Polymer Processing & Battery QC: A Technical Deep Dive

Your question involves two independent technical fields: one is the synthesis process of general "polymer materials,"and the other is the 

quality inspection of "polymer lithium battery" finished products. The current search results exhibit a significant domain mismatch and cannot 

be directly concatenated into a complete process description from "material synthesis to battery packaging."

I will provide a comprehensive analysis in the following two separate parts:

1. General Polymer Manufacturing Process (based on chemical engineering principles and textbooks): Illustrating how polymer materials 

transform from monomers into solid products.

2. Quality Control Details in Polymer Lithium Battery Finished Product Inspection (based on battery factory and third-party testing technical 

data): Revealing the rigorous, often "invisible to the naked eye," controls throughout incoming inspection, cell manufacturing, and 

system-level testing.

Part 1: Description of the General Polymer Manufacturing Process

The preparation of polymers (high molecular materials) is not a single procedure but a deep coupling of chemical synthesis and physical 

molding . Based on product requirements, it is mainly divided into two major technical paths:  "polymerization first, then molding" (e.g., 

modified plastics) and  "simultaneous polymerization and molding" (e.g., cast polymerization).

1.1 Typical Process Flow (Example: Modified Plastics)

This is the most common path in industrial production, involving the physical modification of pre-synthesized resins into granules.

① Raw Material Preparation & Pretreatment

Main Material : Base resin granules or powder such as polyethylene (PE), polypropylene (PP).

Auxiliaries: Functional additives (flame retardants, antioxidants, toughening agents), fillers (calcium carbonate, talc), reinforcing materials 

(glass fiber).

Critical Detail: Some raw materials require pre-drying to remove moisture. Otherwise, moisture vaporization during high-temperature 

extrusion can cause bubbles or degradation in the final product.

② Mixing (Blending/Alloying)

The main material and powdered/liquid additives are fed into a high-speed mixer according to the formulation ratio.

Control Detail: Utilizing high shear force to achieve macroscopic uniform dispersion of materials with different densities. For nano-scale 

fillers, surface modification treatment is required to prevent agglomeration.

③ Melt Extrusion (Core Reaction/Plasticizing Section)

The mixture enters a twin-screw extruder. The barrel is heated in stages (typically 160°C-240°C). Under screw shear and heating, the 

resin transitions from a glassy state to a viscous fluid state.

Engineering Challenge: The vent port must be connected to a vacuum system to remove unreacted monomers, low-molecular-weight 

volatiles, and water vapor. Failure to do so can easily cause "silver streak" defects in the product.

④ Cooling & Pelletizing

The molten material is extruded from the die head as strands, immediately immersed in a water tank for rapid cooling and solidification, 

or cooled via an air-cooled conveyor belt.

A pelletizer cuts the strands into cylindrical granules. A vibrating screen sieves the granules to remove agglomerates and fines.

⑤ Post-Treatment & Homogenization

Granules from different batches are mixed in a homogenization silo to eliminate batch-to-batch variations in color and melt flow index (MFI), 

then finally packaged and warehoused.

1.2 Integrated Polymerization-Molding Process (Example: Cast Polymerization)

For materials like PMMA (acrylic) or MC Nylon, the polymerization reaction occurs directly within the mold, eliminating the pelletizing step.

① Prepolymer Preparation

Monomers (e.g., methyl methacrylate) are mixed with a small amount of initiator in a stirred tank for prepolymerization until the viscosity 

reaches approximately 2000 centipoise (~10% conversion rate). Purpose of prepolymerization: To reduce subsequent volumetric shrinkage 

and shorten mold occupancy time.

② Mold Filling

Silicate glass plates are cleaned, dried, and assembled into a mold cavity. The prepolymer syrup is injected into the cavity. Defoaming is 

critical – the syrup must be poured slowly and tilted to prevent air bubbles that cause optical defects in the final product.

③ Staged Polymerization

Low-temperature slow polymerization (e.g., 25-52°C, 10-160 hours): Controls the exothermic reaction, preventing "runaway" polymerization 

that creates bubbles.

High-temperature post-treatment (boiling water bath for 2 hours): Allows residual monomers to fully react, increasing molecular weight.

④ Demolding & Trimming

After cooling, the mold is disassembled. The gate shrinkage cavity (formed due to significant volumetric shrinkage, up to 15-20%) is cut 

away. Edges are trimmed to obtain the final product.

Part 2: "Detection Details" Behind the Strict Quality Control of Finished Cells

In the field of polymer lithium batteries, a "cell" specifically refers to the energy storage unit that has completed packaging, electrolyte filling, 

and formation. The following inspection details reveal the microscopic battlefield of quality control.

2.1 Incoming Quality Control (IQC): "Moisture & Particles" Invisible to the Naked Eye

Detail 1: ppm-level Moisture Testing

Object: Anode/Cathode materials, separator, electrolyte.

