Views: 0 Author: Site Editor Publish Time: 2026-05-05 Origin: Site
If you work with injection molded parts, surface defects are something you will run into sooner or later.
On paper, the mold design may look correct. The machine is running stable. But once the part comes out, you still see lines, marks, or uneven texture on the surface.
For products like office chairs, these issues are even more obvious, because most of the parts are visible and require a good finish.
This article is not just a list of defects. It’s a practical breakdown of what really causes them and how to deal with them in production.
Surface defects are visible imperfections that appear on plastic parts after molding.
In real production, they usually show up as:
Lines or streaks
Dull or uneven texture
Small dents or shadows
Gloss differences
Some defects are only cosmetic, but others can indicate deeper problems in mold design or processing.
Flow lines look like faint waves or streaks on the surface.
They usually appear when the plastic flow is not stable. For example, when the melt slows down or changes direction.
In many cases, increasing injection speed or adjusting gate position can reduce the issue.
Sink marks are small depressions, often found in thicker areas.
This is very common in parts with ribs or uneven wall thickness.
The material shrinks during cooling, and if the packing pressure is not enough, the surface will pull inward.
Keeping wall thickness consistent is the most reliable way to avoid this.
Weld lines appear where two flow fronts meet.
They are hard to avoid completely, especially in complex parts like chair backs or armrests.
You can reduce their visibility by improving venting and adjusting melt temperature, but the gate position is usually the key factor.
This one causes a lot of confusion.
The part looks fine before texturing. But after applying texture (etching), a visible line suddenly appears, especially on curved areas like armrests.
In most cases, this is not a molding issue.
It usually comes from a combination of:
Small radius design
Uneven texture on curved surfaces
Slight machining or parting line differences
When the radius is too small, the texture breaks the smooth transition and creates a visible “edge”.
A simple fix is to increase the radius or use a lighter texture in curved areas.
Burn marks show up as dark spots, usually near the end of the flow path.
This happens when trapped air cannot escape and overheats.
Improving venting often solves the problem.
These look like shiny streaks or lines.
Most of the time, moisture in the material is the reason.
Drying the material properly before molding usually fixes it.
Sometimes the surface looks patchy — some areas are shiny, others are dull.
This can come from uneven cooling or inconsistent mold surface finishing.
It’s often overlooked, but it affects product quality a lot, especially for visible parts.
From experience, most surface problems come from four areas:
Gate location
Wall thickness
Surface finishing
If the design is not right, no amount of parameter adjustment will fully solve the problem.
Different materials behave very differently.
For example, glass fiber materials (like PA+GF) tend to make surface issues more visible.
Temperature, pressure, and speed all affect surface quality.
But these are usually fine-tuning tools, not the root solution.
Texture (etching) can either improve appearance or make problems more obvious.
A small design issue that is invisible on a polished surface can become very clear after texturing.
If you want to avoid repeated trial and error:
Check surface quality before applying texture
Avoid very small radii on visible areas
Keep wall thickness as uniform as possible
Don’t place parting lines on cosmetic surfaces
These small decisions in early design can save a lot of time later.
Surface defects in injection molding are not always caused by poor processing.
In many cases, they come from design details that only become visible after molding or texturing.
Understanding how mold design, material, and surface treatment interact is the key to getting stable, high-quality parts.
If you are working on office chair parts or other appearance products and facing surface issues, we can help you review the design and suggest practical improvements before tooling.
