Saving Failed Prints: How to Repair Cracked or Shifted 3D Prints

From using epoxy to wielding a soldering iron, makers have developed all kinds of strategies for dealing with damaged prints. This article covers the basics you need to know to keep you from scrapping imperfect models.

Let’s say that you are building an armor set from scratch. You have all the pieces except the center of your chest plate printed and put together. You are 23 hours into that last print when you see a slight layer shift emerge for the final few inches of the print, causing the layers to separate and a small crack to permeate the top layer of the chest plate.

If you are working on a small print and experience print failure due to some poorly tuned settings or a mistake in setting up the file, it might not be the end of the world to toss it and start over. However, a larger print failure can mean that a third to half of a spool of filament and over a day of print time are thrown away along with that print. To avoid wasting time and filament, there are a few ways to rescue failed prints. Also—be sure to check out our troubleshooting guide to ensure that it does not happen again!

(Layer shifting and separation in a test print; source: www.simplify3d.com)

 How to repair broken 3D prints

This guide covers the basic strategies for repairing FDM printed parts. We will cover SLA repairs in another article because resin prints typically require more care and are less likely to be salvageable upon print failure.

The major approaches for fixing broken prints are as listed according to increasing strength of the final repair as follows:

• Superglue
• Plumber’s cement
• Hot glue
• Epoxy
• Plastic welding

It is important to note the limitations of these methods. Superglue is cheap, accessible, and easy to post-process for a smooth print but cannot hold up to major wear.  Epoxy is great for rigid prints but can cause more problems than it solves when working with flexible filaments. We cover everything you need to know about fixing broken prints with these methods below.

Preparation

Before getting started on your repairs, you should ensure that you have a clean, clear, and well-ventilated workspace.

1. Clear off your workstation. Remove all dust and debris.
2. Get your sandpaper and microfiber cloths ready. Having these on hand will help with preparing the parts for re-bonding and cleaning up the final product.
3. Collect all broken parts. If they broke apart before you began the repair process, align everything so you can be certain how it is all going to fit together.
4. Set out rubbing alcohol or a similar cleaning solution with a designated rag. If you end up working with heat, you want to make sure that there is enough ventilation and space to keep all your items separate to reduce the risk of fire and fumes.

Superglue

Superglue is a ubiquitous and easy-to-apply fix for minor repairs or setting materials that will not see heavy use. Unfortunately, superglued parts are unlikely to stand up to heavy wear; although the bond itself can be quite strong, any tension or force on the part, particularly for wearable parts, can twist the bond apart. Moreover, superglue will not bond very well with highly porous or flexible materials.

To fix a part using superglue, start by gently sanding the broken edge to be as flat as possible. If the crack is large or goes in several directions, just ensure that every flat side is sanded down. Repeat this process for the piece that will be joined.

Then, clean each surface with alcohol or your cleaning solution to remove any debris. Let the pieces dry.

Carefully apply the superglue along the edge of one part. Hold the pieces together for at least one minute to allow the parts to bond. Don’t worry about any glue budding out of the crack—it is more important that the bond is formed well than formed cleanly.

Depending on the glue you used, you may have to let the bond cure for anywhere from 2 to 24 hours. Set aside the dried part to cure before doing any post-processing and cleanup as movement, alcohol, and water can disrupt the bond before it is fully cured.

Pipe cement

Unlike superglue, pipe cement functions as more of a solvent that slightly melts the plastic, allowing it to bond to itself rather than to a glue layer. Depending on the extent of the fracture, the infill amount, and the thickness of the part, this method can yield a very strong bond. However, this bond may not hold up well to certain kinds of stress, so be cautious when using pipe cement on wearable prints or tool parts. Moreover, pipe cement is flammable and requires more extensive post-processing than superglue owing to its color.

Sand and clean the edges of the parts you are going to bond. Apply the cement on both parts and hold together until they set. The bonding and curing time can vary according to the manufacturer, but you should not have to hold it together for very long.

Hot glue

Hot gluing parts together is not too different from using pipe cement in terms of resultant strength, but it may require less intensive processing as the glue dries clear. This method is best for thicker parts as around 2 mm is the minimum thickness you will need to extrude to achieve a solid bond. This method is more suitable for layer shifts and separation than those described above because it can quickly fill a crack and form a bond between parts that are not entirely separated.

Working with hot glue is simple; sand and clean the parts and extrude the hot glue between them or in the crack you are aiming to fill. Hold the parts together for a moment, and that’s it. You may have to sand down the remaining glue to reduce the visibility of the repair.

Epoxy

Many makers are familiar with epoxy as a bonding and repair agent for rigid prints. Quick-cure epoxies are great for home printing hobbyists as they typically have a 5–10 minute working time and a short curing time, allowing for same-day post-processing.

Epoxy is slightly more involved than the methods described earlier as most epoxies come with resin and hardener parts, requiring the user to mix the two in a specific proportion before use. Most epoxies will come with instructions for use with different materials. Dried epoxy can be tough, rigid, and withstand a lot of damage if used correctly.

After cleaning and sanding your parts, you simply use the mixed epoxy as a binding material between the parts.

Then, hold the parts together according to the epoxy manufacturer’s instructions. After the bond has set, allow the part to cure before sanding.

One benefit of epoxy is that you can also coat the top and bottom (or inside and outside, depending on the part) of the sealed seam to improve its hold. Most epoxies dry clear and can easily be sanded for a smooth finish.

Plastic welding

The best method for prints that may see heavy use and wear is plastic welding. There are designated tools you can buy that are specially designed for working with thermoplastics, but a simple soldering iron is fine for most applications, though it will produce messier seams and require more cleanup. We recommend having a variety of tips ready to deal with different kinds of fractures. For instance, a flat, angled tip is best for heating a flat part, but a smaller, round tip can be useful for melting auxiliary plastic onto the weld for added strength.

To begin, sand and clean your parts. For this method, it is best to work with clean, straight lines. You may want to take down some material to ensure that the parts can fit cleanly together, and you can always “add” more plastic to the seam while welding.

Fit the parts together. For particularly unwieldy or curved parts, it may be helpful to use a clamp to keep it in place.

Holding the parts together firmly, slowly use the iron to “stitch” the plastic onto itself. For this, we recommend using a wide, flat tip. When repairing small cracks or fixing cosmetic issues, using the existing plastic should be fine. For more strength, flip the part over and use an offprint or extra piece of dried filament to add material by heating it onto the print while applying firm pressure. You have to be careful here—pressing too hard can damage the seam or push through the plastic. For most repairs, a gentle circular motion is optimal for producing a relatively flat and even seam, but some makers find that working in straight lines or zig-zags works best for them.

Because there are a lot of factors influencing how well this method can go, we strongly recommend testing soldering alone as well as soldering with extra material on scraps and offprints until you feel comfortable working on the main piece.

Finishing

Now that your part is repaired, it’s time to clean it up.

Like any print, repaired prints can be cleaned up using increasing sandpaper grits and rubbing alcohol or a cleaner to remove dust and extra material. After ensuring that the seam is flat, a coat of polish or paint can make the repair nearly invisible.