Design and Parameter Optimization of Transverse-Feed Ramie Decorticator

Abstract

In view of the elevated labor intensity and low efficiency of ramie fiber decortication, we designed a simple automatic ramie decortication machine in line with the characteristics of the ramie fiber decortication process, design requirements and market demand through an innovative design and theoretical analysis of key components such as the clamping and conveying device and the fiber detecting device, and identified the key factors and parameters affecting the quality of ramie decortication. We develop a mathematical model of the fiber percentage of fresh stalks and the ramie fiber impurity rate by considering decortication clearance, the drum speed, and the conveyance speed as factors, and determine the effect of operating parameters on ramie decortication and the optimal combination of parameters. Finally, a multi-objective optimization test was performed using the Box–Behnken test. In this paper, based on the results of the multi-objective parameter optimization analysis, the optimal parameters for ramie peeling were determined, namely, a decortication clearance of 3.7 mm, and a conveyance speed of 340 rpm. According to the optimized parameters, the ramie peeling process was experimentally validated. Using the optimized parameters, a validation test of the ramie direction in this study was performed. As indicated by the results, the percentage of fiber in the fresh stalk reached 5.05%, and the impurity rate in the ramie fiber was 1.24%. The relative errors of all metrics and model predictions were less than 5%, thus validating the accuracy of the prediction model. The machine achieved a production efficiency of 78.5 kg·h⁻¹, which is in line with the design specifications. The raw fiber had a gum content of 23.45 percent, and the stripped fiber met the national standard for secondary ramekin fiber. This study lays a theoretical basis while providing technical support for fully automatic ramie decorticators.

1. Introduction

Ramie, a conventional characteristic cash crop, is commonly known as “Chinese grass” worldwide. Ramie fiber products have been favored by the international market for a long time for their characteristics of breathability, moisture absorption, fast heat dissipation, insect and mold resistance, and anti-static properties [1,2,3]. Ramie for fiber can be harvested three to four times a year. It is noteworthy that ramie can only be employed by textile enterprises after being stripped and processed to obtain fibers. Existing statistics suggest that the harvesting and decortication of ramie fibers account for nearly 80% of the labor in the entire production process [4,5,6]. At present, ramie fiber decortication mainly uses pure labor manual decortication and manual reverse pulling mechanical decortication. Several problems have been reported (e.g., high labor intensity and unstable decortication quality). The overall economic benefit is low, and there are production safety issues [7,8]. Thus, a low-loss and efficient ramie decortication machine for use in production should be urgently developed [9].

Ramie decortication entered the era of powered decortication machinery in the early 1980s, and globally, several mature manual reverse-pulling ramie decortication machines (e.g., 6BM-350, 4BM-260, and 6BX-40) were developed [10,11]. Ramie decortication can primarily be divided into single-drum and double-drum types of reverse-pulling decortication machine. The fiber decortication of the base and tip of ramie is completed by successively performing manual feeding and manual reverse pulling. The type of machine described above is characterized by simple structures, and achieves good decortication quality when the fiber percentage of the fresh stalk is over 5% and the original fiber impurity content is lower than 1%, achieving an operation efficiency of 8–15 kg·h⁻¹. However, the labor intensity is high, and there are hidden dangers (e.g., hand injury) [12,13]. Subsequently, direct-feed ramie decortication technology and equipment appeared. This type of machine is a multi-drum automatic decortication machine, where the ramie stem is fed longitudinally along the direction of rotation of the ramie decortication cylinder while realizing fiber decortication by repeatedly bending, rolling, and scraping the stem with multiple groups of ramie decortication cylinders. The NH01, JBM-100, and FL-KB models have been well developed. The above-mentioned machine type possesses advantages including continuous feeding, simple operation, and low labor intensity. The fiber ratio of the fresh stem exceeds 5%, although there are problems (e.g., the unclean decortication of ramie bones at the base and end of the tip). The original fiber exhibits a high content of impurities, and the decortication quality is unstable (i.e., the length of the ramie stem and the rotating speed of the drum), and the decortication efficiency is low [14,15]. Based on the manual reverse-pulling and direct-feed decortication technologies, in the past few years, experts have proposed transverse-feed ramie decortication technology, where mechanical clamping and conveyance are exploited instead of manual reverse pulling. Fiber decortication at the base and tip ends of ramie in the process of ramie stem clamping and conveyance is completed using multiple sets of decortication devices [16,17]. This type of technology is capable of realizing the continuous feeding of ramie, with simple operation and stable decortication quality. It addresses the problems of high labor intensity and poor safety performance of the manual reverse-pulling type of decortication machine, while overcoming the shortcomings of the direct-feed type of decortication machine (e.g., drum winding and unclean base decortication). However, existing research suggests that this type of technology faces a considerable number of technical difficulties (e.g., difficulty in fiber clamping and unstable decortication quality) [18,19].

