Low-Temperature Brittle Cracking of Plastic Connector Housings: Mechanism, Root Causes, and Full-Process Engineering Solutions

In application scenarios such as outdoor photovoltaic power stations in northern winters, new energy vehicle high-voltage systems, cold chain storage equipment, aerospace cryogenic cabins, and polar research equipment, the low-temperature embrittlement and cracking of plastic connector housings are classic, high-risk failure modes in the industry. It is also a significant contributor to electrical faults in winter equipment and operational downtime.

From an operational and maintenance perspective, insulation alarms and functional interruptions due to cracked high-voltage connector housings at low temperatures are common failures in northern winter vehicle high-voltage systems. Brittle cracking of string connector housings in centralized PV plants due to winter cold can lead to string outages, affecting array generation efficiency. Failure cases in medical cold chains and outdoor industrial control, in which connector cracking caused temperature-control failures and equipment shutdowns, impacting material and sample safety, have also been reported.

Low-temperature brittle cracking is far more complex than the simplistic notion of "plastic freezing hard." It is a comprehensive failure resulting from the synergistic effects of microscopic changes in polymer properties, amplified structural stress concentrations, release of internal molding stresses, and service environment factors. This article dissects this industry challenge across four dimensions: material micro-mechanisms, root causes of mass-production failures, full-process engineering solutions, and industry-standard validation, addressing the practical needs of connector R&D, injection molding production, field assembly, and maintenance.

I. Underlying Mechanism: The Microscopic Nature of Low-Temperature Embrittlement

The key performance indicators for the low-temperature mechanical properties of engineering plastics commonly used for connector housings—such as PA66, ABS, PC, PBT, and PPS—are the brittle temperature (Tb) and the glass transition temperature (Tg). The essence of low-temperature brittle cracking lies in the nucleation and propagation of cracks under the combined effects of restricted mobility of polymer chain segments, concentrated internal stress release, and multiple contributing factors.

1. Glass Transition vs. Brittle Transition: The Critical Boundary from Tough to Brittle

Tg is an intrinsic property of the polymer matrix, marking the critical temperature where chain segments in the amorphous regions transition from "freely mobile" to "motion-restricted." It is primarily determined by the molecular structure of the base resin and is not significantly altered by toughening modifications. For toughened blend systems (e.g., PA66+POE, PC/ABS), the material exhibits dual glass transition behavior, with the low-temperature transition characteristics varying with the toughening system.

The impact of these transitions on material performance is clear: Above Tg, chain segments in amorphous regions can move, the material exhibits high elasticity and toughness, and impact energy is absorbed through chain deformation, resisting brittle fracture. Below Tg, chain segment movement is restricted, free volume decreases, and the material enters a rigid glassy state. While tensile strength and hardness may slightly increase, notched impact strength and elongation at break drop significantly, and crack propagation resistance is greatly reduced.

A critical correction for selection: Tg cannot be directly equated to the minimum service temperature. The brittle temperature (Tb) is the key limiting indicator for low-temperature toughness. Tb is the temperature at which 50% of specimens fail in a brittle manner under standard impact conditions, representing the critical value for the material's retained low-temperature impact resistance. For example, standard unmodified PA66 has a Tg around 50°C and maintains good toughness near 0°C; a significant drop in impact resistance typically occurs closer to its brittle temperature range near -30°C.

Table 1 Core Performance Parameters of Common Engineering Plastics for Connectors at Low Temperature

