Design and Experiment of Substrate Grass Seed Blanket Extrusion Device

Abstract

After corn straw and livestock manure are fermented and decomposed, grass seeds are added. The substrate grass seed blanket is made by screw extrusion, applied to park greening, square greening, protective greening, and residential area greening. With this device, the purpose of reducing the labor force and improving space utilization rate can be achieved. The working principle of the substrate grass seed blanket extrusion device is mainly described, and the extrusion process is analyzed including: compaction and filling stage, surface deformation and compaction stage, plastic deformation stage, and molding stage. The main experimental factors are determined through theoretical analysis of screw size and working parameters, including screw pitch, screw length, screw diameter, and screw speed. Using the EDEM simulation analysis method, taking the quality of extruded particles and the uniformity of grass seed mixing as test indexes, and under the condition of the same extrusion time of 30 s using Design-Expert software to carry out an orthogonal quadratic rotation combination test, a significant regression model was obtained. The effects of different parameters and extrusion conditions on grass seed blanket forming influence were studied by response surface analysis. The optimal working parameters were obtained: screw speed 250 r·min⁻¹, screw pitch 120~80 mm, screw diameter 240 mm, and screw length 400 mm. With the same extrusion time of 30 s, extruded pellet mass was 2620 g, calculated mass flow rate was 131 g/s, and the grass seed mixing uniformity was 92.35%. Under the optimal simulation conditions, the prototype was manufactured, and the actual verification test was carried out. The errors between the measured values of extruded substrate quality and grass seed mixing uniformity and the simulation test results were 3.4% and 2.5%, respectively, which met the requirements of the grass seed blanket extrusion molding device.

1. Introduction

Long-term production of conventional lawns can cause land infertile and salinization [1,2]. The main direction of research in lawn production today is soilless lawns, which differ from traditional soiled lawns in that the substrate for its not standard soil. Still, it is manually equipped with waste from industrial and agricultural production and nutrients essential for plant growth [3,4,5]. The substrate, as the basis for lawn growth, is critical to the success of soilless lawn cultivation [6,7,8,9]. The matrix proposed in this paper is a grass seed blanket substrate with corn straw as the main component, with the addition of cow dung, sodium polyacrylate, ferrous sulfate, and other auxiliary materials that are ground, killed, and sterilized [10], which has the advantages of specific resistance to damage strength, good water absorption, and low swelling rate after extrusion [11,12].

There are few experimental studies on the substrate grass seed blanket made of corn straw and livestock manure fermented and decomposed as substrate raw materials and extruded into the green lawn. Mechanized production equipment of soilless carpet, like ecological quilting carpet machine, makes carpet by sewing. However, the content of carpet substrate is low, and the phenomenon of needle and thread breakage is easy. The existing carpet quilting machine has not been developed entirely in China. The complete automatic production line has not been formed, the complex machine makes the processing difficult, and the use of the machine requires more manpower and material resources, resulting in high cost. The current quilting machine cannot achieve the desired effect on non-woven fabrics and crop straw fibers. [13,14]. In addition, substrate bricks or boards produced by compression are used for seedling cultivation and greening. The equipment mainly uses hydraulic pressure to push substrate compression molding, which can be divided into horizontal and vertical compression [15,16,17]. However, the compression pressure is challenging to grasp, and the more significant pressure easily damages seeds, while the smaller pressure is difficult to mold. Moreover, it takes a long time in the pressure maintaining stage, affecting production efficiency. In this context, the core of this research is to design a substrate grass seed blanket molding device by screw extrusion, which can achieve the purposes of extrusion molding of substrate and grass seeds together, convenient storage, and long-distance transportation after drying, suitable paving. The discrete element software EDEM is used to design the screw shaft of the vital component, and the optimal screw size parameters and working parameters are obtained. Finally, the prototype is made and verified. In this research, the grass seed blanket with specific compressive strength, no looseness, no chemical fertilizer, no deformation after drying, low cost, high paving efficiency, and appropriate size can be quickly obtained by the extrusion molding device of substrate grass seed blanket. The greening products can replace the traditional lawn, achieving environmental protection, greenness, convenience, and less or no property maintenance.

2. Structure Design of Extrusion Device for Grass Seed Blanket

2.1. Overall Structure

The overall design of the substrate grass seed blanket extrusion forming device is mainly composed of a frame, driving motor, transmission device, feeding device, feeding mouth, extrusion mechanism, cutting machine, receiving plate, and conveying device. SolidWorks 3D modeling software was used to obtain the complete solid model diagram of the substrate grass seed blanket extrusion molding machine, as shown in Figure 1, where (a) is the 3D model diagram and (b) is the model structure diagram.

The whole machine design parts are many; this paper mainly studies the extrusion part, including motor, transmission device, saw machine bearing seat, feeding mouth, feeding device, extrusion mouth, and material plate. The motor adopts a three-phase asynchronous motor with a power of 2.2 kW and model Y100l-4 to drive the screw shaft to rotate. The three-dimensional modeling software is used to model the machine entity, determine the position of each component, and obtain the structural diagram of the substrate grass seed blanket extrusion molding machine, as shown in Figure 2.

