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Summary of significant accounting policies - Estimated useful lives for property, plant and equipment, net (Details) - USD ($)
12 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Accounting Policies [Line Items]                
Intangible assets estimated useful lives           3 years    
Amortization expenses $ 0 $ 0 $ 0 $ 7,449 $ 32,626 $ 46,085 $ 42,758 $ 3,539
Transportation equipment                
Accounting Policies [Line Items]                
Estimated useful lives           4 years    
Minimum | Plant, machinery and equipment                
Accounting Policies [Line Items]                
Estimated useful lives           5    
Minimum | Office equipment                
Accounting Policies [Line Items]                
Estimated useful lives           3    
Maximum | Plant, machinery and equipment                
Accounting Policies [Line Items]                
Intangible assets estimated useful lives           10 years    
Maximum | Office equipment                
Accounting Policies [Line Items]                
Intangible assets estimated useful lives           5 years