To tackle the problems of the insufficiency of the stem clamping force and the fiber winding drum in ramie decortication and to achieve efficient ramie fiber decortication, a transverse-feed ramie decorticator was developed in this study based on the operation characteristics of transverse-feed decortication technology and the requirements of the ramie decortication process. A mathematical model of the operation parameters and decortication quality indicators was established with the fiber percentage of fresh stalk and the impurity rate of raw fiber as the core evaluation indicators. The optimal combination of operation parameters was obtained via Box–Behnken testing in order to conduct multi-objective optimization testing, and the accuracy of the prediction model and the decortication operation quality were verified via production application testing to provide supporting equipment and technical support for the automatic light and simplified ramie decortication technology.

2. Materials and Methods

2.1. Main Technical Requirements for Ramie Decortication

This study referred to the GB/T20793-2015 (degummed ramie) and the DB43/T332-2007 (ramie fiber processed with decorticator) [20,21]. The operating performance parameters of the transverse-feed ramie decorticator were determined in combination with ramie traits.

Table 1. Main technical parameters of the transverse-feed ramie decorticator.

Parameter	Values
Processed length of ramie/mm	600~1700
Gum content of raw fiber/%	≤28
Impurity rate of raw fiber/%	≤2.0
Fiber percentage of fresh stalk/%	≥4.0
Productivity/(kg·h−1)	≥40
Grade of ramie fiber	Grade 2 and above

lists the main technical specifications of the prototype [22,23].

The grade 2 ramie fiber exhibited good sensory quality (few attached shells and coke tips, soft feel of the fiber), few spots, few radicles, free of mildew, raw fiber impurity rate less than 1.5%, and fiber length greater than 800 mm. Grade 1 ramie fiber exhibited excellent sensory qualities (very few attached shells and coke tips, extremely soft feel of the fiber), fewer spots, very few radicles, free of mildew, raw fiber impurity rate less than 1.0%, and a fiber length greater than 1000 mm [24].

2.2. Structure and Operating Principle of the Transverse-Feed Ramie Decorticator

2.2.1. Overall Structure

The transverse-feed ramie decorticator comprises a feed port, inclined pressure bar, guide plate, fiber decortication device, flexible clamping and delivery device, fiber decortication motor, main motor, fiber output port, frame, control system and other working parts. Figure 1 illustrates the basic structure.

2.2.2. Working Principle

In general, the ramie decortication operation of the transverse-feed ramie sectional decorticator comprises six processes (i.e., stem feeding and conveyance, base fiber decortication, semi fiber stem output, semi fiber stem feeding, tip fiber decortication, and fiber output). During operation, the main motor drives the flexible clamping and conveyance mechanism. The convex pressing block of the upper conveyance chain in the flexible clamping and conveyance mechanism is clipped into the concave clamping plate of the lower conveyance chain to form a clamping and conveyance area. Ramie stems are horizontally fed from the feeding mouth to the clamping and conveyance area to achieve the horizontal clamping and conveyance of the stems. With the clamping and conveyance of the stem, the base end of the stem enters the ramie decortication mechanism under the joint action of the diagonal compression bar and the guide plate, and the ramie decortication area formed by the concave ramie decortication plate and the ramie decortication drum, which is rotating at high speed, completes the base fiber decortication by scraping the base end of the stem. The continuous clamping and conveying performed by the flexible clamping and conveyance mechanism outputs the semi fiber stems, stripped at the base end, from the fiber output port. The semi fiber stems are manually fed from the ramie feeding mouth. The ramie decortication mechanism completes the decortication of the tip end fibers with the clamping and conveyance performed by the flexible clamping and conveyance mechanism. The transversely clamped fibers continue to move to the fiber output port through the operation of the flexible clamping and conveyance mechanism, such that the output of the ramie fibers is achieved.

2.3. Key Part Design and Parameter Determination

2.3.1. Ramie Decortication Mechanism

The flexible clamping and delivery device and the fiber decortication device serve as the key components in the transverse-feed ramie decorticator. References [25,26] put forward the analysis index of number of decortication times (T). The experimental results suggested that the number of decortication times was the primary factor affecting the decortication effect. The general calculation equation is written as follows:

$$T=L\times n\times w/(60 v)$$

(1)

where L is the length of the crop, mm; v is the conveyance speed, m/s; w is drum speed, rpm; n expresses the number of decortication plates. T is the number of times a crop has been scraped by a decortication plate during the decortication process

Properly reducing the number of decortication times can increase machine productivity, while the impurity rate of the raw fiber will be increased, and increasing the number of decortication times will result in a reduction in the fiber percentage of the fresh stalk [27,28]. Based on Equation (1) and experimental study, it can be concluded that the parameters affecting ramie decortication quality are as follows: decortication drum diameter, fiber decortication concave arc length, decortication plate length, number of decortication plates, the decortication clearance, the drum speed and the conveyance speed.

