Rotational molding technology

Rotational molding technologyAuthors: Ji'nan ordnance 53 Zhang Yulong The basic principles of rotational mouldingRotational molding technology also known as rotary or rotary molding technology. It is the plastic powder into the mould, the powder in the mold around the two mutually perpendicular axes, while heating while rotation. Because in the rotation process, powder changes unceasingly, mutual friction and the surface of the mould powder drag and drop between each other, make powder continuously to the mold wall above the movement, until the motion force sufficient to overcome the powder under the action of gravity, powder can cross the powder pool reaches across the die wall, with the mould powder to do a 360 degree rotation. After repeated heating rotary movement repetition, powder will melt and uniformly adhered on the inner wall of the mould fused into one, to finalize the design cooling a desired thickness, hollow products can be taken out from the mould. Compared with the traditional method of hollow plastic molding, rotational molding has incomparable advantages in equipment investment cost and manufacturing cycle, process flexibility, become small batch production of medium and large or super large closed or semi closed hollow seamless containers are the most ideal, the most economical and effective method. Rotational molding process has outstanding characteristics. Rotational molding process and less investment in equipment, mould processing and low cost, the comprehensive cost of products with low volume production. Raw material saving, almost 100% of the material can be converted into products. Mold no runner, gate, does not produce waste, and the product of uniform wall thickness, chamfer at slightly thickened, can give full play to the material performance.Rotational molding technology suitable for the production of large and super large products, suitable for molding a variety of complex hollow shape of parts. In the rotational molding process, material is gradually applied to the mold surface, have very strong ability to copy on the cavity surface pattern and other fine structure, at the same time as the mold without pressure during the whole process, can be directly used precision die casting methods made of a variety of fine and complex in forming such as toys, animal model of all kinds of resin based composite material products.Rotational molding process is also suitable for resin based composite material product production of multilayer material. Rotational molding multi-layer products, just different melting point will be reasonable matching, the resin into the mold, forming process of low melting temperature of the material is first heated to melt, adhesion to the cavity wall, the outer layer formed parts. After the high melting temperature materials and melting, and attached to the outer layer of material is formed on the inner or outer layer, first resin into mould, rotational molding for parts of the outer layer, and then join the inner resin, obtain a multi-layer rotary molding.Rotational molding products made without stress, almost no orientation or residual strain, also do not cause depression, such as deformation and cracking problem.Rotational molding products suitable for multi variety, small batch of resin based composite material production, and easy to transform composite color. In addition, the molding process is simple, strong adaptability, easy to change die. Equipment, chess has the advantages of simple structure, convenient manufacture, rapid, low price, thus to transform the product very favorable.A rotary molding machine, in the production process, but also can product very different size and shape and forming.Rotational molding also has some disadvantages, such as high energy consumption, the molding cycle is longer, molded product size precision is poor, large labor intensity etc..Equipment and mouldRotational molding equipment in general by the transmission system, heating system, cooling system and control system etc.. Rotational molding machine for single arm traditional biaxial (perpendicular) rotation, single station (there are 2 ~ 4 station). Driving system consists of motor, speed governor, vice principal, rotating shaft etc.. The heating system is composed of a heating furnace, burner, an ignition device etc.. The heating medium is liquefied petroleum gas, coal gas, diesel oil etc.. Cooling system consists of water supply pipe and a nozzle. The heated die through air, water mist, water spray, the uniform cooling. The characteristics of rotational molding machine is simple in structure, low price, stable transmission, strong adaptability of products, can meet the needs of medium and small batch production.Compared with injection molding, blow molding and other plastic molding die, rotational molding with mould has the advantages of simple structure, the characteristics of large volume, low weight, simple and cheap production. Rotational mould by ordinary low carbon steel (also available aluminum) welding process, divided into 2.5 modules, the use of fixture is fixedly connected. In order to prevent the deformation of product, mould must open the exhaust hole. On the premise of not affecting the strength and deformation, die thickness as thin as possible, generally use 3 ~ 6mm steel is appropriate. Products flat portion is greater than 400mm, the mold should consider strengthening ribs or corresponding structure design.Molding resinRotational molding with resin often need to pre processing. For example, the solid granular resin often need to pre crushed into powder material, so that the resin price rise. But the rotational molding of less waste, resin prices rise part can thus be compensated. Can be used for resin rotational moulding of polyolefin, polyester, nylon, polycarbonate, EV A, ABS, PS and thermosetting resin, polyethylene powder in