Modelling and simulation of a flexible packaging system for detergents

Abstract. The paper presents how an automated manufacturing system for the packaging of detergents can be modelled using Petri nets. The complexity of the Detergent Packing System and the high level of automation, require the use of efficient modelling and simulation methods to verify the validity of the solutions adopted. For this purpose, a modern modelling and simulation method based on Petri nets is used. The models associated with the system are developed into a hierarchical structure: the model of some modules in a line; the model of a line; system model as a whole. The model of a packing line consists of the following sub-modules corresponding to the modules of the line: filling the bags, grouping the bags in boxes, grouping the boxes in a row, palletizing, supplying with new empty pallets, full pallets evacuation. The six lines of the manufacturing system can work in various ways: they can pack detergent bags of the same mass or of different masses. Under these conditions, the operating mode is checked step by step, and the necessary adjustments can be done as to ensure continuous operation of the system without any blockages.


Introduction
Integrated automatic manufacturing lines for the production of consumer goods are more and more present in the modern industry. These include lines for packing and palletizing detergent, food, drinks.
The paper [1] presents the role of packaging on buying detergent powder. Considering the impact of various elements of packaging on purchasing detergent powder, a conceptual framework was developed by extensive literature review and tested by using structural equation modelling taking 200 usable questionnaires.
The main purpose of the work [2] is to develop a novel Discrete Event Simulation (DES) model to optimize the design and operation of a complex beer packaging system in order to perform a sensitivity analysis to find one or more alternatives to increase productivity levels.
A model using Petri Nets for monitoring the process of bottling and packaging is presents in [3]. This model was used for automation the step of bottling and packaging in the industrial process.
The aim of the study [4] is to simulate and optimize the flow of a flexible robotic manufacturing and packing palletizing cell. In the cell will pack coffee packets in boxes and boxes will be put on euro pallets to create a stack to be foiled and stored.
In [5] is presents a material flow management case study based on our research in modelling, simulation and optimization for a packaging and palletizing system. The studied system allows packaging, palletizing, wrapping and storage of several types of products within a food company.
The paper [6] presents a new challenging modelling approach to support different heuristics to tackle the pallet loading problem (PLP). A discrete event system model to tackle the PLP is specified using the coloured Petri net formalism in order to integrate the model with the industrial context in which the PLP must be solved.
The paper is a synthesis of the researchers who aimed at designing, implementing and putting into operation a system of automatic packaging of detergent. Performance evaluation was done by modelling and simulation with Petri nets.

Description of the manufacturing system
The automatic detergent packaging system consists of six manufacturing lines. Figure 1 shows a schematic diagram of a manufacturing line structure. For each line, the following subsystems can be highlighted: I -the bag filling subsystem; II-the subsystem for grouping bags in boxes; III -the box grouping in a row subsystem; IV -palletizing subsystem; Vfull pallet evacuation subsystem; VI -supply subsystem with empty euro pallets.  They are transferred through the transfer system 3 (conveyor belt). During the transfer, the closing of the bags is also carried out (4closed bag). Under the bag conveyor belt there is a transfer system of roller conveyor type (5) for transferring boxes (6). Through the case packer equipment 7, the filled bags are grouped in boxes. The filled boxes (8) are grouped in rows (9). When a row is complete, it is transferred to pallet 10. The pallet is evacuated from the system. After evacuation of the full pallet, an empty pallet is inserted in the pallet supply subsystem (12).
In figure 2 is presented the transfer system for transferring boxes, and in figure 3 is the case packer equipment.  3 Modelling and simulation of the automatic detergent packaging system

The model
The complexity of the Flexible Detergent Packaging System and the high degree of automation expected for it, require the use of efficient modelling and simulation methods, through which the validity of the solutions adopted can be verified. For this purpose, a modern method of modelling and simulation based on Petri nets is used. This method allows: modelling the functional interdependencies between the different components of the system, the consideration of the durations of the sequences that make up the manufacturing cycle.
In addition to verifying the constructive solutions adopted for the different modules of the system, the simulation of the functioning of the system allows to evaluate its productivity for the different manufacturing tasks.
The models associated with the system are developed in a hierarchical structure: the model of the system as a whole, the model of a line, the model of some modules in the composition of a line.
In these conditions, the operation mode is checked step by step and the necessary corrections can be made until the optimal solution is found. Table 1 shows some of the model transitions and positions. Also, table 1 contains the characteristics of these elements: timing for transitions (d j , j=1,…,79) and the initial token for positions (m 0 (Pi); i=1,…,92)) The model with Petri nets consists, in fact, of six submodels (Figure 6), these correspond to the modules in the line structure: I-the submodel of filling the bags; II-the submodel for grouping bags into boxes; III-the submodel for grouping boxes in a row; IVthe palletizing submodel; V -the submodel for the evacuation of full euro pallets; VI -the sub-model of supply with empty euro pallets.
Two  The grouping of bags in boxes is modelled by using components of generalized Petri nets. Thus, the loading of the arc that starts from position P10 and enters the transition T1 is 12, equal to the number of 1 kg bags entered in a box.  Also, the arc starting from the P2 position and entering the T2 transition has the load 8, that is, the number of boxes on a row.

Simulation
The model is designed so that potential conflicts are avoided. By simulation, it is possible to determine the number of euro pallets that can be made during an 8-hour work shift (Fig.  6). The simulation result of the system operation during the 8 hours is presented in table 2.
In the column Results of simulation -euro pallets appear the quantities of euro pallets evacuated from the system during an 8-hour work shift.
In the column Estimated productivity -euro pallets appears the quantities of euro pallets estimated to be made on each line during an exchange. It is found that between the values resulting from simulation and the estimated values there are no differences. There is an exception, in the case of the line that packs the detergent in 3.3 kg bags, the estimated value is 100 euro pallets/8 hours and the value resulting from the simulation is 99 euro pallets/8 hours.

Conclusions
Modelling and simulation offer the possibility to analyze the functioning of the automated systems.
The paper presented how Petri nets can be used for modelling and simulation: an automated detergent packaging system. Generalized and hybrid timed Petri nets were used. The model was used to evaluate, through simulation, the performance of the automated system. Two situations were studied: packaging on all lines of detergent in six types of bags (bags of different masses) and packaging of detergent on the six lines in different bags.
The simulation highlighted the volume of production considering an interval is for 8 hours. Also, by simulation it was found that during the operation of the system there are no blockages or other types of unwanted events. Future research will consider the use of colour Petri nets in the design of the automated detergent packaging system.