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How to reduce the shrinkage of plastic processing

 

Shrinkage is the enemy facing plastics processors, especially for large plastic product surface quality requirements of the higher, shrinkage is a chronic illness. Therefore, various techniques have been developed, in order to minimize shrinkage, improve the quality of the products.
 
In the injection molding plastic parts with thick position, such as the formation of the ribs or protuberance contraction to be more serious than the adjacent position, which is due to thicker regions of the cooling rate is much slower than the surrounding area. Different cooling rate leads to the formation of depression in the connecting surface, namely the contraction of the familiar marks. This deficiency severely limits the design and molding plastic products, especially for large thick products such as TV casing and a display case. In fact, for household electrical appliances such strict requirements of the product must eliminate shrinkage mark, and the surface quality toys and some other products is not high to allow contraction marks exist. 
 
Reasons for the formation of scar contraction may have one or more, including processing methods, part geometry, material selection and tooling 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 maker mold design factors may affect the shrinkage. Cooling channel design, the types of gate, gate size may produce a variety of effects. For example, a small gate such as pipe gate than cone-shaped gate much faster cooling. The gate early cooling will reduce the cavity filling time, thereby increasing the probability of resulting scar contraction. For the molding of workers, a way to solve the shrinkage problem is to adjust the processing conditions. Filling pressure and time significantly affect the shrinkage. Parts after filling, excess material to fill in the cavity to compensate for material shrinkage. Filling stage is too short will cause shrinkage, and ultimately will produce more or larger shrinkage mark. This method may not be able to shrink itself down to a satisfactory level of marks, but the molding workers who can adjust the filling conditions improved shrinkage mark. 
 
Another method is to modify the mold, there is a simple solution is to modify the conventional core hole, but does not expect this method is applicable to all the resin. In addition, gas assisted method is also worth a try. 
 
GE Polymer Processing Research Center (PPDC) is a 12 month study, to evaluate 8 different methods to reduce scar contraction. These techniques represent a reduction of some new ideas of scar contraction. These methods can be divided into two categories: a class can be called to replace the material method, another kind is the removal of heat method. Replace the material method is to reduce the shrinkage mark by increasing or decreasing the amount of material may shrink the region. The shrinkage in the rapid heat removal may be designed regional heat removal, thereby reducing the possibility of cooling thinner region and thicker regions produce uneven. In this study, a total of 5 substituted material evaluation method: extended convex columns, round convex column, with spring convex column, gas assisted molding and chemical foaming. Three removing heat method: beryllium copper convex column, beryllium copper insert and the special design of the convex column thermal activity. Evaluation of the object is to try the number of shrinkage mark components are produced, tested components for products with triangular convex. All method is compared with the standard as the standard tool -- convex stainless steel column. The testing tool can generate a wall thickness of 2.5mm disc, a convex column height is 22.25mm, diameter 4.5mm, wall thickness of 1.9mm, a triangle 2mm in the chassis.
 
Molding equipment used in the study of 350t level touch hydraulic machine, material is commonly used materials household electronic products, but also the serious problem of material shrinkage, namely GE PC/ABS, Cycoloy CU6800 and PPE/PS, Noryl PX5622. Processing range of the two materials are in the middle of some technical parameters of products range. If shrinkage mark at a minimum, can cut the filling amount to trigger more shrinkage mark, easy to measure and compare with the empirical method. Although the shrinkage mark is usually observed by the naked eye, but these experiments use a machine on the contraction of scar was quantitative measurement. 
 
One of the standard testing technology is extended convex column, namely the standard convex columns protruded into the convex column at the bottom of the wall, thereby reducing the wall thickness and cause the excess material compensation convex column effect. The experiment using two kinds of extended depth of wall thickness, respectively 25% and 50%. Another test used a round head instead of the convex column. This method is not the removal of the convex column region of the material, but makes the transition regions more coherent. There is also a method in the top plate and the convex column between the use of spring. Spring makes part of cooling material convex column under still is under pressure, so that the materials obtained shrinkage compensation effect. Effect of results can be affected by the initial pressure and the spring spring "rigid", test to assess the impact of these two factors. The use of two different spring stiffness, stiffness of each spring exerts a variety of different initial pressure.
 
Chemical foaming agent in the evaluation contents of the test, because the chemical foaming agent has the advantage of not to make any changes to the tool. The method is based on the theory in the thicker is the most likely areas of regional contraction foam, foaming process will produce enough local pressure to prevent shrinkage. Of course, use only a little in the foaming process (0.25%) of the foaming agent (Safoam RPC-40), so as to avoid the formation of surface crack damage parts.
Through the convex column injection nitrogen processed to test gas assisted molding, nitrogen in usually prone to shrinkage region formed bubble, so that it can remove the material of the region filled with gas bubbles.
 
In order to realize the rapid heat transfer, the use of a composed of beryllium copper convex column, heat conduction is much faster than the stainless steel material. The technology also requires convex back column connected with the hot pool is huge, so the heat can be completely removed from the convex column area. Another way this method is the use of standard stainless steel convex column but in the area around the installation convex pillar beryllium - copper plug. This requires the mold cavity full of changes, in the region of processing to install a small tank reinforced rib / convex column structure. Reinforced rib / convex column structure is processed into independent beryllium - Copper cavity plug-in, installed in a small trough. High heat transfer rate plug-ins will completely convex heat absorption column region and into the tool. The first two methods are used in passive heat removal method, "thermal activity convex column" contains a fluid to remove heat from hot areas and dispersed to the cooling device. Use of C/ABS material, to produce a five test methods of shrinkage shrinkage less than standard convex column. Effect of heat removal method of all good, replace the method of convex column only spring loaded material method than the standard convex column effect is good, and the effect of the spring pre-load pressure on the performance is especially outstanding. Gas assisted method results are not decisive: the use of the mold and material, as the wall is too thin products, melting and cooling rate is too high, which is difficult to maintain consistent gas permeability. Foam test did not decisive. Significant part surface cracks show that, this method can not be put on a par with other methods before, should reduce the amount of foaming agent. The use of PPE/PS resin, the convex column spring loaded equally outstanding performance. The other three kinds of substituted materials and methods, including extended convex column and gas assisted molding effect than the standard convex column. For the removal of heat method, only the beryllium copper convex column method is better than the standard convex column method has good effect. The effect of round convex column method for two kinds of materials are not good. The accident is extended convex column method for the case of PC/ABS effect is not very good, but for twenty years, extended convex column has been recommended method. The results of these experiments show that these methods for different materials are not the same.
 
Methods the most interesting results from spring loaded type convex column. For the two kinds of material, the appropriate use of the spring preload, shrinkage are all 50% improvement. Influence of spring steel seems as spring preload large. The pressure is too small, the plastic melt the convex column back end pushed too far, resulting in too many areas remain convex column material, causing contraction. Spring preload is too large, will not be compressed in the melt pressure, effect and the standard convex column. Shrinkage mark measured near the ribs structure, spring loaded method also shows a surprising result. Although the method aims to minimize shrinkage convex column nearby, processing of PPE/PS materials, reinforced rib structure connected contraction has been improved dramatically by. May be a convex column compression effectively into the ribs structure filling materials, thereby reducing the shrinkage.
 
Regardless of the outcome, people also should not underestimate the gas assisted molding method and chemical foaming agent method. For gas assisted molding, mold is not optimized.
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