Updated: Jan 19
Designing for 3D printing will be different from designing for Injection moulding or Subtractive manufacturing. It also depends on the specific 3D printing technology that will be used to produce the part such as FDM, SLS or MJF. It’s good to understand the design rules and guidelines for each 3D printing technology.
When designing a part for FDM printing, the following will need to be considered:
● Minimum Wall Thickness
● Support Walls
● Fillets and Chamfers
● Overhangs and Overhang limits
● Dimensional Accuracy
● Layer Height
● Print Direction
● Print Supports and Types
● Part Infill
Minimum wall thickness
A FDM printer has a nozzle where the material is extruded out from, the diameter of the nozzle will determine the minimum thickness of any given wall, for example for a 0.6mm nozzle each perimeter line will be just over 0.6mm, however it is not ideal to have a single perimeter wall due to the perimeter not having enough strength. Increasing the number of perimeters will increase the wall thickness, and therefore increase the strength of the component. It’s also important to design wall thickness with a derivative of the nozzle size in mind, for example if a part requires a wall thickness of 1.5mm or thicker, and the part was printed using 0.4mm nozzle, it would be advantageous to increase the wall thickness to 1.6mm, which would use 4 perimeters rather than 3 and some infill. It is also important to take wall thickness into account for small detailing, to ensure the detail will print successfully. Another important note is that for 2 or more joining walls should have a consistent wall thickness, this would mitigate the chance of shrinkage and warping within the component as the hot filament cools at different rates. Fillets and chamfers can help transition between two different wall thicknesses, providing gradual cooling across the component area.
Printing tall unsupported vertical walls can sometimes be difficult for an FDM 3D printer, the continuous movement of the nozzle can sometimes cause the wall to be printed at an angle rather than vertical, the upright wall would also lack strength. To prevent this from occurring and to increase the strength of a vertical wall, a support wall or rib can be inserted to help ensure the wall is perfectly upright. A support wall can also be used between two parallel vertical walls. This would help ensure that these two walls remain parallel with each other.
A 3D printed bridge is a horizontal overhanging piece of geometry between two vertical walls. Printing this bridge without support material may cause issues for longer bridges. The rule of thumb is that anything below 5mm long will not require a support material. For longer geometry where there is no underlying support for the bridge, the first few layers of the bridge will droop down and this quality can be seen in the printed object. Support material can be used to support bridges and provide a platform for the bridge to be printed upon.
Fillets and Chamfers
Fillets and chamfers can be used to blend between two surfaces that meet with a sharp edge. The edge where these two walls meet is the weakest part of the joint. Is susceptible to snapping and breaking. To increase the strength of these joints a chamfer or fillet can be added into the geometry, providing a smooth transition from one surface to the other. This can eliminate the weak point in the joint and also increase strength in the overall design, making it less susceptible to snapping when flexed.
Fillets and chamfers can also be used to provide a transition between layers of varying sizes, increasing strength between layers of varying sizes and removing any weak points. This can also be used on overhanging geometry, with a fillet or chamfer at a 45° angle to remove the need for any support material and increase strength. Adding fillets to round corners can reduce any warping filaments might have. Fillets can reduce the stress between layers and allow for printing without layers curling or warping of the bed.
Overhangs are sub-pieces of geometry that extends out from the main geometry without any support underneath. Bridges can be classed as a type of overhang as they extend horizontally from a vertical wall without any support underneath. A bridge is a 90° overhang which pushes beyond the limit of a FDM printer when printing without supports. A general rule of thumb is that the overhang can be printed without support material if the angle from the horizontal is 45° or less. Going beyond this angle will require support material otherwise the quality of the surface on the bottom layer of the overhang will be poor.
General tolerances - Dimensional accuracy
Dimensional accuracy will depend on each material; it relies on how much the material shrinks and warps during printing. For example a 10mm cube printed in PLA may print out at 9.95mm on all sides due to the layer height, nozzle diameter and extrusion thickness. For a general rule of thumb the tolerance of a printed part can be ± 0.5mm, this should be accounted for when designing components that fit together such as holes and pins, or parts that have to fit around an existing component. Clearance holes for fasteners such as a bolt will require a larger hole in the 3D printed component, this is where the ± 0.5mm tolerance will be required to prevent the clearance hole being too tight for the fastener. It’s also important to know that since the Z axis is the most accurate since there is little moment in this axis while each layer is printed, this may be advantageous for having dimensional accuracy in a prefered direction.
