A method for forming a center of gravity stabilizing hollow rotational molded composite material having a non-uniform wall thickness

Abstract

The present application relates to the field of rotational molding, and provides a molding method of a hollow rotational molding composite material with uneven wall thickness and stable center of gravity, which solves the technical problem that it is difficult to obtain a rotational molding product with uneven wall thickness and stable center of gravity without improving a mold in the prior art. The molding process provided by the present application can prepare a rotational molding composite material with stable center of gravity, no material defects, no bubbles, uneven wall thickness and large gradient by adjusting the properties of raw materials, rotational molding parameters and cooling methods. Moreover, the preparation process is simple, does not need to improve the mold, and is easy to realize industrial operation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of rotational molding, in particular to a molding method of a hollow rotational molding composite material with uneven wall thickness and stable center of gravity. BACKGROUND

[0002] Rotational molding is a processing method for processing plastic by using a rotating mold. In the processing process, plastic powder or slurry is poured into the mold, and the closed mold is rotated or oscillated on the equipment while the outside of the mold is heated by open flame or hot air. The plastic powder or slurry turns and flows inside the mold under the heating condition, gradually coats the inner surface of the mold after the temperature reaches the softening point, melts and coagulates, and the mold is cooled in the natural environment or by water, mist, wind and other media, and then the product is taken out after demolding.

[0003] Rotational molding products are generally hollow products with uniform wall thickness. In the prior art, a rotational molding product with uneven wall thickness is often made by wrapping a specific part of the mold with asbestos cloth, but the finished product has the disadvantages of non-obvious wall thickness gradient, unstable center of gravity, and material defects. In addition, with the help of the improvement of the mold, a rotational molding product with uneven wall thickness is obtained, such as the Chinese patent with the authorization announcement number CN209141351U, which discloses an injection mold. By improving the structure of the mold, the problem of molding of difficult products such as uneven wall thickness is solved, but the cost is high by structural improvement, and it still cannot meet the needs of people's production and life, and more methods for preparing products with uneven wall thickness are needed.

[0004] Therefore, it is urgent to provide a preparation method of a rotational molding material with uneven wall thickness and stable center of gravity to solve the problem of molding of difficult products such as uneven wall thickness, so as to meet the needs of people's production and life. SUMMARY

[0005] In order to solve the technical problem of rotational molding without improving the mold to obtain a rotational molding product with uneven wall thickness and stable center of gravity, the present application provides a molding method of a hollow rotational molding composite material with uneven wall thickness and stable center of gravity. The molding method provided by the present application can prepare a rotational molding composite material with smooth surface, no material defects, uneven wall thickness and large gradient, and stable center of gravity by adjusting the properties of raw materials, rotational molding parameters and cooling methods, and the preparation method is simple and does not need to improve the mold.

[0006] The specific technical scheme of the present application is as follows:

[0007] The present application provides a molding method of a hollow rotational molding composite material with uneven wall thickness and stable center of gravity, comprising the following steps:

[0008] (1) 97-99.98 parts of resin matrix and 0.02-3 parts of processing aids are mixed uniformly, granulated, and ground to obtain a powder. The powder flowability of the obtained powder is ≤ 35 s / 100 g, and the melt flow rate is ≥ 10 g / min.

[0009] (2) The powder is put into a mold, and then the mold is put into a rotomolding machine for rotomolding processing. The rotomolding machine parameters are set as follows: the main shaft speed is 12-18 rpm, and the auxiliary shaft speed is 20-30 rpm.

[0010] (3) The mold is cooled in a heating furnace, and kept rotating during the cooling process, demolded, to obtain a gravity-stable rotomolding composite material with uneven wall thickness and no missing material. The rotating parameters are as follows: the main shaft speed is 4-12 rpm, and the auxiliary shaft speed is 10-18 rpm.