2.3.2. Ramie Decortication Device

The fiber decortication device includes a decortication drum, a fiber decortication concave, an introduction plate, a fiber decortication motor, a transmission mechanism, a slack adjuster, and a concave bracket, as well as other working parts. Its basic structure is illustrated in Figure 2. The decortication drum serves as the critical component of the fiber decortication device, and primarily comprises a decortication plate, a spindle, an anti-tangle cover, and a drum bracket. Furthermore, the slack adjuster is primarily composed of a wedge block, a lengthwise regulation bolt, and a broadwise regulation bolt.

The curved area composed of the decortication drum and the fiber decortication concave was the fiber decortication area of the fiber decortication device. In theory, the length of the fiber decortication area should be greater than half of the length of the ramie stem to ensure that the ramie stem can be completely stripped. To reduce the diameter of the decortication drum and the size of the machine, the length of the fiber decortication area is set to be greater than a quarter of the outer arc length of the decortication drum. In other words, the radius of the decortication drum remains unchanged, and longer ramie stems are processed by lengthening the arc region of the fiber decortication concave. In practical application, the average length of ramie stems subjected to decortication processing ranges from 1400 to 1800 mm [29,30]. The diameter of the decortication drum was designed to be 1160 mm, and the length of the arc region of the fiber decortication concave was set as 950 mm, such that the length of ramie fibers for decortication ranged from 910 to 950 mm.

In general, the number of decortication times for ramie is 40~70 times/s. On that basis, the number of decortication plates for the prototype was determined as 5.3~16.8. In line with Reference [31] and the prototype test results, the number of decortication plates was set as 12. The conveyance speed of ramie stems was fixed. An increase in the length of the decortication plate can lead to the increased number of decortication times, such that the fiber percentage of fresh stalk and the impurity rate of the raw fiber can be reduced. The length of the decortication plate was set as 430 mm in accordance with ref [25] and the results of previous bench tests. The decortication plates were produced directly using 10^# angle iron, and the structure is presented in Figure 3. To reduce the instantaneous impact force of the decortication plates on the ramie stalk and increase the fiber percentage of the fresh stalk, the decortication plates were designed to be bell-mouthed to ensure that decortication plates will enter the minimum decortication clearance slowly. Furthermore, the decortication plates were evenly distributed at the outer end of the decortication drum through bolted connections.

The operation principle of the fiber decortication device designed in this study was the same as that of the fiber decortication device described in Reference [25]. The linear velocity of the decortication drum in Reference [25] approached 17.66 m/s, such that the ramie decortication quality can be ensured. The correlation between the drum speed and the linear velocity of the decortication drum is expressed in Equation (2).

$$w=60\times 1000\times v_1/(\mathsf{\pi }D)$$

(2)

where v₁ is the linear velocity of the decortication drum, m/s; D is the diameter of the decortication drum, mm.

Combined with the results of previous experimental research [25], the speed range of the decortication drum was set to 200–500 rpm.

The size of the decortication clearance affects the quality of ramie decortication directly. To adjust the decortication clearance, the position of the wedge block was controlled using the lengthwise regulation bolt to adjust the longitudinal height of the decortication drum; the transverse position of the decortication drum was adjusted using the broadwise regulation bolt. The initial value of the decortication clearance reached 3.0 mm, and the adjustable range was 2.0~5.0 mm.

2.3.3. Flexible Clamping and Delivery Device

The flexible clamping and delivery device comprised a main motor, a side plate, an upper feed system, a lower feed system, a lower drive wheel, a lower driven pulley, an upper driven pulley, an upper tensioning wheel, an upper tightening device, a lower tightening device, an upper pressing mechanism, a lower supporting mechanism, and a frame, as well as other working parts. Figure 4 illustrates the basic structure of this device. In general, the upper feed system comprises the conveyance chain and the convex block (each pair of convex blocks are connected head to tail to form a complete convex compression bar). The lower feed system primarily consists of the conveyance chain and the concave card plate (each pair of concave card plates are connected head to tail to form a complete concave slot). The upper pressing mechanism primarily consists of a chain wheel, a reset spring, and a linear bearing. Furthermore, the lower supporting mechanism is mainly composed of a lead screw, a screw sleeve, and a support frame.