2.2. Working Principle and Analysis of Extrusion Process

By referring to the mixing standard of the substrate for forming seedling clumps, firstly, the substrate and the premix are mixed in proportion, and the material characteristics are analyzed as shown in

Table 1. Analysis of substrate characteristics of grass seed blanket.

Substrate Characteristic Analysis
Index	Total porosity/%	Aeration porosity/%	Water-holding porosity/%	Bulk density/ g cm−3	Electrical conductivity/µs·cm−1	pH value
Parameters	65.73	20.21	58.32	0.43	820~1300	5.8

, with the water content control range of 80% [18]. According to the quality of the substrate, a certain amount of grass seeds are added, and the mixed substrate is sent to the screw feeding device from the feed inlet. The motor drives the screw shaft to drive the chain to rotate so that the substrate is transported to the extrusion outlet for extrusion, and the receiving plate takes over the molded substrate.

According to the different regions of the extrusion process of the substrate grass seed mixture and the differences in deformation states [19], the extrusion process can be roughly divided into four stages, as shown in Figure 3, namely, the compaction filling stage, the surface deformation and compaction stage, the plastic deformation stage, and the forming stage.

(1)

Compaction and filling stage: the substrate and grass seeds are sent to the screw conveyor from the feeding port. The materials naturally fall and accumulate, and the gap between the materials is significant. Due to the continuous falling of the materials and the role of the screw thrust, the positions of the particles are constantly changing and rearranged, in which the water is continuously discharged, and the gap between the materials is reduced and gradually compacted.

(2)

Surface deformation and compaction stage: as the extrusion force of the screw on the material slowly increases, the friction gradually increases, the contact between the particles is compact, and the gap is slowly filled. When the material density reaches a certain level, the surface of the larger substrate particles is deformed and broken. At the same time, the intestinal secretions, intestinal mucosal exfoliations, and fiber structures in the material cow dung have good adhesion and entanglement, making the formation between the materials denser.

(3)

Plastic deformation stage: with the increasing extrusion force of the spiral on the material, the contact area between the particles is significantly increased. When the extrusion force is gradually increased, the particles undergo plastic deformation, and the particles are reordered. The particles are intertwined, staggered, and embedded, making the mutual combination more firm.

(4)

Molding stage: after the molded grass seed blanket is extruded from the discharge port, it will lose the outer shell limit, and some elastic deformation will be restored. At the same time, stress relaxation will occur. The final molded block will achieve permanent deformation and maintain the final density.

3. Stress Analysis and Parameter Design of Screw Conveying Extrusion Device

3.1. Stress Analysis

In the process of conveying substrate by the screw device, under the action of screw rotation, substrate particles do not necessarily move linearly along the screw axis but move spatially along the screw axis in the compound motion of mutual extrusion [20]. Under the combined action of the thrust of the spiral blade, the interaction force between the substrate, the self-weight of the substrate, and the friction between the substrate and the outer shell, the substrate has the trend of axial downward and radial circular motion. Take the substrate particle M at a distance from the spiral axis r as the research object, and the stress analysis of the substrate particle is shown in Figure 4a. During the movement of the substrate inside the screw, the particles are subjected to radial force and axial force, resulting in axial velocity V₁ and circumferential velocity V₂. The substrate material advances in this compound movement and the velocity analysis of the substrate material in the screw conveyor is shown in Figure 4b.

It can be seen from the analysis in Figure 4a that the substrate particles are subjected to two forces in the process of spiral conveying, namely, the thrust P and gravity g of the helical blade on the substrate particles. The thrust P is decomposed into P₁ and P₂; P₁ is the standard component as the substrate interaction force; P₂ is the resistance and the angle between thrust and force β and angle γ. The sum is the friction angle, and S is the pitch. The resistance of substrate particles includes the external friction resistance of substrate particles and spiral contact and the internal friction resistance caused by the relative motion between substrate particles [21]. The analysis shows that the screw pitch S and blade size r are essential to fundamental substrate particles. If the screw pitch S and blade radius r are too large, the extrusion force of substrate particles will be reduced, and resources will be wasted. However, suppose the spiral pitch size S design and blade radius r is too small. In that case, it will cause material accumulation and blockage, affecting operation quality and efficiency. Therefore, when designing, we should create the spiral size according to the manual and consider many aspects of the helical pitch. We should develop relevant tests and select test factors based on the calculation results.

From the analysis in Figure 4b, it can be seen that the substrate material will slide relatively in the process of moving in the spiral outer barrel, which will impact the material. When the substrate material is close to the spiral shaft, the circumferential velocity V₂ of the substrate material will increase, and the axial velocity V₁ of the substrate particles will decrease. On the contrary, when the substrate particles are far from the spiral axis, the circumferential velocity V₂ decreases, and the axial velocity V₁ increases, forming a material flow. When the screw working speed is reasonable, the influence of material flow on the movement of substrate material is not significant [22]. When the rotation speed of the screw exceeds a specific value, the substrate material will move vertically to the conveying direction. At this time, only the stirring will weaken the axial propulsion effect. This situation reduces the conveying efficiency of substrate materials, accelerates the wear of spiral blades, and increases the consumption of spiral power. To avoid this phenomenon, the speed of the screw should not exceed its critical rate. Therefore, the design of screw shaft speed is the primary test factor.