twentieth Century 60's development has injected vitality to the development of rotational molding processing, until today, 90% rotational molding materials used for all grades of polyethylene resin still.Because of the particularity of rotational molding technology, has brought the special requirements for the selection of raw materials. First, since there is no pressure molding, the used raw materials must have good flowability, so the resin particle size is 100 ~ 500 m, the melt flow rate at 4 ~ 8g / 10min is appropriate. Because the molding cycle is longer, the raw material to withstand high temperature, thus better thermal stability. Strength and impact toughness in the selection of the resin is also a very important factor, linear low density polyethylene can satisfy this requirement, so it has been widely used. In recent years, metallocene catalyst polyethylene provides excellent raw materials for rotational molding.Rotational molding technologyThe technological process of rotational molding: ingredients, loading in mould, die rotating and heating, cooling, demoulding and mould rotation.Heating method of rotational moulding with a hot air circulating and direct flame heating, the molten salt heating, heating medium and infrared radiation heating. Hot air circulation is will be circulating air heating as the heat source for abrasives. Direct flame heating method is the direct injection fuel flame to rotary mould. These two methods are widely used, the heating temperature is 200 DEG to 500 deg.. Hot air circulating generally with the gas furnace heating. Direct flameheating with fuel gas, liquefied gas, heavy oil, coal etc.. Molten salt heating is the heating and melting of 200 DEG C to about 300 DEG C molten salt spray directly blowing in rotary die outer surface. The uniform heating, fast heat transfer, so the production cycle is short. Heating medium for the complex structure of rotational molding mould, heating cycle is short, easy temperature control, high heat utilization rate. Infrared radiation heating method is advanced, heating does not require any heat transfer medium, the operation is convenient, fast heating speed, no pollution, the law in Japan has. In short, rotational molding methods are diverse, should be used to determine the heating mode according to the specific conditions of the local areas, departments.Rotational molding cooling method and cooling speed have great influence on the quality of rotational molding products, it is not only related to product size stability and appearance quality, internal performance, but also directly affect the product so, must strictly control.Rotational molding process parameter refers to the molding temperature, time, cooling speed. Because the rotary form and the different heating methods, process parameters vary greatly. Especially because it is more so direct flame heating, temperature control in 250 ~ 350 DEG C. Heating time varied with the product thickness, size, usually 25 ~ 50min, method of cooling by cold air and water binding, generally in about 25 ~ 40min.The main points of the process should pay attention to the rotational molding:There is a close relationship between the rotational molding rotational molding materials - and powder particle size distribution.- rotational molding process have great effects on the performance of forming products, in the process, select the appropriate process parameters is very important.Adopt the heating temperature, the suitable heating time and improved the performance of the material impact.In the higher heating temperature, heating time is longer under the conditions of crosslinking reaction, mainly in surface materials.The production process of products - different crosslinking degree (crystallinity) without significant difference.Rotational molding of thermosetting resin matrix compositeThermosetting composite rotational has two methods: pre mixing method and pre forming method.The following example illustrates with unsaturated polyester premixed legal steps:(1) the preparation of the resin system: according to the conventional formula of thermoset composites, joined the amount of curing agent, catalyst, pigment and other additives in seasoning or unsaturated polyester resin glue solution, then mixing evenly.(2) impregnated fiber reinforced materials: chopped fiber into resin system, fully stirring, mixing evenly after prepared with rotational molding material, this material for short fiber shape. It should be noted, short cut fiber before joining, surface treatment to deal with fiber. Or will cause the resin / fiber interface quality is not high.(3) the mixed material to join rotary abrasive, according to unsaturated polyester molding process conditions of heating, the rotational molding material, to be evenly attached to the cavity, and then finished the resin gel, curing reaction after cooling.(4) to be cooled to room temperature, can open mold, out products.(5) the products after processing, such as removing the flash, burrs, processing heat in, can be contained or put into use.The forming method has the advantages of simple process, low labor intensity, good production environment, but the product of glass fiber content is low, are generally not more than 20%, the length of glass fiber is less than 25mm. So the resulting low product mechanical strength, cannot make bearing structure products.This method is suitable for the preparation of hollow products that use hollow blow molding difficult molding, such as tanks, cans and other containers.The production process of pre forming process is divided into two steps, the first step to use glass fiber mixed with the thermoplastic resin molded blank, then adding unsaturated polyester mixed system, and then the two rotational molding system products. The specific operation steps:(1) the selection of the length of the 25mm surface treated glass fiber, or articles according to size or length of glass fiber length decision.