Why "Thickening the Wall" Causes Ejector Pins to Punch Through Your Parts

You are injection molding a cylindrical plastic housing (like a bezel or a deep cup). Because plastic shrinks as it cools, it naturally grips the steel mold core.

When the press opens, the ejector pins fire—but instead of popping the part off the core, the pins punch right through the plastic, leaving you with a ruined part and a jammed mold.

The most common, intuitive engineering response? "The plastic is too weak. Make the wall thickness larger so the pins don't pierce it."

It sounds logical. It is also mathematically wrong, and will likely make the problem much worse.

To understand why, you must look at the formula for the Contact Pressure (P_c) generated by a plastic cylinder shrinking onto a rigid steel core:

P_c = (E · S) / [ (D / (2 · t)) - ν ]

Where:

• P_c = Contact pressure (the grip force on the core)
• E = Modulus of elasticity of the plastic (at ejection temperature)
• S = Linear shrinkage rate of the plastic
• D = Inner diameter of the plastic part
• t = Wall thickness of the plastic part
• ν = Poisson’s ratio of the plastic

The total Ejection Force required to push the part off is directly proportional to this contact pressure (F = μ · P_c · A).

💡 The Example

Let’s say you have a Polycarbonate housing with an inner diameter (D) of 50.0 mm, and your original wall thickness (t) is 2.0 mm. Assume Poisson’s ratio (ν) is 0.38.

Let's evaluate the denominator of the equation: [ (D / (2 · t)) - ν ]

For a 2.0 mm wall:
50 / (2 · 2.0) - 0.38 = 12.5 - 0.38 = 12.12

Now, you apply the "intuitive" fix and increase the wall thickness to 3.0 mm to stop the pins from punching through.
Let's re-run the denominator:
50 / (2 · 3.0) - 0.38 = 8.33 - 0.38 = 7.95

The Result: Look at what happened. Because the wall thickness (t) is in the denominator of the fraction, increasing the thickness made the entire denominator smaller.

Dividing your numerator by 7.95 instead of 12.12 means your contact pressure (P_c) just increased by 52%.

By adding plastic to "strengthen" the part, you caused it to shrink onto the core with 52% more vice-like grip. The ejector pins will still punch through your new 3.0 mm wall, because the force required to move the part just skyrocketed.

🛠️ The Solution

When you have pin-push-through on a core-bound part, stop adding material. Do this instead:

1. Increase Ejector Surface Area: Do not change the part thickness. Change the tooling. Replace standard cylindrical pins with a stripper ring or blade ejectors to distribute the force over a massive surface area.
2. Increase the Draft Angle: Every degree of draft drastically reduces the distance the part must travel before the contact pressure drops to zero.
3. Drop the Friction (μ): Add a nickel-PTFE coating to the core, or ensure the core is polished completely parallel to the line of draw (draw polishing).
4. Differential Cooling: Run the core coolant 10°C cooler than the cavity coolant. This shrinks the steel core slightly more and creates a more favorable temperature gradient in the plastic prior to ejection.

Great DFM means following the physics, not your intuition.