For previous reading, please refer to Plastic Life and Death Book, Part 2: Layout Decides! How Structural Traps Can Destroy a Product'.
Sudden cracking of plastic products, unexplained surface discoloration, unexpected breakage under load... These "accidents" often hide hidden flaws in design, materials, or processes. To extend life of plastic products, you must become a qualified "plastic forensic pathologist"—precisely identifying root cause through systematic failure analysis. Today, we'll unlock core logic and practical tools of plastic failure analysis, turning every failure into an opportunity for improvement.

Plastic failure is never caused by a single cause; it's more like a "serial case"—material defects, design flaws, processing errors, and operating environment are all potential suspects. Solving case requires a rigorous investigative process:
Site Investigation: First, document failed product's appearance—where did crack originate? Are there any discolorations, bulges, or wear on the surface? For example, phone cases often break at the corners, possibly due to stress concentration during design. Pipeline leaks accompanied by internal corrosion are likely due to erosion by medium.
Tracing medical history: Understanding product's "biography" is also crucial: What materials were used? Were there any abnormalities during processing (such as excessive temperatures)? Was product exposed to direct sunlight, humidity, or chemicals? In one case, a PE water tank leaked during summer. Tracing cause revealed insufficient antioxidants in raw materials, leading to molecular chain breakage due to prolonged exposure to sunlight.
Identifying suspect: Narrowing scope through elimination: If a batch of products fails, it may be due to material or process issues; if individual failures occur, it is more likely due to design flaws or improper use. For example, a batch of ABS toys exhibited brittle cracks. Investigation revealed that excessive proportion of recycled materials contributed to a significant drop in impact strength.

▲ Failure Analysis Checklist for General-Purpose Plastics

Visual inspection alone is far from enough; specialized tools are needed to perform a "physical examination" of plastics. Following types of tests are considered "magic tools" for solving crimes:

Infrared Spectroscopy (IR)
Like a "fingerprint check" for plastics, it can quickly identify material. In one case, a part labeled as PC was actually PS, causing deformation under high temperatures. IR testing immediately exposed "identity fraud."

 

▲ PC Structural Formula and IR Spectrum
Thermogravimetric Analysis (TGA)
Observing weight change of a material at high temperatures can determine filler content. For example, if a PP part fails to meet heat resistance standards, TGA reveals excessive calcium carbonate filling, which impairs heat resistance.

 

▲TGA Plots of Different PP Contents

Tensile/Impact Testing
Simulates the stress conditions of a product. If there is noticeable plastic deformation at fracture site, it may indicate insufficient material toughness. If cross-section is smooth and brittle, it is likely due to aging or stress concentration.
Fatigue Testing
For products subjected to repeated stress (such as gears and springs), cyclic loading is used to determine when they fail. Testing revealed that a certain nylon gear prematurely fractured due to insufficient tooth root fillet, leading to stress concentration and a sharp reduction in fatigue life.
For principles of testing method, see Ultimate Guide to Plastic Material Selection.

Scanning Electron Microscope (SEM)
Observe cross-sectional details at hundreds of times magnification. Fatigue fracture leaves "beach marks," while stress cracking exhibits distinct radial striations. These microscopic traces can accurately reconstruct failure process.

 

▲SEM image of a PA66 sample undergoing fatigue fracture

 

▲SEM image of a fractured PS sample
Polarizing Microscope
Detect internal stress. If a transparent plastic part exhibits colored streaks under polarized light, it indicates severe internal stress, possibly caused by excessive cooling during injection molding.

 

▲ Birefringence pattern of a plastic spoon

Environmental stress cracking (ESCR) test: Expose samples to chemicals under stress and observe for cracking.

 

▲ Internal stress test using a mixture of n-propanol and ethyl acetate
Aging test: Predict lifespan through accelerated aging using UV, humidity, and heat.

 

▲ Double 85 test
Classic case teardown: Finding patterns in failure
Case: Fracture of an ABS syringe needle hub

1. Problem statement: Shortly after molding, an ABS syringe needle hub developed cracks at small-diameter tip. This tip is secured to Teflon tubing via a metal eye inserted after molding. Defect rate was 1-2%.

 

2. History: This project was developed three years ago and has had persistent problems, with more issues last year. Extraction pellets present fewer issues than masterbatch addition.
3. Material: Commonly used ABS Marbon Cycolac T, with a melting point index of 1.5-3.5. Extraction pellets typically present no or minimal problems when using partially recycled material. However, when using masterbatches directly, problems are more frequent, likely due to uneven pigment dispersion. Therefore, use of recycled materials when adding masterbatches is prohibited to mitigate risk.
GPC (gel permeation chromatography) analysis was performed on following:
1. Molded parts
2. ABS raw material
3. Masterbatch
4. Extruded ABS color-modified pellets.
Results are as follows:
a) Color-modified ABS material has a lower molecular weight, likely due to extrusion process.
b) ABS material with masterbatch addition has a higher molecular weight than molded part.
c) Degradation of molded part (regardless of recycled content) is not significant.
All indications suggest that problem stems primarily from excessive residual frozen stress caused by design and processing. Recent increase in defect rates may be related to use of a high-molecular-weight masterbatch carrier, which hinders adequate dispersion and results in increased residual frozen stress, especially at current lower processing temperatures. Furthermore, processing aids may be used in masterbatch compounding process; their content and properties may also contribute to stress cracking.
4. Design: Following figure shows a schematic diagram of ABS needle holder. View A shows the overall structure, with Teflon insert secured to two flat surfaces of plastic part by a press fit. Wide end of part features a dual-gate design (end view B, side view C). In practice, two gate areas have slightly different geometries. At narrow tip (view D), two weld lines are perpendicular to gates and 180° apart. Metal eyelet insert exerts continuous pressure on plastic surface. Due to part design and cavity layout, position of pressure blocks does not correspond to weld lines. Instead, positions of pressure blocks within each cavity are randomly distributed but maintain 180° symmetry. In some finished products, area of pressure blocks subjected to metal insert pressure coincides with weld line.
Ideally, weld line should be symmetrically aligned 180° from beginning to the end of part, assuming uniform melt filling and consistent cooling rates.

