How to reduce shrinkage in plastic processing and improve product quality

Shrinkage is the enemy of plastic processors, especially for large plastic products with high surface quality requirements. Shrinkage is a chronic illness. Therefore, people have developed various technologies to minimize shrinkage and improve product quality.

In the thicker part of the injection molded plastic part, the shrinkage formed by the ribs or protrusions is more severe than that of the adjacent parts, because the cooling rate in the thicker area is much slower than in the surrounding area. Different cooling rates lead to the formation of depressions at the connection surface, which is known as the contraction mark. This kind of defect has severely limited the design and molding of plastic products, especially large-scale thick-walled products such as beveled cases and display cases for television sets. In fact, shrinkage marks must be eliminated for this type of demanding product for household electrical appliances, and for products such as toys that do not require high surface quality, the presence of shrink marks is allowed.

There may be one or more reasons for the formation of shrink marks, including processing methods, part geometry, material selection, and mold design. The choice of geometry and material is usually determined by the raw material supplier and is not easily changed. However, there are many factors on the mold design that may affect shrinkage. Cooling runner design, gate type, and gate size may produce multiple effects. For example, small gates, such as pipe gates, cool much faster than tapered gates. Premature cooling at the gate reduces the filling time in the cavity and increases the chance of shrinkage marks. For molding workers, adjusting the processing conditions is one way to solve the shrinkage problem. Fill pressure and time significantly affect shrinkage. After the part is filled, excess material continues to fill the cavity to compensate for shrinkage of the material. Too short a fill phase will result in increased shrinkage and eventually more or more shrinkage marks. This method itself may not reduce shrink marks to a satisfactory level, but the shaper can adjust the filling conditions to improve the shrink marks.

Another method is to modify the mold. A simple solution is to modify the conventional core hole, but it cannot be expected that this method will work for all resins. In addition, gas-assisted methods are also worth a try.

The Polymer Processing Research Center (PPDC) conducted a 12-month study to evaluate eight different methods aimed at reducing shrinkage marks. These technologies represent some of the latest ideas for reducing shrink 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. Substitution material methods reduce shrinkage marks by increasing or decreasing the amount of material in the area that may shrink. The heat removal method aims to quickly remove the heat of the area where shrinkage may occur, thereby reducing the possibility of uneven cooling generated in the thinner areas and thicker areas.

In this study, a total of five methods for replacing materials were evaluated: protruding bosses, rounded bosses, spring-loaded bosses, gas-assisted molding, and chemical foaming. Three types of heat removal methods: 铍-copper bumpers, 铍-copper inserts, and specially designed thermal active bumps. The object of evaluation is the number of shrink marks produced in the part to be tested, and the part to be tested is a product with triangular protrusions. The standard for all methods is the standard tool—stainless steel bosses. The test tool can produce a disc with a wall thickness of 2.5 mm, with a height of 22.25 mm, a diameter of 4.5 mm, a wall thickness of 1.9 mm, and 2 mm of triangular iron on the chassis.

The molding equipment used in this research is a 350t horizontal touch hydraulic press, which is commonly used in daily-use electronic products. It is also a material with severe shrinkage problems, namely GE's PC/ABS, CycoloyCU6800 and PPE/PS, and NorylPX5622. The processing range of these two materials is at the midpoint of the suggested range of product technical parameters. If the shrinkage marks are at a minimum, the fill amount can be adjusted downwards to cause more shrinkage marks to facilitate measurement and comparison with empirical methods. Although the scars are usually observed with the naked eye, these tests use a machine to quantitatively measure the depth of the scar.

