Common Injection Molding Defects in Office Chairs and Solutions

Office chairs contain many plastic injection molded parts, such as back frames, armrests, chair bases, seat supports, and decorative covers. During mass production, surface defects, dimensional problems, and molding issues are quite common.

These problems not only affect product appearance, but may also influence assembly quality and service life. For office chairs, customers usually pay close attention to both appearance and stability.

Below are several common injection molding defects found in office chair production and some practical methods used to improve them.

1. Weld Lines

Weld lines are one of the most common defects in office chair plastic parts. They appear when two plastic melt flows meet during the injection molding process.

Mesh chair back frames, armrests with holes, and complex structural parts are more likely to show visible weld lines.

Common Causes

• Improper gate location

• Low mold temperature

• Slow injection speed

• Long flow distance

• Poor venting performance

Solutions

• Adjust gate position

• Increase mold temperature

• Improve venting design

• Optimize injection speed and pressure

• Improve product structure

A well-balanced runner system can help reduce visible weld lines significantly.

2. Flow Marks

Flow marks usually appear as wave patterns, uneven gloss, or visible flow traces on the product surface.

This problem is commonly found on large office chair back panels and armrests.

Common Causes

• Unstable melt flow

• Insufficient injection speed

• Low material temperature

• Uneven mold surface finish

Solutions

• Increase injection speed properly

• Adjust melt and mold temperature

• Improve mold polishing

• Optimize gate size

For textured office chair parts, stable molding conditions are especially important.

3. Sink Marks

Sink marks often appear near ribs or thicker sections, showing as small surface depressions.

Many office chair structural parts use ribs to improve strength, so sink marks are relatively common.

Common Causes

• Uneven wall thickness

• Insufficient packing pressure

• Uneven cooling

• Overly thick ribs

Solutions

• Adjust rib dimensions

• Improve cooling channels

• Increase holding pressure

• Reduce local thickness differences

Reasonable product structure design can prevent many sink mark problems at the early stage.

4. Surface Scratches

Office chair parts may get scratched during demolding, handling, or packaging.

Scratches are usually more visible on glossy or textured surfaces.

Common Causes

• Insufficient draft angle

• Mold texture too deep

• Poor ejection performance

• Friction during transportation

Solutions

• Increase draft angle

• Adjust texture depth

• Improve ejection system

• Use protective packaging

Proper mold release design can greatly reduce surface damage.

5. Warpage and Deformation

Large office chair components may bend or deform after molding.

This issue can affect assembly and may even cause installation problems.

Common Causes

• Uneven cooling

• Improper gate location

• Uneven material shrinkage

• High internal stress

Solutions

• Optimize cooling system

• Adjust gate layout

• Maintain stable mold temperature

• Reduce molding stress

For large chair back frames, cooling system design is especially important.

6. Silver Streaks

Silver streaks usually appear as silver or white lines on the product surface and are often related to material conditions.

This problem is common with ABS and PA materials.

Common Causes

• Excess moisture in raw material

• High processing temperature

• Material degradation

Solutions

• Dry raw materials properly

• Control processing temperature

• Reduce material residence time

In many cases, silver streaks are caused by insufficient material drying.

About HUAJI MOULD

HUAJI MOULD specializes in office chair molds and plastic furniture molds, with many years of experience in office chair projects.

We provide mold solutions for office chair back frames, armrests, chair bases, and structural parts, while continuously improving surface quality, mold life, and production stability.

FAQ

Why do office chair parts easily show weld lines?

Office chair components usually have complex structures, holes, and corners, which makes melt flow merging more likely during injection molding.

How can textured surfaces remain consistent?

Stable mold temperature, proper texture processing, and stable injection molding conditions help maintain consistent surface appearance.

What materials are commonly used for office chair parts?

Common materials include PP, ABS, and PA+GF, depending on strength and appearance requirements.

Why is cooling system design important for office chair molds?

A proper cooling system can shorten cycle time, reduce sink marks and warpage, and improve overall product stability.