[0011] The forming process of the rotomolding composite material provided by the application has the following advantages. Firstly, the powder flowability of the powder obtained by grinding the raw material is not more than 35 s / 100 g, and the melt flow rate is not less than 10 g / min. The melt flow rate of the powder for rotomolding needs to be relatively large. The larger the melt flow rate, the better the melt flowability, and the easier it is to flow from the sticking position to other positions under the guarantee of other related conditions, so that a gravity-stable product with uneven wall thickness is more likely to be formed.

[0012] Secondly, in the rotomolding heating forming stage, the main shaft speed of the rotomolding machine is controlled to be 12-18 rpm, and the auxiliary shaft speed is controlled to be 20-30 rpm. The mold is rotomolded at a relatively high speed, and at the same time, the speed is adjusted to a relatively small rate when the mold is cooled, to a main shaft speed of 4-12 rpm and an auxiliary shaft speed of 10-18 rpm. When rotomolding, the powder particles stick to the mold, and the longer the time required for the change of the sticking position under the relatively high rotomolding parameters, the faster the change of the powder melt position is accelerated when the rotation of the mold is adjusted to a relatively small rate in the cooling section. At the same time, as the temperature decreases in the cooling section, the material gradually solidifies. In the two continuous processes of rotomolding heating forming and cooling, the rotomolding mold rotation speed, the cooling mold rotation, and the melt flow rate of the material need to be matched with each other and adjusted and screened to a suitable range, so as to form a gravity-stable product with a smooth inner surface, no missing material, and uneven wall thickness, and the wall thickness is continuously and uniformly changed.

[0013] In addition, the cooling of the mold needs to be carried out at a slower rate, and the heat preservation cooling plays an important role in forming the gravity-stable rotational plastic composite material with uneven wall thickness. As preferred, the heat preservation cooling is carried out by using the waste heat in the heating furnace, which is an economical and feasible cooling method. Specifically, after the heating is turned off, the heating furnace has a high temperature of 250-400 DEG C, the waste heat can continue to heat and heat preservation of the mold, and then slowly cool. Therefore, the mold has a slower cooling rate in the heat preservation cooling in the heating furnace, and has the advantages of economy and green environmental protection.

[0014] Specifically, the resin matrix in step (1) is selected from polyethylene, polypropylene, nylon and polyurethane, or a compound of the above materials, or a modified material with flame-retardant, foaming, weather-resistant, cross-linking, dyeing functions of the above materials.

[0015] Specifically, the processing aid in step (1) is a mixture of antioxidants and release agents, and the mass ratio of the antioxidants and release agents is 1-2:1.

[0016] As preferred, the antioxidant is antioxidant 1076 and / or antioxidant 168, and the release agent is zinc stearate.

[0017] Specifically, in step (1), the granulation method is: adding a double-screw granulator, and granulating at a temperature of 50-300 DEG C, a speed of 400-600 rpm, and a die head pressure of 0.1-0.5 MPa.

[0018] Specifically, the method of grinding in step (1) is mechanical grinding or liquid nitrogen freezing grinding. Further, the method of mechanical grinding is: adding a plastic grinding machine, and grinding with the parameters of "speed 25-300 Kg / h, grinding disc speed 2000-4000 rpm, and grinding disc outlet temperature ≤70 DEG C".

[0019] Specifically, the mixing method in step (1) is: adding a mixer, and mixing at a speed of 5-80 rpm for 3-10 min.

[0020] As preferred, the mold is a spherical or ellipsoidal type.

[0021] Compared with the prior art, the present application has the following technical effects:

[0022] (1) The present application can prepare a gravity-stable rotational plastic composite material with smooth surface, no material defect, uneven wall thickness and large gradient by adjusting the properties of raw materials, rotational plastic parameters and cooling methods.

[0023] (2) The preparation method of the present application has simple steps and does not need to improve the mold, which solves the technical problem that the gravity-stable rotational plastic product with uneven wall thickness cannot be obtained without improving the mold in rotational plastic molding.

[0024] (3) The present application does not need complicated equipment and is easy to realize industrial operation. DETAILED DESCRIPTION

[0025] The present application is further described below in conjunction with examples.

[0026] Example 1

[0027] (1) The weight ratio of antioxidant 1010: antioxidant 168: zinc stearate = 1:2:2 is mixed into a processing aid.