One of the standard techniques tested is a protruding stud, i.e. a standard stud protruding into the wall at the bottom of the stud, thereby reducing the wall thickness and compensating for the effect of excess material in the stud. Two kinds of protrusion depths were used in the test, which were 25% and 50% of the wall thickness respectively. Another test uses a round head rather than a pointed boss. This method does not remove the material of the pillar region, but rather makes the transition of the regions more consistent. There is also a method of using a spring between the ejector plate and the boss. The spring causes the material under the stud to remain under pressure after cooling of the part, so that the material can achieve the effect of compensating shrinkage. The result is influenced by the initial spring pressure and the “rigidity” of the spring. The test evaluates the influence of these two factors. Two springs of different stiffnesses are used, and a variety of different initial pressures are applied to each spring of stiffness.

Chemical foaming agents are also part of the evaluation of this test because the advantage of chemical foaming agents is that they do not require any changes to the tool. The theoretical basis of this method is to foam in thicker areas, ie areas where shrinkage is most likely to occur. The foaming process will generate enough partial 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 prevent cracks from damaging the surface of the component.

The gas-assisted molding was tested by injecting nitrogen gas through the processed bulges. Nitrogen gas bubbles formed in the area where shrinkage normally occurs, so that the material in the area can be removed to fill the area with the gas in the bubbles.

In order to achieve rapid heat transfer, a stud made of bismuth-copper is used, and the heat conduction rate is much faster than stainless steel. This technique also requires that the rear end of the post be connected to a large thermal pool so that heat can be completely removed from the area of ​​the post. Another way of doing this is to use a standard stainless steel stud but with a beryllium-copper insert in the area around the stud. This requires a full modification of the mold cavity, in which a small groove mounting rib/convex structure is machined. The rib/bump structure is machined into a separate helium-copper cavity insert, mounted in a small groove. The high thermal conductivity plug-in will completely absorb the heat from the boss area and introduce it into the tool. The first two methods used a passive heat removal method, and the "thermally active boss" contained a fluid that carried heat away from the hot zone and dispersed it to the cooling device.

Comparison of results

With the PC/ABS material, the five test methods produced less shrinkage than the standard stud. All the heat removal methods work well. In the method of replacing the material, only the spring-loaded stud method is better than the standard stud, and the preload pressure of the spring has a particularly prominent effect on the performance. The results of the gas-assisted method are not decisive: With this type of molds and materials, the gas-penetration is difficult to maintain consistently because the product walls are too thin and the melting-cooling rate is too fast. The foaming test also has no decisive influence. The apparent cracks on the surface of the part indicate that the amount of blowing agent should be reduced before the method can be compared with other methods.

When using PPE/PS resin, spring-loaded bosses also perform well. The other three methods of replacing materials, including the protruding bump method and gas-assisted forming method, are also more effective than standard bumps. For the heat removal method, only the 铍-copper bump method works better than the standard bump method.

The round head method does not work well for both materials. Unexpectedly, the protruding stud method is not very effective for PC/ABS materials, and over the past 20 years, protruding studs have been the recommended method. The results of these tests show that these methods are not the same for different materials.

The most interesting result is still the method of loading the spring-type stud. For both materials, the proper use of the spring preload resulted in a 50% improvement in the shrinkage of the product. The effect of spring steel stiffness does not seem to be as great as the spring preload. When the pre-pressure is too low, the plastic melt pushes the back end of the stud too far, resulting in too much material retention in the stud area, causing shrinkage. The spring pre-pressure is too large and will not be compressed under the pressure of the melt. The effect is the same as that of the standard boss. The spring loading method also showed surprising results when measuring contractions near the rib structure. Although this method aims to minimize the shrinkage near the studs, the shrinkage at the connected rib structure is surprisingly improved when PPE/PS materials are processed. It is possible that the compression of the studs effectively fills the material into the rib structure, thereby reducing shrinkage.

Regardless of the outcome, one should not underestimate the gas-assisted molding method and the chemical blowing agent method. For gas-assisted molding, the mold has not been optimized and is expected to perform well in larger-sized parts because it covers a larger area than the loaded spring post. Moreover, as mentioned earlier, the formulation of blowing agents in these tests has not been optimized.

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