Optimal filament storage conditions and drying methods

Optimal storage conditions for filaments and comparison of drying methods

Hygroscopicity, i.e. the tendency of a solid to absorb moisture, is the great enemy of 3D printing because it is a characteristic feature of almost all thermoplastic materials. Even a small percentage of moisture can negatively affect most of the fibers and therefore the final result of the printing job. This article describes the harmful effects of moisture on polymers and compares traditional methods of drying 3D printing filaments with the effectiveness of BCN3D Smart Cabinet.

Water is one of the most important components of the atmosphere and, depending on geographical factors and weather, it can constitute up to 2% of the volume of the air we breathe. Most polymer materials interact with water by absorbing it, regardless of its physical form. Therefore, thermoplastic raw materials are usually subjected to a so-called dehydration phase, where they are treated at high temperature before use, to ensure that no water remains as bubbles. These water bubbles would remain trapped in the polymer matrix and generate local imperfections that are detrimental to the aesthetic and mechanical properties of the plastic object made 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 only on their surface, which makes them easier to remove by simple heating. On the other hand, hygroscopic materials are able to absorb large amounts of moisture from the air and store it deep in their matrix. Heating the hygroscopic material helps remove water absorbed on the surface, but not water accumulated deep in the matrix. For this reason, hygroscopic polymers require more careful care and storage before processing and storage in a dry and airtight environment.

The most popular 3D printing filaments are formulated from hygroscopic materials, such as PA, TPU, PVA, PET-G or ABS.

Research conducted on wet 3D printing filaments by the BCN3D engineering team

The BCN3D Engineering team conducted research on the impact of moisture on 3D printing filaments with the aim of obtaining basic knowledge about how different materials absorb moisture, how it affects their performance and what are the best methods for drying them. First, they investigated the water absorption characteristics of PA, PVA and TPU filaments and their ability to produce reliable final results when exposed to certain levels of relative humidity, as these are the most hygroscopic materials in our filament portfolio and therefore the most sensitive to incorrect storage conditions.

Table 1: Change in weight of different 3D printing materials at different levels of relative humidity

The results presented above in Table 1 show the amount of water absorbed by these filament spools in relation to the water content in the environment. When the spools were kept in a dry atmosphere (entry 1, table 1), no significant increase in weight was noted, but when they were exposed to higher humidity rates, they absorbed proportionately more water.

As the table shows, each material had a unique absorption profile: PVA is the most hygroscopic of the materials tested and, when stored at 70% RH for 4 days, absorbed the equivalent of 1.22% of its original weight in water. Meanwhile, PA and TPU seemed to behave similarly when exposed to low levels of ambient humidity (items 1-4, Table 1, 10-40% RH), absorbing equal amounts of water. However, while the absorption capacity of TPU flattened out above 40% RH, PA showed a more hygroscopic behavior in high humidity environments (entry 5, table 1).

PA, PVA and TPU filaments – comparison of wet filaments when printing

The team then began printing several simple geometric shapes using these pre-prepared filaments to test whether exposure to moisture negatively affected the materials' printability. The test prints consisted of a thin-walled cylinder and a cuboid, ideal for checking for bubbles, voids and threads.

In the control experiment, humidity was kept below 10%, all spools produced perfect prints, as shown in the figure below, the printed samples show no imperfections.

At 12% humidity, while PA and PVA were still printed as well as in the control experiment, TPU already showed significant threading, meaning that the melt viscosity was reduced by the presence of water acting as a plasticizer .

PA began to produce cloudy surfaces and showed threading at 30% RH, while PVA, despite being the most hygroscopic of the three materials tested, withstood high levels of humidity, maintaining printability at just 40% relative humidity.

The image shows 3D test prints printed by the BCN3D engineering team at RH <10%, RH 12% and RH 70%. From left to right: PVA, TPU and PA.

This experiment sheds light on important information about the behavior of PA, PVA and TPU fibers when stored in environments 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 of the three materials, is also the most sensitive to incorrect storage.

Table 2: Printability limits for the tested materials

The influence of time on the rate of water absorption by 3D printer filaments

The second experiment the team conducted aimed to take into account the effect of time on the rate of water absorption by different materials.

The BCN3D engineering team was also able to calculate the theoretical shelf life at 60% relative humidity in a well-ventilated room by combining the data from the first and second experiments. Based on these calculations, the team was able to determine that PVA needed 12 hours to reach the critical water content of 0.47%, leading to failed prints (Table 3). In the case of 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 no longer passes the printing test (Table 3). These numbers are quite alarming and show how easily humidity and incorrect storage can affect the result and consistency of the FFF 3D printing process.