[0028] (2) 19.6 Kg of polyolefin elastomer (8730, SK, Korea) and 0.4 Kg of the processing aid are mixed by a high-speed mixer at 60 rpm for 4 min, the temperature of each section of the extruder is set to 50-180°C, the screw rotation speed is 400 rpm, the head pressure is 0.2 MPa, and the powder is obtained by melt blending in a twin-screw extruder to form particles and then grinding in liquid nitrogen.

[0029] (3) The powder is put into a mold for rotational molding processing. The rotational molding processing parameters are as follows: the main shaft rotation speed is 15 rpm, and the auxiliary shaft rotation speed is 25 rpm.

[0030] (4) The mold is cooled in a heating furnace, and the main shaft rotation speed is kept at 10 rpm and the auxiliary shaft rotation speed is kept at 14 rpm during the cooling process, and then the rotational molding composite material with stable center of gravity, uneven wall thickness, and no material defects is obtained by demolding.

[0031] Example 2

[0032] (1) The weight ratio of antioxidant 1010: antioxidant 168: zinc stearate = 1:1:2 is mixed into a processing aid.

[0033] (2) 19.9 Kg of polyethylene (8920, Fujian United) and 0.1 Kg of the processing aid are mixed by a high-speed mixer at 80 rpm for 3 min, the temperature of each section of the extruder is set to 120-230°C, the screw rotation speed is 500 rpm, the head pressure is 0.5 MPa, and the powder is obtained by melt blending in a twin-screw extruder to form particles; the plastic powder grinder is added, and the parameters of “speed 25 Kg / h, grinding disc rotation speed 2000 rpm, and grinding disc outlet temperature 70°C” are used for grinding, so as to obtain the powder with a flowability of 33 s / 100 g and a melt flow rate of 20 g / min.

[0034] (3) The powder is put into a spherical mold for rotational molding processing. The rotational molding processing parameters are as follows: the main shaft rotation speed is 12 rpm, and the auxiliary shaft rotation speed is 30 rpm.

[0035] (4) The mold is cooled in the heating furnace, and the main shaft rotation speed is kept at 4 rpm and the secondary shaft rotation speed is kept at 10 rpm during the cooling process, and the product is demolded to obtain the gravity-stable rotational molding composite material with uneven wall thickness and no material defects.

[0036] Example 3

[0037] (1) The weight ratio of antioxidant 1010: antioxidant 168: zinc stearate = 1:2:2 is mixed into a processing aid.

[0038] (2) 19.4 Kg of polypropylene (1250F, Taiwan Polymer) and 0.7 Kg of the processing aid are mixed by a high-speed mixer at 5 rpm for 10 min, the temperature of each section of the extruder is set to 150-240°C, the screw rotation speed is 600 rpm, the die head pressure is 0.1 MPa, and the powder is obtained by melt blending in a twin-screw extruder to form particles and then freezing and grinding in liquid nitrogen, wherein the powder flowability is 32 s / 100 g and the melt flow rate is ≥10 g / min.

[0039] (3) The powder is put into an ellipsoidal mold for rotational molding processing, wherein the main shaft rotation speed is 18 rpm and the secondary shaft rotation speed is 20 rpm.

[0040] (4) The mold is cooled in the heating furnace, and the main shaft rotation speed is kept at 12 rpm and the secondary shaft rotation speed is kept at 18 rpm during the cooling process, and the product is demolded to obtain the gravity-stable rotational molding composite material with uneven wall thickness and no material defects.

[0041] Comparative Example 1 (The main difference from Example 1 is that the powder flowability of the powder obtained by grinding is 40 s / 100 g, and the melt flow rate is 5 g / min)

[0042] (1) The weight ratio of antioxidant 1010: antioxidant 168: zinc stearate = 1:2:2 is mixed into a processing aid.

