Why Weld Lines Appear in Injection Molded Parts

What Are Weld Lines in Injection Molding?

Weld lines, also called knit lines, are one of the most common issues in plastic injection molding.

They appear when two or more flows of molten plastic meet inside the mold cavity but do not fully fuse together.

In some products, the weld line is only a thin visible line on the surface. In structural parts, it may also reduce product strength and durability.

Typical weld line symptoms include:

• thin surface lines

• gloss inconsistency

• whitening marks

• weak bonding areas

• uneven surface texture

For appearance parts, weld lines mainly affect surface quality. For functional structural parts such as office chair components, motorcycle parts, or bucket handles, they may also create structural risks.

Why Do Weld Lines Appear?

1. Plastic Flow Splits and Meets Again

This is the most common reason.

During filling, molten plastic often flows around:

• holes

• ribs

• inserts

• screw bosses

• complex product structures

After the flow separates, it joins together again inside the cavity. If the material cannot bond properly at that point, a weld line appears.

For example, visible weld lines are very common near handle holes on plastic chair backs.

Solution

• optimize product structure

• reduce unnecessary holes or sharp flow changes

• improve gate location

• balance the material flow path

2. Melt Temperature Is Too Low

If the melt temperature is too low, the flow fronts cool too quickly before fully combining together.

This usually causes:

• visible weld marks

• poor bonding

• lower strength

Materials such as PP, ABS, PA, and PC/ABS are usually more sensitive to weld line problems.

Solution

• increase melt temperature properly

• improve material plasticizing

• avoid unstable barrel temperature

• choose material grades with better flow performance

3. Mold Temperature Is Too Low

When the mold surface temperature is too low, the plastic loses heat rapidly before the two flow fronts fully merge.

This may result in:

• rough weld lines

• whitening

• poor surface finish

• texture inconsistency

Solution

• increase mold temperature

• improve cooling system balance

• keep mold temperature stable during production

• use mold temperature controllers if necessary

4. Poor Gate Design

Gate location directly affects flow direction and filling balance.

If the gate position forces the material to split several times before filling the cavity, weld lines become more obvious.

This problem is very common in large plastic furniture molds.

Solution

• optimize gate position

• shorten flow distance

• reduce multiple flow intersections

• choose suitable gate type and gate size

• perform flow analysis before mold manufacturing

5. Injection Speed Is Too Slow

When injection speed is too low, the melt fronts may partially cool before meeting together.

As a result:

• bonding becomes weak

• weld lines become more visible

• product strength decreases

Solution

• increase injection speed properly

• improve cavity filling efficiency

• optimize injection pressure and holding pressure

• avoid unstable machine settings

Common Areas Where Weld Lines Appear

Weld lines are commonly found near:

• holes

• side core areas

• ribs

• logo positions

• insert areas

• sharp corners

In office chair molds, common locations include:

• armrest holes

• screw bosses

• mesh fixing areas

• handle openings

These positions usually involve flow separation during filling.

How HUAJI MOULD Reduces Weld Line Problems

HUAJI MOULD has many years of experience in office chair molds, plastic furniture molds, and household molds.

During mold design, we pay close attention to:

• gate layout

• flow balance

• venting structure

• cooling system

• product wall thickness

• weld line position control

For products with high appearance requirements, we try to move weld lines to less visible areas whenever possible.

For structural parts, we focus more on weld line strength and molding stability.

By combining mold optimization with processing support, many weld line problems can be significantly improved before mass production.

Are Weld Lines Always Defects?

Not necessarily.

Small weld lines that do not affect product appearance or performance are usually acceptable in production.

However, if the weld line causes:

• cracking

• weak strength

• surface defects

• easy breakage

then it becomes a weld line defect and should be improved through mold modification or process adjustment.

Conclusion

Weld lines are a normal phenomenon in injection molding, especially in products with complex structures.

Understanding material flow, gate design, mold temperature, and molding conditions is important for reducing weld line problems and improving product quality.

A well-designed mold not only improves product appearance quality, but also helps increase structural strength and production stability.

HUAJI MOULD focuses on plastic injection molds, especially office chair molds and plastic furniture molds, providing practical mold solutions for different product structures and production requirements.