Abstract: The successful application of arc spraying technology to mold making has opened up a new field of application of arc spraying technology. The use of arc spraying to make molds has the remarkable characteristics of short cycle, low cost and high efficiency, especially for some cavity molds with complex shapes and various decorative patterns, which have obvious economic effects and practical value.
Key words: arc spraying; mold; spray coating; substrate
1. Principles and characteristics of arc spraying to make molds
The arc spray molding technology is to continuously transport two charged special-purpose wires through a conduit, the wires intersect at the front end of the spray gun to form an arc, and the wire is melted by an arc, and the molten metal is melted under the action of compressed air. Atomized into metal particles and sprayed onto the surface of the mold at a certain speed, stacked on top of each other and stacked to form a high-density, high-bonding metal spray coating, that is, a shell (or entity) of the mold cavity. The inner wall shape of this layer of the casing completely conforms to the surface of the mold to form the desired mold cavity. The metal shell formed by spraying is filled and reinforced with other base materials, combined into a whole, and then combined with other components, that is, a complete mold is formed. This molding method is similar to electroforming, but it is not completely similar. The molding process is simple, easy to master, the mold making cycle is short, and the cost is low. Compared with steel molds of the same shape, the molding cycle and mold cost can be reduced by more than 60%.
2. The basic structure of the arc spray mold
The basic structure of the arc spray mold can be divided into three parts, namely a metal sprayed layer portion, a base portion and a steel structure portion. The basic structure of the arc spray mold is described by taking the decorative strip mold for interior decoration as an example.
(1) Metal spray coating: The product of the mold is a high-hardness polyester plastic product, which requires a certain strength, hardness, surface roughness and good thermal conductivity. For this reason, we use a special alloy material as the metal spray material. The mold cavity in the illustration is formed by arc melt spraying.
(2) Mold base: Item 6 is a filled base. The base material of the mold is a mixture of an organic binder and a metal powder. It can be firmly integrated with the metal spray layer, and has good strength and thermal conductivity, which can fully meet the working requirements of the mold.
(3) Steel structural parts: such as mold frames, inserts, injection nozzles, hinges, etc., are all machined, and then assembled and assembled on the mold.
3. Molding process of arc spraying mold
The arc spraying mold molding process can be adjusted according to the requirements of the product and the structure of the mold.
(1) Spraying mold
The characteristic of the arc spray mold is that the mold cavity size and geometric precision are completely determined by the sample mold. The design of the sample mold should be determined according to the process requirements of the product forming process. In order to facilitate demolding after spraying, the mold can be designed in the form of a block combination.
There are many materials for making molds: gypsum, graphite, resin, ceramics, wood, metal, etc., and can also be directly used as a sample mold.
(2) Release agent, release agent
The use of arc spraying technology to make molds requires both metal particles to be uniformly and densely applied to the surface of the mold, and it is required to form a shell to be smoothly and completely separated from the mold, and to protect the mold during the spraying process. Its surface pattern can withstand the impact of sprayed metal particles. At the same time, it can withstand the high temperature during spraying and prevent heat from being transmitted to the mold.
(3) Spray parameters
1 Arc power: The arc power is determined by the power source, which is the main factor affecting the quality of the mold cavity. The power is small, the arcing is difficult or the arc is broken, and the sprayed metal material does not melt well, which affects the bonding strength of the coating. If the power is too large, the sprayed metal material is easily overheated, the surface of the metal particles is seriously oxidized, and the mechanical properties of the sprayed layer are lowered. The zinc alloy is used to spray the molding material with an arc voltage of 36V and an arc current of about 110A.
2 Spraying distance: The spraying distance refers to the distance between the nozzle and the mold when spraying. The spray distance directly affects the quality of the spray coating. If the distance is too large, the temperature and speed of the metal particles on the surface of the sample will decrease, the kinetic energy will decrease, the plasticity will be poor, the coating structure will be loose, the strength will decrease, and the deposition efficiency will decrease. When the distance is too small, the temperature of the sprayed layer rises, and the mold cavity is prone to thermal deformation, which affects the accuracy of the mold. Through the test, the spraying distance between the nozzle and the sample mold is preferably 150-200 mm.
3 Spray angle: The spray angle refers to the angle between the nozzle airflow axis and the sprayed mold during arc spray forming. The angle is preferably perpendicular to the surface of the mold, and should not be less than 45° when it is necessary, otherwise a "shading effect" will occur.
4 Spray pressure: Arc spray molding uses compressed air to atomize molten metal into particles and spray it onto the surface of the mold. Therefore, it is required that the compressed air has a certain stable pressure, and the air is to be dried and cleaned. Through the test, we have selected an air pressure of 0.5 to 0.6 MPa.