3.2. Design of Spiral Blade Dimension Parameters

As the most important part of the extrusion device, the spiral shaft should be designed taking into account the influence of processing and installation. According to the above force analysis, it can be seen that the screw pitch and screw blade diameter have a greater interference with the conveyed material, so the impact of different sizes should be fully considered in the design calculation. The spiral blade diameter design is based on the characteristics of the substrate used for the 150 mm seed blanket. By looking up

Table 2. Relationship between types of bulk materials and comprehensive characteristic.

The Lumpiness of the Material	The Abrasive Cut of Material	Application	K Value
The powdery	There was no grinding cut Half a mill cut	Flour, Graphite Lime, Soda ash	0.0415
The powdery	Grinding cut sex	Dry furnace powder, Cement, Gypsum powder	0.0565
Granular	There was no grinding cut Half a mill cut	Grain, Sawdust, Coal slime, Granular salt	0.0439
Granular, Small block < 60 mm	Grinding cut sex There was no grinding cut	Molding soil, Molding sand, Coal, Limestone	0.0600 0.0537

, it can be known that the comprehensive characteristic coefficient of the substrate material is K = 0.0439.

In the design, the width of the final grass seed blanket is taken as a reference, and the diameter of the spiral blade is calculated as follows [23]:

$$\mathrm{D}=\mathrm{K}·\sqrt[2.5]{\frac{\mathrm{Q}}{\mathsf{\phi }\gamma \mathrm{C}}}$$

(1)

where:

K is a comprehensive characteristic coefficient of materials;

φ is the Filling factor;

γ is the Bulk density of materials, unit t/m³;

C is the inclination traffic; in science, the feeding device is placed horizontally, C = 1;

Q is conveying capacity of the screw conveyor, unit t/h, Q = 2t/h according to the target requirements.

Substituting the data into the calculation formula of spiral diameter, because of the unique characteristics of substrate materials, many factors such as substrate moisture content, extrusion force, and molding effect need to be considered in the calculation, so the range of spiral diameter D can be calculated from the formula as 180~300 mm.

The design and calculation formula of the axial diameter of the spiral shaft is shown in (2) [24]:

$$\mathrm{d}=\mathsf{\delta }\mathrm{D}$$

(2)

where:

d is diameter of helical shaft, unit: mm;

$\mathsf{\delta }$ is axial diameter coefficient, usually about 0.2–0.35;

D is spiral diameter, the range of spiral diameter according to formula (1), in mm.

According to Formula (1), D = 250 mm, the axial diameter coefficient δ is usually about 0.32, and the value is put into Formula (2), so it can be concluded that the axial diameter of the spiral axis is 80 mm.

Calculated by Formula (3), the equal pitch screw pitch [24]:

$${\mathrm{S}=\mathrm{K}}_1\mathrm{D}$$

(3)

where:

S is screw pitch, unit: mm;

K₁ is the diameter coefficient of the spiral shaft;

K₁ is the spiral diameter coefficient, which usually takes the value of 0.6~0.1. When materials are arranged obliquely or transported with poor fluidity, K₁ ≤ 0.6 [25]. The substrate used in the substrate grass seed blanket extrusion molding device usually has certain moisture content and viscosity, so it is best to choose the material comprehensive characteristic coefficient of 0.4~0.6. Through calculation, the pitch size of a central spiral is 100 mm or 120 mm, which meets the size requirements of the spiral design manual.

The pitch spiral blade is essentially composed of a number of tangents to the pitch spiral, which consists of a number of points formed by the uniformly accelerated upward movement of the bottom endpoint of any line on the vertical side of the cylindrical bottom surface during its rotation around the axis, which is expanded into a parabola. [26]. The expansion equation of the intersection line of variable pitch conveying shafts is [27]:

$$\mathrm{Y}=\frac{{\mathrm{S}}_0}{{2\mathrm{N}\left(\mathsf{\pi }\mathrm{D}\right)}^2}{\mathrm{X}}^2$$

(4)

where:

• Y is an axial spiral process, mm;
• X is the turning length of the spiral shaft, mm;
• S₀ is the final pitch, mm;
• N is several turns with variable pitch.

It can be obtained that the pitch S_n of the nth turn of the cross line of the variable pitch conveying shaft is [27]:

$${\mathrm{S}}_{\mathrm{n}}{=\mathrm{Y}}_{\mathrm{n}}-{\mathrm{Y}}_{\mathrm{n}-1}=\frac{\left(2\mathrm{n} - 1\right){\mathrm{S}}_0}{2\mathrm{N}}\left(2 \le \mathrm{n} \le \mathrm{N}\right)$$

(5)

According to the calculation results of the central screw and the size of the feed inlet, the pitch of the variable pitch screw can be designed. The final pitch S₀ is determined to be 120~100 mm, and the number of turns N of variable pitch is selected to be 3~5. Substituting it into Formulas (4) and (5), and considering the factors such as too small pitch, which is inconvenient to process, the rise of each turn of the cross line of the variable pitch conveying shaft is calculated from the angle of 100 mm is determined as 120~80 mm, and considering the change of the rise of the screw, the pitch is reversely designed to ensure the accuracy of the design, so the angle of the reverse variable pitch screw is 80~120 mm. When the rise of 110 mm is brought in, the variable pitch screw is 130~70 mm, and its reverse pitch is 70~130 mm; when the angle of 120 mm is brought in, the variable pitch screw is 140~60 mm, and its reverse slope is 60~140 mm. SolidWorks, a three-dimensional modeling software, models the calculation results, and the spiral pitch size design, as shown in Figure 5, is obtained. The design range of spiral length is 300~500 mm through the calculation of pitch and turns. The thickness of the spiral flights should not be too thick or too thin, according to structural considerations usually take 3~5 mm [28]; this paper takes 4 mm.

