Mold builders can increase the quality of a finished product and reduce manufacturing costs by minimizing irregularities or imbalances in the cooling process with improved thermal performance of injection and blow molds.
The limitations of traditional cooling channel geometries and their placement are often not optimized for rapid heat extraction. Enter conformal cooling, where intricate passageways are created that closely follow the shape or profile of the mold core or cavity. With this approach, you can inject melted resins more rapidly and at higher pressures, significantly decreasing cycle times, increasing throughput and improving product quality.
To achieve high-quality engineered plastic components, the mold must use a uniform thermal distribution pattern, which requires the coolant channels (or heating elements) to be very near the mold’s edge and mirror the component’s geometry.
Although the benefits of conformal cooling are established, the larger question is how to achieve this result in the most efficient, effective moldmaking approach possible. To date, there has been little consensus on the best approach for cooling channel design.
Mold builders have machined cooling channels into molds in straight lines for many years. When more intricate channels were required, they could make the mold in multiple layers with pre-machined channels and then braze them into a final assembly.
Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint. The filler metal flows into the gap between the layers through capillary action. Although brazing can be used to join similar or dissimilar metals with considerable strength, it has drawbacks when internal passageways are involved. For example, brazing can cause small “fillets” to form in the passageways that obstruct flow and break off during use.
Brazing alloys typically used with tool steel molds are nickel-based, which can lead to a variation in thermal performance that make it difficult to provide temperature uniformity across intricate or delicate polymer-based components during processing.
Diffusion bonding is an alternative to making complex molds, as this process efficiently joins multiple layers of high temperature-resistant tool steels and stainless steel without brazing material. Moreover, it is not necessary to add a different material to join the layers, and one ends up with a mold optimized for thermal performance.
For many years, manufacturers have used diffusion bonding to join high-strength and refractory metals that are either difficult or impossible to do so by other means. The process—which involves applying high temperature and pressure to similar or dissimilar metals mated together in a hot press—causes the atoms on solid metallic surfaces to intersperse and bond. Due to the molecular bonding of the layers, the final part often shows no interface lines or striations, as the interface of one material is blended into the other one, and vice versa, even with dissimilar materials.
In addition, this technique allows moldmakers to build molds from the ground up using thin 1-2-millimeter metal and alloy sheets, as well as map out sophisticated internal channels or passageways with 3D modeling software before cutting each layer. A mold builder can incorporate more complex cooling channels into injection or blow molds using a multilayer design, for example, enabling high pressures. Once the mold layers are bonded, the final external shape can be machined.
3D printing is also being used to create conformal cooling channels. Although this approach accomplishes similar results, 3D metal-printed parts are limited in size and struggle with yielding a high-quality surface finish on machined channels and finished parts. Diffusion bonding produces finished molds that have an external geometry as large as 900 millimeters (35.43 inches) x 1,250 millimeters (49.21 inches) x 500 millimeters (19.69 inches), including smaller molds in batches.
Advances in Diffusion Bonding Equipment
Advances in high vacuum hot presses enable superior pressure control and rapid cooling systems to improve the bond, increase yields and significantly decrease cycle time.
For example, in the case of the pressure applied, integrated, single-cylinder hydraulic presses can apply a consistent, measurable amount of force. However, this provides very little control over large parts with more complex geometries. To improve force distribution, thick graphite pressing plates (10-15 inches in height) mate the metal layers together at a more consistent pressure. Unfortunately, this takes up furnace space while adding to the time to heat the surfaces of the metals.
The first law of thermodynamics states that energy cannot be created or destroyed, but only transferred from one place to another. If you think of an injection mold as a closed thermodynamic system, you can account for the energy entering and leaving the system. A shot of molten plastic injected into a mold is the energy input, and energy leaves by various mechanisms. Some of the energy (heat) is carried away in the ejected parts and some is removed by the cooling system. It is less obvious, but some energy will also go into the surrounding environment when a mold is heated substantially above room temperature.
For simplicity, we can ignore any heat loss due to elevated mold temperature. This leaves us with three relatively simple tasks to quantify the energy flow and estimate cooling requirements, assuming a traditional water-circulation cooling system:
1. Understand the thermal properties of polymers and learn the method for calculating energy values and flow for both amorphous and semi-crystalline materials.
2. Calculate the (cooling) energy that must be removed from the molded parts to change the temperature from the processing (melt) temperature to a safe ejection temperature.
3. Using the cooling energy requirement as a starting place, determine the diameter and length of cooling circuits, the coolant ∆T value, and the coolant flow rate required to be turbulent and to remove the required amount of heat (energy)