Plastic Product Design: Part 2

In this installment I talk about optimization and analysis. This applies equally to mass production and one-offs, and is really the critical set of skills that sets apart engineers from the many other people involved in successfully manufacturing a product. That said, anyone involved in manufacturing benefits from understanding how these tools are used, even if they won’t use them directly.

Note this isn’t a how-to for FEA, there are lots of those available on Youtube and elsewhere.

Plastic bending calculations

As I discussed in my first post, I used BASF’s guide to write my own calculator. To test it, I compared my results with with finite element analysis performed in Fusion 360. I also confirmed my results with a hand calculation, however that was just to confirm my math was setup correctly.

Finite Element Analysis (not just pretty pictures)

Any engineer reading this can probably think of a time that they’ve seen an FEA analysis that was really just a pretty picture, either because it wasn’t set it up correctly, or because the thing analyzed was not worth the time. The goal of this post is to help you understand how to do FEA and get useful information, not just pretty pictures.

The first step in all this is understanding what data you want to get out of the analysis, and why you can’t get it by other means. The second step is to understand the importance of boundary conditions, mesh size, and loads.

Identifying the problem

I expect the issue here to be a stress concentration where the clip bends away from the base. This is simply a matter of experience, but anyone can detect potential stress concentrations by looking for geometry with sudden changes in cross section. These are always potential failure points.

In this case, we only have an interest in structural FEA, but there are many other types of FEA for fluid flow, thermodynamics, electromagnetics, etc. Solving problems with them can be approached the same way, but requires a different knowledge base.

Boundary conditions

The boundary conditions are physical limits that we know or can assume are true. These are absolutely critical to getting a useful solution, and are often the most difficult part of any FEA analysis. Today’s example is very simple, mostly because the part is simple. In cases requiring dynamic analysis, or with many components, or with odd physical limits, some or all of any analysis may be garbage regardless of how it is run, and it’s up to the user to identify which parts are useful and which are not. That’s why you pay a professional for this kind of work.

Once I’ve run the simulation and determined the magnitude of the stress, I want to confirm that it won’t cause the design to break. Given that, I need to go back and determine what the stress at the elastic limit is for my material (aka the yield strength). I performed my analyses using ABS, which is a common plastic for both 3D printing and injection molding. One thing to keep in mind is that 3D printed material is anistropic (the strength between layers is significantly lower than the strength of each layer, which is equivalent to injection molded part strength). Basically, it wants to delaminate because the layers aren’t held tightly together.

The yield strength of ABS is quoted at anywhere from 4-6,000 PSI, depending on the test standard (ASTM D638 is the most common) and who performed it. It’s common in the 3D printing world to assume that Z-axis (the direction that layers are stacked in) strength is 30% of the specification yield strength. So I want to stay below 1,300-2,000 PSI in the Z-direction to prevent delamination.

Mesh Size

First things first, a mesh is the structure of points that is being analyzed. It looks like, a mesh net or fence, hence the name. It’s built using a series of polygons, usually triangles but there are other options. The actual math is performed at the locations where the triangles meet (the nodes), and the system is basically iterating through until the change in value gets very small relative to the value. If you’re interested in understanding more of the math behind it, I suggest finding a book on numerical methods or contacting your nearest university to take classes.

When it comes to sizing the mesh, we have some areas of interest and some areas that are not interesting. Large, flat or otherwise geometrically identical surfaces usually will not tell us anything of real value (that can’t be calculated by hand relatively quickly, for example). Usually there are a few features of the model that are really of interest, and those require finer meshing. I’ll just jump straight into an example.

Below is the meshed holder in Fusion360. The software has done some automatic optimization of the mesh size in different areas, so you can see the corners where it has made the mesh significantly finer in order to get useful values.


The tradeoff in meshing is time versus the helpfulness of the result. You can make a mesh that has tiny elements which takes forever to solve and gives you high granularity, but it will take longer to run and may not help give you better answers. See the mesh below, which has about 10 times the mesh density of the one above, but most of the mesh is now being solved in areas that are not helpful or interesting.


