# A Review of the High-Mix, Low-Volume Manufacturing Industry
## 4 Optimisation of the HMLV Layout and Flow
### Loading Jobs/job Release  
Due to the dynamic production flow, the HMLV industry often has a job-shop environment. CONWIP and paired-cell overlapping loops of cards with authorization (POLCA) are two of the most common job release techniques for job shop with complex material flow discussed in the literature. These job release systems control the amount of work in a work centre, control the lead time, and monitor the backlog. Constant work-in-process (CONWIP) is a continuous shop floor release method. The workflow is regulated by job number-specific cards, rather than part-number-specific cards. Therefore, the card stays with the specific product or batch throughout the process, providing a way to handle high variations. However, the CONWIP hierarchical control architecture may need items to be grouped into families of common routings, which is limited in HMLV. To set the CONWIP limit in HMLV, [PERSON] and [PERSON] [43] devised aggregate modelling to reduce the development time and computational requirements of HMLV simulations. The VSM approach is used to create an aggregate simulation model, which is used to set the limit of the CONWIP dynamically. [PERSON] and [PERSON] [44] developed a DES for evaluating single-loop and multi-loop CONWIP control mechanisms. With a simulation based on the end-of-line process of a semiconductor company, a multi-loop approach shows better robustness with a lower cycle time and easier implementation. With the multi-loop system, cards could be returned faster, and starvations caused by bottleneck loops could be minimised. In a job shop environment, CONWIP may lack the workload balancing required [18]. [PERSON] and [PERSON] (2016) evaluated this, which showed that CONWIP controls the maximum throughput at a lower WIP level than POLCA. Both CONWIP and POLCA in the simulation have an equivalent lead time. However, POLCA prevents WIP accumulates in one area, which CONWIP was unable to accomplish. POLCA is a push-pull card-based signalling system that aims to reduce lead times, cut production costs, increase delivery date adherence, and reduce scrap and rework. This system was mainly devised for highly engineered production, small batches and high product variety [45]. In POLCA, cards are in pairs where one card travels in the direction of the job, and the other returns the information. This approach might be useful in a job shop multi-directional flow and variable routing by placing two cars between a pair of cells to allow work and feedback data to travel in both directions [7]. Variations of POLCA with a centralised and decentralised job release were also compared [46], showing centralised job release benefits in general flow shop and pure flow shop environments. Workload control (WLC) is a production planning and control mechanism devised for a high variety of production, such as the MTO industry. By using a pre-shop pooling of orders to reduce shop floor congestion, shop more is made more manageable with a series of short queues. The shop's performance is stabilised by making them independent of variations coming in from the order stream. In most WLC cases, the jobs are only released onto the shop floor if the released workload levels do not exceed pre-set maximum limits, ensuring jobs do not stay in the pool too long to reduce lead times and meet delivery date objectives. The WLC is based on two control mechanisms: input control (IC) and output control (OC). IC uses information about jobs in the system to decide when incoming orders can be released to the production process--order review and release (ORR). At the same time, OC uses information about the production capacity to regulate the workload in the system. [PERSON], et al. [47] worked on integrating the order review and release (ORR) and output control (OC) with information about production capacity. Considering worker allocation as part of the OC mechanism, this work considered different labour-related parameters, such as flexibility and efficiency levels for workers.