Here is a painful lesson learned by one of our clients from their previous supplier: they provided us with an issue report stating that excessive addition of color masterbatch could lead to abnormal increases in MFI (Melt Flow Index), resulting in brittleness in plastic products. However, we remain only half-convinced of the authenticity of their report.
It is true that adding color masterbatch during the compounding process can degrade the mechanical properties of the original plastic. Based on experience, the increase in MFI after coloring should generally be controlled within 50%, or at least within 100%.
However, the report indicated that adding 2% color masterbatch to their PC material caused the MFI to increase to 2.6 times the original value, while adding 5% resulted in a sharp rise to 6.2 times the original value—an absurd deviation. If this isn’t due to excessive use of recycled material, what else could it be?
When analyzing the defective samples sent by the client, we first examined the fracture surfaces of the main body and the hook under a microscope. The surfaces showed smooth brittle fractures, unlike the rough and jagged fractures typical of normal plastic breaks. This observation further supported our suspicion that recycled material might have been added to the plastic product.
Test and Analysis Results for Side Cover MFI and Push Test
Material Used for Side Cover
MFI Test Value (g/10min)
MFI Change Rate
Push Test Result
Remarks
Standard: Transparent PC2805 + 2% Masterbatch
26.2
2.62
OK
Transparent PC2805 Standard: 7.8–12.5 g/10min; Actual: 10 g/10min
Experimental: Transparent PC2805 + 5% Masterbatch
62.4
6.24
NG
Raw Material: Transparent PC2805
11.2
1.12
OK
Improved Product: Sabic Black PC223S-BK1G335
18
1.07
OK
Black Raw Material Standard: 15–19 g/10min; Actual: 16.8 g/10min
Consequently, we advised the client to demand a detailed analysis report and corrective action plan from their previous supplier. Notably, the supplier conspicuously avoided addressing the most critical issue—the excessive addition of recycled material, which they dared not acknowledge.
What are the causes of brittle fracture of injection molded parts?
Processing-Related Causes
Insufficient Melt Temperature: If the material is not fully melted, it can lead to poor molecular bonding and brittleness.
Excessive Cooling Rate: Rapid cooling can cause internal stresses and reduce the material’s ability to absorb impact.
Over-Packing or High Injection Pressure: Excessive pressure can create internal stresses, making the part more prone to brittle fracture.
Inadequate Holding Pressure or Time: Insufficient holding pressure or time can result in poor packing and weak molecular structure.
Mold Temperature Too Low: Low mold temperatures can increase residual stresses and reduce ductility.
Design-Related Causes
Sharp Corners or Notches: Stress concentrations at sharp corners or notches can initiate brittle fractures.
Inadequate Wall Thickness: Thin sections are more prone to brittle failure due to insufficient material strength.
Poor Gate Location: Improper gate placement can lead to uneven flow, weld lines, or weak areas in the part.
Environmental Factors
Chemical Exposure: Exposure to chemicals or solvents can degrade the material, making it brittle.
UV or Weathering: Prolonged exposure to UV light or environmental conditions can cause material degradation.
Temperature Extremes: Operating in very low or high temperatures can reduce the material’s toughness.
Quality Control Issues
Contamination: Foreign particles or contaminants in the material can weaken the part.
Inconsistent Material Properties: Variations in material batches can lead to unpredictable performance.
Improper Drying: Hygroscopic materials that are not properly dried can become brittle.
Post-Processing Factors
Annealing or Stress Relief: Lack of proper annealing or stress relief can leave residual stresses in the part, increasing brittleness.
Mechanical Damage: Scratches, dents, or other surface defects can act as stress concentrators.