To make a fairly long story short, the required deflection of this design creates super high stress at that corner. I tried again using relief notches, but they didn’t help much. Ultimately it was quicker and easier to go back to the drawing board and make up a new model.


This switches the mounting side away from the flexure, and it’s also a little more compact and robust. In addition, it was easier to tweak this to make the holding force closer to the flashlight’s weight. This requires about 1 pound of force to deflect to the point required to allow the flashlight to be retained or removed.

End-on mount - Rev 11 - 1.PNG

End-on mount - Rev 11 - 2.PNG

Something wasn’t quite right about my measurements, as round 1 didn’t fit. So, I made some slights adjustments.


Better fit, but I wasn’t happy with how difficult the bevel was making it to insert the flashlight. Time for one more revision.


Much easier to insert, with no retention issue.



Plastic Product Design: Part 1

Today I’m going to start a short series on manufacturing plastic parts, using a flashlight snap-fit holder as an example. I’ll talk about design with 3D printing in mind, and go into analysis for injection molding and part optimization in later posts.

Design Process

Any time you’re designing a physical part, there are two critical aspects:

  1. What problem does this part solve?
  2. How can I manufacture this part?

There are addendums to these questions, like reliability, ergonomics, cost, lead time, and a host more depending on application, but if you start with those two you will build a good foundation. To answer these, we have to ask two sets of people questions: the customer, who has the problem and the manufacturer, who is going to make the part. If you work in manufacturing, you’ll often hear these referred to as VoC (Voice of the Customer) and VoP (Voice of the Process). That’s just Lean/Lean Six Sigma’s nomenclature.

For this project, the problem is that I have a flashlight banging around in the dash (there’s a small compartment underneath my radio it lives in), and I’d like it to at least stay put and if possible, be somewhere more accessible but still out of the way.

When I go to think about manufacturing, I have a bunch of options readily available to me: CNC machining, 3D printing, buying a bunch of parts online (for a spring-loaded assembly, for example). Ultimately, I’m going to start by 3D printing because it’s easy and quick, and because creating a retention holster requires fewer parts when using plastic than when using metal (depending on reliability, cost, etc. — you can really dig deep when you talk about optimizing products, but we’ll do that in a later post).

A note about workflow

Pretty much any time I’m making something to interact with a known part, I’ll start by making the known part. So in this case, I’ve modeled the flashlight. This gives me a convenient source of dimensions and an easy way to test fitup. In general, building the model gives me better ideas for how to make the part I’m designing.

Tangent on modeling software (mostly for beginners)

I generally design in Solidworks, using a mouse, keyboard, and 3dconnexion SpaceMouse Pro. I occasionally do work in Fusion360, but I primarily use it for CAM. I’ve also used ProE, which is a nice program with a lot of really great features, but not enough that I’m willing to relearn the work flow from Solidworks. There are a lot of decent free CAD packages available right now if you’re a hobbyist, drafter or entry-level engineer who just wants to learn how to design, and some great tutorials available on Youtube for virtually every one of them. Fusion360 is probably the most common, but there’s also OnShape and FreeCAD. I strongly suggest you start in a 3D, parametric (meaning dimensions define the geometry) CAD software. Stay away from SketchUp (it plays poorly with everything else and is hard to get data out of), and only learn AutoCAD or DraftSight if you have to for your industry.

I’ll note that as use most software exclusively as an engineer. I rarely do renders or animations, I’m not interested in making marketing materials, and I primarily work on commercial rather than consumer products. Those of you who are more interested in consumer products may need a different work flow or set of tools, and you are probably going to be optimizing more for cost than quality, which is usually the goal of commercial products.

Starting the design

Now I’ve got the model of my flashlight and I’m ready to begin building the holder itself. If you’re new to all this, then the best place to start is usually looking at designs that are already on the market. Figure out how and why they designed their part the way they did, and you can incorporate it into your own design.