Table 3: Calculation of the theoretical shelf life of various 3D printing materials based on experiments conducted by the BCN3D engineering team

They also examined the behavior of materials inside the BCN3D Smart Cabinet, an environment designed to be protected against moisture, as explained later in this document. The graph below shows the environmental performance of the Smart Cabinet in operation with an outdoor relative humidity of 55%.

Chart 1: Smart Cabinet BCN3D maintains internal humidity levels between 15% and 20%, even with an external relative humidity of 55%

BCN3D Smart Cabinet maintains filaments in a low-humidity environment, significantly reducing the risk of printing failure due to excessive water content. As shown in Chart 6, even with an external humidity of 55%, Smart Cabinet ensures an internal humidity between 15° and -20% to maintain the printability of the fibers, extend their shelf life consumption and reduce aesthetic defects in the final result.

Effectiveness of traditional filament drying methods compared to Smart Cabinet BCN3D

There are several methods of drying the filament that are well known in the 3D printing industry. However, these methods have various disadvantages that may even damage the polymer.

Traditional methods of drying the filament

Baking in the oven

Oven-dried fibers can incur high energy costs while reducing tensile strength and even melting them if they are too hot. It is also a very time-consuming process.

Air conditioner or dehumidifier

Using an air conditioner or dehumidifier to dry the filament can also be quite expensive, and it cannot dry the filament below 40% relative humidity. It is also ineffective when the ambient temperature is low.

Desiccants

The relative humidity cannot be controlled and this method requires constant replacement and maintenance.

Other traditional methods of drying the filament

Continuously heating the filaments to dry them can also involve high energy costs, while only drying a few spools of filament at a time.

Professional filament drying methods

Adsorption dryers

Adsorption dryers are a common method of drying solids and polymers that tend to absorb moisture. Their action is based on the combination of adsorbents with water; they effectively capture water particles from the air, which significantly reduces air humidity. These adsorbents usually come in the form of granules or beads and are made of silica, alumina, or special clays that have the ability to absorb large amounts of water from the air and can be regenerated. The drying process successively increases the rate of water evaporation from surrounding solid surfaces, thereby reducing the total water content. After taking a certain amount of water from the air, the adsorption material becomes saturated and its effectiveness quickly decreases. By isolating the absorbent material from the heating chamber and raising its temperature, we can release all the absorbed moisture into the environment and regenerate the material.

The BCN3D Smart Cabinet also works based on this method, alternating between drying and regeneration cycles, thus maintaining a constant dry environment around the stored reels and protecting them from sudden external changes.

Hot air drying process

This simple process involves circulating hot and dry air through a layer of plastic pellets. Pellets are usually moved mechanically, and their final water content depends on the air temperature and residence time in the tank (Fig. 2). This process is most effective for non-hygroscopic and high-melting materials because heat treatment would affect materials such as PLA.

Figure 1: Hot air drying process. Source: www.process-heating.com

Vacuum process

Vacuum drying of the filament means that the vapor pressure and boiling point of the liquid depend on the ambient pressure. By lowering atmospheric pressure, you can lower the boiling point of water. For example, if the pressure dropped to one tenth of normal atmospheric pressure (from 1.0 to 0.1 atm), the boiling point of water would change from 100 ° C to 33 ° C. Thus, it is possible to remove liquids from solids without actual heating. For this reason, vacuum drying is considered a very gentle and effective way to reduce the water content of solids. However, the great disadvantage of vacuum drying is the cost of the equipment and the ongoing maintenance needed to ensure safe and long-term operation.

BCN3D Smart Cabinet

Compared to all previously mentioned methods, Smart Cabinet BCN3D has low energy consumption ( average 12W / 100W max), while it can keep the filament below 40% RH, which is optimal for most 3D printing materials.

Can dry up to 8 small spools of filament (from 750 g to 1 kilogram) or 4 large spools (up to 2.7 kilograms per spool) without heat, keeping in thus the tensile strength of materials.

BCN3D Smart Cabinet protects filaments for a long time and significantly reduces printing errors caused by moisture. The silica gel inside, open to the drying environment, absorbs moisture from the air in the chamber. Once saturated, the gel is isolated from the materials and heated until it releases moisture, removing it from the system. After cleaning, the silica gel was refreshed and ready to collect more moisture. This process effectively keeps the filament dry and optimal for use.

Wet filament and 3D printing - summary of research by BCN3D engineers

As the above results of all experiments conducted by the BCN3D engineering team show, moisture-rich filaments can spoil the final result of the printing job, as well as seriously damage the 3D printer itself.

Therefore, it is extremely important to always store 3D printing materials in an airtight place where the level of relative air humidity can be controlled. This is where BCN3D Smart Cabinet comes in handy, effectively extending the life of materials by storing them in optimal conditions, storage maintained even during the printing process, ensuring a flawless 3D printing experience.

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