Tool: Karl Fischer Coulometric Titrator.

Logic: Polymer batteries are extremely sensitive to moisture. Water reacts with lithium salts in the electrolyte to generate HF (hydrofluoric 

acid), which corrodes the electrodes and produces gas, leading to battery swelling. Moisture content must be controlled at the parts per 

million (ppm) level. Sampling and testing must be conducted in a dry room with a dew point below -40°C.

Detail 2: Separator Puncture Strength & Air Permeability

Misconception: Many assume separator testing only measures thickness.

Reality: A tensile testing machine must be used to measure puncture strength (typically required ≥300gf), simulating the risk of anode/cathode 

burrs piercing the separator. A Gurley densometer measures air permeability (Gurley value), reflecting the uniformity of micropores, which 

directly affects ionic conduction impedance.

Detail 3: Powder Specific Surface Area (BET) & Particle Size Distribution

The specific surface area of anode/cathode active materials directly affects rate capability. Tested via nitrogen adsorption method. Too high a 

specific surface area leads to excessive side reactions; too low results in insufficient power.

Laser Diffraction for particle size: A D50 value fluctuation exceeding ±1μm can cause surface streaking defects during coating.

2.2 "Physical Examinations" After Cell Production

Detail 4: K-Value (Voltage Drop) Screening

Phenomenon: After the cell is fully charged and left to rest, the voltage drop (ΔV) is measured over a 24-hour period.

Logic: Micro-short circuits are a silent killer for polymer batteries. Even a microscopic, invisible pinhole in the separator or tiny burrs on the 

electrode sheets penetrating the separator will cause excessively high self-discharge rate (large K-value).

Automated production lines use high-temperature aging/resting combined with high-precision voltage acquisition to reject cells with abnormal 

K-values. This is the most rigorous screening step before cell matching.

Detail 5: Thickness Change Rate (Swelling Monitoring)

Unlike steel or aluminum cans, polymer batteries (pouch cells with aluminum-plastic film) undergo expansion during charge/discharge cycles.

Requirement: The thickness expansion rate after cycling is typically required not to exceed 110% (relative to original dimensions). 

Uncontrolled expansion indicates significant gas generation or unstable SEI (Solid Electrolyte Interphase) film.

Detail 6: "Blunt Needle" and Speed in Nail Penetration Test

National/UL standards specify a steel nail diameter of 3mm-5mm penetrating the cell vertically at a specific speed (e.g., 25mm/s).

Subtle Difference: The sharpness of the nail tip and the surface roughness of the nail significantly impact test results. For the same cell, the 

probability of ignition is drastically different between using a brand-new needle and a used, blunted needle. International transport certification 

(UN38.3) has strict calibration requirements for this.

Detail 7: Temperature Ramp Rate in Hot Box Test

The cell is placed in an oven, and the temperature is increased to 130°C or 150°C and held at a controlled ramp rate of 5°C ± 2°C/min.

Purpose: Simulates extreme high-temperature environments. The heating rate must be strictly linear. If the cell is abruptly placed into a 

high-temperature chamber (thermal shock), the cell's thermal inertia might prevent an explosion. However, linear ramp testing rigorously 

examines the separator's shutdown characteristics and the thermal stability of the anode/cathode, making thermal runaway more likely to 

be triggered.

2.3 Battery Pack Level: Consistency and Intelligent Warning

Detail 8: Multi-Channel Voltage Consistency

After assembly into packs, a 32-channel or even 128-channel voltage monitoring system is used to monitor the voltage of each individual 

cell in series/parallel in real-time.

Significance: An alert is triggered if the voltage difference exceeds 10mV. This prevents the "weakest link" effect – where one cell is 

over-discharged first and subsequently reverse-charged by others, leading to catastrophic failure.

Detail 9: Insulation Resistance & Dielectric Withstand

Simulating humid environments (40°C, 93% RH), the insulation resistance between the positive/negative terminals and the casing 

(aluminum-plastic film) is tested.

Pain Point: The edge sealing area around the tabs of polymer pouch cells is a weak point. Microscopic cracks in this area absorb moisture 

under humid heat conditions, degrading insulation resistance. In actual operating conditions, this can lead to protection circuit board (BMS) 

misjudgment or leakage current.

Summary:

The polymer manufacturing process is a chemical engineering discipline, centered on controlling reactions and phase transitions. In contrast, 

the quality control of polymer lithium battery cells is a precision manufacturing engineering discipline, centered on using extremely precise 

instruments to capture microscopic defects. From the drop of water measured by the Karl Fischer titrator to the standardized steel nail on 

the penetration test stand, every "small detail" validates an industry consensus: A defect not detected before shipment is a 100% 

guaranteed failure in the hands of the customer.