 

▲Schematic diagram of an ABS injection needle holder
However, in some finished products, weld line is not straight as shown in (C) and (D), but rather curved as shown in (E), resulting in unusually close weld lines at the tip. This phenomenon is difficult to detect in original product, even with a magnifying glass. However, after part is heat-treated at 135℃ for 10 minutes to allow for shrinkage, uneven melt filling and internal stress caused by double gate become clearly apparent. After stress relaxation heat treatment, weld line becomes more pronounced.
All injection molded parts exhibit residual flow orientation or frozen stress along plastic flow direction. To ensure strength, orientation effects must be minimized to ensure uniform stress distribution and avoid localized stress concentrations that can cause overall imbalance. During heat treatment, some parts exhibit asymmetrical shrinkage (as shown in Figure G), with weld line distribution shown in Figures (E) and (F). Mold cooling process follows a horseshoe-shaped pattern, resulting in slight temperature differences in double gate area.
Additional Notes:
These non-ideal design features may not cause problems if processing conditions are met, such as high-temperature, uniform flow of molten plastic, mold temperature and molding cycle time provide sufficient stress relaxation time for part. However, fundamental flaws in current part and mold design result in a very low processing tolerance.
5. Processing: Injection temperature of 200-205℃ is lower than manufacturer's recommended 205-260℃. This temperature results in excessively high melt viscosity and insufficient mold relaxation, which in turn causes uneven filling and high internal stresses.
6. Assembly: When insertion force is reduced from 24 lbs to 12 lbs, incidence of metal earring insertion-induced cracking decreases but does not completely eliminate it. At lower insertion forces, cracks develop away from weld line. Clearly, weld line cracks are primary cause at higher insertion forces. When stress is no longer concentrated at weld (due to reduced insertion force), other weak areas begin to become failure points. Tolerances between outer diameter of metal earring and inner diameter of narrow tip result in differential stress in part. In cases where weld line is close to flat plate and stress is concentrated, a larger metal outer diameter and/or smaller part inner diameter may result in higher stresses, which may explain why not all parts cracked.
7. Environment: Environmental stress cracking is not a major factor, unless it is caused by process additives used during extrusion pelletizing. However, this is unproven and unlikely due to major design and handling issues.
8. End Use: Not a relevant factor; failure occurred shortly after molding or in storage.
9. Corrective Action:
Material: (a) Use a carrier resin with a melt viscosity equal to or lower than that of ABS resin in masterbatch.
(b) Regrind can be used as long as its flowability is acceptable.
Part Design: Modify mold to optimize position of flat portion inside tip relative to weld line.
Mold Design: (a) If necessary, modify injection point to balance flow on both sides of part.
(b) Inspect cavities to ensure that spacing of eyelet insert flats is uniform across cavities.
Processing: Increase molding temperature to approximately 230℃, perform quality control testing (see below) to ensure and maintain good processing conditions.
Assembly: Check diameter tolerance of metal inserts to reduce possibility of stress caused by excessive spacing.
Quality Control: To minimize shrinkage and weld lines, perform internal stress testing on improved product to determine optimal injection method and weld line location.
10. Conclusion: Failure was caused by multiple factors, including material, product design, mold design, processing, and assembly.
Molding temperature was too low.
Weld line was located in a high-stress area during mold design.
Unbalanced flow during injection molding process led to abnormal weld lines.
Masterbatch carrier resin had a higher molecular weight than main resin, resulting in uneven dispersion.
Avoidance Guide: Make Failure Analysis More Efficient.
Preserve "Evidence": Failed products should be stored in their original condition to avoid secondary damage. One team once haphazardly cleaned fracture surface, making it impossible to trace crack source using SEM.
Comparative Testing: Comparing with qualified products often reveals obvious differences. For example, if you suspect a batch of material is defective, compare its melt index (MI) with that of qualified material. An abnormally high MI indicates molecular chain fracture.
Apply this to real-world scenarios: Laboratory testing should simulate real-world usage conditions. For example, when testing corrosion resistance of water pipes, you can't just use pure water; you need to add actual medium being transported (such as chlorinated tap water).
Plastic failure analysis is like a puzzle. Every piece of test data and every microscopic trace is a fragment, and only through systematic integration can the truth be revealed. Mastering this method not only helps resolve existing problems but also helps prevent potential risks in advance—after all, extending life of plastic products is our ultimate goal.