The clamping and delivery device had insufficient gripping force for ramie stalks and fibers, which could easily cause problems (e.g., missing fibers). There will be certain damage to the fiber during the clamping and delivery operation. To address the above problems, the transverse-feed ramie decorticator adopts the flexible clamping technology of the coordinated operation of a convex compression bar and a concave slot, in order to guarantee sufficient clamping force without damaging the ramie fiber.

When working, the convex compression bar is stuck into the concave slot to form the clamping area, the ramie stalk is fed into the clamping area from the feeding port, the main motor drives the rotation of the lower drive wheel to drive the lower conveyance chain, and the frictional force between the convex compression bar on the upper conveyance chain and the concave slot on the lower conveyance chain synchronously drives the upper feed system. In the clamping and conveying of the clamping conveyance mechanism, the ramie stem base enters the fiber decortication device to undergo fiber decortication. Moreover, with the continuous operation of the flexible clamping and delivery device, ramie stems that have completed fiber decortication are output at the base. Likewise, the fiber decortication is completed for the ramie tips once again, such that the fiber decortication is performed for the whole ramie stem. Conforming to the design requirements regarding the productivity of the machine, the flexible clamping and delivery device achieved a minimum conveyance speed of 0.24 m/s, while excessive conveyance speed led to an increase in the impurity rate of the raw fiber, such that the adjustment range of the designed conveyance speed was 0.24~0.48 m/s.

2.4. Bench Test of Ramie Decortication

2.4.1. Test Conditions and Test Equipment

To obtain optimal operation parameter combination of the transverse-feed ramie decorticator, the ramie decortication test of the transverse-feed ramie decorticator was conducted in National Ramie Germplasm Nursery, Wangcheng District, Changsha City. The experiment time was from June 2021. The ramie variety used for the experiment was Zhongzhu No. 1, test ramie from the 11 years old ramie garden, ramie stalks harvested the same day were selected for the experiment, 100 kg ramie was decortication stripped in the respective test. The test was repeated fourfold, and the average value was selected as the test value. The length of ramie tip resection was set at 1800 mm, the ramie stem shorter than 600 mm was removed directly, the average stem length of the testing ramie reached 1680~1800 mm, and the base diameter of the testing ramie was obtained as 11.90~15.27 mm [32]. The average moisture content of fresh stem reached 79.46%, the average moisture content of fresh bark was determined as 80.23%, the average thickness of fresh bark was obtained as 0.57 mm, and the average thickness of fiber reached 0.23 mm. The average moisture content of fresh stem, the average moisture content of fresh stem, the average thickness of fresh bark and the average thickness of fiber were 3.1%, 2.4%, 1.9%, and 1.4%, respectively.

The test instrument and equipment comprised the experimental prototype of the transverse-feed ramie decorticator, an electronic scale (a range of 20 kg and accuracy of 0.1 g), a XMA-600 automatic electric drying oven, an micrometer, a vernier caliper, a tape measure, a meter gauge, a stopwatch, and so forth.

2.4.2. The Assessment Index

To further test the decortication performance of the transverse-feed ramie decorticator, this study refers to the GB/T20793-2015 China (Degummed ramie) [20] and the DB43/T332-2007 China (Ramie-fiber processed with decorticator) [21], and conducts field experiments based on the assessment indices of the fiber percentage of fresh stalk and the impurity rate of raw fiber. The productivity of the prototype, the gum content of raw fiber and the bundle breaking tenacity of ramie fiber were examined (Among them, commissioned The Quality Supervision, Inspection and Test Center of Best Fiber Products, Ministry of Agriculter and Rual Affairs to test the gum content of raw fiber and the bundle breaking tenacity of ramie fiber).

(1)

Fiber percentage of fresh stalk

$$Z=W_r/W_j$$

(3)

where Z is fiber percentage of fresh stalk, %; W_r is mass of ramie fiber with moisture content of 14%, g; W_j is mass of ramie stem with out leaves, g.

(2)

Impurity rate of raw fiber

$$I=(W_1-W_2)/W_1$$

(4)

where I is impurity rate of raw fiber, %; W₁ is mass of ramie fiber before removing impurities, g; W₂ is mass of ramie fiber after removing impurities, g.

(3)

Production efficiency

$$E=W_r/t$$

(5)

where E is production efficiency, kg/h; t is the decortication time, s.