Through the above calculation, the size parameters of each part of the spiral axis are shown in Figure 6, and the direction of material flow and spiral rotation is indicated.

3.3. Working Parameter Design of the Helical Blade

According to the above particle velocity analysis, it is clear that one of the key factors affecting the conveying capacity of the screw conveying extrusion unit is the screw shaft speed. Generally, the higher the rotating rate of the screw shaft, the greater the conveying capacity, but the rotating speed should be limited. With the increased rotating speed, the circumferential speed will increase, leading to increased centrifugal force on the substrate. It is easy to break the substrate and roll over at the outlet, which is not conducive to conveying the substrate. Moreover, the screw conveying and extruding device will be damaged over time [29]. Therefore, there will be a critical rotational speed (that is, the centrifugal force on the substrate is equal to the gravity on the substrate). The calculated critical rate is [30]:

$${\mathrm{n}}_{\max }=\frac{30}{\mathsf{\pi }}\sqrt{\frac{2\mathrm{gcos}\mathsf{\theta }}{{\mathsf{\mu }}_r\mathrm{D}}\left[\tan \left({\mathsf{\alpha }}_0{+\mathsf{\phi }}_{\mathrm{s}}\right)-{\tan \mathsf{\theta }\sin \mathsf{\phi }}_{\mathrm{t}}\sqrt{{\mathsf{\mu }}_{\mathrm{r}}^2+1}\right]}$$

(6)

where:

G is gravitational acceleration, m·s⁻²;

φ_S is friction angle between soil and spiral blade, 15°~40°;

μ_r is the coefficient of friction between the substrate and the shell, 0.2.

According to the calculation, the screw speed is n_max = 350 r·min⁻¹, and the screw speed is not allowed to exceed the critical rate, so n ≤ n_max. Through the calculation results, the experimental factors can be selected. The maximum value of the screw speed is 350 r·min⁻¹, which decreases in turn.

4. EDEM Simulation Test Analysis

4.1. Purpose and Method of Simulation Test

Based on the above calculations, the range of values of dimensional and working parameters can be derived. In order to obtain the optimal combination of screw dimensional and working parameters in the substrate grass seed blanket extrusion forming device and to obtain grass seed blankets that meet the requirements, the screw pitch, screw diameter, screw length, and screw speed are selected as the main factors for evaluating the screw extrusion effect in combination with the above theoretical analysis. The EDEM discrete element software was used for the simulation test with the quality of extruded particles and the uniformity of grass seed mixing as the evaluation indexes to determine the optimal parameter combination of the screw extrusion device [31].

4.2. Spiral Feed Extrusion Device and Simulation Model Construction of Substrate Particle Model

After the actual measurement of substrate materials and the calibration of substrate parameters in reference literature [32], this paper selects the spherical particles with a radius of 2 mm and a contact radius of 2.5 mm to construct the nutrient soil particles in the substrate. Four spherical particles with a radius of 1 mm and a contact radius of 1.5 mm were selected to build grass seeds. The parameters are as follows: Poisson’s ratio of substrate material is 0.43, the shear modulus is 1.08 × 10⁸, and density is 1620 kg·m⁻³; Poisson’s ratio of grass seed is 0.4, shear modulus is1.0 × 10⁶, and density is 441 kg·m⁻³. The primary material used in the screw extrusion device is steel, with Poisson’s ratio of 0.28, a shear modulus of 2.06 × 10¹¹, and a density of 7850 kg m⁻³. HertzMindlin with JKR contact model is used as the simulation particle of substrate material in this paper. To ensure an accurate simulation, it is necessary to set the contact parameters between substrate particles, grass seed particles, and steel [33], and determine the contact parameters of the simulation model, as shown in

Table 3. Contact parameters of the simulation model.

Parameter	Numerical Value
Substrate particle—Substrate particle to particle recovery coefficient, e	0.30
Substrate particles—Static friction coefficient between substrate particles, μs	0.65
Substrate particle—Dynamic friction coefficient between substrate particles, μr	0.43
Substrate particles—Intergranular recovery coefficient of grass seeds, e	0.20
Substrate particles—Static friction coefficient between grass seeds, μs	0.52
Substrate particles—Dynamic friction coefficient between grass seeds, μr	0.05
Grass seed granule—Intergranular recovery coefficient of grass seeds, e	0.25
Grass seed granule—Static friction coefficient between grass seeds, μs	0.30
Grass seed granule—Dynamic friction coefficient between grass seeds, μr	0.01
Substrate particles—Recovery coefficient between steels, e	0.28
Substrate particles—Static friction coefficient of steel, μs	0.30
Substrate particles—Dynamic friction coefficient of steel, μr	0.20

.

