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Fasten your Design!

You've created some plastic parts and are now unsure how they will fasten together. Maybe this blog will help.

Many plastic parts contain specialised features such as snap hooks, mounting bosses and a gap at the joint where two components come together.  Let see how we can do this inside SOLIDWORKS.  There's a handy set of tools located in the menu, Insert > Fastening Features.

 

 

The model is first split into multiple bodies to identify the upper and lower housing. The shell tool can be used to hollow out the part. It will leave the faces you select open and create thin-walled features on the remaining faces.  

We will use the Lip/Groove feature to create an edge. It is often used in plastic parts to prevent an edge-to-edge joint between two parts.

Select the Lip/Groove feature. Highlight the upper housing to receive the groove and the lower housing to receive the lip.

For the Groove Selection select the face and the edge you wish to apply. Repeat the same procedure for the Lip Selection. You can then define dimensions in the parameters.

Snap Hooks are common features in plastic parts enabling quick assembly without the need for tools or fasteners.

 

First create sketch points where you wish locate the snap hooks. Select Snap Hook feature, pick the sketch point. Select a reference plane to define the vertical direction of the snap hook. Select another reference plane to define the direction of the hook (use Reverse direction if the hook is pointed the wrong way). Define the dimensions in the Snap Hook Data. Repeat the same procedure for each snap hook.     

 

Next is to create the Snap Hook Groove. The dimensions of the snap hook groove are driven by the snap hook. You can change the offsets or clearances through the PropertyManager. Again, repeat the procedure for the other snap hook.

Mounting boss can be added for hardware fasteners. When you create a mounting boss it can either be a through clearance hole for the fastener or one with a blind hole for the threads.

First we will create the through clearance hole boss as shown below. You can reposition the mounting boss by editing the 3D sketch.

Now that we have a clearance hole boss, it will be easier to create the corresponding blind pilot hole boss because we can reference some of the existing geometry to orient the boss and define its height.

To define the height, select the mating face option and select the planar face on the mounting boss of the lower housing. 

For added support you can set the fin parameters. A reference plane can be used to orientate the fins. The number of fins will be equally spaced out.

 

 

I hope you find this information useful and please subscribe to keep informed of future blog posts.

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Using a Shell Mesh with Thin Components

I’ve always been clumsy when decorating my Christmas tree, consistently working my way through my decorations at a rate of two to three baubles per year.  This inconvenience of having to replace broken baubles has taken its toll and I’ve decided to design the world first structurally sound bauble. 

How can SOLIDWORKS Simulation help? Well stage one will be to determine how much force one of these baubles will withstand before breaking, that way we will have a basis for comparison.  In order to do this we’ll set up a simple stress test to begin with, with a fixture to simulate my hand holding the top of the bauble and a force to represent the moment when I idly bump it into something and turn another shiny ornament to a glittery mess on the floor.  This is where I face problem one:

In order to achieve accurate results, it is recommended to have at least two high quality mesh elements across the thickness of your part, with our wall thickness of 1mm we therefore require an element size of 0.5mm.  This would result in an incredibly dense mesh across the surfaces of the model, which in turn would take an unreasonable amount of time to both mesh and solve for a part of this simplicity. 

We can therefore use an alternate mesh type for thinned walled parts (such as our bauble) known as a shell mesh.  By utilizing a shell mesh, it is possible to drastically reduce the mesh complexity resulting in significantly faster solve times in models which feature a thin cross section.

To define your thin walled solid bodies as shell’s follow this process:

Step 1: Right click your chosen body in the Simulation tree and select ‘Define Shell by Selected Faces’

Step 2: Select the faces you would like to define as a shell.  Also input the thickness of your material as well as the material offset to determine where the top and bottom faces lie.

Step 3: Re-mesh your model to create your shell mesh.  Ensure you remove any small element size you may have added trying to achieve two solid elements across the material thickness as these are not necessary for a shell mesh.  Once meshed you will notice both the ‘inside’ faces and ‘outside’ faces are highlighted in different colours, ensure the ‘outside’ and ‘inside’ colour is consistent and doesn’t swap between faces. 

If you have a mismatch of shell colour select the face, right click the mesh in the simulation tree and choose to ‘Flip Shell Elements’

Step 4: Run your simulation and get your results in a fraction of the time it would have taken using Solid Elements.

Whilst I continue my design quest of the unbreakable bauble (You hear it here first…) why not give shell elements a try for yourself and see how this technique could save you analysis time.

Merry Christmas from everyone at TMS CADCentre!

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