2.4.3. Box–Behnken Test Scheme

Ramie decortication testing was performed using a three-level and three-factor Box–Behnken test design method. In this study, the decortication clearance, the drum speed, and the conveyance speed were employed as the test factors, and the fiber percentage of fresh stalk and the impurity rate of raw fiber served as the assessment indices. Notably, the decortication clearance was 2.0~5.0 mm, the rotation speed test of the drum was performed with speeds ranging from 200~500 rpm, and the conveyance speed ranged from 0.24~0.48 m/s. The significance analysis of the factors of the test indicators was conducted in accordance with the test results, and the respective parameter combination was optimized following the mentioned actual demand and the parameter range, and a more appropriate factor combination was achieved.

During the experiment, 100 kg ramie stalks were stripped in their respective groups, the experiment was repeated 4 times, and the average of the results was taken.

Table 2. Experimental factors and levels.

Levels	Experimental Factor
e	Decorticating Clearance x1/mm	Drum Speed x2/(r·min−1)	Conveying Speed x3/(m·s−1)
−1	2.0	200	0.24
0	3.5	350	0.36
1	5.0	500	0.48

lists the coding of test factors.

3. Results and Discussion

3.1. Experiment Scheme and Results

Table 3. Box–Behnken experimental design.

Number.	Experimental Factor			Fiber Percentage of Fresh Stalk Y1/%	Impurity Rate of Raw Fiber Y2/%
	Decortication Clearance x1 (mm)	Drum Speed x2 (rpm)	Conveyance Speed x3 (m·s−1)
1	−1	−1	0	5.07	1.21
2	1	−1	0	5.68	1.89
3	−1	1	0	4.61	0.85
4	1	1	0	5.48	1.71
5	−1	0	−1	4.77	0.97
6	1	0	−1	5.54	1.74
7	−1	0	1	4.83	1.05
8	1	0	1	5.65	1.85
9	0	−1	−1	5.04	1.33
10	0	1	−1	4.76	0.97
11	0	−1	1	5.21	1.42
12	0	1	1	4.85	1.05
13	0	0	0	5.13	1.23
14	0	0	0	5.16	1.19
15	0	0	0	5.12	1.25
16	0	0	0	5.09	1.21
17	0	0	0	5.13	1.22

Note: x₁ and x₂ and x₃ indicate significance in the levels of decortication clearance, drum speed and conveyance speed, respectively. The same as below.

lists the test scheme and results. Design-Expert software was employed to conduct quadratic regression analysis and multiple regression fitting on the Box–Behnken test results [33,34,35]. A mathematical model was built between the fiber percentage of fresh stalk (Y₁), the impurity rate of raw fiber (Y₂) and the decortication clearance (x₁), the drum speed (x₂), and the conveyance speed (x₃) to examine their significance and analyze the interaction effect rule.

3.2. Analysis of Test Results

3.2.1. Regression Model and Significance Analysis of the Impurity Rate of Raw Fiber

Multiple regression fitting was performed on the test results, and a ternary quadratic polynomial regression model was built, comprising the fiber percentage of fresh stalk (Y₁), and the decortication clearance (x₁), the drum speed (x₂), and the conveyance speed (x₃). After the removal of insignificant items, the regression equation was yielded, expressed as in Equation (6). The significant items of the regression equation were examined (

Table 4. Variance analysis of fiber percentage of fresh stalk.

Source of Variance	Sum of Squares	Degrees of Freedom	Mean Square	F Value	p Value
Model	1.58	9	0.18	241.79	<0.0001 **
x1	1.18	1	1.18	1618.6	<0.0001 **
x2	0.21	1	0.21	290.24	<0.0001 **
x3	0.023	1	0.023	31.75	0.0008 **
x1x2	0.017	1	0.017	23.22	0.0019 **
x1x3	6.25 × 10−4	1	6.25 × 10−4	0.86	0.3849
x2x3	1.60 × 10−3	1	1.60 × 10−3	2.20	0.1817
x12	0.11	1	0.11	144.87	<0.0001 **
x22	0.023	1	0.023	31.89	0.0008 **
x32	0.032	1	0.032	43.53	0.0003 **
Residual error	5.10 × 10−3	7	7.28 × 10−4
Lack of fit	2.58 × 10−3	3	8.58 × 10−4	1.36	0.3741
Pure error	2.52 × 10−3	4	6.30 × 10−4
Total	1.59	16
R2 = 0.9968, R2adj = 0.9927, CV = 0.53%, Adeq precision = 52.799

Note: ** indicate significance at the 0.01 (p < 0.01) levels, respectively. R², R²_adj, CV and adeq precision indicate the significance of the coefficient of determination, the adjusted determination coefficient, the coefficient of variation, and adeq precision. The same as below.

).

