Analysis of the movement mode between parts in APS

In the batch production type enterprises of processing and assembly, due to the variety of parts, the process routes, processing methods and technical equipment are very different, so there are many ways of circulation of products. Generally speaking, there are three main ways for parts to move between processes: sequential movement, parallel movement and parallel sequential (smooth) movement .

Assume that a customer orders a product with a batch size of 4 pieces, which needs to be processed through 4 processes, and the time for each process is $t_1$=10min，$t_2$=5min，$t_3$=15min，$t_4$=10min, regardless of the transportation time and preparation time between processes. The following will calculate the processing cycle of producing this batch of products under different moving modes.

(1) Sequential movement mode

Sequential movement means that a batch of parts is transferred to the next process for further processing after the previous process is completed , as shown in the figure below.

It is easy to know that the processing cycle of a batch of parts in the sequential movement mode is equal to the sum of the processing time of the batch of parts in all processes, that is:

$$T_{}=n\sum _{i=1}^m{t}_i\text{\hspace{1em}(1)}$$

In the formula:

$T_{}$: The processing cycle under the sequential movement mode of a batch of parts;

$n$ : batch of parts;

$t_i$: part in $The\; working\; hours\; of\; a\; single\; piece\; in\; the\; i$ process;

$m$ : the number of processes of part processing;

For the above case, substitute the data into formula (1) to get:

$$T_{}=n\sum _{i=1}^m{t}_i=4\times \left(10+5+15+10\right)\min =160\min$$

In the sequential movement mode, because the parts are moved in batches between each process, the organization work is relatively simple ; the parts are processed and transported in a centralized manner, which reduces the equipment adjustment time and transportation workload ; during the processing period, the equipment of each process does not stop Turn, the equipment has high work efficiency. However, most products are waiting for processing and transportation, and the production cycle is long ; the capital turnover is slow, and the economic benefits are poor . It is usually suitable for situations where the batch size is small, the single-piece process time is short, and the distance between processes (work places) is long .

(2) Parallel movement method

The parallel movement mode refers to a production mode in which a batch of work in progress is transferred to the next process immediately after processing a part in the previous process , without waiting for the entire batch to be processed before moving to the next process. A batch of parts can be processed in parallel in different processes at the same time, so it is called parallel movement (or flow movement) , as shown in the figure below.

It can be seen from the above figure that the longest processing path of a part is composed of the total man-hour of the process with the longest man-hour of a single piece and the man-hour of a single piece of other processes (red arrow part), so the calculation formula of the processing cycle of the parallel movement method is as follows:
$$T_{}=\sum _{i=1,i\ne i_{\max }}^m{t}_i+n{t}_{\max }=\sum _{i=1}^m{t}_i+\left(n-1\right)t_{\max }\text{\hspace{1em}(2)}$$

In the formula:

$T_{}$: The processing cycle under the parallel movement mode of batch parts;

$T_{max}$: The processing time of a single piece of the longest working procedure in each procedure;

$i_{max}$: The index of the process with the longest processing time for a single piece.

For the above case, substitute the data into formula (2) to get:

$$T_{}=\sum _{i=1}^m{t}_i+\left(n-1\right)t_{\max }=\left(10+5+15+10\right)\min +\left(4-1\right)\times 15\min =85\min$$

In the parallel movement mode, since the waiting between processes is reduced to a minimum, its processing cycle is the shortest , the WIP reserve between processes is also greatly reduced , and the occupation of working capital is also reduced. However, when the processing time of the front and back processes is not equal, if the time of the latter process is less than the time of the previous process, the equipment and workers will . , it is difficult to make full use of; if the processing time of the previous process is less than the time of the subsequent process, there will be a phenomenon of parts waiting for processing. And it will increase the cost of handling and replacement due to frequent handling and switching . It is usually applicable to the situation where the unit time of each process is basically equal and the distance between processes (work places) is relatively short .

(3) Smooth movement

The parallel sequential movement method is a method that combines the sequential movement method and the parallel movement method , that is, a batch of parts or products, which not only maintains the parallelism of each process, but also maintains the continuous operation movement method. When the entire batch of parts has not yet completed the processing of the previous process, some of the completed parts are transferred to the next process for processing. The lead time for transferring parts to the subsequent process shall be subject to maintaining the continuous processing of the batch of parts in the subsequent process , as shown in the figure below.

