Laser cut living hinges for neater designs
Adding flexibility to sheet material with DesignSpark Mechanical
A living hinge is a flexible joint made from the same material as the parts that it joins together. These are commonly found on disposable or low-cost injection moulded plastic parts, one example is the hinged clips that are found on some designs of food storage boxes, shown below.
Image: Yorkshire Trading
It is also possible to make living hinges from thicker sheet material with a laser cutter. I first came across this technique with Snijlab laser cut notepad cover, and have since employed it with good effect in several projects.
Whilst laser cut living hinges can be of limited durability, they are interesting and provide a somewhat elegant solution for some designs. In this post, we will design several versions of the living hinge within DesignSpark Mechanical (herein referred to as DSM), before making a simple case enclosure for the Raspberry Pi.
Sketching out the hinge design
Looking at photographs of the laser cut living hinge, it is apparent that the design is made up of a series of parallel lines, overlapping each other rather like brickwork. By changing the spacing between these parallel lines, it appears that we will change the flexibility and strength of the hinge. Thus it seems logical to sketch out several different designs for a physical test, before choosing the optimal design for our Pi enclosure.
Interesting reading on the layout and design of these hinges, particularly the length and width of cuts can be found in these links:
Taking note of the work in the above links, I chose to try several different approaches to the design of the hinges: parallel lines, similar to those that appear to make up the design of the Snijlab notepad cover, and parallel rectangles, similar to those detailed in the two links above.
To start, a simple rectangle was sketched on a plane within the drawing area. This will be the main body of our test samples. A series of lines were then drawn for the hinge design. I used the Move tool, holding Ctrl to create copies of the lines, to speed up the drawing process.
Note the position of the lines relative to the edges of the sample – there are two row types – one made of a single line with tabs at either end, and the second consisting of tabs closer towards the middle, and cuts at either end.
DSM user interface top tip
When selecting different entities within a design, the way you drag the mouse cursor can change the method of selection. For example, when dragging a selection box from left to right, only entities that are fully enclosed in the selection box are selected.
In contrast, dragging a selection box from right to left selects all entities touched by the box, not just those completely enclosed (note the vertical lines are also selected). This is a useful distinction that is important to know, in this case when selecting lines to be moved and copied, as shown above.
The spacing between these rows of lines will also affect the flexibility of the hinge, and so several versions of the design are made, with differing spacings.
For the hinge design involving rectangle cut outs, the layout process is much the same; with two row types, and differing spacing between the rows.
Note that this sketch is all done in 2D, since we will be exporting a 2D DXF design file to be laser cut. See a previous post on more information and tips for exporting your DSM designs for laser cutting.
Satisfied with the first sketches of hinge designs, I exported the design in DXF format ready for laser cutting.
The sample designs were cut from acrylic and MDF, to test how each material responded to the different layouts.
MDF is a favourite material of mine for prototyping, particularly since it is low cost and relatively forgiving compared to acrylic. As shown above, several of the hinge designs worked well in this material.
Acrylic is a rather brittle material and I was surprised when I first saw living hinges cut from sheets as thick as 5mm working well, without the immediate breakages I would have expected! It is also a popular material for DIY electronics projects, given that it laser cuts rather well with aesthetically pleasing results, in a wide variety of colours and finishes.
We can see that some of the hinge designs also worked well when cut from 3mm acrylic sheet. However, some warping can be seen when the cut lines are 1mm apart, which does not look particularly good, and suggests that repeatability and reliability may be an issue.
However, placing cut lines closer together allows for a tighter bend radius, which may prove advantageous for some designs. To help assess the inner bend radius, I sketched out a template: a square with each corner a different radius.
Bending the acrylic and MDF test hinges around this template helped me to determine a suitable inner radius for each material and hinge design, and choose which to use for this project.
Designing the enclosure
With these test cuts in mind, I set about modelling the Raspberry Pi enclosure, beginning by downloading a 3D model of the Pi from the RS parts library, accessible via the interface within DSM or the RS website itself. This allowed me to work from a known model, saving time rather than drawing the Pi myself.
I drew a rectangle around the Pi to determine the size and shape of the base, then added a radius to each corner using the Create Rounded Corner tool.
A limitation of the laser cut living hinge is the larger corner radius required, compared to simply placing two sheets perpendicular to one another. Although I chose to use the hinge design consisting of lines rather than rectangles, allowing for a tighter corner radius, the walls of the enclosure still had to be some distance from the Pi. I used the Pull tool to increase the size of the base accordingly.
Following this, I drew a series of 6mm x 3mm rectangles around the base, that would act as locating holes for the wall section.
Next, I drew rectangles on each side of the base – the beginnings of the wall section. I could then copy the locating holes in the base, to add corresponding tabs to the wall parts. Next, rotating each wall part by 90 degrees, I used the 3D Pi model with the Combine tool to cut holes in each wall, for the ports and connectors on the Pi. This was a simple and quick operation that gave great results!
Since most cables / plugs are larger than the ports they connect to, I quickly measured them and used the Pull tool to enlarge the cut outs as required.
To make up the continuous wall of the enclosure, I then made copies of each wall piece and arranged them in a row. This allowed me to add extra length between each wall for the corners, before adding in the lines to create the hinges.
Returning to the base, I again used the Pi model to add the mounting holes, before adding four extra holes for screws and spacers that would hold the entire assembly together. Finally, I made a copy of the base with added holes for ventilation, to make the lid of the enclosure.
Cutting and revising
Despite initial tests, my first attempt at the hinged enclosure proved less flexible than I had hoped. Attempting a compromise of spacing between the hinge cut lines resulted in a design that did not work.
The first cut also revealed something I hadn’t noticed during the design process: I had forgotten to add locating tabs to the tops of the walls! This was quickly and easily rectified by making a copy of the walls, rotating them about 180 degrees, modifying the copies with the Pull tool and then combining them with the original wall parts, as shown in the screen shot below.
Making these changes was a quick and simple task, and given that the laser cutter was close to hand, fixing these problems in the design and re-cutting took a matter of minutes. Before long, I had a great looking enclosure fit for a Pi.
Despite the limitations of having to accommodate the larger corner radii, the living hinge design is aesthetically pleasing and makes for an easy to assemble enclosure. The fragility of the hinge with such small spacing between the cut lines is also negated somewhat when sandwiched between the top and bottom layer, in fact the corners are fairly tough once assembled.
Laser cut living hinges are well worth considering when it comes to design for laser cutting, be it for making simple corners as demonstrated here, or adding elegant curves without the need for heat bending and/or cutting many small parts.
DesignSpark Mechanical provides a great platform in which to draw up such designs, and use for rapid prototyping, with the added bonus of being able to import 3D models of popular products available through the RS website, saving valuable time when designing and building projects.