The three-dimensional model of the screw feeding device is established by using SolidWorks software [34]. To improve the simulation calculation efficiency, the model is simplified, and the parts that do not affect the operation effect are removed and saved as a (.stp) file is imported into EDEM software. The simulation time is set to 30 s, of which about 10 s is used for material filling and about 20 s for material extrusion, and the material extrusion time starts to be calculated when the material reaches the extrusion outlet. The substrate material particle model is established. The simulation process shown in Figure 7a is the simulation process of two kinds of particles (yellow is substrate particles, purple is grass seed particles); Figure 7b is the linear flow process of particles, and the figure shows some particle motion trajectories at the time of 12 s. It can be found that particles from aggregation at the extrusion port.

The simulation results at different times are shown in Figure 8a, the particle movement speed at 2 s, and Figure 8b, the particle movement speed at 18 s. It can be seen that the particle speed decreases gradually near the outlet, and the rate decreases to the minimum of 0.1 s m⁻¹ during extrusion. Figure 9 is the velocity vector diagram at 2 s. It can be found that the particles move linearly and circularly when they fall, while Figure 10 is the velocity vector diagram at 18 s. It can be seen that the particles move regularly, and the speed is stable when they are transported. The velocity vectors are intertwined at the extrusion port, which indicates that the extrusion molding is completed.

Figure 11a,b presents cross-sectional views of the molded grass seed blanket in the X direction, which can clearly show that the grass seeds are mixed with the substrate, and there are grass seeds in the grass seed blanket.

4.3. EDEM Simulation Test

4.3.1. Test Plan

Through the analysis of the above theory, the main influencing factors of screw on the forming quality of grass seed blanket are screw pitch, screw diameter, screw length, and screw speed. Therefore, to obtain the best screw design parameters, the simulation experiment was carried out by EDEM software with screw pitch, screw diameter, screw length, and screw speed as test factors, taking the quality of extruded particles and the uniformity of grass seed mixing as evaluation indexes. According to the design principle of Central Composite Design in the Design-Expert software [35], a four-element quadratic orthogonal rotation combination experimental design combined with resa response surface analysis was carried out for the extrusion molding of grass seed blanket in consideration of interaction to explore the optimal parameters of the extrusion molding device design of grass seed blanket, and to determine the codes of experimental factors as shown in

Table 4. Coding table of test factor level.

Factor Levels	Factors
	Spiral Speed X1/r·min−1	Spiral Pitch X2/mm	Spiral Diameter X3/mm	Spiral Length X4/mm
+2	100	70~130 mm Variable pitch screw	180	300
1	150	80~120 mm Variable pitch screw	210	350
0	200	120~80 mm Variable pitch screw	240	400
−1	250	130~70 mm Variable pitch screw	270	450
−2	300	140~60 mm Variable pitch screw	300	500

.

4.3.2. Test Index

(1) Extruded particle mass

To evaluate the extrusion effect, the EDEM software post-processed the Grid Bin Group function in Setup Selections, divided the extrusion outlet into grids, and recorded all the changes in particle quality through the grid. The extrusion particle quality results were obtained under different conditions by extracting the data of particle quality through the extrusion outlet.

(2) Mixing uniformity of grass seeds

The evaluation index of the uniformity of matrix grass seed mixing was evaluated by statistical method, which was represented by the difference in the number of particles in the space region at the extrusion exit. The experiment was repeated five times to compare the difference in the number of grass seed particles in the spatial area and to evaluate the mixing uniformity of the grass seed matrix under different experimental conditions. The site is shown in Figure 12.

The larger the number of grass seed particles in the space, the more evenly the substrate and grass seed are mixed. The corresponding values X1, X2, X3, …, Xn of grass seed particles was measured in the spatial area, and the average value $\stackrel{-}{\mathrm{X}}$, standard deviation S, its variation coefficient, and mixing uniformity M were calculated. The variation coefficient (CV) measures the difference in the number of grass seed particles in different spatial areas under other experimental conditions [36,37]. Refer to the calculation method of coefficient of variation (CV), and follow Formulas (7) and (8):

$$\mathrm{CV}=\frac{\mathrm{S}}{\stackrel{-}{\mathrm{X}}}\times 100\%$$

(7)

$$\mathrm{M}=1- \mathrm{CV}$$

(8)

where:

$\stackrel{-}{\mathrm{X}}$ is the mean value of the cumulative sum of the number of grass seeds in the spatial area;

S is the standard deviation of the number of grass seed particles in the spatial area;

CV is the coefficient of variation of the number of grass seed particles in the spatial area;

M is mixing uniformity of grass seeds.

Theoretically, the smaller the CV value, the better the mixing uniformity of grass seed particles. The coefficient of variation of absolutely uniform substrate grass seed mixture should be close to 0.

4.3.3. Results and Analysis

The effects of screw pitch, screw length, screw diameter, and screw speed on the substrate grass seed blanket performance index were analyzed using the single factor test method. According to multivariate analysis, the experimental results are shown in Table 4. Within the range of experimental parameters, the mass of extruded particles varies from 2289~2751 g, and the uniformity of grass seed mixing varies from 81.02~94.96%. The regression analysis of the influence of screw pitch, screw length, screw diameter, and screw speed on the quality of extruded particles and the uniformity of grass seed mixing was carried out using the Design-Expert10.0 software [35]. The results of variance analysis of the regression equation are shown in

Table 5. Test arrangement and results.

