Injection Molded Part Tolerances

Legos

Continuing the discussion of geometric variation on mechanical components, it’s time to talk about plastic. Before getting into details of injection molded plastic part tolerances, you might want to have a look at these two posts that provide a foundation for the following discussion:
Injection Molded Plastic Parts Checklist
EDM Tolerances

Let’s have a look at the major factors that affect plastic part tolerances.

  • Part Design
    As a general rule the achievable tolerance on any given dimension is a function of feature size; roughly ±0.2% to ±0.5% depending on some other factors we will discuss below. It’s easier to hold tighter absolute tolerances on smaller parts and features than it is on larger parts.
  • Material
    All engineering thermoplastics shrink as they cool, providing a challenge for the tool designer who must create a mold at the size the part will be at the time of molding. The adjustment can be made via non-uniform scaling of design geometry based on the material specified. Since each material has a unique shrink rate it can be difficult or impossible to run a different material without significant tool modifications.
  • Tool Design and Construction
    Injection mold tools can be incredibly complex and sensitive and designing them is an art. The design must compensate for the material shrink as mentioned above, and that’s just getting started.  Tools can require mechanisms to provide transverse motion, water lines and heaters for thermal control, and geometry to accommodate the molding press. Tools with multiple cavities for the same part are quite common. Depending on how each cavity is cut and the tolerances required there can be significant variation between cavities.
    A complete description of tool design concerns is far beyond the scope of this article. Understanding the complexity and importance of the tool design and construction is the point. So give your tooling engineers a cookie next time you see them.
  • Processing
    A mold press can be quite complicated with many moving parts, sensors, heaters, fluid channels, and control systems. The process of getting all these parameters dialed in to produce a good part can be tricky. Once dialed, any deviation may cause a change in part geometry after the part cools. Like tool design, a complete description of processing is far beyond the scope of this article and simply understanding the complexity and importance of processing is the point.
  • Equipment
    Just as variations in processing result in part geometry variation, variations in equipment will also result in more variation in geometry. Expect wider tolerances on parts coming off of older, more worn equipment. The same goes for older tools; variation increases with tool-wear.
  • Cost
    Tight processes typically require longer cycle times which results in less capacity (parts-per-hour per tool) so you can expect to pay more. On the other hand, if you run the process fast you get more capacity but with higher geometric variation. Finding the sweet spot between cost and tolerance range takes time and effort. So give your tooling engineers another cookie next time you see them.
    You now know that part geometry drifts as tools wear out. Newer tools produce more consistent parts but new tools are expensive and the expense of tooling usually gets rolled into part cost.

All of these aspects will play a part in the overall geometric variation. You won’t know the actual limits of your part size until tools have been cut and T0 parts have been run and inspected. So where do you begin when you’re in the early design phase? The Form Loves Function Plastic Part Design Checklist is a good place to start.

There are several good resources available:

{image: Roberto Bouza}

Have a comment? Post in the forum.

5 Axis CNC Machining of Helmet from Aluminum Billet

Amazing work with this 5-axis CNC. I was impressed with the deep undercuts achievable with long cutters and clever programming and the fact the the finish-pass was done so cleanly in one setup.

Company: Daishin
Material: A7N01-T6 Aluminum
Cycle Time: I’d love to know

Hat tip: C. Sven of ReBang

EDM Tolerances

That’s a trick title. Most of the time design engineers aren’t concerned with the tolerances associated with EDM process directly and I will tell you why.

First some background. EDM, or Electrical Discharge Machining (thanks again Wikipedia), is a material removal process wherein a large amount of electrical energy is delivered to the workpiece, burning material away. Energy is delivered through an electrode, either a block of conductive material (known as sinker, cavity, or volume EDM), or a wire (known as wire EDM). In both types, tools are mounted on machines with CNC controls.

EDM'd Moldbase and Electrode
EDM'd Moldbase (left) and Electrode (right)
Image via Eroda Tools, Ltd.

EDM processes are typically very slow and not suitable for volume production. However, EDM is typically very accurate and very effective on hard materials making it a great process for making plastic, sheetmetal, and other types of tooling. This is why design engineers are not usually concerned with the tolerances introduced by the EDM process; they are usually most interested in the tolerances of the parts coming off the tooling.

In this ongoing discussion of part tolerances (start here) I wanted to bring up EDM before talking about injection molding, metal stamping, metal forming, and other tooling-dependent processes because EDM is where the tooling starts. And you know how I love the fundamentals.

Tolerances and Material Removal Processes

If you haven’t already, I recommend reading and understanding my previous post about tolerances before digging in here.

Material removal processes are often used to build tooling for other manufacturing processes such as plastic molds, dies and punches for metal stamping and forming, extrusion dies, EDM electrodes, and many others.  Understanding material-removal process capabilities will be invaluable in understanding capabilities of downstream processes.

Common material-removal processes include lapping and honing, grinding, boring, turning, broaching, reaming, milling, planing and shaping, and drilling.

Engineering Toolbox has a good summary of tolerance limits for different material-removal processes:

Tolerance Grades
4 5 6 7 8 9 10 11 12 13
Lapping and Honing
Cylindrical Grinding
Surface Grinding
Diamond Turning
Diamond Boring
Broaching
Reaming
Turning
Boring
Milling
Planing and Shaping
Drilling

ansi-standard-tolerances

As you can see, lapping and honing give the tightest tolerances while milling, planing and shaping, and drilling have wider tolerances.  Also notice that  the tolerances get bigger as the part size gets bigger, regardless of process.

This is great information, but it doesn’t tell you anything about cost.  In general, as tolerances get smaller the parts gets more expensive regardless of process.  That is, a milled part with a dimension of 1.00 +/-0.05mm will be more expensive than a part spec’d at 1.00 +/-0.20mm.  How much more?  It’s impossible to know for certain because there are so many other factors that affect cost.  The take-home message is simply that better parts are more expensive.

Another key piece of information you do not get from this table and chart is any indication of the applications for these processes.  If lapping gives me the tightest tolerances, why don’t I just make all of my parts by lapping them?  Well, lapping only works on flat surfaces.  I suggest clicking through the links above and checking out what wikipedia has to say about each process.  All of the overviews are pretty good.

We will discuss applications of these process as they relate to mechanical components in later posts.

Tolerances

I’m writing this post to help those in the audience that aren’t familiar with detailed mechanical design. A basic understanding of tolerances is essential to follow subsequent discussions here.

First, a quick definition:

Engineering tolerance is the permissible limit of variation in

  1. a physical dimension,
  2. a measured value or physical property of a material, manufactured object, system, or service,
  3. other measured values (such as temperature, humidity, etc).
  4. in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel (see structure gauge and loading gauge).

Thanks Wikipedia.

Every manufacturing process has some variation on dimensional output and a sound mechanical design needs to account for these variations. If you ask a machinist to make you a block that’s 1″ by 1″ by 1″ you might get a block that’s 1.012″ by 0.923″ by 1.103″. Is that close enough?

Could the machinist have done a better job getting closer to the 1″ target? Probably, but since we didn’t specify a tolerance, technically it’s close enough. If we wanted something closer to 1″ per side we’d need to specify how close. That’s the tolerance. We’d say 1″ plus or minus 0.010″, for example. The 1″ dimension is called the nominal value and the 0.010″ is called the tolerance.

Later I’ll talk about tolerances associated with different manufacturing processes and environmental conditions and how mechanical engineers and product designers account for them in their designs.