Cooling Systems In Injection Molding
Injection molding is a cyclical process wherein melted polymer is forced into a cold metal mold under high pressure and cooled until it is solid enough to be ejected from the mold. Cooling is a crucial part of the injection molding cycle because it has a major impact on both the quality of the part and the productivity of the mold. The time needed to cool a part in the mold is also a significant factor in determining the cycle time needed for the part, and makes up a majority of the total cycle time.
Many defects such as residual stresses, shrinkage and warping are caused by non-uniform cooling. These defects affect the quality of the final part in appearance and precision. Cooling can last more than two-thirds of the whole injection cycle, and the design of an efficient cooling channel can reduce the cooling time and improve the productivity of injection. Therefore the design of an optimized cooling system would achieve minimum cooling time and balanced (uniform) cooling. In order to have a better understanding of the injection molding cooling process, computer aided cooling analysis has been used as a helpful tool for the design of cooling systems and the evaluation of cooling conditions.
In the design of cooling lines, the goal is typically to achieve uniform cooling in relation to the part cavity. In multi-cavity molds, this also includes maintaining reliable temperature control across all cavities. Because temperature and resin properties such as viscosity and molecular weight go hand-in-hand, temperature monitoring is essential to keep parts within tolerance. In a uniform thickness part, cooling lines will generally be the same distance from the part across the surface, however in a part with uneven thickness cooling lines will be closer to the surface in thicker sections.
Residual Stresses: Shear stresses created by pushing the plastic into the mold can create residual stresses in the final part. The melt temperature and gate location also has a significant impact on residual stress by creating inconsistent properties across the part including non-uniform temperature and flow. In some parts, some residual stress is acceptable in order to reduce cycle time especially in high volume production.
Shrinkage: Part shrinkage can occur from insufficient cooling before ejection from the mold. In some cases, shrinkage in opposing directions can cause stress cracks in parts.
Warping: In addition to non-uniform cooling, warpage can be caused by ejecting a part from the cavity before it has sufficiently cooled. It can also be caused by a cooling system that is too cold.
Sink Marks: Sink marks are created when one area of the part shrinks more than the adjacent area, pulling material away from thinner areas and creating a defect.
Jetting: Jetting is be caused by variations in speed and cooling of the plastic flow inside the mold.
Water or oil is pumped through the mold, and cooled in an external heat exchanger before re-entering the cooling channels. The most common cooling fluid is a mixture of ethylene glycol and water which helps to prevent rust inside the mold. The liquid is pumped at a rate high enough to achieve turbulent flow within the mold, which is more effective in drawing heat from the mold than laminar flow.
In the injection molding process, resin is heated, injected into the mold, held at pressure, and cooled before ejecting the part. The resin is held at pressure during cooling in order to make sure that the cavity is completely filled, and prevent backflow while resin solidifies. If there is insufficient cooling or the part is ejected too early, ejector pins can damage the part in addition to a variety of other defects such as shrinkage, warping, and sink marks.
Mold temperature and cooling time are dependent on many factors, including the type and quality of resin, part size and thickness, and injection time. Melt temperatures that are too high can lead to degradation of the resin and longer cycle time. In difficult cooling situations, devices such as baffles, bubblers, and thermal pins can be used to increase the mold cooling efficiency. Baffles include a “blade” that divides a wider cooling channel in half to create a U-shaped turn. This increases the surface area and therefore increases the cooling in that area. In a bubbler, coolant flows through a tube and flows or “bubbles” out the top like a fountain. Thermal pins are used in slender cores, and have a sealed cylinder containing a fluid which vaporizes near the tip and condenses on the far end releasing heat into the main coolant channel.
Cooling analysis software was developed using the BEM approach and the theory of heat transfer. From this cooling analysis program, temperature distribution can be computed for parts and molds during injection molding. The program can also calculate the temperature profile of the part after being ejected from the mold. Experiments were carried out to verify the accuracy of the simulations compared to the physical process.
The cooling analysis of injection molding is adopted to predict the temperature distribution in both mold and part, using heat transfer theory and BEM to simulate the cooling of mold and part during injection molding, as well as the continued cooling of the part after being ejected from the mold. Experimental results obtained with an injection molded plate of ABS can validate the numerical predictions of the cooling analysis software. Although the numerical simulation results are acceptable, there is still much room for improvement. In order to get more accurate predictions, realistic thermal boundary conditions during cooling, crystalline material and material properties dependent on temperature and time should be taken into consideration when a theoretical mathematical model is being built.
Here is a brief description of the development of the simulation of plastic injection molding cooling. The pioneering work of Dusinberre focused on the prediction of temperature and pressure fields on rather simple geometries, and the one-dimensional transient model Finite Difference Method (FDM) was used to calculate the temperature distribution. Later, Keing, Kamai and Singh applied the two-dimensional Finite Element Method (FEM) to simulate the cooling process. Barone, Cauik, Burton and Rezayat first applied the Boundary Element Method (BEM) to calculate the temperature field, but it was limited to two-dimensional analysis. Since most injection molded parts are of three-dimensional complex geometrical configuration, in order to calculate temperature distributions based on three-dimensional cooling analysis, some researchers used middle-plane BEM to simulate the cooling of injection molding .
Conformal Cooling and Additive Manufacturing
Using metal additive manufacturing technology (such as DMLS), inserts can be produced with complex geometry including conformal cooling channels. Often this geometry is not possible to manufacture using machining because of internal geometry that is unreachable with cutting tools. In a conformal cooling channel, the geometry of the cooling can follow the shape of the part to achieve more uniform results. These cooling channels can be analyzed and optimized using Finite Element Analysis (FEA) programs.