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How to reduce shrinkage plastics processing
日期:2017-04-18

??Contraction plastics processors are facing the enemy, especially for large-scale high surface quality plastic products, shrinkage is a chronic illness. Therefore, various techniques have been developed to minimize shrinkage, improve product quality. Thick injection molded plastic member in position, such as the ribs or protrusions are formed at positions adjacent to shrinkage than the more serious because it is much slower than the cooling rate of the thicker area of the peripheral region. Different cooling rate leads to the ion of the connecting surface of the recess, is the familiar shrinkage marks. This deficiency severely limits the design and molding of plastic products, especially large thick-walled products such as televisions and monitors slant cabinet shell. In fact, for this type of household electrical appliances must eliminate the strict requirements of product shrinkage marks, and surface quality for toys and some other less demanding products allow contraction marks exist. Reasons for the ion of scar contraction may have one or more including processing methods, part geometry, material selection, and mold design. Geometry and material selection which is usually determined by the raw material suppliers, and not easy to change. But there is still a lot of mold manufacturer on mold design factors may affect contraction. Cooling flow design, gate type, gate size may produce a variety of effects. For example, the small gate as cooling tube gate much faster than the tapered gate. The gate early cooling will reduce the cavity filling time, thereby increasing the chances of scar contraction produced. For the molder, adjusting the processing conditions is a solution to the problem of shrinkage. And filling pressure was significantly affect shrinkage. After the filling member, the excess material continues to fill the cavity to compensate for shrinkage of the material. The filling phase is too short will result in increased shrinkage, will eventually produce more or larger shrinkage marks. This approach may not be able to shrink scar itself reduced to a satisfactory level, but you can adjust the fill molding conditions improve worker contraction marks. Another method is to modify the mold, there is a simple solution is to modify the conventional core hole, but did not expect this method is applicable to all resins. In addition, gas-assisted method is also worth a try. Column, the gas bubble GE Polymer Processing Research Center (PPDC) conducted a study of 12 months, to assess eight different ways to reduce shrinkage marks. These technologies represent a reduction in shrinkage marks some of the latest ideas. These methods can be divided into two categories: one method can be called to replace the material, and the other method is to remove the heat. Substituted method is to reduce the shrinkage of the material by increasing or decreasing the scar area may shrink material usage. Heat removal methods designed to quickly remove the heat shrinkage may occur in the region, thereby reducing the possibility to produce a thinner region and a thicker region of the cooling unevenness. In this study, a total of five kinds of assessments to replace the material law: any projecting lug, lug round, with spring lug, gas-assisted molding and chemical foam. Three kinds of heat removal method: Beryllium - Copper lugs, beryllium - copper inserts and a specially designed thermal activity lugs. Rate is the number of objects to be scar contraction generated in the test member, the member to be tested with a triangular projection products. All the standard method of comparison to the standard tools - stainless steel lugs. The test tool can produce a disk of 2.5mm thickness, lug height 22.25mm, diameter 4.5mm, thickness is 1.9mm, 2mm on the chassis with the triangle. The Institute molding equipment used for the horizontal touch 350t hydraulic machine, the material is commonly used in household electronics materials, but also a serious problem shrink material that GE's PC / ABS, Cycoloy CU6800 and PPE / PS, Noryl PX5622. Range of processing these two materials are in the recommended range of technical parameters interte points. If the mark is the smallest state of contraction, can lead to more down to fill the amount of shrinkage marks to facilitate measurements and compared with empirical methods. Although shrinkage marks are usually observed by the naked eye, but these tests using a machine to mark the depth of the contraction was quantitatively measured. One standard technology test content test is extended convex column, namely, the standard lug lug extending into the bottom of the wall, the wall thickness is reduced and thus compensate for the excess material protruding column caused effects. Experiments using two projecting depth of the wall thickness were 25% and 50%. Another test used instead of a round pointed lugs. This method is not removing material lug area, but making the transition in the regions more coherent. Another way between the top of the plate and a spring lug. Kyphosis column beneath the spring so that the cooling material parts are still in the state of stress in order to obtain compensation for the material shrinkage effect. The results will be the initial pressure and the influence of the spring of the spring, "rigid", the test evaluated the effects of these two factors. Two different spring stiffness, the stiffness of each spring is applied to a variety of different initial pressures. Chemical blowing agents are the contents of this evaluation test, because the advantages of chemical blowing agents that do not make any changes to the tool. The theoretical basis of the method is that the thicker region which is the most likely region of foam shrinkage foaming process generates a partial pressure sufficient to prevent shrinkage. Of course, only a small amount (0.25%) of the foaming agent (Safoam RPC-40) during the foaming process, so as to prevent damage to the part surface cracks. By processed lug to test nitrogen gas-assisted injection molding, nitrogen in the area is usually prone to shrinkage of the ion of bubbles, so that you can use to remove material in the region where the gas bubbles to fill the area. In order to achieve rapid heat transfer, use a beryllium - copper lug, the thermal conductivity is much faster than stainless steel. This technique also requires connection to a rear end lug huge pool heat so that heat can be removed completely from the area of the lug. Another way is to use the method of standard stainless steel mounting lug, but in the area around the lug beryllium - copper plug. This requires modification of the mold cavity fully, a small groove machined mounting the ribs / lug structure in the region. The ribs / convex cylindrical structure processed into separate beryllium - copper cavity plug installed in a small slot. High rate of heat transfer heat plugin will lug area and completely absorbed into the tool. The first two methods are used in a passive heat removal method "lug thermal activity" comprises a fluid heat away and dispersed in the hot zone to the cooling means. When the results were compared using PC / ABS material, shrinkage of the five test methods produce less shrinkage than standard lug generated. All with good heat removal method, the method of Method substituted material only spring-loaded lug to lug better results than the standard, the impact force of the spring pre-load especially on performance. The results of gas-assisted method is not decisive: the use of such materials and mold, the thin wall products, melt - fast cooling rate and thus the gas permeability is difficult to maintain consistency. Foaming test is not a decisive influence. Showed significant cracking of the component surface, prior to the present method can not be compared with other methods, should reduce the amount of blowing agent. When using PPE / PS resin, spring loaded lugs same outstanding performance. The other three methods to replace materials, including extending convex column gas-assisted molding method and the effect is better than the effect of standard convex column. For heat removal method, only beryllium - copper lugs method is better than the effect of standard lug method. The round convex column method for the effect of the two materials are not good. Surprisingly convex column method for projecting PC / ABS material terms of the effect is very good, but two decades, has been extended convex column is the recommended method. These results indicate that these methods for the purposes of the effect is not the same type of material. The most interesting result is spring-loaded lug method from. For both materials, the proper use of the spring preload, the shrinkage of 50% have been improved. Steel springs seem as great as the impact of the impact of the spring preload. Pre-pressure is too small, the back end of the plastic melt lug push too far, resulting in too much material retention lug area, resulting in contraction. Spring preload is too large, the melt will not be compressed under pressure, the same effects and standard lug. When measuring muscle contraction marks near the rib structure, the spring-loaded method also shows amazing results. Although the method is designed to lug around minimize shrinkage processing PPE / PS materials, the structure of the contraction of the ribs have been attached to a surprising improvement. May be an effective way to structure the material filled into the ribs when the lug compression, thereby reducing shrinkage. Regardless of the outcome, people should not be underestimated for this gas-assisted molding method and the method of chemical blowing agents. For gas-assisted molding, mold has not been optimized, is expected to play a very good effect in a larger size components, because it covers a larger area than the range of spring loaded lugs coverage. Moreover, as described above, the blowing agent formulations in these tests has not been optimized.

 
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