Number	Spiral Speed X1	Spiral Pitch X2	Spiral Diameter X3	Spiral Length X4	Extruded Particle Mass Y1/g	Uniformity of Grass Seed Mixing Y2/%
1	−1	−1	−1	−1	2520	86.21
2	1	−1	−1	−1	2548	94.96
3	−1	1	−1	−1	2590	86.14
4	1	1	−1	−1	2645	91.05
5	−1	−1	1	−1	2418	82.52
6	1	−1	1	−1	2709	84.28
7	−1	1	1	−1	2456	83.25
8	1	1	1	−1	2585	87.64
9	−1	−1	−1	1	2326	83.11
10	1	−1	−1	1	2598	83.67
11	−1	1	−1	1	2442	81.21
12	1	1	−1	1	2657	82.68
13	−1	−1	1	1	2312	83.86
14	1	−1	1	1	2549	82.01
15	−1	1	1	1	2351	82.31
16	1	1	1	1	2578	85.63
17	−2	0	0	0	2289	81.02
18	2	0	0	0	2751	85.81
19	0	−2	0	0	2425	84.58
20	0	2	0	0	2435	85.55
21	0	0	−2	0	2679	94.22
22	0	0	2	0	2532	88.95
23	0	0	0	−2	2623	89.01
24	0	0	0	2	2414	82.65
25	0	0	0	0	2674	93.97
26	0	0	0	0	2685	93.22
27	0	0	0	0	2669	92.91
28	0	0	0	0	2690	92.86
29	0	0	0	0	2689	92.84
30	0	0	0	0	2629	90.45

. According to the variance analysis of the regression equation in

Table 6. Variance analysis of extruded particle quality and grass seed mixing uniformity.

Parameters	Extruded Particle Mass		Uniformity of Grass Seed Mixing
	F Value	p-Value	F Value	p-Value
Model	21.90	<0.0001	28.27	<0.0001
X1	149.19	<0.0001	74.52	<0.0001
X3	11.56	0.0040	55.42	<0.0001
X4	30.55	<0.0001	53.00	<0.0001
X1 × 4	7.94	<0.0001	7.25	0.0167
X2 × 3	5.13	0.0388	9.61	0.0073
X3 × 4	-	-	8.77	0.0013
X12	25.47	<0.0001	80.75	<0.0001
X22	64.18	<0.0001	84.54	<0.0001
X32	4.79	0.0415	14.71	0.0917
X42	25.97	<0.0001	66.35	<0.0001
Fitting	3.98	0.702	2.65	0.1558

, the p-value of each model is less than 0.0001. The p-value of the model’s mismatch test is more significant than 0.05, indicating that the correlation between independent and dependent variables is good, the regression equation fits well, and the model can predict two test indexes well, with high model accuracy.

4.3.4. Interaction Analysis of Test Factors

The significant two-factor interaction was analyzed by response surface methodology. The combined effect of various factors on the quality of extruded particles is shown in Figure 13. As seen from Figure 13a, when the screw length is constant, the mass of extruded particles increases with the increase of screw speed. The reason is that the screw speed which causes particle blockage is excluded in the single factor test, so the rapid rotation of the screw increases the movement speed in the particle space and the mass of extruded particles. When the screw speed is constant, the quality of extruded particles improves at first and decreases with the increase of screw length. Still, the trend fluctuates gently because, with the rise of screw length in the early stage, the particles are looser and move faster in a more extensive range, and when the size increases, the movement time of particles in the barrel increases in the later stage, which affects the extrusion quality. When the screw speed is 230~250 r·min⁻¹, and the screw length is 390~410 mm, the mass of extruded particles is the highest. As seen from Figure 13b, when the screw diameter is constant, the group of extruded particles increases first and then decreases with the change of screw pitch. The reason is that the size change of variable pitch screw directly affects the movement speed of particles. The shift in screw pitch from big to small makes particles move more loosely at the inlet, which leads to faster particle movement and more extrusion quality. The size of the screw pitch decreases at the extrusion outlet, resulting in better particle aggregation and a better extrusion effect. When the screw pitch is constant, the extruded particle mass increases at first and then decreases with the increase of screw diameter, and the trend is relatively stable. With the rise of screw diameter, the movement space between particles increases in the early stage, and the contact friction resistance decreases; with the rise of screw diameter, the space increases in the later stage, and the larger movement space of particles affects the extrusion quality. The screw pitch is 120~80 mm, the angle is 130~70 mm, the screw diameter is 240~260 mm, and the extruded particles have the highest quality.