Material type	Glass transition temperature Tg	Room temperature cantilever beam notch impact strength	-40℃ Cantilever beam notch impact strength	embrittlement temperature Tb	Conventional minimum applicable temperature
Ordinary ABS	85 ~ 95 ℃	10 ~ 15 kJ/M²	< 2 kJ/M² (apparently embrittlement)	- 18 ~ 20 ℃	10 ℃ above
Unmodified pure PA66 (dry)	45 ~ 55 ℃	5 ~ 8 kJ/M²	< 3 kJ/M²	- 28 ~ 30 ℃	20 ℃ above
30% glass fiber-enhanced common PA66	50 ~ 60 ℃	6 ~ 9 kJ/M²	< 2.5 kJ/M²	- 20 ~ 22 ℃	10 ℃ above
30% to 40 ℃ level toughening glass fiber PA66	48 ~ 58 ℃	15 ~ 20 kJ/M²	8 or more kJ/M²	- 50 ℃	40 ℃ or less
Hardy PC/ABS alloy	110 ~ 130 ℃	~ 20 kJ/M²	10 kJ/m or more squared	- 45 ℃	40 ℃ or less
30% glass fiber-enhanced common PPS	90 ~ 100 ℃	5 ~ 8 kJ/M²	< 1.5 kJ/M²	-- 30 ~ 40 ℃	20 ℃ above
Toughening 30% glass fiber PPS	88 ~ 98 ℃	10 ~ 13 kJ/M²	5 kJ/M²	to 40 ℃	35 ℃ or less

2. Crystallinity and Differential Shrinkage: The Inherent Source of Internal Stress

Semi-crystalline plastics like PA66, PBT, and PPS contain both crystalline and amorphous regions. At low temperatures, the differing shrinkage rates between these regions create microscopic internal stresses. If crystallization is uneven during injection molding, localized shrinkage differences can amplify into macroscopic internal stress. While material toughness may mask this stress at room temperature, once the material embrittles in the cold, this internal stress can readily induce cracks, sometimes even causing spontaneous cracking without external force.

A frequently overlooked accelerator: The moisture absorption characteristics of polyamide (PA, Nylon) materials are a significant factor in winter low-temperature cracking. When saturated, moisture acts as a plasticizer for PA66, notably improving its notched impact strength. The effect of moisture absorption on the toughness of glass-fiber reinforced PA66 is less pronounced than for pure PA66. In the dry winter air of northern climates, nylon housings can quickly lose moisture. The low-temperature impact strength of this dry state is markedly lower than in the conditioned state. This is a key reason why some connectors molded in summer might crack spontaneously during winter storage, with residual molding stress remaining the root cause.

3. Low-Temperature Effects of Modified Formulations: The Balance of Toughening and Reinforcement

Connector housings often use glass fiber (GF) and tougheners to balance properties. The formulation directly dictates low-temperature toughness:

The Dual Nature of Glass Fiber Reinforcement: GF increases tensile strength, heat resistance, and dimensional stability but reduces notched impact toughness. At low temperatures, the interfacial bond between GF and the polymer matrix weakens, increasing the likelihood of debonding and crack initiation. For low-temperature applications, GF treated with specialized silane coupling agents should be used to enhance interfacial stability. A reasonable GF ratio with high-quality interfacial treatment can balance stiffness and low-temperature toughness—GF does not inherently degrade low-temperature performance.
The Core Role of Tougheners: A key modification for cold-resistant materials is the addition of compatibility tougheners, such as maleic anhydride-grafted POE. These additives have extremely low Tg themselves, remaining elastic at low temperatures. They absorb impact energy through mechanisms such as crazing and shear yielding, thereby inhibiting crack propagation and lowering the material's brittle temperature. Core-shell MBS tougheners have limited temperature resistance and are unsuitable for high- and low-temperature cycling scenarios, such as automotive and PV applications.
The Negative Impact of Recycled Material: Recycled material undergoes molecular weight degradation and the loss of effective modifiers after multiple heat-processing cycles, leading to a significant drop in low-temperature impact strength. High proportions of recycled material are a common cause of batch cracking in low-cost connectors during winter.

II. Full-Chain Causes: Core Root Causes of Low-Temperature Cracking

In mass production and application, incorrect material selection alone accounts for only part of low-temperature failures. More often, it is the combined result of structural design, molding process, and assembly/service factors, with low temperature merely being the final trigger.

