## 2 Theoretical Foundation
### Lean Implementation in the Upholstery Manufacturing Industry  
When searching the literature for case studies that addressed the topic, several interesting and relevant studies were identified, which are presented in Table 4.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Authors** & **Title** & **Methods** & **Results** \\ \hline \hline \multirow{8}{*}{[PERSON] (2008) [36]} & & & Increase in productivity by 11.2\%; \\  & & & reduction of direct labor by 36.4\%; \\  & The Toyota Production System & Re-engineering of the production & reduction in walking distance; easy \\  & Applied to the Upholstery & line into several “D”-shaped work & communication; reduction of physical \\  & Furniture Manufacturing & cells; Kanban system for inventory management; & space; reduction of the level of stock \\  & Industry & production control system. & between sub-cells; drive continuous \\  & & & improvement efforts; reduction of \\  & & & inventory costs of work-in-progress; \\  & & & ergonomic improvements. \\ \hline \hline \end{tabular}
\begin{tabular}{c c c} \hline \hline \multirow{8}{*}{[PERSON] et al. (2016) [37]} & & & Identification of causes, effects, and \\  & & & solutions; streamlining \\  & & & communication; reduction in time \\  & & & spent on activities caused by \\  & & & inefficient organization of the factory \\  & & & environment; motivation of workers \\  & & & (due to a more hygienic and \\  & & & suggestion system; 5S; Checklists & organized workplace); promotion of \\  & & & for eliminating communication \\  & & & errors and Standardization; & communication between the office \\  & & & questionnaire. & and the shop floor; elimination of \\  & & & & conflicts causing defects; the \\  & & & questionnaire concluded that the \\  & & & need for Lean strategies in the daily \\  & & & life of the organization was voted as \\  & & & very high by 50\% of employees and \\  & & & as high by the remaining 50\%. \\ \hline \hline \multirow{8}{*}{[PERSON] and [PERSON] (2019) [38]} & & & VSM; a Pull system combined & \\  & & with a Kanban-based stock & \\  & & management support system; & synchronization of cycle time \\  & & & with Takt Time *; combination of \\  & & & processes; modification and \\  & & & simplification of activities with 5S \\  & & & principles; modification of the \\  & & & layout; reorganization of the work \\  & & & carried out at the workstations; \\  & & & training on Lean and \\  & & & Kaizen approaches. \\ \hline \hline \end{tabular}
\end{table}
Table 4: Case studies identified Lean implementation in the upholstery manufacturing industry.  
As it can be concluded from this subsection, it appears that in the literature there are few studies of the implementation of Lean tools for production optimization in the upholstery industry and, more specifically, regarding the upholstery maintenance/production of vehicle seats as most of the studies found address productive processes in the furniture industry. However, the case studies identified and presented provide an important basis for the best tools--namely, 5S, Standardized Work, Kanban, Kaizen, VSM, and layout studies (Spaghetti Diagram)--and how to proceed in terms of implementation steps. They also provide a guiding thread of methodology to follow in a practical approach similar to the one carried out in these studies. Moreover, the reviewed studies provide a guide in as to what kind of results to expect from the implementation of the named tools and Lean practices mainly, the reduction of waste and, consequently, of the production Lead Time, while improving workers' motivation through a clean and organized workplace and improving work ergonomics and safety.