**1. Introduction**

Seedling transplanting is an important process in vegetable and economic crop production, and mechanical automation and intelligent production is an imperative portion of its development trend. However, at present, there still exists some problems, such as the rupture and fall of the seedling substrate or the damage of seedling stems and leaves, which will affect the quality, efficiency, and reliability of the transplantation [1]. Except for the structural design and stability of the transplanting machine, the quality of seedlings also has a significant effect on the seedlings' transplanting performance. A strong and unified seedling is expected to reduce management costs, and it is also suitable for mechanical operation. There are three types of seedlings for transplanting: bare root, plug trays, and pot/substrate block seedlings. The latter two are most used for mechanized transplantation [2,3]. The substrate block seedlings have better functional and developed root systems that are capable of withdrawing more water and nutrients, and they have higher survival rates than bare seedlings and plug seedlings [4,5]. Additionally, they also have relatively regular characteristics for the automatic mechanized transplanting than pot seedlings, which would help to expand the fully automated transplanting [6].

Although the substrate block is suitable for seedlings and mechanical operation, it also has stricter requirements than other types due to its compressed structure. In addition to the chemical properties (pH, EC, nutrients, etc.) provided by the components of the substrate materials, the physical characteristics have put forward higher requirements for the compressed substrate block, such as its mechanical strength, density, porosity,

**Citation:** Fu, J.; Cui, Z.; Chen, Y.; Guan, C.; Chen, M.; Ma, B. Simulation and Experiment of Compression Molding Behavior of Substate Block Suitable for Mechanical Transplanting Based on Discrete Element Method (DEM). *Agriculture* **2023**, *13*, 883. https:// doi.org/10.3390/ agriculture13040883

Academic Editor: Jacopo Bacenetti

Received: 17 March 2023 Revised: 7 April 2023 Accepted: 14 April 2023 Published: 17 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

water availability, air capacity and expansion rate [7]. Those are mainly dependent on the compression process. Some studies have investigated the moisture content of the raw materials, component formula, and compression parameters (compression stress, temperature) on the quality of the substrate block and the seedling's performance [8–10]. In addition to the above factors, the processing machinery of the substrate block is also one of the key factors affecting its performance. However, there is currently no certain standard for the compression molding of the substrate blocks, and there is no uniform manufacturing standard for molding equipment. Nowadays, the existing equipment used in China still has some problems, such as substrate shedding or sticking to the mold. Hence, studying the working mechanism between the substrate and molding machinery will help to improve the operation performance of the processing equipment and further improve the quality of substrate blocks. The force and movement process between the machine and substrate particles are complex. The combination of a simulation method and a practical experimental method is usually the commonly used and effective method. In the research on agricultural products and equipment, the discrete element method (DEM) is widely employed to capture the dynamic behavior of the particle materials and the interaction mechanisms between the materials and tools [11–14]. For the substrate block seedlings, understanding the microscopic processes of the particle flow and compression is critical for the quality control of the substrate block. The application of DEM can be beneficial to reduce the risk of failure and cost of production.

In this work, the commonly used peat was used as the growth substrate. The calibration of the DEM parameters of the peat was determined by a combination of a physical test and a simulation test. The stacking angle was adopted as a response value, and the optimal parameters were obtained by Plackett–Burman test, the steepest climbing test, and Box–Behnken test methods. Then, those calibrated parameters were applied into the DEM simulation of the compression molding of the substrate block. The effects of the loading speed on the compression behavior of the substrate block were evaluated from the screen of the distribution movement of the pressed particles. Finally, the reliability and accuracy of the simulation parameters were verified through experiments. It was expected to provide a theoretical basis for the simulation of experiment research and the preparation of substrate blocks with good qualities that are suitable for mechanical transplanting. It also provided an improvement direction for the design of a substrate block forming machine.
