Serial No. 9 (Friends who are interested can follow Gud Mold and view previous serials from historical messages)

 

Heat transfer efficiency of cooling system has a great influence on product deformation. Due to complexity of product structure, an absolutely uniform cooling effect cannot be achieved. In actual operation, connecting water lines in heat accumulation area separately and using a lower temperature cooling medium will help to achieve uniform cooling. There are two ways to improve cooling effect of heat accumulation area: first, increase flow of cooling water in heat accumulation area, usually by connecting heat accumulation area to a separate water channel; second, reduce temperature of cooling medium in heat accumulation area. Usually in practice, a combination of the two is used, i.e. lower temperature is connected in series alone.
Generally speaking, temperature of mold surface is affected by two factors: first, heat input by melt; second, heat taken away by cooling system. In one cycle, heat input and heat output in each area of mold are approximately equal, which means the entire mold is cooled evenly. If local input is greater than output, mold temperature will rise and form a hot spot. In this case, it is necessary to strengthen cooling of hot spot area and quickly remove heat to obtain an overall uniform cooling rate. Cooling water circuit temperature determines mold temperature but is not equal to mold temperature, thus affecting shrinkage of product. It should be pointed out that during an injection molding cycle, temperature of mold changes and is not constant. We need to look at mold temperature from a dynamic perspective. Mold temperature value measured after product is ejected can only be used as a reference. Actual mold temperature during injection pressure holding and cooling process is higher than measured mold temperature. High mold temperature means that product has a longer time to shrink and a greater shrinkage rate; but from another perspective, melt viscosity is lower under high mold temperature and pressure-holding effect is better, which can better compensate for shrinkage. Low mold temperature means that product solidifies quickly and shrinkage time must be shortened. Therefore, shrinkage degree is lower than that of high mold temperature, size is generally larger at higher mold temperature. However, if better pressure holding effect of high mold temperature is considered, result is not certain.
By artificially adjusting local temperature difference of mold, purpose of improving local uneven shrinkage of product is of great significance in actual operation. If mold designer can deeply understand this relationship and face products with complex structures, they can optimize mold waterway design and optimize heat accumulation area waterway so that they can be connected individually to achieve enhanced cooling of local areas, which is of great positive significance for improving product deformation. Otherwise, only relying on downstream molding process to solve this problem will achieve twice the result with half effort. At the same time, designers should start from this point and put forward suggestions for product improvement and optimization. By improving product structure from source, the best quality mold can be obtained.
It is generally believed that increasing mold temperature can increase shrinkage of material. Because high mold temperature plastic materials take longer to cool and solidify, they shrink more and product sizes tend to become smaller. This is true for both crystalline and amorphous plastics. However, in actual injection molding production, when product size is too large, countermeasures of increasing mold temperature are not always effective, and sometimes size becomes larger. This is because on the one hand, high mold temperature can promote shrinkage, but on the other hand, high mold temperature provides better conditions for pressure holding, so pressure holding effect becomes better. If space released by shrinkage of plastic can be fully compensated by melted glue added by pressure maintaining in time, shrinkage of plastic will become smaller. Therefore, we must clearly understand these relationships before we can truly solve problem.

Cooling time required for injection molding is determined by thickness of product glue level, mold temperature, melt temperature, and heat dissipation effect of heat accumulation area. If material thickness is thick, mold temperature is high, melt temperature is high, and heat dissipation effect in heat accumulation area is poor, cooling time required is long; conversely, cooling time is short. The longer cooling time required means higher injection molding production costs.
Cooling time determines how long injection molded parts remain in mold cavity. We know that mold is a shaper, which plays a role in shaping and constraining shrinkage of injection molded parts. The longer cooling time, the greater shaping effect of mold on product, and the longer time to restrain free shrinkage of product. Therefore, cooling time has a great impact on shrinkage of product.
Usually in actual operation, if size of product is too small, appropriately extending cooling time setting can inhibit shrinkage of product and make size larger; if product warps seriously, this method can also be considered appropriately. However, we know that deformation of plastic parts is essentially result of internal stress. If internal stress cannot be effectively reduced, this method of improvement by shaping is usually ineffective and requires high costs. Using a jig to correct deformation after product is ejected can also improve deformation to a certain extent. Using a jig correction method, it is necessary to shorten cooling time. That is, when internal solidification of product is not sufficient, but it has reached a state that can be ejected, a jig is used to deform product. When internal molecules of product are completely solidified, jig is released. But this is a very helpless approach, indicating that there is no other way out, and it is the last resort.

 

As mentioned above, extending cooling time prolongs shaping effect of mold, which is also limiting effect of mold. For analysis of deformation, mold restriction effect is a factor that cannot be ignored. Restricting effect of mold can inhibit free shrinkage of product. However, in mold cavity, mold has different restrictions on various directions of each area of product, which in turn leads to shrinkage anisotropy.
Structure of product determines restricting effect of mold. For a simple flat product, mold has almost no restricting effect on product; but if flat plate is covered with many holes, restricting effect is different; results will be different if holes are evenly distributed or there are local holes. Deep cavity parts such as trash cans (those with flanges and those without flanges) will shrink differently in Z direction.

Introduction to ambient temperature affects effectiveness of heat conduction because heat will eventually be radiated to surrounding environment. For precision injection molding production, maintaining a constant temperature environment is very important. Ambient temperature will also directly affect post-shrinkage of product after it is released from mold, thus affecting deformation.

Performance of injection molding machine mainly refers to: accuracy of screw; response sensitivity of control system; mechanical accuracy of moving/fixed template, and stability of machine, etc. It is reflected in: difference in screw position of each shot; difference in amount of glue per shot; difference in temperature control; and pressure transmission, etc. These factors will affect degree of shrinkage of injection molded parts.

Characteristics of plastic are an important factor affecting deformation of injection molded parts. Even if it is same company and same brand of plastic raw materials, there will be subtle differences in properties if factory batches are different. Main influences on deformation include differences in fluidity, mechanical strength, shrinkage, etc. Usually these variables will not significantly affect deformation of product, but when molding window of injection molding is extremely narrow, then some small differences in materials will cause product quality to exceed standards.
Difference in fluidity affects molding pressure and degree of pressure distribution in mold cavity; difference in mechanical strength affects performance of internal stress. For same degree of residual internal stress in product, deformation performance will be different if material strength is different.
Here is the whole series of causes and countermeasures of deformation of injection molded parts: 1, 2, 3, 4, 5, 6, 7, 8, 9.