How to reduce shrinkage in plastic processing

2021-03-23 15:22:41 admin

How to reduce shrinkage in plastic processing




Shrinkage is a major enemy faced by plastic processors, especially for large plastic products with high surface quality requirements. Shrinkage is a stubborn disease. Therefore, various technologies have been developed to minimize shrinkage and improve product quality.


  In the thicker part of the plastic part of the injection molded part, the shrinkage formed at the ribs or protrusions is more serious than the adjacent position, because the cooling rate of the thicker area is much slower than that of the surrounding area. The difference in cooling rate results in the formation of depressions at the connecting surface, which are known as shrinkage marks. This defect severely restricts the design and molding of plastic products, especially large thick-walled products such as beveled cabinets and display housings for televisions. In fact, shrink marks must be eliminated on products with strict requirements such as household appliances, while shrink marks are allowed for products with low surface quality requirements such as toys.


   There may be one or more reasons for the formation of shrinkage marks, including processing methods, part geometry, material selection, and injection mold design. The geometry and material selection are usually determined by the raw material supplier and are not easy to change. However, there are many factors related to the design of the injection mold for injection mold manufacturers that may affect the shrinkage. Cooling runner design, gate type, and gate size may have multiple effects. For example, small gates such as tube gates cool much faster than tapered gates. Premature cooling at the gate will reduce the filling time in the cavity, thereby increasing the chance of shrinkage marks. For molding workers, adjusting processing conditions is a way to solve the shrinkage problem. Filling pressure and time significantly affect shrinkage. After the part is filled, the excess material continues to be filled into the cavity to compensate for the shrinkage of the material. Too short a filling stage will result in increased shrinkage and eventually produce more or larger shrinkage marks. This method itself may not reduce the shrinkage mark to a satisfactory level, but the molding worker can adjust the filling conditions to improve the shrinkage mark.


   Another method is to modify the injection mold. A simple solution is to modify the conventional core hole, but this method cannot be expected to be suitable for all resins. In addition, the gas-assisted method is also worth trying.


  Columns, Gases and Foams The GE Polymer Processing Research Center (PPDC) conducted a 12-month study to evaluate 8 different methods aimed at reducing shrinkage marks. These technologies represent some of the latest ideas for reducing shrinkage marks. These methods can be divided into two categories: one can be called the substitution material method, and the other is the heat removal method. The substitution method is to reduce shrinkage marks by increasing or reducing the amount of material in the area that may shrink. The heat removal method aims to quickly remove heat from areas that may shrink, thereby reducing the possibility of uneven cooling in thinner and thicker areas.   In this study, five alternative material methods were evaluated: extending convex column, round head convex column, spring convex column, gas-assisted molding and chemical foaming. Three heat removal methods: beryllium-copper bumps, beryllium-copper inserts and specially designed thermally movable bumps. The object of evaluation is the number of shrinkage marks generated in the part to be tested, and the part to be tested is a product with triangular protrusions. The standard for comparison of all methods is the standard tool-stainless steel boss. The test tool can produce a disc with a wall thickness of 2.5mm, the height of the convex post is 22.25mm, the diameter is 4.5mm, the wall thickness is 1.9mm, and there is a 2mm triangle iron on the chassis. The molding equipment used in the institute is a 350t horizontal touch hydraulic press. The materials are commonly used in daily electronic products and are also materials with serious shrinkage problems, namely GE's PC/ABS, Cycoloy CU6800 and PPE/PS, Noryl PX5622. The processing range of these two materials is in the middle point of the recommended range of product technical parameters. If the shrinkage marks are at a minimum, the filling amount can be lowered to induce more shrinkage marks to facilitate measurement and comparison with empirical methods. Although shrinkage marks are usually observed with the naked eye, these tests use a machine to quantitatively measure the depth of shrinkage marks.