Y₁ = 5.13 + 0.38x₁ − 0.16x₂ + 0.054x₃ + 0.065x₁x₂ + 0.16x₁² − 0.074x₂² − 0.087x₃²

(6)

As depicted in Table 4, when p < 0.0001, the regression model exerted a notable effect. The loss of fit term p = 0.3741 (p > 0.1) suggested that the regression equation of the soil fragmentation exhibited a high fitting degree, confirming that there were no other major factors of the test indices. A significant quadratic relationship was identified between the test indices and the test factors, and the analysis results were reasonable. The coefficient of determination was R² = 0.9968, suggesting that the regression equation model applied to 99.68% of the test data. The coefficient of determination was R² = 0.9968, and the adjusted R square reached R²_adj = 0.9927, both of which approached 1, suggesting that the fitting equation can take on a certain degree of significance.

As depicted in Table 4, the decortication clearance (x₁), the drum speed (x₂), and the conveyance speed (x₃) notably affected the fiber percentage of the fresh stalk (Y₁), and the quadratic term of the decortication clearance (x₁²), the quadratic term of the drum speed (x₂²), and the quadratic term of the conveyance speed (x₃²) significantly affected the raw fiber percentage (Y₁), while the inclination angle of decortication clearance and the drum speed (x₁x₂) remarkably affected the raw fiber percentage (Y₁). In a comprehensive analysis, the order of the degree of influence on the raw fiber percentage is: decortication clearance (x₁) > drum speed (x₂) > conveyance speed (x₃).

The response surface of the effect of significant and relatively significant interactions among the linear velocity of decortication clearance (x₁), the drum speed (x₂), the conveyance speed (x₃) on the test indices of the raw fiber percentage (Y₁) was obtained (Figure 5) through data processing with Design-Expert V8.0.6.1 software. The effects of the interactive factors x₁x₂, x₁x₃ and x₂x₃ on Y₁ were analyzed, respectively. At a constant decortication clearance (x₁), the raw fiber percentage (Y₁) decreased with drum speed (x₂), and the fiber percentage of fresh stalk (Y₁) decreased with conveyance speed (x₃). At a constant drum speed (x₂), the raw fiber percentage (Y₁) increased with decortication clearance (x₁), and the raw fiber percentage (Y₁) decreased with conveyance speed (x₃). At a constant conveyance speed (x₃), the raw fiber percentage (Y₁) increased with decortication clearance (x₁), and the raw fiber percentage of fresh stalk (Y₁) decreased with drum speed (x₂). At a conveyance speed (x₃) of 0.36 m/s, the decortication clearance (x₁) and the drum speed (x₂) reached 5 mm and 200 rpm, respectively, and the maximum value of the fiber percentage of fresh stalk (Y₁) was 5.7%.

3.2.2. Regression Model and Significance Analysis of the Raw Fiber Percentage

Multiple regression fitting was performed on the test results, and a ternary quadratic polynomial regression model was built, comprising the impurity rate of raw fiber (Y₂), the decortication clearance (x₁), and the drum speed (x₂), as well as the conveyance speed (x₃). After the removal of insignificant items, the regression equation was obtained, expressed as in Equation (7). The significant items of the regression equation were examined (

Table 5. Variance analysis of impurity rate of raw fiber.

Source of Variance	Sum of Squares	Degrees of Freedom	Mean Square	F Values	p Values
Model	1.61	9	0.18	187.39	<0.0001 **
x1	1.21	1	1.21	1267.88	<0.0001 **
x2	0.2	1	0.2	211.43	<0.0001 **
x3	0.016	1	0.016	16.99	0.0044 **
x1x2	8.10 × 10−3	1	8.10 × 10−3	8.49	0.0225 *
x1x3	2.25 × 10−4	1	2.25 × 10−4	0.24	0.642
x2x3	2.50 × 10−5	1	2.50 × 10−5	0.026	0.8759
x12	0.17	1	0.17	181.06	<0.0001 **
x22	2.37 × 10−4	1	2.37 × 10−4	0.25	0.6335
x32	1.68 × 10−3	1	1.68 × 10−3	1.77	0.2255
Residual error	6.68 × 10−3	7	9.54 × 10−4
Lack of fit	4.68 × 10−3	3	1.56 × 10−3	3.12	0.1504
Pure error	2.00 × 10−3	4	5.00 × 10−4
Total	1.61	16
R2 = 0.9959, R2adj = 0.9906, CV= 2.37%, Adeq precision = 46.234

Note: ** and * indicate significance at the 0.01 (p < 0.01) and 0.05 (p < 0.05) levels, respectively. R², R²_adj, CV and adeq precision indicate the significance of the coefficient of determination, the adjusted determination coefficient, the coefficient of variation, and the adeq precision. The same as below.

).