The significant two-factor interaction was analyzed by response surface methodology. The combined effect of various factors on the uniformity of grass seed mixing is shown in Figure 14. As can be seen from Figure 14a, when the screw length is constant, the mixing uniformity of grass seeds increases with the change of sc seen from Figure 14a; when the screw length is continuous, the mixing uniformity of grass seeds increases speed. The reason is that the increase in screw speed affects the movement speed of grass seeds in space. With the rise of screw speed, the movement speed of grass seeds in the substrate is faster and faster, and with the increase of screw speed, grass seeds are more easily dispersed, making the mixing of grass seeds more uniform. When the screw speed is constant, the mixing uniformity of grass seeds increases at first and then decreases slowly with the change of the screw length. The reason is that the space between grass seeds and substrates is smaller with the smaller screw length, and grass seeds are not easy to disperse. With the increased screw length, the overall space gradually increases, and the particles are loose. Grass seeds have a particular movement space, which is not easy to contact with each other. Excessive length will lead to too many substrates in the area and decrease the uniformity. When the screw speed is 230~250 r·min⁻¹, and the screw length is 390~410 mm, the uniformity of the grass seed mixture is the highest. As seen from Figure 14b, when the diameter of the screw is constant, the mixing uniformity of grass seeds increases at first and decreases with the change of the screw pitch, and the trend fluctuates wildly. The reason is that the size change of the variable pitch screw affects the joint movement of particles, and the screw pitch changes from big to small. When the screw pitch increases at the entrance, the particle movement speed is accelerated to move to the exit. Still, when the screw pitch decreases at the extrusion outlet, the particles can keep their original position again to ensure uniformity. When the angle is 120~80 mm, the rise is 130~70 mm, and the diameter is 220~240 mm, the uniformity of grass seed mixing is high. As seen from Figure 14c, when the screw diameter is constant, the mixing uniformity of grass seeds increases at first and then decreases with the increase of the screw length. The reason is that the initial speed of grass seeds at the entrance of the screw is more significant; when the screw length is shorter, the movement range of the particles is smaller, and it moves forward at a certain speed, which makes the space between the particles smaller. With the increase of the screw length, the movement range of the particles increases so that the movement space of grass seeds is easy to accumulate and decreases; at the later stage, the movement space of the particles is too large due to the long screw length. When the screw diameter is 210~230 mm, and the screw length is 390~410 mm, the uniformity of grass seed mixing is high.

4.3.5. Parameter Optimization

To obtain the optimal parameter combination of the screw extrusion device, the optimization module in the design expert software is used to optimize the above regression model. The constraint conditions of the test factors are as follows: The screw speed is 100~300 r·min⁻¹; The screw pitch shall be 70~130 mm variable pitch screw, 80~120 mm variable pitch screw, 120~80 mm variable pitch screw, 130~70 mm variable pitch screw, and 140~60 mm variable pitch screw; The spiral diameter 180~300 mm; The spiral length is 300~500 mm. The process parameters were further optimized by using the software Design-Expert10.0. The optimal results were as follows: the screw speed was 233 r·min⁻¹, the screw pitch was 120~80 mm variable pitch screw, the screw diameter was 232 mm, the screw length was 371 mm, the mass of extruded particles was 2721 g, and the mixing uniformity of grass seeds was 94.96%. The optimized parameters are used to conduct simulation tests in EDEM, and the average value is taken for three simulation times. The test results are shown in

Table 7. Verification of the simulation test results.

Parameters	Spiral Speed X1/r·min−1	Spiral Pitch X2/mm	Spiral Diameter X3/mm	Spiral Length X4/mm	Extruded Particle Mass Y1/g	Uniformity of Grass Seed Mixing Y2/%
Test value	250	120~80 mm	240	400	2620	92.35
Predicted value	233	120~80 mm	232	371	2721	94.96
Error					0.0371	0.0274

. With the same test time of 30 s, the relative errors between the predicted value and the test value of the extruded particle quality and the extrusion force of the substrate grass seed blanket spiral extrusion device are 3.71% and 2.74%, respectively. It shows that the experimental value of the regression equation is close to the predicted value, and the model’s prediction accuracy is high. Further, the mass flow rate can be calculated as 131 g/s based on the extruded mass of 2620 g.

5. Verification Test

The actual verification test is carried out according to the optimal parameters obtained from the simulation test. As shown in Figure 15, the prototype of the substrate’s extrusion forming device for the grass seed blanket is taken as the test index. Five groups of repeated tests are carried out with the optimal parameter combination: 120 mm~80 mm variable pitch screw, 400 mm screw length, 240 mm screw diameter, and 250 r·min⁻¹ screw speed.

The experiment was conducted in Shenyang Agricultural University’s comprehensive training center. The test materials were from a ten-mile river town in Sujiatun district of Shenyang, at a farmers’ home. Maize straw was collected in October 2021; the strips length was 5~20 mm. After harvest, natural air drying, and crushing, we then mixed with the method of aerobic fermentation rotten after cow dung. The average outdoor temperature was 24 °C, and we covered the straw with the black plastic sheeting. A thermometer was used to test the internal temperature, and the straw was turned over when it reached 60 °C. After 20 days, the material could be held by hand and molded as a premix for reserve. The total premix prepared had a mass ratio of 80%. The premix solvent consisted of tap water and was added the mass balance of 3% polyamide epichlorohydrin, 3% polyacrylic acid sodium, 1% ferrous sulfate, 1% borax, 1% amino acid, 1% magnesium sulfate, and a certain proportion of pesticides and fungicides. Polyamide epichlorohydrin is a wet strength agent used in paper making, and sodium polyacrylate is a flour additive used to improve the bond strength of substrate bricks in the damp state and to ensure that water absorption is not deformed and loose. Other trace elements are environment-friendly additives, which do not impact the ecological environment [10]. The premix was prepared in proportion, and the grass seed was added to the mixture to get the substrate grass seed blanket. The molding grass seed blanket was obtained by the extrusion device of the substrate grass seed blanket. As shown in Figure 16, it was placed outdoors and watered once a day at the initial stage. After seven days, the seeds germinated, and the watering interval was appropriately extended. When the surface of the grass seed blanket becomes dry and white, it is necessary to water it once in an irregular period of about 3–7 days due to the influence of temperature, rain, and other factors. The growth trend of the substrate grass seed blanket in 45 days is shown in Figure 17, in which Figure 17a is the result of incubation for 15 days, and Figure 17b is the result of pregnancy for 45 days.

