Fused Deposition Modeling (FDM) Design Guidelines
This tutorial is designed to give you a foundation of what to consider when designing and printing a part for Fused Deposition Modeling (FDM).
FDM is an additive manufacturing technology that extrudes thin layers of thermoplastic layer by layer until a part is completed. FDM produces robust production parts in engineer-grade thermoplastic, with the added benefits of speed and design freedom possible with 3D printing. The cutting-edge technology is utilized across industries like aerospace, energy and transportation for production components.
Step 1: FDM Design Considerations
The following information builds from conventional plastic part design to explain design considerations for manufacturing high-quality FDM parts.
Step 2: Shrinkage
This is when your material shrinks to a degree in the printing process. This happens because your selected material (whether it be ABS, ASA, PLA, Nylon, etc) is melted down to a liquid, extruded to build your part, and finally cools as a solid.
This comes into play if you have a 3D printer that is completely enclosed versus an open one. Stratasys printers already take shrinking factors into account so you don't have to worry about your part not being the correct dimensions.
Step 3: Warping
This can occur if you create a part with walls too thin or too high without adding in supports to your part such as ribs. The reason why this occurs is because your part contracts/shrinks as it cools.
Once again this is heavily dependant on the kind of 3D printer you have (open build tray vs. enclosed)
Step 4: Holes
Holes are subject to many factors that affect their general usability when printed:
- Shrinkage can cause holes to be undersized.
- Printing a hole in a vertical orientation can cause a misshapen hole due to how FDM printing has a stair stepping effect on the part.
- Holes can also be prone to being abnormally weak because of weak neighboring walls. Many of these end up being drilled out for accuracy.
GrabCAD Print however does allow for the insertion of heatset, helical, or custom inserts to mitigate this issue.
Step 5: Columns and pins
Both of these aspects can be affected by shrinking and warping. When designing, it's a good idea to print out these joining pieces exclusively to test out how much tolerance to give so that you have the desired fit.
Step 6: Wall Thickness
This is the distance between one surface of your part and the opposite surface. In general printing a part with thinner walls makes a part more susceptible to shrinkage and warping.
This brittleness can be either fixed while designing or mitigated by changing the orientation of your part while printing.
Step 7: Threads
Designing and printing threads can be tricky. Printing very thin threads can make them too brittle to function as intended. It’s important to keep in mind:
- Keep in mind the contour width
- Slice thickness
- Material choice
- Part orientation.
When designing built-in threads, avoid sharp edges and include a radius on the root. Sharp edges can be stress concentrators in plastic parts. Creating an ACME thread design with rounded roots and crests has been found to work well when using FDM.
Also, use a “dog point” head of at least 1/32 in. (0.8 mm). This dog point design makes starting the thread much easier.
Small threads produced from the FDM process are not recommended and not possible for holes or posts smaller than a 1/16 in. (1.6 mm) diameter. An easy alternative is to use a tap or die to thread holes or posts.
Step 8: Undercuts/Overhangs
Overhangs that are greater than 45 degrees need support material so while printing, your part doesn’t flop over. If surface finish is a concern, printing orientation also plays a large role.
Stair Stepping can not only make your part look choppy and unappealing, it can negatively alter how well your part fits if that is its purpose.
Step 9: Fillets
Fillets are rounded interior or exterior corners of a part. This can relieve stress and add strength to a part such as a tall wall.
Step 10: Size
Making a part very large or exactly to scale may use up large amounts which may increase your cost. When making prototypes, it may be beneficial to print out your tests at a smaller scale to save on cost and money.
Learn more on how to lower your printer material cost.
Step 11: Orientation
Designers should note that extruded plastic has its strongest strength in the tensile mode along the x-y plane. Since the layers are held together by “hot flow” across the strands (one strand is cooling while the other is laid upon it), the lowest strength is in the Z-direction for both tensile and shear modes.
Changing the orientation can also have an effect on the amount of material being used and the time spent to print your part.
Step 12: Assemblies
Proper clearance should be given between mating assembly parts to prevent them from fusing together. The standard guideline for creating clearances on assemblies being produced fully assembled is a minimum Z clearance of the slice thickness.
- The X/Y clearance is at least the default extrusion width based on a suggested minimum wall thickness.
- The minimum clearanceT needed for mating parts, when not producing the components fully assembled, is equal to the tolerance of the FDM machine itself.
If under a time constraint, a minimum clearance of 0.4mm-0.5mm is a safe estimate to start.
Step 13: Sectioning Parts
There are many benefits of sectioning a part:
- At the surface, it can allow you to print a part that is too large for your respective printer.
- It can also make post processing your part more manageable and safer if there are any fragile features that need to be preserved.
- It also gives you more control over the printing orientation that can save you additional support material use.
Step 14: Living Hinge
Living hinges are thin webs of plastic that connect two or more solid sections of a part. It is important to take into consideration the printing orientation so that the hinge prints along the width moving up in the z-axis. This will allow it to actually move as intended.
Depending on the material you choose, it may survive few or multiple uses.
Step 15: Fastening Hardware
To protect your part, use cap screws or washers that can help spread the stress over a larger surface area so that it is not concentrated over such a tiny area.
Using the Apply Insert Tool (picture below) in GrabCAD Print can automatically alter the structure of your part to accept hardware without having to go back into your respective design software.
Step 16: Bosses and Ribs
Many times the design of FDM parts can be solid or have a suitable infill rather than using a hollowed out design supported by bosses and ribs. This can reduce build time and use less support material.
It is not necessary to reduce the wall thickness of a boss, rib, or gusset in FDM parts. Generally bosses can be the same size as the part thickness or up to 0.02 in. (0.5 mm) less. It is also important to use gussets or ribs to support the bosses in FDM parts. This will increase the amount of stress the feature can withstand.
Step 17: Text
Minimum suggested text size on the top or bottom build plane of a FDM model is 16 point boldface. Minimum suggested text size on vertical walls is 10 point bold.
In most cases the supports generated to support text on a vertical wall can be eliminated to save time and material.
Step 18: Finishing and Secondary Options
Since the FDM process uses engineering-grade thermoplastics, the parts produced are capable of withstanding a number of post-manufacturing processes, including machining operations such as:
- Drilling and tapping
(Note that heat is easily built up in plastic parts, so removing the material slowly and using coolant keeps the part from distorting.)
Other post processing operations may include:
Learn more about finishing options for FDM 3D printed parts.
Step 19: In Summary
FDM parts are meant for prototyping your parts or replacing them entirely depending on the material used. Where FDM might lack in overall aesthetics, it makes up in toughness and the access to multiple options material wise to suit the needs of many industries. When designing to replace your part with a 3D printed one, strength should be a high priority if not the main consideration. Incorporating one or all of these considerations and methods facilitate and highlight this aspect and can help you end up with a durable and hopefully long lasting part.