(2) the selection of excellent adhesion properties of thermoplastic resin, such as ABS, acrylic acid, polystyrene, polyurethane and other resin. The thermoplastic resin not only has excellent adhesion and fiber, but also has good compatibility of unsaturated polyester.(3) the glass fiber prepared with 30% content join cavity, and then adding the thermoplastic resin cavity, and the hybrid 5~10min, mixed evenly.(4) according to the selected thermoplastic resin process conditions, the mixture is heated, the heating time is 10~15min.(5) a preform thickness up to 3~5mm, can be cooled to finalize the design, cooling time is 3~5min, the stand-by thermosetting resin impregnated.(6) the selection of viscosity of 0.6Pa * s unsaturated polyester catalyst, adding proper amount of room temperature curing, selection of methyl ethyl ketone peroxide, heating and solidifying the selection of benzoyl peroxide, after 3~5min mixing evenly add cavity is provided with a preform.(7) rotates the unsaturated polyester glue impregnation and curing 15~20min.(8) the product is fully cured and then cooled and demoulded, take out the products, the forming period of about 45~60min.(9) the article in a 120 DEG ~160 DEG 6~24h heat treatment.(10) cleaning mold, prepared for rotational molding next time.The main advantage of pre formed a:- can use long more than 25mm of fiber, fiber content can be increased to 40%~50%, fiber distribution, no product performance orientation, can be comparable with the spray molding and hand lay up molding products.Can be large and complex shape product molding.- a good production environment, labor intensity is low, the workplace without harmful gas and glass fiber dust.- products can be used for heating and solidifying, shorten the molding cycle.Hollow products, - rotational molding of large containers and other mechanical properties, dimensional stability are better than the blow molding products, can be used as load bearing structure.Rotational foaming molding technologyContinuous rotational foaming molding is generally divided into two steps, multi step and step. 1, two step methodTwo step method of rotational foaming molding technology is developed on the basis of intermittent rotational foaming molding on. The two step forming process of most of the input boxis placed a pre loaded rotational moulding foaming resin powder in the. When the non foaming resin powder mold covering the mold wall began to melt, open the input box, release the foaming resin powder, foaming inner core can be formed in the non foaming layer of dense on. The operator can also use high melting point of the thermoplastic plastic into the bag, bag in investment in advance into the foaming resin powder, when the mold temperature reaches the melting temperature of thermoplastic plastic bag, put into the bag melt rupture, releasing resin powder.Two step although not interrupt processing, but how to correctly open input box or how to accurately control temperature makes the input bag complete rupture, the full release of non foaming resin is the most difficult part of the method. At the same time, processing and manufacturing into the box or put into bags will undoubtedly increase the cost of production, and put forward higher requirements of control system of the equipment.2, multi step methodMulti step method is generally used for the production of multi-layer products, the working principle and the two step is similar. If the foam on the inner wall and a layer of foam layer, second input box or a plurality of thermoplastic into the bag is used in foaming, already formed in the surface of the core layer release of non foaming resin, forming a non foam layer.The two step and multi step problems still exist, the need for a large number of complex equipment, transformation of the old equipment rotomolding process conditions difficult to control. In order to maximize the use of the original rotational molding equipment, people developed a one-step rotational foaming molding technology.3, a stepRotational molding rotational foaming molding equipment one-step method basically can directly use traditional, without the need for additional equipment transformation, production process is relatively simple. Compared with the traditional rotational molding equipment, one-step rotational foaming equipment without the need for large-scale transformation, but the heating and cooling, the temperature control system is put forward higher requirements. Requirements for accurate temperature control, high heat conduction efficiency. At present, due to a step rotational foaming molding technology has obvious advantages in the aspects of equipment, technology and quality products, has developed rapidly in recent years, will become the rotational foaming molding technology in the future is the most important, most has the development prospect of application. In order to obtain a dense non foaming and foaming inner core shell, one step rotational foaming is mainly products processing production using different foaming and non foaming resin, resin of the heat capacity of different density, different particle size characteristics. At present, there are mainly 3 kinds of process route and forming method:Responding to use non foaming resin powder and different foaming resin powder density characteristics obtained non foaming and foaming inner core shell. Non foaming resin is the method chosen density than foaming resin, in feeding time also joined the two resin. Die rotating into the heating furnace, because of the different density of the resin, under the action of centrifugal force density larger non foaming resin from the cavity wall recently, the first heat began to melt, forming a non foam layer. Then the foam in the inner wall of non foaming layer on resin began to melt, foaming agent to release the gas, forming