The Silent Killer of Plastic Assemblies: Press-Fit Boss Fracture

Pressing a metal pin or bearing into a plastic boss seems like the simplest assembly method in the world. No screws, no adhesives, just pure mechanical interference.

But fast forward 24 hours, and the assembly line is reporting cracked bosses. Why? Because plastic is viscoelastic, and a press-fit isn't just a physical interference—it's a continuous application of hoop stress. If you guess the interference dimension, you are gambling with the structural integrity of the part.

To design a press-fit that holds tight without fracturing, you must treat the plastic boss as a thick-walled cylinder and calculate the maximum allowable diametral interference (I_max).

When pressing a rigid metal shaft into a plastic hub, the foundational equation is:

I_max = d · (σ_y / E) · [ (D² + d²) / (D² - d²) + ν ]

Where:

• I_max = Maximum allowable diametral interference
• d = Outer diameter of the metal shaft
• D = Outer diameter of the plastic boss
• σ_y = Design stress limit of the plastic (yield strength)
• E = Modulus of elasticity of the plastic
• ν = Poisson’s ratio of the plastic

💡 The Example

You are pressing a 4.0 mm steel pin into a Polycarbonate (PC) boss with an outer diameter of 8.0 mm. For your specific PC grade: Yield Stress (σ_y) = 60 MPa, Modulus (E) = 2400 MPa, Poisson’s ratio (ν) = 0.38.

Let’s calculate the bracketed geometry factor (W) first:

W = [ (D² + d²) / (D² - d²) + ν ]
W = [ (8² + 4²) / (8² - 4²) + 0.38 ]
W = [ 80 / 48 + 0.38 ]
W = 1.667 + 0.38 = 2.047

Now, calculate the maximum interference (I_max):

I_max = d · (σ_y / E) · W
I_max = 4.0 · (60 / 2400) · 2.047
I_max = 4.0 · 0.025 · 2.047 = 0.20 mm

The Result: The absolute maximum interference you can safely design into this joint is 0.20 mm (meaning the boss hole should be no smaller than 3.80 mm). If your tooling engineer defaults to a 3.70 mm hole (0.30 mm interference) simply because it "feels tighter," the hoop stress will exceed the 60 MPa yield limit, and the boss will inevitably split.

🛠️ The Solution

1. Do the Math First: Never guess the interference. Run the thick-walled cylinder equation during the CAD phase.
2. Beware the Weld Line Penalty: If your injection mold design places a weld line at the boss (highly common), the local strength of the plastic can drop by up to 50%. You must reduce σ_y in your calculation accordingly, which drastically lowers your allowed interference.
3. Add a Lead-In: Always include a chamfer on both the pin and the hole. Without it, the metal shaft will broach (shave) the plastic during insertion, reducing the actual interference and creating stress concentrators that lead to premature failure.

Great engineering isn't just about making things fit; it's about predicting how materials behave under continuous stress over time.