[0043] (2) 19.6 Kg of polyolefin elastomer (8730, SK, Korea) and 0.4 Kg of the processing aid are mixed by a high-speed mixer at 60 rpm for 4 min, the temperature of each section of the extruder is set to 50-180°C, the screw rotation speed is 400 rpm, the die head pressure is 0.2 MPa, and the powder is obtained by melt blending in a twin-screw extruder to form particles and then freezing and grinding in liquid nitrogen, wherein the powder flowability is 40 s / 100 g and the melt flow rate is 5 g / min.

[0044] (3) The powder is put into a mold for rotational molding processing, wherein the main shaft rotation speed is 15 rpm and the secondary shaft rotation speed is 25 rpm.

[0045] (4) The mold is cooled in the heating furnace, and the main shaft rotation speed is kept at 10 rpm and the sub-shaft rotation speed is kept at 14 rpm during the cooling process, and the mold is demolded to obtain a rotational molding composite material.

[0046] Comparative Example 2 (The main difference from Comparative Example 1 is that the main shaft rotation speed of the rotational molding machine is 22 rpm and the sub-shaft rotation speed is 35 rpm during the rotational molding process)

[0047] (1) The antioxidant 1010:antioxidant 168:zinc stearate = 1:2:2 by weight are mixed into a processing aid.

[0048] (2) The polyolefin elastomer (8730, SK, Korea) 19.6 Kg and the processing aid 0.4 Kg are mixed by a high-speed mixer at 60 rpm for 4 min, the temperature of each section of the extruder is set to 50-180°C, the screw rotation speed is 400 rpm, and the die head pressure is 0.2 MPa. The melt blending is made into particles by a twin-screw extruder, and the particles are ground by liquid nitrogen to obtain a powder with a flowability of 33 s / 100 g and a melt flow rate of 30 g / min.

[0049] (3) The powder is put into a mold for rotational molding. The rotational molding parameters are that the main shaft rotation speed is 22 rpm and the sub-shaft rotation speed is 35 rpm.

[0050] (4) The mold is cooled in the heating furnace, and the main shaft rotation speed is kept at 10 rpm and the sub-shaft rotation speed is kept at 14 rpm during the cooling process, and the mold is demolded to obtain a rotational molding composite material.

[0051] Comparative Example 3 (The main difference from Comparative Example 1 is that the main shaft rotation speed of the rotational molding machine is 8 rpm and the sub-shaft rotation speed is 15 rpm during the rotational molding process)

[0052] (1) The antioxidant 1010:antioxidant 168:zinc stearate = 1:2:2 by weight are mixed into a processing aid.

[0053] (2) The polyolefin elastomer (8730, SK, Korea) 19.6 Kg and the processing aid 0.4 Kg are mixed by a high-speed mixer at 60 rpm for 4 min, the temperature of each section of the extruder is set to 50-180°C, the screw rotation speed is 400 rpm, and the die head pressure is 0.2 MPa. The melt blending is made into particles by a twin-screw extruder, and the particles are ground by liquid nitrogen to obtain a powder with a flowability of 33 s / 100 g and a melt flow rate of 30 g / min.

[0054] (3) The powder is put into a mold for rotational molding. The rotational molding parameters are that the main shaft rotation speed is 8 rpm and the sub-shaft rotation speed is 15 rpm.

[0055] (4) The mold is cooled in the heating furnace, and the main shaft rotation speed is kept at 10 rpm and the sub shaft rotation speed is kept at 14 rpm during the cooling process, and the product is demolded to obtain a rotational molding composite material.

[0056] Comparative Example 4 (The main difference from the Example 1 is that the mold cooling method is water mist cooling outside the furnace, and the mold does not rotate)

[0057] (1) The antioxidant 1010:antioxidant 168:zinc stearate = 1:2:2 by weight is mixed into a processing aid.

[0058] (2) The polyolefin elastomer (8730, SK, Korea) 19.6 Kg, the processing aid 0.4 Kg are mixed by a high-speed mixer at 60 rpm for 4 min, the temperature of each section of the extruder is set to 50-180°C, the screw rotation speed is 400 rpm, the head pressure is 0.2 MPa, and the powder is obtained by melt blending into particles by a twin-screw extruder and grinding by liquid nitrogen.

