier to dry filament can also be quite expensive, while it cannot dry the filament below 40% of relative humidity. It is also ineffective when the environment’s temperature is low.
The relative humidity rate cannot be controlled, and this method needs constant replacements and maintenance.
Other traditional filament drying methods
Constant heat baking filaments in order to dry them can mount up to high energy costs, as well, while it only makes it possible to dry a few spools of filament at a time.
Professional filament drying methods
Adsorption dryers are a common way to dry solids and polymers that have a tendency to absorb moisture. The way they work is based on the affinity of the adsorbing agents to water; what they effectively do is seizing water molecules from the air, greatly reducing atmospheric humidity. These adsorbing agents are normally in the form of pellets or spheres, and are made of silica, alumina, or special clays, which have the ability to absorb great amounts of water from the air, and can be regenerated. The drying process consecutively increases the water’s evaporation rate from the surrounding solid surfaces thus reducing their total water content. After absorbing a given amount of water from the air, the adsorption material becomes saturated and its effectiveness quickly decays. By isolating the absorption material from the heating chamber and increasing its temperature, we can release all the absorbed humidity into the surrounding environment and regenerate the material.
The BCN3D Smart Cabinet also works based on this method, alternating between drying cycles and regeneration cycles, thus maintaining a constant dry environment around the stored spools and protecting them from sudden external changes.
Hot air drying process
This simple process consists of circulating hot and dry air through a bed of plastic pellets. The pellets are normally moved by mechanical means and their final water content depends on the temperature of the air and the time of residence in the hopper (Figure 2). This process is most effective with non-hygroscopic and high-melting materials, as materials such as PLA would be affected by the thermal treatment.
Vacuum drying is based on the fact that vapor pressure and the boiling point of liquids are dependent on environmental pressure. By reducing the atmospheric pressure, it is possible to lower the boiling point of water. For example, if the pressure would be reduced to one-tenth of the normal atmospheric pressure (from 1.0 to 0.1 atm) the boiling point of water would change from 100 ºC to 33 ºC. This way it is possible to remove liquids from solids without actually heating. For this reason, vacuum drying is considered a very mild and effective way to reduce the water content of solids. However, a great disadvantage of vacuum drying is the equipment cost and constant maintenance needed to ensure a safe and long-lasting operation.
BCN3D Smart Cabinet
Compared to all previously mentioned methods, the BCN3D Smart Cabinet has a low energy consumption (12 W Avg / 100 W Max.) while it can keep the filament below the 40% relative humidity rate, which is optimal for the majority of the 3D printing materials.
It can dry up to 8 small spools of filament (between 750g to 1 kilogram), or 4 big spools (up to 2,7 kilograms per spool), without heat, therefore maintaining the materials’ tensile strengths.
The BCND Smart Cabinet protects the filaments long-term and significantly reduces print failure due to moisture. The silica gel inside, open to the dehumidifying environment, absorbs the moisture from the air in the chamber. Once it becomes saturated, the gel is isolated from the materials and gets heated up until it releases the moisture, purging it out of the system. After the purge, the silica gel has been refreshed and ready to collect more moisture. This process effectively keeps the filament dry and in optimal condition for its use.
As demonstrated by the above-mentioned results to all experiments conducted by the BCN3D Engineering team, moisture-rich filaments can spoil the end result of the print job as well as they can also seriously damage the 3D printer itself.
Therefore, it is of crucial importance to always keep 3D printing materials in a sealed place where the air’s relative humidity rate can be controlled. This is where the BCN3D Smart Cabinet can come in handy, as it effectively extends the service life of materials by storing them in optimal conditions, storage maintained even during the printing process, providing a flawless 3D printing experience.
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