Meeting Cosmetic Standards for Plastic Parts

When it comes to achieving cosmetic standards for plastic parts, CNC machining provides the most consistent, high-quality finishes and tight tolerances. Injection molding and additive machining (3D printing) are prone to certain cosmetic defects that can mar the appearance, fit, and function of plastic parts. Secondary CNC machining, however, can remove these defects and bring parts into compliance.

What are the Cosmetic Standards for Plastic Parts?

Cosmetic standards for plastic parts are broken down into 5 different grade levels:

• Grade 1: Highly polished or clear transparent
• Grade 2: Low-grade polish, non-textured & textured or clear transparent
• Grade 3: Low-grade polish, non-textured
• Grade 4: Painted parts
• Grade 5: Ink stamped, printed, silk-screened, or other decorated parts

Each grade has specific standards and allowances for defects. For example, Grade 1 parts cannot have scratches greater than 0.100” on the top/most often viewed side of the part, and no more than one scratch per 4”x4”. Grade 2 parts, on the other hand, can have scratches no greater than 0.200” on the top/most often viewed side of the part, and no more than two scratches per 4”x4”. Both grades, however, cannot have flash (excessive plastic found at edges where the mold comes together) greater than 0.005 or display any pitting (crater-like holes on the surface of the part).

Common Cosmetic Defects in Injection Molded and 3D Printed Parts

Injection molded parts are prone to cosmetic defects such as:

• Sink – surface depression caused by a non-uniform cooling rate or an injection pressure that’s too low
• Shorts – incomplete filling of the mold cavity, likely due to insufficient shot volume or low flow rate of material
• Orange peel – a ripple appearance, often a sign of shorts or sink
• Pulling – distortion caused by plastic catching on the mold when ejected
• Weld lines – where two or more fronts of molten plastic meet (also called flow lines or knit lines)

3D printed parts have a unique set of cosmetic defects, including:

• Overly wide base layer – A print’s initial base layer is often programmed with extrusion rates greater than 100% in an effort to ensure the part adheres to the bed, resulting in a base layer that is wider than designed
• Poor surface finish – 3D printed parts are known for poor surface quality – most commonly due to the “staircase effect” – which typically requires secondary machining to correct
• Z-Wobble – Improperly calibrated machines and/or out-of-tolerance mechanical components result in this “wobble” phenomenon that causes defects.

Achieve Cosmetic Standards for Plastic Parts with Reading Plastic

As an expert in CNC services for nearly four decades, we often provide secondary operations to correct cosmetic defects in injection molded or 3D-printed parts and get them to hold tight tolerances. If your plastic parts require smooth finishes and close tolerances, call us today at 610-926-3245 to start talking solutions.