[0059] (3) The powder is put into the mold for rotational molding processing. The rotational molding processing parameters are that the main shaft rotation speed is 15 rpm and the sub shaft rotation speed is 25 rpm.

[0060] (4) The mold is cooled by water mist outside the heating furnace, and the main shaft rotation speed is kept at 10 rpm and the sub shaft rotation speed is kept at 14 rpm during the cooling process, and the product is demolded to obtain a rotational molding composite material.

[0061] Comparative Example 5 (The main difference from the Example 1 is that the mold does not rotate when the mold is kept warm in the heating furnace)

[0062] (1) The antioxidant 1010:antioxidant 168:zinc stearate = 1:2:2 by weight is mixed into a processing aid.

[0063] (2) The polyolefin elastomer (8730, SK, Korea) 19.6 Kg, the processing aid 0.4 Kg are mixed by a high-speed mixer at 60 rpm for 4 min, the temperature of each section of the extruder is set to 50-180°C, the screw rotation speed is 400 rpm, the head pressure is 0.2 MPa, and the powder is obtained by melt blending into particles by a twin-screw extruder and grinding by liquid nitrogen.

[0064] (3) The powder is put into the mold for rotational molding processing. The rotational molding processing parameters are that the main shaft rotation speed is 15 rpm and the sub shaft rotation speed is 25 rpm.

[0065] (4) The mold is cooled in the heating furnace, and the product is demolded to obtain a rotational molding composite material.

[0066] Comparative Example 6 (The main difference from Comparative Example 1 is that the main shaft rotation speed is 16 rpm and the sub shaft rotation speed is 20 rpm during the heat preservation cooling in the heating furnace of the mold)

[0067] (1) The weight ratio of antioxidant 1010:antioxidant 168:zinc stearate = 1:2:2 was mixed into a processing aid.

[0068] (2) 19.6 Kg of polyolefin elastomer (8730, SK, Korea) and 0.4 Kg of the processing aid were mixed by a high-speed mixer at 60 rpm for 4 min, the temperature of each section of the extruder was set to 50-180°C, the screw rotation speed was 400 rpm, the head pressure was 0.2 MPa, and a double-screw extruder was used for melt blending to produce granules, which were then ground by liquid nitrogen, thereby obtaining a powder with a flowability of 33 s / 100 g and a melt flow rate of 30 g / min.

[0069] (3) The powder was put into a mold for rotational molding processing. The rotational molding processing parameters were that the main shaft rotation speed was 15 rpm and the sub shaft rotation speed was 25 rpm.

[0070] (4) The mold was cooled in the heating furnace, and the main shaft rotation speed was kept at 16 rpm and the sub shaft rotation speed was kept at 35 rpm during the cooling process, and then the mold was demolded, thereby obtaining a rotational molding composite material.

[0071] Comparative Example 7 (The main difference from Comparative Example 1 is that the main shaft rotation speed is 2 rpm and the sub shaft rotation speed is 5 rpm during the heat preservation cooling in the heating furnace of the mold)

[0072] (1) The weight ratio of antioxidant 1010:antioxidant 168:zinc stearate = 1:2:2 was mixed into a processing aid.

[0073] (2) 19.6 Kg of polyolefin elastomer (8730, SK, Korea) and 0.4 Kg of the processing aid were mixed by a high-speed mixer at 60 rpm for 4 min, the temperature of each section of the extruder was set to 50-180°C, the screw rotation speed was 400 rpm, the head pressure was 0.2 MPa, and a double-screw extruder was used for melt blending to produce granules, which were then ground by liquid nitrogen, thereby obtaining a powder with a flowability of 33 s / 100 g and a melt flow rate of 30 g / min.

[0074] (3) The powder was put into a mold for rotational molding processing. The rotational molding processing parameters were that the main shaft rotation speed was 15 rpm and the sub shaft rotation speed was 25 rpm.

[0075] (4) The mold was cooled in the heating furnace, and the main shaft rotation speed was kept at 2 rpm and the sub shaft rotation speed was kept at 5 rpm during the cooling process, and then the mold was demolded, thereby obtaining a rotational molding composite material.

