Polycarbonate (PC) is widely used because it offers a rare mix of toughness, transparency, heat resistance, and dimensional stability. Yet PC injection molded parts can still crack unexpectedly, especially after assembly, cleaning, storage, or long-term service. In most cases, cracking is not caused by material weakness alone. It usually comes from the interaction between internal stress, part design, molding conditions, and environmental stress cracking.
For a broader comparison of plastic choices, see our injection molding materials selection guide. This article focuses specifically on why polycarbonate parts crack and how to reduce that risk during design and molding.
What is Polycarbonate?
Polycarbonate belongs to the amorphous family of thermoplastics, which means its molecular chains do not form a regular crystalline structure. Polycarbonate (PC) refers to a high-molecular-weight polymer containing carbonate groups in its molecular chain. It is widely used in transparent covers, housings, electrical components, automotive parts, and functional plastic parts that need both rigidity and impact resistance.
Properties of Polycarbonate
Mechanical Properties: Polycarbonate represents a synergistic combination of rigidity and toughness. Generally speaking, a material with high rigidity tends to be brittle, but PC can absorb impact while maintaining shape and structural strength.
Thermal Resistance: It features high glass transition and melting temperatures, resulting in excellent heat resistance; its decomposition temperature exceeds 300°C, and it can sustain a long-term working temperature around 120°C depending on grade and application.
Flowability: Due to the difficulty with which its molecular chains slide past one another, the polymer melt exhibits high viscosity and poor flow characteristics, making it somewhat challenging to process during injection molding.
Dimensional Stability: PC possesses exceptional creep resistance among thermoplastic engineering plastics. Dimensional changes caused by water absorption or long-term loading are typically lower than many alternatives.
Optical Properties: The large molecular chains of PC resist orientation and crystallization, maintaining the polymer in an amorphous state; this structural characteristic gives it excellent transparency and light transmission.
Electrical Properties: Characterized by low molecular polarity, a high glass transition temperature, and low water absorption, polycarbonate exhibits superior electrical insulation properties.
Flame Retardancy: PC possesses inherent flame-retardant properties, achieving a UL94 V-2 rating even without the addition of external flame retardants. Flame-retardant grades can be selected for electrical housings and safety-focused components.
Why Do PC Parts Crack So Easily?
Despite these impressive traits, polycarbonate (PC) plastic parts are surprisingly prone to cracking. Three main factors usually combine to cause failure:
- PC is an amorphous plastic.
- PC parts often retain internal stress after injection molding.
- PC is sensitive to environmental stress cracking, or ESC.
Amorphous Structure
In the world of plastics, crystallinity versus non-crystallinity (amorphous nature) is a fundamental classification that largely determines a material’s performance. Because PC is amorphous, it has excellent transparency and dimensional stability, but its molecular chains can retain orientation stress after processing. When this stress is not released, cracking can occur later under load or chemical exposure.
Internal Stress in Molding
During injection molding, PC melt is pushed through the runner, gate, and cavity under high temperature and high pressure. As the material cools, different regions shrink at different rates, and molecular chains may freeze before they relax. These locked-in stresses become weak points inside the part.
Common sources of internal stress include:
- High shear near gates and sharp corners.
- Uneven cooling across thick and thin sections.
- Surface layers freezing faster than the core.
- Excessive packing or holding pressure.
These stresses may not be visible, but they weaken the part from within. Once the part is exposed to load or chemicals, cracks can start at weak points such as weld lines, gate areas, ribs, or sharp transitions. Similar molding defects are also discussed in our guide to knit lines in injection molding.
Environmental Stress Cracking
Environmental stress cracking is one of the most important reasons PC parts crack in service. ESC happens when residual stress or external stress combines with a chemical environment that attacks the material surface.
Typical stress-cracking agents include alcohols, oils, detergents, surfactants, solvents, and some cleaning fluids. The chemicals penetrate the surface, weaken intermolecular bonds, and initiate micro-cracks. The cracks then grow under stress.