Y₂ = 1.22 + 0.39x₁ − 0.16x₂ + 0.045x₃ + 0.045x₁x₂ + 0.2x₁²

(7)

The analysis presented in Table 5 suggests that when p < 0.0001, the regression model exerted a significant effect. As indicated by the loss of fit term p = 0.1504 (p > 0.1), the regression equation exhibited a high fitting degree, confirming that there were no other major factors for the test indices. A significant quadratic relationship existed between the test indices and the test factors, and the analysis results were reasonable. The coefficient of determination reached R² = 0.9959, suggesting that the regression equation model applied to 99.59% of the test data. The coefficient of determination was R² = 0.9959, and the adjusted R square was R²_adj = 0.9906, both of which approached 1, suggesting that the fitting equation possessed a certain significance.

As indicated by the analysis presented in Table 5, the decortication clearance (x₁), the drum speed (x₂) and the conveyance speed (x₃) notably affected the impurity rate of raw fiber (Y₂), the quadratic term of the decortication clearance (x₁²), the quadratic term of the drum speed (x₂²) and the quadratic term of the conveyance speed (x₃²) exerted a notable effect on the impurity rate of raw fiber (Y₂), and the inclination angle of the decortication clearance and the drum speed (x₁x₂) significantly affected the impurity rate of raw fiber (Y₂). In a comprehensive analysis, the order of the degree of influence on the raw fiber percentage was: decortication clearance (x₁) > drum speed (x₂) > conveyance speed (x₃).

In this study, the response surface of the effect of significant and the relatively significant interactions among the linear velocity of the decortication clearance (x₁), the drum speed (x₂), and the conveyance speed (x₃) on the test indices of the impurity rate of raw fiber (Y₂) was determined (Figure 6). The effects of the interactive factors x₁x₂, x₁x₃ and x₂x₃ on Y₂ were analyzed, respectively. Under a constant decortication clearance (x₁), the impurity rate of raw fiber (Y₂) decreased with drum speed (x₂), and the impurity rate of raw fiber (Y₂) decreased with conveyance speed (x₃). At a constant drum speed (x₂), the impurity rate of raw fiber (Y₂) increased with decortication clearance (x₁), and the impurity rate of raw fiber (Y₂) increased with conveyance speed (x₃). At a constant conveyance speed (x₃), the impurity rate of raw fiber (Y₂) increased with decortication clearance (x₁), and the impurity rate of raw fiber (Y₂) increased with drum speed (x₂). At a conveyance speed (x₃) of 0.36 m/s, the decortication clearance (x₁) and the drum speed (x₂) reached 2.0 mm and 500 rpm, respectively, and the minimum value of the impurity rate of raw fiber (Y₂) was obtained as 0.8%.

3.3. Parameter Optimization and Verification Test

3.3.1. Parameter Optimization

The raw fiber percentage should be high, and the impurity rate of raw fiber should be low in order to achieve the optimal productivity of the transverse-feed ramie decorticator. Since the effect of the respective variables on the target value was not consistent, the global multi-objective optimization should be determined. With the fiber percentage of fresh stalk and the impurity rate of raw fiber as the objective function, the decortication clearance, drum speed and conveyance speed of the transverse-feed ramie decorticator were optimized, and the optimization constraint conditions were written as:

$$\left\{\begin{array}{c}\max Y_1\left(x_1,x_2,x_3\right)\\ \begin{array}{l}\min Y_2\left(x_1,x_2,x_3\right)\\ \mathrm{st}\left\{\begin{array}{c}2.0\le x_1\le 5.0\\ 250\le x_2\le 450\\ 0.24\le x_3\le 0.48\end{array}\right.\end{array}\end{array}\right.$$

(8)

The parameters were optimized using the optimization module of Design-Expert V8.0.6.1 software to achieve the optimal combination of working parameters. Under a decortication clearance in the transverse-feed ramie decorticator of 3.69 mm, the drum speed of the transverse-feed ramie decorticator reached 340.3 rpm, and the conveyance speed of the transverse-feed ramie decorticator was determined to be 0.37 m/s; the fiber percentage of the fresh stalk accounted for 5.19%, and the impurity rate of the raw fiber was 1.29% during the operation of the transverse-feed ramie decorticator. In accordance with the model optimization results and with the suitable adjustment of the testing parameters, the decortication clearance reached 3.7 mm, the drum speed was 340 rpm, and the conveyance speed was 0.37 m/s; the fiber percentage of fresh stalk still reached 5.19%, and the impurity rate of raw fiber was 1.29%.