In the parallel sequential movement mode, due to the different sequence of long and short processes, there are two different arrangement methods:

(1) When the processing time of the previous process is less than or equal to the processing time of the subsequent process, each finished part should be transferred to the subsequent process in time, that is, transferred piece by piece in parallel .

(2) When the processing time of the previous process is longer than the processing time of the subsequent process, the parts completed in the previous process are not immediately transferred to the subsequent process for processing, but wait until enough to ensure that the subsequent process can be continuously processed Only at the right moment, all the completed parts are transferred to the subsequent process, which can avoid intermittent equipment downtime in the subsequent process, and use the scattered downtime in a centralized manner.

The processing cycle of the parallel sequential movement method can be obtained by subtracting the time of each overlapping part (shown by the red arrow in the figure) from the processing cycle of the sequential movement method . The processing time of the current process ( $t_{}$) is less than the processing time of the subsequent process ( $t_{}$), the overlapping part of the processing time of the batch of parts in the two processes is $(n-1)t_{}$; When the processing time of the previous process is greater than the processing time of the subsequent process, the overlapping part of the processing time of the batch of parts in the two processes is $(n-1)t_{}$; When the processing time of the front and back processes is equal, the overlapping part of the processing time of the batch of parts on the two processes is $(n-1)t_{}$or $(n-1)t_{}$. In each case above, $t_{}$, $t_{}$Both are relatively short working hours, so ( n − 1 ) t short (n-1)t_{short} can be used uniformly$(n-1)t_{}$express.

In the parallel sequential movement mode, the processing cycle of a batch of parts can be calculated by the following formula:
$$T_{}=n\sum _{i=1}^m{t}_i-\left(n-1\right)\sum _{i=1}^{m-1}{t}_{}\text{\hspace{1em}(3)}$$

In the formula:

$T_{}$: The processing cycle under the parallel sequential movement mode of batch parts;

$t_{}$: The shorter one-piece man-hour of two adjacent processes.

For the above case, substituting the data into the formula can be obtained:
$$\begin{array}{l}{T}_{}=n\sum _{i=1}^m{t}_i-\left(n-1\right)\sum _{i=1}^{m-1}{t}_{}\\ =4\times \left(10+5+15+10\right)\min -\left(4-1\right)\times \left(5+5+10\right)\min =100\min \end{array}$$

The production cycle of parts is shorter under the parallel sequential movement mode , which eliminates the downtime of the equipment to a certain extent, concentrates the downtime of the equipment, and is convenient for other work. However, this method of organization and management is more complicated, and is generally applicable to object-specific organization forms .

(4) Comparison and selection of three moving modes

The above three moving methods have their own advantages and disadvantages, and their comparisons are shown in the table below.

compare items	sequential movement	parallel movement	Parallel sequential movement
Processing cycle	long	short	middle
Transport times	few	many	middle
equipment utilization	good	Difference	good
organization management	Simple	middle	complex

From the perspective of processing cycle, parallel movement and parallel sequential movement are better; from the perspective of organizational form, sequential movement is simpler, and enterprises must choose according to specific conditions. When choosing a method of moving parts between processes, the following main factors need to be considered.

1) Specialized forms of production units . If the production unit is formed according to the form of process specialization, the method of sequential movement should be adopted because the parts are inconvenient to be transported in one piece; if the production unit is formed according to the form of object specialization, it is appropriate to adopt the method of parallel or parallel sequential movement.

2) The size of the process labor and the weight of the parts . If the labor load of the process is large and the workpiece is heavy, the parallel movement method should be adopted; if the labor force of the process is small and the workpiece is light, the sequential movement method should be adopted.

3) The amount of labor required for equipment adjustment . If changing the processing object and adjusting the equipment requires a lot of labor, sequential movement should be considered, otherwise, parallel movement or parallel sequential movement can be considered.

4) Type of production . Although the above discussion is aimed at mass production types, it can also be extended to other types of production. If it is single-piece small batch production, there are many types of parts, the number of each part is small, and the process is very different. It is appropriate to use sequential movement. , if it is a mass production, it is advisable to use parallel or parallel sequential movement.

5) The urgency of the task . If the processing task is urgent, parallel movement should be adopted.

In short, the adoption of parts moving methods between processes is affected by many factors. When designing, we should proceed from the characteristics of the enterprise and use one or a combination of several methods to achieve the purpose of rationally organizing the production process.