Warm Global Customers
With China Plastic Machinery
With China Plastic Machinery
The so-called vacuum back suction molding is a molding method that heats the plastic sheet to a viscoelastic state, then adheres to the mold surface through vacuum suction, and obtains the desired shape after cooling. The vacuum back suction molding process is: heat the plastic sheet to a certain temperature, then evacuate the mold, form negative pressure through the small holes on the mold, and the sheet is attached to the outer wall of the mold under atmospheric pressure, and is formed after cooling.
The essence of Vacuum Forming molding is the behavior of stretching and deforming the sheet when it is heated (generally radiant heating) and softened. In this way, there is a certain relationship between tension and elongation. Studying and mastering this relationship is the key to Vacuum Forming molding technology.
The vacuum Vacuum Forming unit consists of upper and lower heaters, temperature controller, material receiving plate, material pressing frame, vacuum pump, vacuum chamber, mold temperature control machine, cooling device, etc., among which there are two methods of temperature adjustment: manual (potentiometer) and automatic (PLC). After configuring the computer, it is more convenient to debug the parameters and store various parameters. When producing any type of parts, just call out the parameters.
Note that the heating temperature is capped at the lowest range of the raw material melting temperature (one-time molding). This allows the raw material molecules to flow freely without changing the shape given by the one-time processing and molding. Secondly, not only does the appearance leave behind the shape remnants before the Vacuum Forming molding, but there are also remnants inside the material itself. Since the deformation during Vacuum Forming molding is the thermal motion (segment motion) of the material molecules, part of the energy required for deformation is stored inside the raw material. When the molded product (sheet materials such as PVC, ABS, PP, etc. after one-time molding) is heated again, it will return to its original shape and show restoring power. Such a frozen and restored force is a feature of Vacuum Forming molding products.
The following is a specific analysis of the vacuum Vacuum Forming molding of ABS resin.
The pressing frame fixes the ABS board on the receiving plate to ensure that the edge of the board does not leak when blowing bubbles. The length and width of the pressing frame are determined by the size of the board. The types are:
¢Ù Flat pressing, the entire pressing frame is assembled from a 20mm thick steel plate, and the characteristics are high bottom flatness and flat strength. The disadvantage is that it needs to be processed by milling, planing and other processing equipment, and the production cost is high. When the receiving plate is bent and deformed, it cannot be pressed evenly on the board.
¢Ú Point pressing, like a comb shape, press the board. It can be fixed on the angle steel with countless screws to form a square frame. The screw spacing is about 50mm. The pressing plate is preferably pointed, or a cylinder is used to push the connecting rod mechanism, just like playing the piano with both hands to press the board tightly, divided into arrays, with a simple structure and can make up for the shortcomings of flat pressing.
¢Ù Heating methods
There are infrared heating, resistance wire heating, etc. The most advanced heating method currently uses the patented metal heating tile of Japan's Asano Company. The heating time is extremely short, and it only takes 10 seconds to heat from room temperature to 500¡ãC, so it is more convenient to debug than other heating tiles. When heating, the upper and lower parts are generally heated at the same time, in order to heat the upper and lower surfaces of the material evenly. The adjustment of the heating temperature is mainly determined by the shape and thickness of the parts. When the effect of temperature adjustment is not obvious, other methods can be used, such as sticking aluminum foil on the edge of the pressing frame to reflect the heat to the position where the plate needs to be heated. The smaller the heating unit (infrared heating tile), the better. Each heating tile is controlled individually, and the temperature control is precise.
¢Ú Heating temperature
The temperature of ABS board from softening to forming is 127¡æ ~ 180¡æ, which varies with the vacuum degree and the shape of the product. When using fast vacuum to form low-draw products, the forming temperature is about 140¡æ, and the temperature of deep-drawn products is about 150¡æ. Only more complex products take the upper limit forming temperature of 170¡æ. In addition, heating tiles are easily affected by voltage fluctuations and cause temperature fluctuations. A voltage-stabilized power supply is required to ensure constant temperature.
¢Û Heating time
The thicker the board, the longer the heating time, but the heating time must correspond to the heating temperature. The lower the temperature, the longer the heating time; the higher the temperature, the shorter the heating time. It cannot be simply understood as shortening the heating time and improving production efficiency by increasing the heating temperature. Because the material heating temperature has a certain range, on the other hand, a longer heating time is good for the material to be heated evenly. This is very effective for thick boards with a thickness of more than 3mm, especially those with slight moisture.
