Do You Know Defects of Injection Molding (II)-Plastic Introduction-LFT-G

Do You Know the Defects of Injection Molding? (II)

Weld Line

info-3872-1088

Causes of formation

1. The two molten materials met at the end of the flow, with lower temperatures and insufficient pressure, resulting in incomplete fusion.
2. Mold structure issue: For instance, if the flow channel position is incorrect, it can cause the molten material to separate and then merge.
3. Inappropriate process parameters: low injection pressure, slow speed, low temperature, insufficient holding pressure, etc.
4. Material factors: using an excessively high ratio of recycled material leads to poor fluidity and low melt strength.

Control scheme

1. Optimize Mold Design:
Optimize gate locations to minimize melt flow separation and convergence. Hot runner systems or multi-gate injection can be applied when necessary to improve melt filling balance and reduce weld line formation.

2. Improve Processing Parameters:
Increase injection pressure, injection speed, and melt temperature to enhance melt flowability. Extend the packing time to improve fusion strength at the weld line.

3. Optimize Material Selection:
Select materials with good flowability, control the proportion of regrind materials, and add impact modifiers when required to improve weld line performance.

4. Improve Mold Venting:
Ensure proper mold venting to prevent trapped air from obstructing melt fusion and causing weak weld lines.

5. Optimize Part Design:
Avoid design features such as abrupt wall thickness transitions and excessive ribs that cause melt flow splitting. Where possible, position weld lines in non-critical or non-visible areas to minimize their impact on appearance and performance.

Notes for Attention The welding marks will affect the appearance of the product and reduce its strength (especially in the direction of force application). The transparent parts will affect the light transmittance. Therefore, comprehensive control needs to be carried out in both design and process.

Surface Scratches and Scuff Marks

info-681-161

I. Phenomenon Description

There are fine elongated scratches, abrasions and texture scratches on the surface of the injection molded products, which affect the appearance quality.

II. Common Cause Analysis

Mold-Related Causes

Scratches, drag marks, or surface damage on the mold cavity
Improper positioning of mold ejection components (such as ejector pins and ejector blocks), or burrs on their surfaces
Improper mold installation causing interference during part ejection

Equipment-Related Causes

Improper gripping or handling by robots or part removal systems
Wear or burrs on moving components of the equipment
Insufficient lubrication or foreign particles on equipment guide rails and sliders

Material-Related Causes

Impurities, black spots, or contaminants in the raw materials
Excessive material hardness or high brittleness leading to poor surface resistance
Excessive use of regrind materials, resulting in unstable material quality

Process-Related Causes

Excessive ejection speed or ejection force
Insufficient cooling of molded parts, resulting in inadequate strength during demolding
Collision or friction during part removal, handling, or packaging processes

III. Control Measures

Cause Category	Improvement Measures	Detailed Actions	Expected Benefits
Mold-Related Control Measures	Repair mold surfaces and optimize mold structures	Polish or repair the mold cavity surface to remove scratches and burrs Inspect and adjust ejection components such as ejector pins and ejector blocks Ensure proper mold installation to prevent interference during part ejection	Reduce surface scratching caused by mold contact
Equipment-Related Control Measures	Maintain equipment and optimize part removal methods	Inspect robot grippers and select suitable fixtures (such as soft gripping materials) Regularly maintain equipment guide rails and sliders to ensure proper lubrication and cleanliness Replace worn components and remove burrs from moving parts	Reduce the risk of scratches during equipment operation and part handling
Material-Related Control Measures	Improve material quality and ensure stable material properties	Use clean and high-quality raw materials Control the regrind material ratio (recommended below 20%) Properly dry materials before processing to prevent contamination	Reduce surface scratch defects caused by material-related issues
Process-Related Control Measures	Optimize processing parameters and operating procedures	Adjust ejection speed and ejection force to avoid excessive force or rapid ejection Ensure sufficient cooling time before part ejection Standardize handling procedures to minimize collision and friction during handling and packaging	Reduce scratches and surface damage during the molding process

Delamination and Peeling

Injection molding case study

Possible Causes	Specific Manifestations	Box Size/cm
Material-Related Issues	High moisture content and excessive volatile components in raw materials Poor compatibility between different materials Excessive regrind/recycled material ratio or material degradation	Use properly dried and qualified raw materials according to processing requirements Select materials with good compatibility or add compatibilizers when necessary Control the regrind material ratio and avoid using degraded materials
Injection Molding Process Issues	Excessively high melt temperature causing polymer degradation Excessive injection speed causing shear heating Excessive back pressure causing material degradation under high shear stress	Set appropriate melt temperatures and maintain balanced temperature settings across heating zones Reduce injection speed to minimize shear heating Properly reduce back pressure to lower excessive shear stress on the material
Mold-Related Issues	Poor mold venting, resulting in trapped gas inside the melt Mold temperature being too high, too low, or unevenly distributed Rough or damaged mold surface conditions	Optimize the mold venting system to ensure smooth gas evacuation Control mold temperature to avoid excessive or insufficient heating Polish mold surfaces and repair damaged areas
Part Design Issues	Uneven wall thickness or significant thickness variations Sudden structural changes causing weld lines or stress concentration Improper rib design resulting in localized stress concentration	Optimize wall thickness design to ensure uniform transitions Avoid sudden structural changes and optimize weld line locations Design ribs properly to prevent stress concentration
Environmental Factors	High humidity or excessive temperature in the production environment Improper storage of raw materials or molded parts causing moisture absorption	Control workshop temperature and humidity Store raw materials in sealed conditions and ensure sufficient drying before processing
Equipment-Related Issues	Screw wear or damaged check ring Abnormal heating system performance or inaccurate temperature control	Inspect and replace worn screws and check rings when necessary Check the heating system and temperature control instruments to ensure accurate temperature regulation

⭐Preventive Measures

Material Drying
Process Optimization
Mold Maintenance
Part Design Optimization
Environmental Control and Storage
Equipment Maintenance

LFT-G^® will help you learn more about the potential problems that may occur during the injection molding process and provide you with solutions. We will continue our exploration in the next chapter~