Case 1: Automotive air vent frames, commonly made from ABS or PC/ABS alloy, may develop cracks after being cleaned with alcohol wipes and exposed to retained molding stress.
Case 2: PE caps for syrup bottles or pesticide containers can appear sound after injection molding but crack after storage or use because chemicals and internal stress work together over time.
This phenomenon occurs because changes have taken place within the injection-molded parts.
Stress Effects: Internal residual stresses within the molded part, resulting from uneven cooling or molecular orientation, or external mechanical stresses from assembly and use, create the driving force for cracking.
Environmental Medium Penetration: Certain chemical substances, such as alcohols, esters, or surfactants, penetrate the surface of the plastic, thereby weakening the intermolecular forces between polymer chains.
Synergistic Effects: The presence of stress accelerates the penetration of the environmental medium, while the medium reduces the material’s localized strength. This interaction leads to cracking and eventual failure.
How To Reduce PC Parts Cracking?
When the material grade cannot be modified, the primary approach to mitigating failure is to minimize internal residual stress. This is achieved by optimizing the gating system design and adjusting injection molding parameters.
1. Minimizing Orientation Stress
Orientation stress occurs when polymer chains are stretched during the melt-flow phase and fail to relax before solidification. This is a primary cause of post-molding cracking in PC parts.
- Increasing Melt Temperature: Raising the melt temperature reduces melt viscosity and shear stress, which accelerates molecular stress relaxation. PC has a relatively narrow thermal processing window, so overheating must be avoided.
- Optimizing Injection and Holding Pressures: Excessive injection or holding pressures induce high levels of internal strain. Parts may show immediate micro-cracking or develop delayed cracking after assembly.
- Modifying Gating System Design: If radial cracking occurs around a central gate, localized stress concentration is likely. Increasing the gate size or changing the gate position can reduce shear and pressure loss.
- Controlling Mold Temperature: Elevating the mold temperature promotes molecular de-orientation. Excessively high mold temperatures, however, extend cycle times and may cause demolding deformation.
- Optimizing Wall Thickness: Where structural design permits, increasing wall thickness reduces flow resistance and shear stress, thereby minimizing orientation alignment.
2. Reduce Temperature Stress
Thermal stress is induced by temperature gradients and non-uniform cooling rates within the molded part. Balanced cooling channels, stable mold temperature, and enough packing time before gate freeze help reduce internal stress.
3. Reduce Shrinkage Stress
Parts with abrupt thickness changes are more likely to crack. Smooth transitions, uniform wall thickness, and careful rib design help the part cool more evenly. Large parts are especially sensitive because the distance between the gate and far-end flow areas can create pressure and cooling differences.
4. Reduce Demolding Stress
Mechanical stress applied during the demolding phase can introduce micro-fractures that compromise the final product. Make sure the mold has enough draft angle, smooth surfaces, and balanced ejection. Avoid aggressive undercuts and sharp corners that force the part to deform during release.
5. Improve Part Design
Proactive design adjustments during the Design for Manufacturing (DFM) phase significantly reduce the risk of structural failure. Avoid sharp corners, notches, sudden thickness jumps, and aggressive snap fits. If snap features are needed, review the design principles in our guide to snap fits in plastic parts.
Conclusion
PC injection molding is not simply melting plastic and injecting it into a mold. Behind every instance of cracking lies an imbalance between temperature, pressure, time, material behavior, and structural design. By reducing internal stress, improving gate and cooling design, avoiding chemical exposure risks, and adjusting part geometry early, manufacturers can significantly lower the risk of cracking.
As an experienced custom plastic parts manufacturer, Flexiparts offers end-to-end support from product development to mass production. For PC housings and transparent protective structures, our custom polycarbonate enclosures page shows how polycarbonate can be applied in functional enclosure designs. We also manufacture a wide variety of plastic enclosures and custom molded plastic parts using PC, ABS, PP, PE, POM, PA, and other engineering plastics.