The heated and softened sheet is stretched by blowing bubbles, and then placed on the Vacuum Forming mold. A seal is formed between the sheet and the mold. The vacuum is drawn through the vacuum holes on the mold to form a negative pressure, and the sheet is attached to the mold surface.
¢ÙBlowing bubbles: Compressed air is blown out through the vacuum chamber, and the heated sheet is stretched evenly like a balloon. The bubble height is generally 2/3 of the mold height.
¢ÚAuxiliary molding: Some products have complex shapes, so an auxiliary pressing frame is required to pre-form the sheet.
¢ÛVacuum degree: Usually the vacuum degree is controlled at more than 0.1MPa. In order to remove the air in the mold in a very short time, a vacuum tank with a larger volume is required.
¢ÜVacuum hole: The diameter is generally 0.5-1.2mm. The size and number of vacuum holes are related to the thickness of the sheet. When the sheet thickness is less than 1.5mm, the diameter is 0.5-0.8mm, and when the sheet thickness is more than 3mm, the diameter is 1.2mm. If the aperture is too large, there will be traces of vacuum holes after vacuum forming, which will affect the appearance. The more vacuum holes there are, the easier it is to form.
¢Ý The vacuuming time depends on the size of the workpiece and the thickness of the plate. When the thickness is 1.5mm, the controllable time is 10s. When the thickness is not less than 3mm, the time should be about 25s. The longer the time, the better the material will be shaped. During production, vacuuming can be carried out at the same time as cooling.
¢Þ Mold temperature: controlled by the mold temperature controller. For parts with a relatively small width and depth, such as refrigerator doors, the mold temperature can be controlled at 40 to 50¡ãC; for parts with a relatively large width and depth, such as refrigerator inner boxes, the temperature is 80 to 90¡ãC. Especially for square parts, if the four corners are wrinkled, it means that the material cools too quickly after contacting the mold, and the mold temperature must be increased, but the maximum should not exceed 95¡ãC.
After the sheet is shaped, the workpiece is cooled by a centrifugal fan. Because the sheet shrinks during cooling, it wraps around the mold and makes demoulding difficult. At this time, compressed air is blown back through the vacuum hole of the mold to separate the part from the mold. Sometimes, a demoulding agent can be sprayed on the surface of the mold to help demoulding.
There are two types of forming methods: male mold forming and female mold forming.
Advantages of female molds: the inner surface (A surface) does not contact the mold, so the surface is smooth and suitable for parts with inner surfaces as the use surface, such as the box of refrigerators and freezers; in addition, under the premise of ensuring the size of the inner surface, the thickness of the sheet can be reduced, thereby saving materials and making molding easier. Disadvantages: It is difficult to grasp the mold size parameters, the internal size of the mold is easy to expand and difficult to reduce, and auxiliary molding tooling is required during vacuum forming.
Advantages of male molds: the mold size is easy to reduce, the B surface is smooth, and it is suitable for parts that require the B surface to be exposed to the outside. Disadvantages: The material thickness is thicker than that of the concave die molding, the material stretches greatly, and the A surface is rough, while the A surface of the refrigerator liner or bathtub requires a smooth surface.
(1) Bubbling points on the surface of the product: The material is damp and needs to be heated at 100¡æ for about 5 hours.
(2) Net points on the surface of the product: The material heating temperature is too high or the material stretches too much.
(3) Uneven thickness of the product: The heating temperature is uneven or the bubble is not enough, and the material stretches too much locally.
(4) Poor molding of the product:
¢Ù The vacuum degree is not enough, the material and the mold are not sealed and leak, and the material cannot be well attached to the mold;
¢Ú Problems in mold design and manufacturing, such as unreasonable structure, especially the vacuum holes in the dead corners are blocked or the number is insufficient or the hole diameter is too small.
(5) Local wrinkles on the product: It is most obvious in the dead corners, which may be due to the low mold temperature in this area. The solution is to increase the mold temperature or add vacuum holes in the dead corners.
(6) Partial ribs on the product: mainly appearing in the step area, the bubble is too high, or the heating temperature of this area is too high. The auxiliary pressing frame can be used to pre-position the rib area to avoid excessive bubble size.
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