Shrink plastic processors are facing the enemy, especially for high surface quality requirements of large plastic products, shrinkage is a chronic illness. Therefore, various techniques have been developed to minimize shrinkage, improve product quality.
Thick injection molded plastic parts in position, such as the ribs or protrusions formed at positions near the shrinkage is more serious than that, this is due to the thicker area of ??the cooling rate is much slower than the surrounding area. Cooling rate causes depression formed at the connecting surface, 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 household electrical appliances demanding products in this category must eliminate sink marks and surface quality for toys and some other less demanding products allow the presence of scar contraction.
????Causes the formation of sink marks may have one or more, including the processing methods, part geometry, material selection and die design. Including geometry and material selection is usually determined by the raw material suppliers, and less easy to change. However, there is still a lot of mold makers mold design on factors that may affect the contraction. Cooling flow channel design, the type of gate, gate size may produce a variety of effects. For example, if a small gate gate than the conical tubular gate cooling much faster. Premature cooling will reduce the gate cavity fill time, thereby increasing the probability generating sink marks. For molding workers, adjusting the processing conditions is to solve the problem plastic case in a way contraction. Filling pressure and time significantly affect shrinkage. Member filled, the excess material continues to fill the cavity in the shrinkage compensation material. Filling phase is too short will cause contractions intensified, eventually will produce more or larger shrinkage marks. This method may not be able to shrink scar itself reduced to a satisfactory level, but the molding clamshell packaging conditions have improved worker can adjust the fill shrinkage 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 suitable for all resins. In addition, the gas-assisted method is also worth a try.
?????Columns, gas and foam
?????GE Polymer Processing Research Center (PPDC) conducted a 12-month study to evaluate the clear plastic box eight different ways to reduce shrinkage marks. These techniques reduce shrinkage marks represent some of the latest ideas. These methods can be divided into packaging company two categories: one method can be called instead of material, and the other method is to remove the heat. Method is a substituted material may shrink by increasing or reducing the amount of material in the area to reduce shrinkage marks. Heat removal method to quickly possible to remove the heat shrink region, thereby reducing the thinner regions and a thicker zone, resulting in the possibility of uneven cooling.
????In this study, a total of five kinds of substitute materials assessment method: projecting lug, round lug with spring lugs, gas-assisted molding and chemical foaming. Three plastic package kinds of heat removal method: Beryllium – Copper lugs, beryllium – copper inserts as well as a specially designed thermal activity lugs. Evaluation objects are generated in the part to be tested number of sink marks, to be tested with a triangular member is convex products. Comparison of standard all standard tools – stainless steel lugs. The test tool can produce a disc thickness of 2.5mm, lug height 22.25mm, diameter 4.5mm, thickness of 1.9mm, 2mm on the chassis of the triangle.
????The molding equipment used in the study of the horizontal touch 350t hydraulic press, the material is commonly used in household electronic products material, but also a serious shrinkage material, i.e., GE’s PC / ABS, Cycoloy CU6800 and PPE / PS, Noryl PX5622. These two materials are in the range of processing technical parameters suggest the mid-point range. If the shrinkage marks in the minimum state, you can cut the amount of fill to raise more shrinkage marks to facilitate measurement and comparison with empirical methods. Although shrinkage marks are generally observed by the naked eye, but these tests using a machine of sink marks a quantitative measurement of the depth.
????Test content
????One standard technique for testing projecting lug, which is the standard lug projecting into the inside wall of the convex bottom of the column, thereby reducing the wall thickness and to compensate for the excess material to cause lug effect. Tests using two out depth of the wall thickness were 25% and 50% Another experiment used a round rather than a pointed lugs. This method is not material to plastics products remove lug area, but making the transition regions more coherent. There is a way out of the top plate and a spring between the lugs. Spring makes the parts cool kyphosis column under the pressure of the material is still in the state in order to obtain compensation for the material shrinkage effect. The results will be the initial pressure spring and the spring “rigid” effect, the test assesses the impact of these two factors. Used two different spring stiffness, the stiffness of each spring are applied to a variety of different initial pressures.
????Chemical blowing agents also evaluate the content of the trial, because the advantage of chemical plastic box blowing agents without any changes to the tool. The theoretical basis of the method is in the thick region is the area most likely to shrink foam, the foaming process may generate sufficient partial pressure to prevent shrinkage. Of course, during the foaming process can only use a small amount (0.25%) of the foaming agent (Safoam RPC-40), so as to prevent damage to the part surface cracks.
????Processed through the tabs to experiment with nitrogen gas-assisted injection molding, nitrogen is usually prone to shrinkage in the area to form bubbles, so that you can remove the material in the region where the gas bubbles to fill the area.
????In order to achieve rapid heat transfer by the use of a beryllium – copper lugs formed, much faster than the thermal conductivity of stainless steel. This technique also requires the rear lug with a huge hot pool connections, so that heat can be completely removed from the lug area. Another way of this method is the use of standard stainless steel lugs but the area around the mounting lug beryllium – copper plug. This requires modification of the mold cavity sufficiently in the region of a small bay processing of ribs / boss structure. The ribs / boss structure processing into independent beryllium – copper cavity plug-in installed in a small slot. High rate of heat conduction plugin will lug area and completely absorbed heat into the tool. The first two methods using a passive heat removal method, “lug thermal activity” includes a fluid heat away from the hot zone and dispersed into the cooling device.
????Comparison of results
????Using the PC / ABS material, the five test methods to produce the contraction produced than standard lug less shrinkage. All heat removal method works well, the method of replacing only the material of the spring-loaded lug lug than the standard plastic trays method is good, and the spring pre-loading pressure is particularly prominent effect on performance. The results of gas-assisted methods are not conclusive: the use of that mold and the material, due to thin wall products, melting – cooling speed is too fast, so the gas permeability is difficult to maintain consistency. Foaming test is not a decisive influence. Showed significant cracking packaging design of the part surface, the method can not be compared with other methods, you should reduce the number of blowing agent.
????Use PPE / PS resin, spring loaded lugs same outstanding performance. Substitute materials other three methods, including projecting lugs and gas assisted molding method is better than the standard lug good effect. For heat removal method, only beryllium – copper lug lug than the standard approach works well.
????The round lug method for the effect of the two materials are not good. Surprisingly method for projecting lug PC / ABS material terms of the effect is very good, and the last two decades, has been projecting lug is the recommended method. These results indicate that these methods for different materials, the effect is not the same purpose.
????The most interesting result comes from the spring-loaded lugs approach. For both materials, the proper use of the spring pre-stress, shrinkage resistance are obtained a 50% improvement. The impact of spring steel spring preload seems less impact. Pre-pressure is too small, plastic melt will lug on the back side push too far, resulting in too much material retaining lug area, resulting in contraction. Spring pre-pressure, the pressure of the melt will not be compressed, the same effects and standard lugs. Measuring muscle contraction marks near the rib structure, the spring loading method also shows amazing results. Although this method is designed to lug around minimize shrinkage, processing PPE / PS material, attached to the ribs structure has also been shrinking at an astonishing improvement. May be efficiently compressed lug filled into the ribs of the material structure, thus reducing shrinkage.
????Whatever the outcome, plastic packaging one should not underestimate this gas-assisted molding method and the method of chemical blowing agent. For gas-assisted molding, the mold has not been optimized for the larger size components is expected to play a very good effect, because it can cover wider area than spring loaded lug greater coverage. Moreover, as described above, a blowing agent formulation in these tests has not been optimized.