ABS in Rapid Prototyping: Common Properties and Managing Moisture Permeability

Acrylonitrile Butadiene Styrene (ABS) is one of the most widely used materials in rapid prototyping, particularly in processes like Fused Deposition Modeling (FDM) and CNC machining. Its popularity stems from an excellent balance of properties, manufacturability, and cost.

I. Common Properties of ABS in Rapid Prototyping

• Strength and Durability: ABS offers good mechanical strength, stiffness, and impact resistance. This makes it suitable for functional prototypes that need to withstand stress, testing, or handling.

• Good Toughness: The butadiene component provides flexibility and durability, preventing the material from being overly brittle.

• Ease of Post-Processing: This is a key advantage. ABS can be easily sanded, machined, primed, and painted. It is also renowned for its ability to be chemically smoothed using acetone vapor, which dissolves the surface layer to create a seamless, glossy finish.

• Heat Resistance: ABS has a higher glass transition temperature (~105°C) than PLA, making prototypes suitable for applications involving moderate heat (e.g., inside electronic enclosures).

• Cost-Effective: It is a relatively inexpensive material, both for filament and solid billet stock.

II. Moisture Permeability (Hygroscopicity) in ABS

A significant and often problematic characteristic of ABS is that it is hygroscopic, meaning it readily absorbs moisture from the surrounding air.

1. Causes and Effects of Moisture Absorption:

Moisture does not just sit on the surface; it permeates the bulk of the material. This leads to two major issues:

• During FDM 3D Printing:

• Steam Explosion: When wet ABS filament is fed into a hot nozzle, the trapped moisture instantly turns to steam. This causes bubbling, popping sounds, and inconsistent extrusion.

• Poor Print Quality: This results in a rough, porous surface texture, layer splitting, poor layer adhesion, oozing, and a catastrophic loss of dimensional accuracy and mechanical strength.

• Stringing and Blobbing: The compromised melt state leads to excessive stringing between parts and blobs on the surface.

• In Finished Prototypes:

• Dimensional Instability: Absorbed moisture can cause the prototype to very slightly swell or warp over time, which is critical for parts requiring precise dimensions.

• Reduced Mechanical Properties: Water molecules act as a plasticizer, weakening the inter-layer bonds and making the part more flexible and less strong overall.

• Preparation for Painting: If a damp part is painted, the trapped moisture can lead to poor paint adhesion, pinholing, or blistering later on.

2. How to Avoid and Prevent Moisture Absorption

Prevention is always better than a cure. The goal is to minimize the exposure of ABS to humid air.

• Proper Filament Storage:

• Use Airtless Containers: Store ABS filament spools in sealed plastic bags with desiccant packs (silica gel) when not in use.

• Invest in a Drybox: For frequent printing, use a dedicated drybox (a sealed storage container that maintains a low-humidity environment) or a heated drybox that actively warms the filament to drive off moisture.

• Control the Printing Environment: Avoid printing in humid environments (e.g., a damp basement). If necessary, use a dehumidifier in the room.

3. How to Solve and Fix Wet ABS

If your ABS filament or a finished prototype has absorbed moisture, here is how to solve it:

• Drying Filament:

• Use an accurate thermometer to verify the temperature.

• Set the oven to the lowest possible setting, ideally no higher than 70°C (158°F).

• Do not leave unattended.

• Filament Dryer: The most effective and recommended solution. Dedicated filament dryers are inexpensive appliances that heat the spool to a precise temperature (typically 65-75°C / 149-167°F) for several hours, driving out the moisture without melting the plastic.

• Food Dehydrator: A popular DIY alternative. Ensure it can maintain a consistent, controlled temperature.

• Oven (Use with Extreme Caution): If no other option is available, you can use a conventional oven. However, most ovens have poor temperature control at low ranges and can easily exceed the glass transition temperature of ABS, melting the spool into a useless lump. If attempting this:

• Drying Time: Typically takes 4-6 hours, but severely wet filament may need longer.

• Post-Processing a Finished Prototype:

• Primer and Paint: A high-quality primer and paint system will create an excellent moisture barrier.

• Clear Coat: A spray-on acrylic or polyurethane clear coat will seal the surface.

• Epoxy Resin Coating: For parts that require maximum protection (e.g., those used outdoors), a thin coating of epoxy resin is highly effective.

• Specialized Sealants: Products like XTC-3D are specifically designed to coat and seal 3D printed parts.

• Drying the Part: If dimensional stability is critical, you can gently dry the finished prototype in a low-temperature oven or a food dehydrator using the same cautious method described above. Note: This can sometimes induce stress or warping, especially in thin-walled parts.

• Sealing the Part: The most common and effective solution for finished prototypes is to create a barrier against moisture.

Summary

By understanding ABS's hygroscopic nature and implementing these storage, drying, and sealing practices, you can consistently produce high-quality, dimensionally stable, and strong prototypes.