The Hidden Cost of “Just Make It Thicker” in Plastic Part Design

Imagine you're deep into a time-sensitive rollout, like the M0200_EnrolEze kiosk project. You hit a structural snag during manufacturing, and a colleague—let's say it's Krunal—suggests a seemingly simple fix: just increase the material thickness of the enclosure from 2.0mm to 3.0mm to gain rigidity.

It solves the mechanical issue on paper. But it might just destroy your production budget. Here’s the exact science on why.

In injection molding, cooling time often dictates 50% to 80% of the entire molding cycle. The relationship between wall thickness and cooling time isn’t linear—it’s quadratic.

If we look at the 1D transient heat conduction equation for polymer cooling (extensively detailed in Kazmer’s Injection Mold Design Engineering), we get:

t_c = (h² / π²α) * ln[ (4/π) * (T_m - T_w) / (T_e - T_w) ]

Where:

• t_c = Cooling time
• h = Nominal wall thickness
• α = Thermal diffusivity of the specific polymer
• T_m, T_w, T_e = Melt, mold wall, and ejection temperatures

The most critical variable in that entire equation is h². Cooling time is proportional to the square of the wall thickness.

💡 The Example

If you accept the suggestion to increase your enclosure wall from 2.0 mm to 3.0 mm (a 50% increase in material thickness), look at what happens to the math:

(3.0)² / (2.0)² = 9 / 4 = 2.25

You haven't increased your cooling time by 50%. You’ve increased it by 125%. If your original cooling phase was 20 seconds, it is now 45 seconds. Over a moderate production run of 50,000 units, you didn't just add a little plastic—you just added nearly 350 hours of press time. At standard machine rates, that completely alters the ROI of the product.

🛠️ The Solution

When dealing with plastic enclosures, never use uniform wall thickness to solve a stiffness problem.

1. Maintain Nominal Wall: Keep the base wall thickness as thin as the flow length and material viscosity will allow.
2. Add Proportioned Ribs: Increase the area moment of inertia using ribs. To prevent sink marks on your cosmetic surfaces, rib thickness should be kept to 50%–70% of the nominal wall thickness.
3. Utilize Gussets: Support load-bearing areas (like bosses or snap-fit bases) with gussets rather than thickening the entire local geometry.

Great Design for Manufacturability (DFM) means finding structural integrity without paying the thermal penalty on the manufacturing floor. Every second you shave off the cooling cycle is pure margin added back to the project.