The newly extruded grass seed blanket is conducive to cutting, which can simply meet the requirements of the shape of the lawn landscape. Figure 18 shows the cut lawn landscape, which can be applied to other conditions.

To shorten the simulation time and improve the efficiency, the particle volume was three times larger than the actual size during the simulation, and the measured mass value calculated by the density formula should be less than three times the simulation value. The measured value was obtained by the optimal parameter combination of the simulation results in the actual test, and the measurement results are shown in

Table 8. Verification test results.

Parameters	Test Serial Number					Average
	1	2	3	4	5
Extrusion substrate mass/g	2570	2550	2580	2520	2575	2559
Uniformity of grass seed mixing/%	90.8	89.2	91.4	90.3	89.7	90.28

. According to the simulation test also in 30 s time, the average mass of extruded substrate was 2559 g, the average uniformity of grass seed mixture was 90.28%, and the errors respectively were 3.4% and 2.5%, the calculated mass flow rate results in 127.9 g/s. The results showed that the parameters combination and optimization results obtained by simulation were almost the same as the actual ones, and the simulation results were reliable.

6. Discussion

Commonly used optimization design methods include analytic, orthogonal, homogeneous, and response surface design, among which response surface design is widely used in process or product optimization studies [38].

Nguyen Huu Loc’s team studied the effect of different parameters on the efficiency of helical gears using a responsive design approach and concluded from the analysis that, among the relevant parameters, speed, number of teeth, gear ratio, helix angle, and torque the parameters that have a greater effect on the efficiency of helical gears are gear ratio and number of teeth [39]. The same Gurkirat Kaur team, in their study of cooking with twin-screw extrusion, also used the response surface design approach to investigate and compare the effect of barrel temperature, feed moisture, and screw speed on mechanical energy, expansion ratio, and stack density, respectively, and concluded that screw speed has a direct relationship with mechanical energy and expansion ratio [40]. The above two teams used the response surface method mainly for the selection of important influencing parameters. Meanwhile, the response surface method can be used for more detailed optimization of parameters, such as Chunguang Wang’s team in the study of kneading corn straw screw conveying device to optimize the screw pitch, screw shaft speed, and feeding volume of three parameters, and finally obtain the optimal combination of parameters, so that the overall performance of the device can be improved [41]. In the same way, Wanzhang Wang’s team also used the response surface method to optimize the vibration parameters in the lifting device of the tiger fruit harvester to obtain the optimal parameters of vibration frequency, vibration amplitude, lifting speed and lifting angle, and the results of the study provide theoretical references for the design optimization and simulation analysis of the tiger fruit harvester [42].

In this paper, response surface design is used to optimize the spiral shaft size parameters and process parameters, and the optimal parameters of the spiral are obtained by response surface analysis: spiral speed 250 r·min⁻¹, spiral pitch 120~80 mm, spiral diameter 240 mm, and spiral length 400 mm.

7. Conclusions

(1)

According to the technical parameters of grass seed blanket, combined with mechanical analysis, the extrusion molding device of grass seed blanket is designed, which can realize the extrusion molding after mixing the substrate and grass seeds. The molding thickness is about 12 mm, which is convenient to transport and easy to cut. Grass seeds are evenly mixed in the substrate, and the germination effect is good.

(2)

Through theoretical analysis and design of the size and working parameters of the screw in the screw conveying and extruding device, it is determined that the main factors affecting the quality of the extruded substrate and the uniformity of grass seed mixing are screw pitch, screw length, screw diameter, and screw speed. The simulation model of extrusion molding device of substrate grass seed blanket was established in EDEM software. Taking the quality of extruded particles and grass seed mixing uniformity as test indexes and the screw pitch, screw length, screw diameter, and screw speed as test factors, the four-element quadratic orthogonal rotation combination simulation experiment was carried out. The optimal parameter combination was obtained: variable pitch screw with a screw pitch of 120~80 mm, screw length of 400 mm, screw diameter of 240 mm, and screw speed of 250 r·min⁻¹. Under the optimal parameters, the extrusion time was 30 s, the mass of extruded pellets was obtained as 2620 g, and the grass seed mixing uniformity was 92.35%.

(3)

To ensure the accuracy of the simulation test results, the actual verification test was carried out. The errors between the measured values of extruded substrate quality and grass seed mixing uniformity and the simulation test results of 3.4% and 2.5% met the grass seed blanket extrusion molding requirements.

8. Patents

A patent has been applied for in China for the extrusion molding machine of grass seed blanket in this manuscript (Patent No. CN216058571U; Application No. CN2021215585649).