Metal Additive Manufacturing Enables Conformal Cooling for Plastic Injection Molding

When using plastic injection molding (PIM), it’s important to control heat.

The majority of injection mold tools and dies are made with cooling channels that have been drilled or milled from a block of steel. Cold water flows through these channels to cool the core and draw heat away from the plastic. Traditionally, these paths are made by drilling straight holes. However, the effectiveness of the method is limited when there is a complex shape to be molded, as channels can only follow straight line-of-sight pathways.

The practice of conformal cooling overcomes this limitation. With conformal cooling, the channels can be designed to follow the shape of the part evenly, hence, they ‘conform’ to any shape for optimum efficiency and faster cycle times.

At Star Rapid, we’ve been conducting research to explore how metal 3D printing can be utilized for conformal cooling. It was decided that the best way to evaluate the efficiency of pairing conformal cooling with 3D printing first-hand was to apply the method to an existing PIM customer project – Marco Beverage Systems’ hot water reservoir.

Marco Beverage Systems designs and manufactures specialty coffee makers, water boilers, filters and coffee grinders. The company uses a transparent and high-quality reservoir cup to supply hot water to the coffee making system. This is a part that is created using plastic injection molding and had previously been manufactured using traditional straight-line cooling channels.

Below is an outline of the steps taken to apply conformal cooling methods to the plastic injection molding tools in an attempt to streamline the production process, while achieving a high-quality final part.

Making the Inserts

Creating conformal cooling channels using metal 3D printing takes longer than conventional methods, but costs can be recouped due to improved cycle times and product quality, thereby lowering production costs for large volumes. The conformal cooling channels were designed to closely follow the contours of the part, while staying close to the internal walls of the tool for greater efficiency.

The inserts were printed from maraging steel and oriented vertically on the print bed of a Reinshaw DMLM AM250 powder bed metal printer. Vertical printing avoided the need for additional supports and prevented the holes from collapsing.

3D printed cores and inserts mounted onto the plastic injection molding base plates.

After printing, the parts still had a rough surface texture, which is to be expected when 3D printing methods are used as the part is built layer-by-layer. The inserts were later CNC post-machined and polished to achieve a high gloss. Although the total volume of coolant required for the new insert is roughly the same between the original insert that used a straight-line channel, the new 3D printed insert has a channel follows the taper of the tool more closely and has a greater exposed surface area against the tool wall.

Creating the Cores

Two different CAD designs were created for the conformal cooling cores – one by Star Rapid’s engineers and one from LBC Engineering. LBC, a subsidiary of Renishaw, specializes in advanced design solutions for complex 3D printing applications. Star Rapid’s design improved cooling efficiency, but the location of the channels didn’t leave enough room for other supporting architecture in the mold.

The mold in its fully closed position.

The inserts were mounted on the B-Side of the plate, below (right). This is the plate that opens and closes for each cycle. The A-side (left) held the core, which remained fixed during each molding cycle.

It was important to carefully polish the outer surface of the tools. This created a glossy finish on the transparent parts and helped the parts to slide off the tools easily after molding.

Results

With the new injection mold tools, a series of test runs were conducted to compare results to the benchmark studies. The 3D printed cores and inserts performed significantly better than conventional mold tools. Not only was conformal cooling able to help greatly improve the product quality, but it reduced the cooling time by 38 seconds, or 60 percent. This eliminated the need for additional manual air cooling.

This reduction in cycle time can be a significant cost-savings for longer production runs, and may go a long way to offsetting the relatively larger expense that comes from developing such a tool. Working closely together, with Renishaw and LBC Engineering, we created a solution that greatly reduced cycle time while improving quality. We will continue to refine this process for future advanced applications, as R&D efforts are expanded.

Chloe Kow is DMLM Manager for Star Rapid.