  Test content


One of the standard techniques of    test is the protruding stud, that is, the standard stud protrudes into the wall at the bottom of the stud, thereby reducing the wall thickness and compensating for the effect caused by the excess material in the stud. Two extension depths were used in the test, which were 25% and 50% of the wall thickness. Another experiment used a round head instead of a pointed boss. This method is not to remove the material in the bump area, but to make the transition of each area more coherent. There is another way to use a spring between the ejector plate and the boss. The spring makes the material under the convex column still under pressure after the part is cooled, so that the material can obtain the effect of compensating for shrinkage. The result will be affected by the initial pressure of the spring and the "rigidity" of the spring, and the effects of these two factors have been evaluated by the experiment. Two springs of different stiffness are used, and a variety of different initial pressures are applied to the springs of each stiffness.


   Chemical foaming agent is also included in the evaluation content of this test, because the advantage of chemical foaming agent is that there is no need to make any changes to the tool. The theoretical basis of this method is to foam in the thicker area, that is, the area most likely to shrink. The foaming process will generate enough local pressure to prevent shrinkage. Of course, only a small amount (0.25%) of foaming agent (Safoam RPC-40) can be used in the foaming process to avoid cracks and damage to the surface of the part.


   Test gas-assisted molding by injecting nitrogen into the processed convex post. Nitrogen forms bubbles in the area that is usually prone to shrinkage, so that the material in the area can be removed and the gas in the bubble can be used to fill the area.


   In order to achieve rapid heat transfer, a convex pillar made of beryllium-copper is used, and the heat conduction speed is much faster than that of stainless steel. This technology also requires the rear end of the boss to be connected to a huge heat pool, so that the heat can be completely removed from the area of the boss. Another way of this method is to use standard stainless steel bosses but install beryllium-copper inserts in the area around the bosses. This requires sufficient modification of the mold cavity of the injection molded part, and a small groove is machined in this area to install the rib/column structure. The rib/convex column structure is processed into an independent beryllium-copper cavity insert and installed in a small groove. An insert with a high heat transfer rate will completely absorb the heat in the boss area and guide it into the tool. The first two methods use passive heat removal methods. The "thermally active stud" contains a fluid that takes away the heat from the hot zone and distributes it to the cooling device.


   Comparison of results


  When using PC/ABS material, the shrinkage produced by the five test methods is less than the shrinkage produced by the standard convex post. All the methods of removing heat are effective. Among the methods of replacing materials, only the method of loading the spring-loaded stud is better than the standard stud, and the preload pressure of the spring has a particularly prominent effect on the performance. The result of the gas-assisted method is not decisive: using this kind of injection mold and material, because the product wall is too thin, the melting-cooling rate is too fast, so it is difficult to maintain consistent gas penetration. The foaming test also has no decisive influence. The obvious cracks on the surface of the part indicate that the amount of foaming agent should be reduced before this method can not be compared with other methods.


  When using PPE/PS resin, the spring-loaded boss also performs well. The other three methods of replacing materials, including the protruding stud method and the gas-assisted molding method, are also better than the standard stud. For the heat removal method, only the beryllium-copper bump method is better than the standard bump method.


  The round head bump method is not good for both materials. Surprisingly, the extended boss method is not very effective for PC/ABS materials, and the extended boss has been the recommended method for two decades. These test results show that these methods are not the same for different materials.


   The most interesting result comes from the method of loading the spring-type boss. For the two materials, proper use of the spring preload, the shrinkage of the product has been improved by 50%. The effect of spring stiffness seems to be not as great as the effect of spring preload. If the pre-pressure is too small, the plastic melt will push the back end of the convex column too far, causing too much material to stay in the convex column area, resulting in shrinkage. The spring preload is too large and will not be compressed under the pressure of the melt, and the effect is the same as the standard convex post. When measuring the shrinkage marks near the rib structure, the spring loading method also showed amazing results. Although this method is designed to minimize the shrinkage near the boss, the shrinkage at the connected rib structure is also surprisingly improved when processing PPE/PS materials. It may be that when the convex column is compressed, the material is effectively filled into the rib structure, thereby reducing the shrinkage.