Compression moulds can be classified into three types: mobile, semi-fixed and fixed according to the state of the compression moulds with which they are fitted and installed on the hydraulic press.
Mobile compression mold is not fixed on the hydraulic press, but in the machine to add material, close the mold (there are inserts to install the inserts before adding material), and then put into the machine heating and pressure (hydraulic press on the upper and lower template are installed with electric heating plate, so there is no heating device on the mold). After forming, the upper and lower mold plates are separated from the cavity plate with the mold unloading frame, and the products are removed to clean the mold. After that, the next cycle of molding is carried out.
The mobile compression mold is completely manual, labor-intensive and inefficient. The mold is opened by hitting the mold with the mold unloader and closed by hand, which increases the wear and tear on the mold. Therefore, it is only suitable for making a small number of samples or for testing when making new products, and is generally not used for production.
Semi-fixed compression mold, the upper mold is fixed on the hydraulic press template, while the lower mold is equipped with a pair of ball bearings on each side and placed in the slot of the rail frame, which can be moved in the slot of the rail, so that the lower mold can be pulled out of the machine along the rail with the handle after opening the mold, and the cavity plate and the lower mold plate can be separated by the mold unloading frame or the small cylinder installed vertically under the rail frame to remove the product and clean the cavity and the convex mold. Put the insert or material outside the machine, then push it into the hydraulic pressure along the guide rail, use the positioning parts to position the upper die guide column to align with the lower die guide column hole, then close the mold and repeat the next cycle of molding.
Semi-fixed compression mold is suitable for small batch production, it has short manufacturing cycle, simple structure, low labor intensity compared with the former, and it is convenient to load material and inserts outside the mold.
The upper and lower die of fixed compression mold are fixed on the hydraulic press. The process of opening, adding, closing and pushing out the products are carried out in the machine. High efficiency, easy operation, low labor intensity. The mold opening and closing is smooth and shock-free, and the mold life is long, which is suitable for mass production. Because its structure is more complicated than the first two types, it requires high precision and high cost.
For experimental or trial products, choose mobile compression mold, which is simple and fast, low cost and short cycle. For small batch production, choose semi-fixed compression mold. Batch and mass production, should undoubtedly choose a fixed mold, and mass production of products should take pre-pressing process, can achieve fully automatic (adding materials, pressing, out of the mold) production, can greatly improve efficiency, reduce costs.
Compression molds are sometimes classified according to the form of mold closure.
The overflow compression mold has no filling cavity and the total height of the cavity is basically the height of the plastic part. Since the concave mold and the convex mold have no mating part, the excess material is easy to be abused during compression molding, and the mating between the concave mold and the convex mold is completely positioned by the guide pillar, so the radial wall thickness of the plastic part is not highly accurate. In addition, the accuracy of the filling amount is not high, and the filling amount is generally more than 5% ~ 9% of the mass of the plastic part.
The advantages of the overflow compression mold are simple structure, low cost, durability, easy demolding, and convenient installation of inserts; the disadvantage is that because there is no filling cavity, the loading volume is limited, and it is not suitable for molding plastics with high compression ratio. In addition, the density of the parts is often low, and the strength and other mechanical properties are not good.
The overflow compression mold is suitable for molding flat parts, especially for products without strict requirements on strength and size, and not for molding thin-walled parts and parts with high requirements on wall thickness uniformity.
The filling chamber of the non-overflow compression mold is a continuation of the upper section of the cavity, with no extrusion surface and little plastic overflow. Therefore, the clearance between the convex mold and the filling chamber should not be too small, generally the clearance is about 0.075mm on one side. If the clearance is too small, the gas in the cavity cannot be eliminated smoothly during compression molding; if the clearance is too large, it will cause more overflow and affect the quality of plastic parts.
Since there is no extrusion surface in the non-overflow compression mold, theoretically the pressure of the press will be fully transferred to the molded part when compressing, so it can obtain a tightly organized molded part. When forming, the amount of plastic charge should be accurate, because it directly affects the thickness of the part and the size of the part. The non-overflow compression mold should not be designed as a multi-cavity mold, because a slight imbalance in the charge will cause unequal pressure in each cavity and cause underpressure in some parts. Another disadvantage of the non-overflow compression mold is that there is friction between the convex mold and the side wall of the filling cavity, and the side wall is easily damaged, and the surface of the plastic part is easily damaged when the plastic part is ejected. The non-overflow compression mold must be equipped with an ejector device, otherwise it is difficult to release the molded part.
The non-overflow compression mold is suitable for compressing plastics with large volume and poor flowability, such as cotton, glass cloth or long-fiber filled plastics, and it is also suitable for compressing complex shapes and thin-walled plastic parts.
The semi-overflow compression mold has a filling cavity and a convex mold. The gap between the convex die and the cavity or the overflow slot can make the excess plastic overflow, and the overflow slot also has the function of excluding gas, and the clearance between the convex die and the cavity on one side is often 0.025~0.075mm.
With semi-indiscriminate compression mold, the shape of the convex mold can be determined without the shape of the plastic part, so it is simpler than the non-overflow compression mold. Because the size of the filling cavity is larger than the cross section of the plastic part, the convex mold does not rub along the side wall of the mold cavity and does not scratch the surface of the cavity, so the outer surface of the plastic part is no longer damaged during ejection. In addition, the compactness of the part is better than that of the overflow compression mold, and it is easier to ensure the accuracy of the height direction.
Where the pressing accuracy and strength requirements are not high, the products are not large, the requirements for feeding accuracy is not high, small batch production, flat or thin-walled products, can be used overflow cavity compression mold. Where the shape is more complex, high dimensional accuracy requirements of products, or better flowability, compression is relatively large plastic, optional half overflow cavity compression mold. Where the shape is complex, thin wall, deep cavity requires high density products or long process products, as well as poor liquidity, large specific volume, high unit specific pressure requirements (such as fibers, debris as filler) plastic products, can choose not overflow structure of compression mold.
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