In this case, there are several options for mounting orientation in the car and for how to retain the flashlight. I’ve illustrated these below, but usually I will go through and mentally eliminate all but the one or two designs I think will be best. In this case, I would eliminate the tailcap-holding design entirely for ergonomic reasons: I don’t see a mounting location where this would end up in my hand the right way. Similarly, the dual clip version will get pretty large to get my hand between the flashlight and the base, so I would eliminate it. The center clip might work out, and it’s really simple (which would be a big plus when we get around to talking about injection molding), but ultimately I see the bezel mount fitting this application better.


Detailed design method

At this point (when I’m down to the final one or two models) I will start fleshing out the details of the design until I see a significant flaw with one or they’re complete. This is initially just basic mechanics: how do you attach two things together? In this case, I know I’m using a flexible joint in advance so that has informed my design to some extent.

Designing snap fits

BASF has a lot of great literature on plastic snap fits (and plastic design in general), as well as a calculator here:

Personally, I built my own version of the calculator for several reasons: it forced me to learn the details of the physics involved (which aren’t terribly complex), and my application doesn’t fit their calculator. It also allows me to easily scale if I’m using snap fits with changing geometry, multiple snap fits of different geometry, or need to develop a snap fit that falls outside the guidelines BASF provides.

Ultimately for engineers, you should be confirming that you can get similar results to any piece of software you use, and that they make sense. Whether you’re checking it by hand, in Excel, or with an industry standard of some sort, you should never blindly trust any piece of software regardless of source. For hobbyists and others whose designs will not really have an impact because they’re not widely shared, this is a good exercise as well, but more so because it will save you aggravation, time, and money if things are wrong.

Excel pic.png

First iteration

End-on mount - Rev 2 - 1.PNG

Ultimately everything can (and should) start from simple geometry. In this case, I’m basically building a cap for the flashlight, which will then require an added feature to hold the flashlight in the cup.End-on mount - Rev 2 - 2.PNG

As I go along here adding features I’m constantly trying to think a step or two ahead. I tend to work in a lot of disciplines, often on large projects, and sometimes miss details, which is why it’s important to look back on my design and try to critique it. I think you will find it similarly useful in your own work.

At this point in the design, I’m basically just shaping the retention clip to the flashlight shape. I’m already thinking about how I will need to reshape the overall design to make it easy to insert the flashlight, but I haven’t modeled that yet. For now I’ll focus on the retaining clip.

End-on mount - Rev 2 - 3.PNG

I’ve extended the clip length by cutting down into the cup part. This increases the maximum deflection of the clip and can be used to reduce the required force.End-on mount - Rev 2 - 5.PNG

I’ve tapered the full clip length, in order to maximize the deflection it’s capable of. I’ve also added a chamfer along the length of the non-deflecting edge. These will both make it much easier to get the flashlight into the holder.

End-on mount - Rev 2 - 6.PNG

End-on mount - Rev 2 - 7.PNG

Now, we may have some issues with stress concentrations where the clip attaches to the non-moving portion of the holder. I will save detailed discussion of that for a future post, where I can talk a little bit about analyzing the part in both Solidworks and Fusion 360 (and again, checking by hand!).

At this point, I have a functional design (at least theoretically). I could print this and try it out, as it has a mounting point (the flat bottom) and will at least ostensibly hold the flashlight in place.


Thin wall - 1.PNG

Switching to the thinner wall saves about 6% in quantities of 1, and about 13% in quantities of 10 or more. For my purposes, there’s not much value in that 6%, so I’ll stick with the thicker wall.

End-on mount - Rev 5 - 1.PNG

To improve mounting this, I want to add some better surface area to apply double-sided tape (3M VHB RP25 – I love this stuff for automotive work in particular). Despite having 12% more volume, this is only 5% more expensive than the original thick wall version.

Note that I’ve tied in the mounting point at the top and bottom, but not connected them to the clip — if I had, the clip would be mounted to the surface and it would be very difficult to work with. Putting the mounting point at some other orientation would simplify the design, but in general I think this will work best ergonomically.