[0076] Performance test

[0077] The performance test and characterization were carried out on the rotational molding composite materials prepared from examples 1-3 and comparative examples 1-7, and the results are shown in Table 1. Among them, the thickest part wall thickness / average wall thickness, that is, the ratio of the thickest part wall thickness to the average wall thickness, the greater the value, the more stable the center of gravity. The evaluation standard of the stability of the center of gravity is that the value of the thickest part wall thickness / average wall thickness is greater than 3.0, which is excellent, the value of the thickest part wall thickness / average wall thickness is in the range of 2.0-3.0, which is good, and the value of the thickest part wall thickness / average wall thickness is less than 2.0, which is poor.

[0078] Table 1

[0079]

[0080] Data analysis

[0081] ①From the performance test results of each example, it can be seen that the rotational molding composite material prepared by the present application has uniform thickness variation, excellent center of gravity stability, and surface performance of no material shortage and no bubbles.

[0082] ②Compared with example 1, the powder flowability of the rotational molding material powder of comparative example 1 is greater than 35s / 100g, and the melt flow rate is less than 10g / min. The rotational molding composite material prepared therefrom has uniform thickness and poor center of gravity stability, indicating that in order to prepare a rotational molding composite material with uneven wall thickness and large gradient and excellent center of gravity stability, the powder flowability of the rotational molding powder should be controlled to be not greater than 35s / 100g, and the melt flow rate should be not less than 10g / min.

[0083] ③Compared with example 1, the rotational molding process of comparative example 2 has a main shaft rotation speed of 22rpm and a secondary shaft rotation speed of 35rpm, which is too fast. The rotational molding composite material prepared therefrom has uniform thickness and poor center of gravity stability, indicating that in order to prepare a rotational molding composite material with uneven wall thickness and large gradient and excellent center of gravity stability, the rotational molding machine rotation parameters should be adjusted within a certain range, and should not be too fast.

[0084] ④Compared with example 1, the rotational molding process of comparative example 3 has a main shaft rotation speed of 8rpm and a secondary shaft rotation speed of 15rpm, which is too slow. The rotational molding composite material prepared therefrom has a steep change in thickness and poor center of gravity stability, indicating that in order to prepare a rotational molding composite material with uniform wall thickness variation and large gradient and excellent center of gravity stability, the rotational molding machine rotation parameters should be adjusted within a certain range, and should not be too slow.

[0085] ⑤Compared with example 1, the mold cooling method of comparative example 4 is water mist cooling outside the furnace, which is too fast, and the mold does not rotate during the cooling process. The rotational molding composite material prepared therefrom has uniform thickness and poor center of gravity stability, indicating that in order to prepare a rotational molding composite material with uneven wall thickness and large gradient and excellent center of gravity stability, the cooling speed should not be too fast, and the mold should be kept rotating.

[0086] Compared with Example 1, the mold of Comparative Example 5 does not rotate during the heat preservation and cooling in the mold heating furnace, and the rotational plastic composite prepared thereby has uneven surface, material defects, abrupt thickness change and poor center of gravity stability, indicating that, in order to prepare the rotational plastic composite with uniform wall thickness change, large gradient and excellent center of gravity stability, in addition to slow cooling rate of the mold, the rotation of the mold should also be maintained.

[0087] Compared with Example 1, the mold of Comparative Example 6 rotates at a main shaft rotation speed of 15 rpm and a secondary shaft rotation speed of 25 rpm during the heat preservation and cooling in the mold heating furnace, and the rotational plastic composite prepared thereby has material defects, uniform thickness and poor center of gravity stability, indicating that, in order to prepare the rotational plastic composite with non-uniform wall thickness change, large gradient and excellent center of gravity stability, in addition to slow cooling rate of the mold, the rotation speed of the mold should also not be too fast.