foam layer. However, if not the density difference of non foaming resin and foaming resin case, the difference between the foam layer and the non foam layer is not very obvious. Several hot plastic density difference is not toobig, so to get enough centrifugal force, the mold must be high speed rotation. But in the rotational molding process, the rotating speed of mold is generally low, to ensure that the inner surface of resin powder can well cover mould, get qualified product appearance.By using the properties of non - foaming resin powder and foaming resin powder heat different obtain non foaming and foaming inner core shell.The heat capacity of non foaming resin the method selection of the specific heat capacity of foaming resin powder in small, feeding time also joins the two resin. The mold into the heating furnace, due to non foaming resin capacity is small, first began to melt in the mold, the inner wall adhesion. At the same time, foaming resin heat capacity is large, began to melt when the non foaming resin, foaming resin or granular, with the mold rotation movement, gradually gathered in the middle of die mold, with the temperature rising, the foaming resin began to melt, foaming agent began to release the gas decomposition, so that the inner wall of the non foam layer on the the formation of foam layer. But this method of processing conditions and temperature control system with high request.By using the properties of non - foaming resin powder and foaming resin particle size is different to obtain non foaming and foaming inner core shell. Non foaming resin the method selected size is more obvious than the small size of foaming resin. Foaming resin pre and foaming agent are mixed evenly, preparation into larger particles. Feeding time at the same time to join the two resin. The mold is to send people to the heating furnace, due to the smaller size of non foaming resin, away from the wall of the heating furnace is close, the first heat began to melt, forming a non foam layer. Then the foam in the inner wall of non foaming layer on resin began to melt, foaming agent to release the gas, forming foam layer.At present, the research and development of an important aspect of rotational foaming molding is foaming resin, polymer resin should be selected with the melt elasticity must, must be able to resist the pressure of gas inside the bubble. If the elasticity of the melt is too small, the bubble will burst. And, in the resin grinding into powder before blowing agent and resin mixed foaming agent, to ensure that the powder can be uniformly dispersed in the polymer powder. Now, mostly combining resin density, heat capacity, resin size and chemical foaming agent and other factors, develop foaming resin raw materials suitable for rotational foaming molding.In theory, any type of thermoplastic plastics can be used for rotational foaming molding, but because of its special process, the selection of raw materials have some special requirements. First, since there is no pressure molding, rotational foaming material must have good fluidity, make every corner can flow into the mold smoothly like a liquid, to obtain the product with uniform wall thickness. Secondly, because of the longer cycle of rotational foaming molding, raw material to withstand high temperature, so it is better thermal stability. Finally, in order to be able to resist the pressure of gas bubbles within the melt elasticity plastic raw materials requires a certain. If the elasticity of the melt is too small, the bubble bursts, bubble influence quality.Impact strength or toughness is a very important consideration in choosing the material, is also the main requirements for the rotational foaming products. Linear low density polyethylene can satisfy this requirement, so it is widely used in rotational foam molding. PVC plasticized powder, vinyl acetate copolymer (EV A), ethylene butyl acrylate copolymer (EBA) and nylon, ABS, ethylene - three fluorine ethylene copolymer (ECTFE) also for special applications. With the development prospects of the material further comprises polyphenylene ether (PPO), ABS-, PEEK, PPS and polycarbonate alloy glass fiber reinforced nylon etc.. In recent years, metallocene catalystpolyethylene provides excellent raw materials for the rotational foaming polyethylene. Generally speaking, the foaming agent with physical foaming agent and foaming agent. However, due to rotational foaming molding is basically no pressure molding, not by the physical foaming agent. Because of the use of physical foaming agent to improve the mold pressure, which will greatly increase the cost of equipment, therefore in the rotational foaming molding in general the use of chemical blowing agent. Chemical foaming agent within a specific temperature range start decomposing gases. The use of a chemical foaming agent needs to pay attention is the blowing agent decomposition temperature range must be higher than the polymer melt temperature. If the foaming agent has began to decompose and melt phase of the polymer melt is just beginning, the gas is easy to overflow, not the formation of bubbles in the melt. In addition to the decomposition temperature of the foaming agent, but also pay attention to the decomposition of blowing agent series, produced or the ability to absorb energy, gas production. Because foaming the final structure are related to these factors, the choice of suitable blowing agent is the key of polymer foam molding, foaming agent commonly used at present are CelogenOT and AZ.In addition to the foaming agent outside, also need to consider the various other additives. Need to pay attention to is some colorants, especially carbon black or iron oxide pigment. These additives may absorb gas foaming agent generated, or will and chemical foaming agent to react, reducing the efficiency of the foaming agent, sometimes can bring serious consequence. (end)。