How to reduce shrinkage in plastic processing to improve product quality?

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

Shrinkage at the thicker locations of injection molded plastic parts, such as ribs or protrusions, is more severe than adjacent locations because the thicker areas are cooled much more slowly than the surrounding area. The difference in cooling rate results in the formation of a depression at the joint surface, which is a familiar shrinkage mark. Such defects severely limit the design and molding of plastic products, especially large thick-walled products such as the beveled casing and display casing of television sets. In fact, for the demanding products such as household appliances, the shrink marks must be eliminated, and for products such as toys, which have low surface quality requirements, shrink marks are allowed.

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

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

The Polymer Processing Research Center (PPDC) conducted a 12-month study to evaluate eight different methods designed to reduce shrinkage marks. These techniques represent some of the latest ideas for reducing shrinkage marks. These methods can be divided into two categories: one can be called a substitute material method, and the other is a heat removal method. The replacement material method reduces shrinkage marks by increasing or decreasing the amount of material that may be in the area of ​​shrinkage. The heat removal method is designed to quickly remove heat from areas that may cause shrinkage, thereby reducing the likelihood of uneven cooling that occurs in thinner and thicker areas.

In this study, five alternative material methods were evaluated: extended studs, round studs, spring studs, gas-assisted molding, and chemical foaming. Three heat removal methods: 铍-copper studs, 铍-copper inserts, and specially designed thermal active studs. The object of the evaluation is the number of shrink marks produced in the part to be tested, and the part to be tested is an article with a triangular protrusion. The standard for comparison of all methods is the standard tool - stainless steel studs. The test tool can produce a disc with a wall thickness of 2.5 mm, a height of 22.25 mm, a diameter of 4.5 mm, a wall thickness of 1.9 mm, and a 2 mm triangular iron on the chassis.

The molding equipment used in this research is a 350t horizontal touch hydraulic press. The material is commonly used in daily electronic products. It is also a material with serious shrinkage problems, namely GE's PC/aBS, CycoloyCU6800 and PPE/PS, NorylPX5622. The processing range of these two materials is in the middle of the range of product technical parameters. If the shrink marks are at a minimum, the fill level can be lowered to induce more shrink marks for ease of measurement and comparison with empirical methods. Although the shrink marks are usually observed by the naked eye, these tests used a machine to quantitatively measure the depth of the shrink marks.

One of the standard techniques of the test is the extended 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 effects of excess material in the stud. Two extension depths were used in the test, which were 25% and 50% of the wall thickness, respectively. Another test used a rounded head instead of a pointed stud. This method is not to remove the material of the stud area, but to make the transition of each area more consistent. Yet another method uses a spring between the ejector plate and the stud. The spring causes the material under the studs to remain under pressure after the components are cooled, so that the material is compensated for shrinkage. The result is affected by the initial spring pressure and the "rigidity" of the spring, and the test evaluates the effects of these two factors. Two springs of different stiffness are used, and a variety of different initial pressures are applied to each of the stiffness springs.

Chemical blowing agents are also included in the evaluation of this test because the advantage of chemical blowing agents is that no changes are made to the tool. The rationale for this method is to foam in thicker areas, that is, areas where shrinkage is most likely to occur, and the foaming process produces sufficient partial pressure to prevent shrinkage. Of course, only a small amount (0.25%) of the foaming agent (Safoam RPC-40) can be used in the foaming process to prevent cracks from damaging the surface of the part.

The gas-assisted molding is tested by injecting nitrogen through the processed studs, and the nitrogen forms bubbles in the areas where shrinkage is usually easy, 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 consisting of beryllium-copper is used, which has a heat transfer rate far exceeding that of stainless steel. This technique also requires that the rear end of the stud be connected to a large thermal pool so that heat can be completely removed from the area of ​​the stud. Another way of this method is to use a standard stainless steel stud but install a beryllium-copper insert around the stud. This requires a full modification of the mold cavity, in which a small groove mounting rib/crest structure is machined. The rib/bump structure is machined into a separate 铍-copper cavity insert that is mounted in a small slot. A plug with a high heat transfer rate fully absorbs heat from the stud area and introduces it into the tool. The first two methods use a passive heat removal method. The "thermal activity stud" contains a fluid that carries heat from the hot zone and disperses it into the cooling device.

Comparison of results

When using PC/aBS materials, the five test methods produced less shrinkage than the standard studs. All methods of removing heat work well. In the method of replacing materials, only the method of loading the spring-shaped studs is better than the standard studs, and the preloading pressure of the springs has a particularly significant effect on performance. The results of the gas-assisted method are not decisive: the use of such molds and materials, because the product walls are too thin, the melt-cooling rate is too fast, so gas permeation is difficult to maintain consistent. The foaming test also had no decisive influence. Significant cracks on the surface of the part indicate that the amount of blowing agent should be reduced before the method can be compared to other methods.

The spring-loaded studs also perform well when using PPE/PS resin. The other three alternative material methods, including the extended bump method and the gas-assisted molding method, also perform better than standard studs. For the heat removal method, only the 铍-copper bump method works better than the standard bump method.

The round-headed column method does not work well for both materials. Unexpectedly, the extended stud method is not very effective for PC/aBS materials, and for over two decades, extended studs have been the recommended method. 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 stud. For both materials, the preload of the spring was used appropriately, and the shrinkage of the product was improved by 50%. The effect of spring steel does not seem to be as great as the spring preload. The pre-stress is too small and the plastic melt pushes the back end of the stud too far, causing too much material to remain in the stud area, causing shrinkage. The spring preload is too large and will not be compressed under the pressure of the melt, the effect is the same as the standard stud. The spring loading method also shows surprising results when measuring the shrinkage marks near the rib structure. Although this method is intended to minimize shrinkage near the studs, the shrinkage at the associated rib structure is surprisingly improved when processing PPE/PS materials. It may be that the stud is effectively filled with material into the rib structure when compressed, 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 is not optimized and is expected to perform well in larger size parts because it covers a larger area than the loaded spring studs. Moreover, as mentioned earlier, the formulation of the blowing agent in these tests was also not optimized.

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