3.3.2. Verification Test

In this study, we aimed to verify the accuracy of the mathematical model by means of Box–Behnken testing, along with the reliability of the optimization results, and to examine the productivity of the transverse-feed ramie decorticator, as well as the stability and reliability of the transverse-feed ramie decorticator. The ramie decortication testing of the transverse-feed ramie decorticator was performed at the National Ramie Germplasm Nursery, Wangcheng District, Changsha City (Figure 7). The experiment took place in July 2021. Zhongzhu No. 1 was the ramie variety employed for the experiment, and ramie stalks harvested the same day were selected for the experiment. A total of 1000 kg ramie was stripped during testing. The test was repeated four times, and the average value served as the test value.

Based on the model optimization results, the decortication clearance was 3.7 mm, the drum speed was 340 rpm, and the conveyance speed was 0.37 m/s. As indicated by the test results in

Table 6. Verification test results.

Items	Value of Validation Test	Predicted Value	Relative Errors/%
Fiber percentage of fresh stalk/%	5.05	5.19	2.7
Impurity rate of raw fiber/%	1.24	1.29	3.9

, the fiber percentage of the fresh stalk reached 5.05%, and the impurity rate of raw fiber was obtained as 1.24%, with relative errors with respect to the simulated values of 2.7% and 3.9%, respectively. Thus, the mean testing values were consistent with the optimized values, since the relative errors were all smaller than 5%, suggesting that the regression models were accurate. The productivity of the transverse-feed ramie decorticator was determined to be 78.5 kg/h. The impurity rate of raw fiber and the gum content of the raw fiber of the stripped ramie fiber were 1.24% and 23.45%, respectively, suggesting that ramie fibers of grade 2 had been obtained.

4. Conclusions

The transverse-feed ramie decorticator was designed in accordance with the operation characteristics of transverse-feed decortication technology and in combination with the processing requirements of ramie decortication to efficiently strip the ramie fibers.

To guarantee sufficient clamping force on the stalks and fibers without damaging the fibers, the device adopts a flexible clamping and conveyance device to realize the clamping and conveyance of ramie stalks and fibers. To improve the efficiency of ramie decortication, the rotation of the ramie decortication drum was matched to the concave ramie peeling plate to realize the automatic decortication of ramie fibers. In general, the ramie decortication operation of the transverse-feed ramie sectional decorticator comprises six processes, i.e., stem feeding and conveyance, base fiber decortication, semi fiber stem output, semi fiber stem feeding, tip fiber decortication, and fiber output. This study lays the theoretical basis and provides technical support for the development of a fully automatic ramie decorticator.

(1)

The mathematical regression model between the test indices and the factors was built through the bench test of the separation of ramie decortication. A three-factor and three-level regression orthogonal test was designed and then performed using the Box–Behnken central combination method, and a quadratic polynomial regression model was built on the basis of the three factors of the fiber percentage of fresh stalk and the impurity rate of raw fiber as the assessment indices of ramie decortication. The optimal parameters were determined through experiments, and comprised a decortication clearance of 3.7 mm, a drum speed of 340 rpm, and a conveyance speed of 0.37 m/s.

(2)

The ramie decortication testing of the transverse-feed ramie decorticator was performed using the optimized parameters. As indicated by the results of this study, the fiber percentage of fresh stalk reached 5.05%, and the impurity rate of raw fiber was 1.24%, with the relative errors of the simulated values reaching 2.7% and 3.9%, respectively. Thus, the mean testing values were consistent with the optimized values, since the relative errors were smaller than 5%, suggesting that the regression models are accurate.

(3)

The productivity of the transverse-feed ramie decorticator reached 78.5 kg·h⁻¹, which is 6~8 times that of the small ramie decorticator currently employed in the market. The impurity rate and the gum content of the raw fiber of the stripped ramie fiber were determined to be 1.24% and 23.45%, respectively, suggesting that the ramie fibers were of grade 2. The results conformed to the design requirements of the transverse-feed ramie decorticator, and the quality of the stripped fiber satisfied the requirements of textile enterprises and the market.

5. Discussion

The transverse-feed ramie decorticator developed in this study aimed at being able to perform the decortication of fibers of ramie stems without crushing them. The smoothing and feeding of the ramie stems were performed manually, the fiber decortication of the ramie stem tips can only be achieved by manually reversing the stems after the fiber decortication of the base of the ramie stem.

In a follow-up study, research and experiments on the continuous operation of these devices will be systematically performed in depth (e.g., fully automatic ramie smoothing device, fully automatic ramie crushing device, fully automatic ramie feeding device, and fully automatic ramie stem end changing device) to achieve the fully automatic operation of ramie decortication and increase the productivity of the transverse-feed ramie decorticator.

6. Patents

Patents for the transverse-feed ramie decorticator reported in this manuscript have been applied for in China (ZL202010113593.8, ZL202010112856.3, ZL202010112891.5; Application No. CN109355712A; CN109338478A).