5 Filling the base body: After spraying to form the shell, the base material is filled and solidified to support and reinforce the shell. The choice of the base material is mainly based on the working conditions of the mold and the material of the sprayed layer, and different materials and processes are selected. The requirements of the mold on the base material should have a certain strength, which can meet the force requirements of the working state of the mold and be easy to form. Since the shape of the sprayed shell is complicated, the shape of the base must be closely matched with the coating. Therefore, the filled base must have the characteristics of easy forming, a certain thermal conductivity and heat resistance, and at the same time, its thermal conductivity and thermal expansion. The coefficient should be similar to that of the metal spray coating, especially when the mold operating temperature is high.
6 inlaid metal parts: Some parts of some molds have some special requirements. If it is difficult to spray, metal inserts can be used. The sprayed metal inserts are directly part of the mold cavity after the mold is removed. These special parts generally refer to structures with deep depressions, protrusions or ribs on the mold, and the height of the protrusions or depressions is greater than the width thereof, and the inserts should be placed in these parts. For some mold parts with large force and heavy wear, metal inlay can also be used to improve the life of the mold.
According to the working conditions of the mold, the force and other factors, different insert materials, such as steel, cast iron, aluminum, zinc alloy, copper, etc., can be selected.
7Assembling test mode: The arc spray forms the cavity of the mold. After filling and solidification, the sample mold is taken out from the cavity, and cleaned to meet the requirements and the corresponding accessories are assembled to form a complete mold.
4, arc spray molding process application
Taking the PVC plastic female sole injection mold as an example, the arc spray forming process is summarized as follows:
(1) The model is based on the popular PVC plastic soles purchased by the market.
(2) Determine the mold parting surface according to the product requirements, and apply the release agent and release agent. The release agent and release agent should be applied evenly, and each application should be carried out after the last application and drying.
(3) After the release agent and the release agent are dried, the first spraying is performed to form a metal sprayed concave mold casing.
(4) Level the casing as required, install the die frame, and cast the die base filling material. Care should be taken to seal the perimeter during pouring to prevent leakage during pouring.
(5) After turning solidification, the release agent and the release agent are applied on the parting surface mold, and the inlaid metal parts are assembled on the corresponding parts.
(6) After drying, a second spray is applied to form a metal sprayed punch housing.
(7) Install the punch frame and cast the punch base filling material, also pay attention to the peripheral closure.
(8) After solidification, the convex and concave molds are separated according to the parting surface, and the mold is taken out, that is, the arc spray forming of the mold cavity is completed.
(9) Clamp the upper and lower planes to ensure the parallelism of the mold.
(10) Connect the upper and lower stencils and assemble steel parts such as injection nozzles.
(11) Injection molding on the machine.
5, the economic effect of arc spraying to make mold
(1) The process of making arc by arc spraying is simple. Based on the sample, the arc is used to melt the metal, and the compressed air atomizes the metal and sprays it onto the surface of the mold to form a mold cavity. This method is simpler than machining or electroforming, especially the mold cavity with complicated shape and difficult machining, and the effect is more obvious.
(2) The mold making cycle of the arc spraying production mold is short. Conventional mold processing requires a long processing cycle from casting (or forging), roughing, finishing, profiling, and fitter to the finished mold. Some cavity molds with complex shapes are directly processed. Difficulties, having to adopt a mosaic form of the mosaic, so that the design and manufacturing cycle of the mold is correspondingly lengthened. The mold is made by arc spraying, and as long as the mold is provided, the mold that meets the requirements can be produced in a short time. A pair of projection area of ​​1900mm × 1200mm, a three-dimensional decorative pattern on the surface and a double-head mold for plastic molding were used, which took only one week. A pair of molds of the same size can be completed in more than five months using the conventional method of cast aluminum processing.
(3) The cost of arc spraying mold is low. The main materials used to spray the mold are sprayed wire and matrix filler. The amount of spinning is very small, only a thin layer, and the proportion of the mold cost is very small. The bedside mold mentioned above only uses less than 20kg of spinning wire, and the value is more than 500 yuan. The cost of the whole mold is more than 1,200 yuan, while the casting cost of the conventional method is nearly 7,000 yuan, and the whole mold cost is nearly 15,000 yuan.
Arc spraying molding is a typical rapid molding technology. It has the characteristics of simple molding process, short production cycle and low mold cost. It is especially suitable for small batch and multi-variety production and use, especially in the current market competition. In the case of arc spray molding technology, it provides a new molding method and shortcut for product replacement. This technology will be more and more valued and applied by people.
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