[0088] Compared with Example 1, the mold of Comparative Example 7 rotates at a main shaft rotation speed of 2 rpm and a secondary shaft rotation speed of 5 rpm during the heat preservation and cooling in the mold heating furnace, and the rotational plastic composite prepared thereby has uneven surface, material defects, abrupt thickness change and poor center of gravity stability, indicating that, in order to prepare the rotational plastic composite with uniform wall thickness change, large gradient and excellent center of gravity stability, in addition to slow cooling rate of the mold, the rotation speed of the mold should also not be too slow.

[0089] The raw materials and equipment used in the present application are conventional raw materials and equipment in the art, unless otherwise specified; the methods used in the present application are conventional methods in the art, unless otherwise specified.

[0090] The above description is only a preferred embodiment of the present application, and does not limit the present application in any way. Any simple modification, change and equivalent transformation of the above embodiment based on the technical essence of the present application are still within the protection scope of the technical solution of the present application.

Claims

1. A method of forming a gravity stabilized hollow roto-molded composite material having a non-uniform wall thickness, the method comprising: The method comprises the following steps: ​ (1) uniformly mixing the rotational molding raw material, granulating, and powdering to obtain a powder with a powder flowability of ≤ 35 s / 100 g and a melt flow rate of ≥ 10 g / min; (2) putting the powder into a mold, and then placing the mold into a rotational molding machine for rotational molding processing; The rotational molding machine is set at a main shaft rotating speed of 12-18 rpm and a secondary shaft rotating speed of 20-30 rpm; (3) cooling the mold in a rotating state, demolding, and obtaining a gravity-stable hollow rotational molding composite material with uneven wall thickness and no material defects; The rotating state is that the main shaft rotating speed of the rotational molding machine is 4-12 rpm and the secondary shaft rotating speed is 10-18 rpm.

2. A method of molding a biaxially oriented hollow roto-molded composite material having a non-uniform wall thickness and a center of gravity stabilization according to claim 1, characterized in that: In step (1), the rotational molding raw material comprises a resin matrix and a processing aid.

3. A method of molding a biaxially oriented hollow roto-molded composite material having a non-uniform wall thickness and a center of gravity stabilization according to claim 2, characterized in that: The resin matrix is selected from polyethylene, polypropylene, nylon, and polyurethane, or a compound of the above materials, or a modified material with the functions of flame retardation, foaming, weather resistance, crosslinking, and dyeing of the above materials.

4. A method of molding a biaxially oriented hollow roto-molded composite material having a non-uniform wall thickness and a center of gravity stabilization according to claim 2, characterized in that: The processing aid is a mixture of an antioxidant and a release agent, and the mass ratio of the antioxidant to the release agent is 1-2:

1.

5. A method of molding a biaxially oriented hollow roto-molded composite material having a non-uniform wall thickness and a center of gravity stabilization according to claim 4, characterized in that: The antioxidant is antioxidant 1076 and / or antioxidant 168.

6. A method of molding a biaxially oriented hollow roto-molded composite material having a stable center of gravity with non-uniform wall thickness as defined in claim 4, wherein: The release agent is zinc stearate.

7. A method of molding a biaxially oriented hollow roto-molded composite material having a stable center of gravity with non-uniform wall thickness as defined in claim 1, wherein: In step (1), the granulation method is as follows: adding a double-screw granulator, and granulating at a temperature of 50-300 ℃, a rotating speed of 400-600 rpm, and a die head pressure of 0.1-0.5 MPa.

8. A method of molding a biaxially oriented hollow roto-molded composite material having a stable center of gravity with non-uniform wall thickness as defined in claim 1, wherein: In step (1), the powdering method is mechanical powdering or liquid nitrogen freezing powdering.

9. A method of molding a biaxially oriented hollow roto-molded composite material having a stable center of gravity with non-uniform wall thickness as defined in claim 1, wherein: In step (1), the mixing method is as follows: adding a mixer, and mixing at a rotating speed of 5-80 rpm for 3-10 min.

10. A method of molding a biaxially oriented hollow roto-molded composite material having a center of gravity stabilization according to claim 1, characterized in that: The mold is a spherical or ellipsoidal mold.

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