1. Three Common Material Selection Pitfalls

Pitfall 1: Focusing only on room temperature strength, ignoring low-temperature notched impact strength and brittle temperature (Tb). Selecting based solely on tensile strength and heat resistance without verifying toughness at extreme low temperatures (e.g., -40°C/-55°C) is the most common error.
Pitfall 2: Blindly increasing glass fiber content, assuming more GF means higher strength. High GF content exacerbates low-temperature brittleness. Low-temperature applications require a balanced approach with proper GF interfacial treatment and toughening systems, rather than simply pursuing a high GF percentage.
Pitfall 3: Neglecting Environmental Stress Cracking (ESC) resistance. Materials like PA and PC, when in contact with media like oils or antifreeze, experience reduced cohesive energy and are prone to accelerated cracking under stress. While media penetration ability generally weakens at low temperatures, reducing ESC risk, thermal cycling can accelerate both media penetration and stress loading, still increasing cracking risk. Some low-temperature cracking failures in automotive/industrial connectors are related to ESC under thermal cycling conditions.

2. The Stress Amplification Effect of Structural Design

With greatly reduced crack resistance at low temperatures, structural stress concentrations become natural origins of cracking. Common issues include:

Non-Uniform Wall Thickness & Sharp Corners: Adjacent wall thickness differences should be controlled within 20%. Excessive differences cause uneven cooling shrinkage during molding, creating residual stress. Sharp corners have a much higher stress concentration factor than fillets. Insufficient fillet radii at the roots of latches or screw posts are high-frequency failure locations.
Weld Lines in Stress Areas: Weld lines formed by converging melt fronts are weak points, with strength often only 50%-70% of the base material at room temperature, further reduced in the cold. If located in stressed areas like latches or mounting holes, a fracture is likely.
Thermal Expansion Mismatch with Metal Inserts/Pins: The coefficient of thermal expansion (CTE) of plastics is anisotropic. For example, 30% GF-reinforced PA66 has a CTE of 15–25×10⁻⁶/°C in the flow direction and 30–40×10⁻⁶/°C in the transverse direction, which is significantly higher than that of copper alloys. At low temperatures, plastic shrinkage is much greater than that of metal. Excessive interference fit without compensatory design creates sustained tensile stress, which can cause cracking in embrittled plastic.
Excessive Latch Cantilever Ratio: When the latch cantilever length-to-thickness ratio exceeds 4:1, bending stress at the root is significantly amplified. With reduced low-temperature flexural strength, latches become prone to brittle fracture during mating/unmating.

3. Internal Stress Risks from Injection Molding Processes

Excessive residual internal stress from improper molding is a critical, often uncontrolled factor in mass production:

Low/Uneven Mold Temperature: Low-temperature toughened materials require adequate mold temperature for proper crystallization and molecular chain alignment, minimizing internal stress. For example, conventional toughened PA66 typically requires 80–100°C, high-crystallinity grades 100–120°C, and PC/ABS 80–100°C. Low mold temperature causes rapid quenching, freezing in high stress. Uneven cooling exacerbates localized shrinkage differences.
Improper Packing Parameters: Excessive packing pressure or time can cause overpacking, creating compressive internal stress. Insufficient packing leads to voids/sink marks, creating weak points.
Ejection Defects: A standard draft angle of 0.5°–1° is often sufficient for connector housings, increasing to 1.5° for deep cavities or high surface roughness. Insufficient draft or uneven ejection can cause scratches or distortion. Microscopic scratches become crack initiation sites at low temperatures. Forceful ejection must be avoided.
Lack of Annealing Post-Processing: Annealing can effectively relieve molding residual stress. Products for low-risk applications benefit from proper annealing. Precise processes like high mold temperature and multi-stage injection can also control stress at the source. Note: Solvent-based stress testing (like the crazing test) is only suitable for amorphous plastics like PC and PMMA, not for semi-crystalline materials like PA66.
Excessive Use of Mold Release Agents: These agents can reduce the material's surface cohesive energy, increasing the risk of Environmental Stress Cracking (ESC). Their use should be strictly controlled for low-temperature connectors.