Inserts can be used to incorporate a fastener into the 3D printed design. A lot of plastic 3D printing materials are too soft to have a thread tapped into, designing a reliable thread within the 3D printing design can be a challenge and if the material is too soft, the integrity of the thread will fail over time. Inserts such as a captive nut can be placed into a design during the printing process, this will incorporate the strength of the metal captive nut into the design rather than 3D printing the nut geometry within the design. Threaded inserts are another fastener component that can be inserted straight into a part after it has been printed, this produces a thread that can be used with a bolt. A threaded insert requires a smaller hole to be printed than the intended thread, the insert component is then drilled into a 3D printed component where the teeth of the insert bite into the plastic hole walls. The recommended dimensions for a 3D printed hole for an insert can be found on the threaded inserts manufacturers website.
During printing the dimensional accuracy of the FDM printing process may lead to the dimension of the printed hole to be different to the intended hole size. Shrinkage and warping of the material during printing is one reason why hole sizes vary. To produce an accurate hole, it would be best to design a smaller hole than required and then drill out the hole to the correct size. The direction the hole is printed in is important, the way a vertical hole is printed would be different to a horizontally printed hole, this is because for a vertical hole each layer would consist of a circular cutout. A horizontally printed hole requires the roof of the hole to be supported due to it deforming during printing, however not all holes can be supported as it may prove difficult for the support material to be removed afterwards, and therefore a horizontally printed hole won’t be circular. One way to overcome this is by compensating for the bowing within the design by changing the hole profile to a teardrop shape, which won’t sag as much during printing. So the best way to produce a precise horizontal hole is to drill out a larger hole to the exact dimensions.
In an FDM 3D printed part each layer is deposited one layer at a time, with each layer having the same layer height, denoted in either mm or microns. Layer height directly impacts the print time and extrusion thickness, the range of the layer height will depend on the nozzle size so that the material extruded remains ball-like, rather than flat or stringy. Decreasing the layer height, does increase the strength of the overall part as the layer adhesion over a given area is better. Decreasing the layer height also gives a better wall surface finish, removing the bumpy unevenness that would be more noticeable when using a larger layer height. A general rule of thumb is the layer height should be 80% of the size of the nozzle diameter. So a 0.6mm nozzle would have an optimum layer height of 0.48mm, and then a 0.25mm nozzle would have an optimum layer height of 0.2mm. For smaller intricate components with thin walls it would be suitable to use a small nozzle and therefore a small layer height. For a large object with solid walls, a larger nozzle and larger layer height would be suitable for print time optimization. The print time is proportional to the layer height, an object printed at 0.1mm would take double the time to print than the same object printed at a 0.2mm layer height.
The orientation an object is printed in depends on many factors such as bridging, supports, support material, hole direction, layer finish and optimum print time. The orientation is typically a compromise between these factors to give the printer the best chance to successfully print the object while the printed part satisfying the requirements.
In the case where bridging, holes and layer finish does not matter; having the print direction where no support material is used would significantly reduce the print time. Support material should be used if there is no suitable orientation for a printed part.
For printed parts that have a force exerted on it may benefit from having an optimum print direction. If a force is applied in parallel to the print direction of the part, it is more likely for that part to fail due to the separation of layers. When solely considering the orientation of a printed part that will have a force exerted on it, a good rule of thumb is to have the print direction be perpendicular to the direction of force applied.
Print Supports and Support types
When the print direction is decided and support material is required, there are several support options for a printed object. Some objects may only require support when the angle is beyond a threshold, for example overhangs that exceed 45°. Other options for support material include supports that only interact with the print surface and the first few layers, this may be beneficial for making sure the first few layers that have small detailing adhere to the print bed. Another option for support material to be used everywhere that would help ensure all aspects of the part are printed correctly, this increases print time significantly and the post process support removal may lead to end part damage.
There are different types of supports such as linear supports that build up from beneath the structure that they are supporting, other types of supports include tree supports with “grow” around the structure making it easier to remove support material, in some cases this increases the print time.
An object that is 3D printed is not always solid, having a solid 3D print increases print time significantly and also is not required in most cases. Having a solid infill (100% infill) means that the entire volume of a 3D part consists of the extruded polymer material. The amount of material extruded within the core volume of a 3D part is referred to as infill. 0% infill means there is no material internally and 100% material means the entire internal structure is full of material. In most cases a default 20% infill is used for 3D printing parts, this reduces the print time and weight of the part however this may not have enough strength for some applications. Increasing the infill will help increase the strength within the part but will also increase print time and cost. For parts that will incorporate a hardware fastener, having a higher infill density will help.
The infill shape can also be customised, this is the way the internal geometry is printed from the bed up. The most common and standard shape infill is rectangular, increasing the infill density would decrease the distance between the rectangular lines increasing the strength. Triangular infills are also common, however it generally takes longer to print. Triangular infill increases the strength of the part compared to a rectangular infill. Other types of infill shapes exist such as wave, honeycomb and wiggle each with their own strength and flex characteristics.