Plastic Products Wall Thickness: The Most Recommended Reference

Plastic Products Wall Thickness: The Most Recommended Reference

Introduction

Injection-molded parts can be complex to manufacture. For best results, designers must understand the molding process, design for manufacturing (DFM) best practices, and the importance of material selection. One of the key factors to consider is the part’s wall thickness.

Why is Wall Thickness Important?

The optimal wall thickness for an injection-molded part balances cost and moldability. A single part may be relatively inexpensive at scale, but even a small bit of unnecessary material can have a large impact on overall project costs. Optimizing wall thickness reduces overall part mass, but must be done carefully. Otherwise, thin walls may not receive proper material flow in the mold, while walls that are too thick can end up with sink marks and warped surfaces. Both cases lead to excessive rejection rates, but by following recommended DFM guidelines, you can create parts with walls that are just right — neither too thick nor too thin. Optimized walls also cool more quickly and reduce cycle times. The shorter the cycle time between parts, the lower the production cost because more parts can be molded per hour of machine time. So, since excessively thick walls take longer to cool, they increase molding costs. Ultimately, wall thickness has a direct impact both on cost and on project timelines.

Defects Caused by Incorrect Wall Thickness

Wall thickness optimization prevents multiple types of injection-molding issues. The most common problems that stem from wall thickness are listed below:

Sink Marks: Sink marks can have multiple causes, but the most common one is a wall that’s too thick. Because resin cools from the outside, the material in the interior will remain fully molten even after the outer surfaces have solidified. As the core slowly cools, it contracts and pulls the outer faces inward, creating indentations. The effect is particularly apparent on flat surfaces. In most cases, thinner walls supported by ribs are better than thicker walls. Ribs won’t precipitate sink marks and the structure uses less material, too.

Short Shots: Short shots occur when plastic solidifies before completely filling the mold. There are many potential causes for this, including wall thickness. If a wall is too thin, the plastic will not flow quickly enough to fill the whole mold before solidifying.

Warping: Injection-molded parts will warp if their internal stresses are not balanced. These imbalances can be caused by non-uniform wall thicknesses. For example, when a thin wall section is abruptly followed by a significantly thicker section, the two will cool at different rates. The thicker section will cool more slowly than the thinner section, induce unequal shrinkage, and deform the structure.Below is a sample:

Wall Thicknesses for Various Materials

Flow Length Ratios

Each material has a maximum flow length for a given injection pressure and wall thickness — this is known as the flow ratio. Thin walls of greater length require greater pressure, which requires larger injection molding machines and more robust tooling, both of which increase costs.Below you’ll find flow length ratios for some common plastics.

Design Guidelines for Part Wall Thickness

At minimal wall thicknesses, there are a number of design techniques that can optimize part strength. These include the use of ribs and gussets, bosses, draft angles, corner rounding, and smooth thickness transitions.

Ribs/Gussets: Ribs and gussets are used to support walls, bosses, and other free-standing features in injection-molded parts. Ribs primarily enable the reduction of wall thickness without sacrificing part strength.

Bosses: Bosses are cylindrical features that reinforce fastener holes. They should follow the same thickness guidelines as standard walls.

Draft Angles: Draft allows parts to be properly ejected from molds. Walls and ribs must have draft angles of between 0.5% and 1% per side.

Corners: Corners should be rounded to prevent excessive stress concentrations.

Thickness Transition: Walls must transition smoothly between thicknesses to prevent issues such

as sink marks and warping.

Conclusion:

The wall thickness of an injection-molded part is a critical factor that affects both the quality and cost of production. By following recommended DFM guidelines, designers can create parts with walls that are just right — neither too thick nor too thin.