4. Synergistic Failures from Assembly and Service Environment

"Cold" Assembly: Assembling or mating connectors directly at low temperatures when material toughness is insufficient. Normal mating force or torque can crack latches or housings.
Dynamic Load Fatigue: Under vibration, low temperatures reduce material fatigue life. Sustained impacts cause rapid propagation of micro-cracks.
Thermal Cycling Fatigue: Repeated expansion and contraction during high- and low-temperature cycles (e.g., -40°C to +125°C) induce cyclic loading of internal stresses, leading to thermomechanical fatigue cracking.

III. Failure Modes and On-Site Hazards of Low-Temperature Cracking

Cracking triggers a cascade of failures across structural, electrical, and safety dimensions:

Latent Failure: Microscopic cracks compromise IP67/IP68 seals. Moisture ingress leads to contact corrosion, insulation degradation, and signal interference. Progressive crack growth leads to structural failure.
Apparent Failure: Latch breakage or housing cracking causes connector loosening/disconnection, transmission interruption, and equipment shutdown. Cracking in high-voltage connectors also reduces creepage/clearance distances, increasing the risk of insulation breakdown and short circuits.
Critical Failure: Cracking in automotive high-voltage connectors can trigger faults in the high-voltage system. Failures in aerospace or polar equipment can cause functional loss. Cracking in cold chain connectors leads to temperature-control failures, compromising material safety.

IV. Full-Process Engineering Solutions

Addressing the full chain of causes, here are actionable engineering guidelines across four dimensions.

1. Material Selection: Core Guidelines for Low-Temperature Applications

Priority Order: Low-Temperature Notched Impact Strength > Brittle Temperature (Tb) > Glass Transition Temperature (Tg) > Room Temperature Tensile Strength

Table 2 Material Selection Specifications for Connector Housings in Low-temperature Scenarios

Minimum working temperature range	Recommended material performance	Performance requirements	Not recommended materials
0℃ to -10℃	Toughened PA66, PC, PC/ABS, and common PBT	cantilever beam notch impact strength of ≥5 kJ/m² at -10℃	Tb≤-20℃ high-filled recycled materials, ABS, untoughened high glass fiber PBT
-10℃ to -20℃	high-toughness PA66, cold-resistant PC/ABS, toughened PBT	-20℃ cantilever beam notch impact strength ≥6 kJ/m², Tb≤-30℃	unmodified PA66, ordinary 30% glass fiber reinforced PA66
-20℃ to -40℃	-40℃ grade super tough glass fiber PA66, cold-resistant PC/ABS, toughened PA6T/PA9T	-40℃ cantilever beam notch impact strength ≥8 kJ/m², Tb≤-50℃	untoughened engineering plastic, ordinary high glass fiber PPS
-40℃ to -55℃	Super tough nylon 12, toughened PEEK, PI special engineering plastics	-55℃ cantilever beam notch impact strength ≥10 kJ/m², Tb≤-60℃	Common PA66-based materials, general engineering plastics
below -55℃	High-performance PEEK, PI, metal casings (aluminum alloy/stainless steel)	show no obvious embrittlement at ultra-low temperatures and are long-term stable	general-purpose thermoplastic engineering plastics

Selection red line: For low-temperature, high-risk, automotive/medical/photovoltaic, and other certification scenarios, the use of high proportions of recycled materials is strictly prohibited. New materials should be given priority. The glass fiber is made of special low-temperature-resistant glass fiber treated with a silane coupling agent to ensure strong bonding at the low-temperature interface. It needs to pass the environmental stress cracking test, and the retention rate of room-temperature impact strength after medium immersion should be ≥80%. The low-temperature notched impact strength of raw material batches needs to be randomly inspected. Those that fail the inspection are prohibited from being stored in the warehouse.

2. Structural Design: 8 Golden Rules for Resistance to Low-Temperature Cracking

Uniform Wall Thickness: Adjacent wall thickness difference ≤20%. Minimum wall thickness ≥1.2mm for low-temperature use. Use gradual transitions, avoid sudden changes.
Full Radii Design: Radius at stress points R≥0.8mm; at non-stress points R≥0.5mm. Eliminate sharp corners.
Weld Line Avoidance: Optimize gate design to avoid stress areas. If unavoidable, add reinforcing ribs nearby.
Thermal Expansion Matching: Calculate and provide clearance for differential expansion/contraction. Design an appropriate plastic wall thickness around metal inserts to reduce stress.
Latch Optimization: Cantilever length-to-thickness ratio ≤4:1. Root fillet radius ≥1/2 thickness. Use stress-dispersing geometries.
Sealing Compensation: Account for low-temperature shrinkage in o-ring/gasket groove design. Maintain 25%-35% compression.
Rib Design Rules: Rib thickness ≤ 2/3 of main wall thickness. Rib height ≤ 3x wall thickness. Use filleted roots.
Drop Protection: Use rounded corners. Add impact-resistant ribs for applications prone to low-temperature drops.

3. Injection Molding Process: Production Control to Eliminate Internal Stress

Mold Temperature Control: Conventional toughened PA66: 80–100°C; High-crystallinity grades: 100–120°C; PC/ABS: 80–100°C. Ensures proper crystallization.
Multi-Stage Injection/Packing: Use slow-fast-slow injection profiles. Apply multi-stage packing with progressively reduced pressure to avoid over-packing.
Uniform Cooling: Temperature difference between core and cavity ≤10°C. Eject at a temperature below the material Tg.
Annealing Control: Implement annealing for high- and low-temperature-risk products. Use controlled furnace cooling. Employ appropriate internal stress detection methods for the specific material.
Standardized Ejection: Standard draft angle 0.5°–1°; ≥1.5° for deep cavities. Ensure consistent ejection force, avoid surface damage, and use mold-release agents judiciously.

4. Assembly & Maintenance: Operational Standards for Low-Temperature Environments

Low-Temperature Pre-Conditioning: Allow connectors to acclimate to room temperature for >2 hours before assembly if the ambient temperature is below 0°C.
Torque Management: Reduce assembly torque by 10-15% compared to room-temperature specs for low-temperature assembly. Use a calibrated torque tool.
Proper Mating: Mate connectors straight, without angular misalignment. Avoid forceful mating/unmating. Ensure latches are fully engaged.
Maintenance Inspection: During winter inspections, use magnification to check for micro-cracks. Perform air-tightness tests to verify seal integrity. Replace components showing anomalies promptly.

V. Reliability Testing & Validation: Industry Standards and Essential Tests

Table 3: Mandatory Reliability Test Items and Judgment Criteria for Connectors in Low-temperature Scenarios

Test Item	Test Parameter Specification	Acceptance Criteria
Low-Temperature Notched Impact Test	Soak at the minimum service temperature for ≥4 hours, ISO 180 / GB/T 1843	Impact strength meets the standard, no brittle fracture
Brittle Temperature Test	GB/T 5470, ISO 974-1:2019 Gradient Test	Brittle temperature is lower than the product's minimum service temperature (recommended to be at least 10℃ lower)
High and Low Temperature Cycle Test	Automotive: -40℃~125℃, 1000 cycles; Industrial: -40℃~85℃, 500 cycles	No cracking or deformation, sealing meets standard, contact resistance fluctuation ≤10%
Low-Temperature Drop Test	Soak at low temperature for ≥4 hours, 1m six-sided drop	No cracking, no buckle fracture, normal functionality
Low-Temperature Vibration Test	Low-temperature environment, 10~2000Hz, 15g, 24h, IEC 60068-2-6	No cracking or loosening, stable contact resistance
Low-Temperature Mating Life Test	Soak at low temperature for ≥2 hours, rated mating cycles	No buckle fracture, mating force fluctuation ≤20%, qualified resistance
Environmental Stress Cracking Test	GB/T 25712, ISO 22088, medium immersion + 80℃/168h	No cracking, impact strength retention rate ≥80%

Note: Low-temperature impact testing corresponds to standards such as IEC 60068-2-27, and vibration testing to standards such as IEC 60068-2-6. These are typically executed as separate, specific tests within a validation plan.