Curing the Porosity Problem in Additive Manufacturing

Additive manufacturing, also known as 3D printing, allows the fast and cost-effective production of complex high-quality components in a range of materials. The rise of this technology has been fast, and it is rapidly altering the manufacturing landscape.

Additive manufactured plenum chamber vacuum impregnated to prevent pressure loss through component porosity.

In 2019, the global additive manufacturing market size was valued at $11.58 billion and is predicted to grow at a CAGR (compound annual growth rate) exceeding 14% from 2020 to 2027 (GVR). Additionally, research from Deloitte shows that additive manufacturing is empowering industry 4.0.

3D printing was once a revolutionary piece of technology, but it has now become a mainstream process in a number of industries. From the automotive industry relying on additive manufacturing for jigs, fixtures and grips, to aerospace using it for functional aircraft components, 3D printing has seen a significant growth in applications recently. Due to its clean and simple process, additive manufacturing produces high-quality components and removes the need for expensive tooling and machining. 3D printing is also a cost-effective, quick way to produce prototypes, one-offs and components in low volumes, and is particularly suited to making small and intricate parts.

An Invisible Porosity Problem

While some industry pain points are removed as manufacturing technology evolves, one legacy challenge still remains: porosity. During the additive manufacturing process, microscopic holes that are invisible to the naked eye are formed within the body of the part. Known as porosity, this is already an inherent issue with diecast components, and while the cause and application might be different, the result is the same with components ending in scrappage.

In most cases, porosity is caused either by the powder used in the process, or the printing process itself. These microscopic voids reduce the density of components, leading to leaks, fatigue and cracks. For parts that go into applications that need to be air or fluid tight - for example in fuel or cooling systems – this can be an especially critical issue.

Vacuum Impregnation

A technician loads a batch of components into the hot cure module.

Vacuum impregnation is one way that additive manufacturing businesses can reduce the waste, cost and productivity impact of porosity. By partnering with sealing experts, 3D printing manufacturers can prevent gases and fluids from leaking through a component by sealing any voids with a chemically and thermally resistant polymer sealant.

3D printing consultant, Graphite AM, wanted to ensure their customers were getting the best product quality and realized they needed to work with a porosity sealing partner in order to do so. They chose to partner with Ultraseal International, a company that specializes in the development, manufacture, and supply of porosity sealing chemicals, impregnation equipment and services.

Overhead view of Ultraseal International top-loading impregnation equipment.

Porosity Sealing for Complex Components

Graphite AM specializes in complex designs and high-performance components in tailored materials, and primarily uses SLS (selective laser sintering), an additive manufacturing process that deploys lasers to sinter powdered material, binding it together to create a solid structure.

While the majority of 3D printing bureaus use standard or glass-filled nylon materials - such as PA11 and PA12 -Graphite AM has developed its own range of unique SLS blends, including the use of fine graphite particles. As well as having impressive anti-static properties, the use of graphite also improves impact and thermal resistance (up to 170°C), meaning it is particularly suited to lightweight applications where strength and performance are critical factors. This includes components for automotive applications, turbo system components, plenum chambers, oil and water pipework and manifolds, fuel cells, and electric vehicle (EV) battery cooling systems. Graphite AM also produce components for mission critical applications including environmental monitoring systems and unmanned aerial vehicles (UAVs).

Graphite AM chose to partner with Ultraseal International due to their track-record in the automotive industry and unrivalled experience and understanding of the component impregnation process. Ultraseal’s solution involves sealing the component using vacuum impregnation, a process that uses three key stages to ensure the highest-quality component sealing.

First, components are placed into an autoclave containing Ultraseal PC504/66 resin, a high performance thermocure sealant. Once the components are submerged, a vacuum is applied to evacuate the air within the autoclave. After 20 minutes the vacuum is released; through the resultant change in pressure, the sealant penetrates into the micro-porosities and leak paths within the 3D printed part and seals them.

The second stage involves a cold wash using standard tap water to remove the excess sealant from the component surfaces and critical features such as threaded holes.

The third process stage is a hot cure cycle. The hot water cure uses demineralised water in a 3,000-liter capacity heated tank. This solution contains a corrosion inhibitor to provide some corrosion protection for certain metallic materials. The water operates at 95°C and parts are submerged for ten minutes to ensure the Ultraseal sealant polymerises, making it an integral part of the component.

An additively manufactured pipe base.

Quality Assurance Testing

Once this process is complete, pressure testing is completed by either conventional air under water testing, or hydraulic testing, depending on the customer's request. There are two tests that can be conducted – either a ‘proof’ or ‘leak’ test. A proof test is to ensure that the component is structurally capable of withstanding a certain pressure. Meanwhile, the leak test is to detect porosity and leaks within the component.

Ultraseal design and manufacture ‘test tooling kits,’ which are required to blank off holes and ports, while delivering air or water into the part to search for leaks. Every component requires its own test kit in accordance with its geometry and features. The amount of pressure the component is subject to varies as per customer request; this is often determined by the operating pressure of the component when in the field. Historically these parts can be tested from a range of 5 PSI to 10,000 PSI.

These pressure tests produce critical information as to whether the parts are leak free and fit for purpose, or whether porosity is present, which would result in a failure in the field. Vital information can also be communicated to the manufacturer during this stage, such as leak location and severity, which aids knowledge and understanding of component quality going forward. As additive manufacturing becomes commonplace in manufacturing supply chains, so too will porosity sealing technology, allowing manufacturers to drive up throughput, reduce scrappage, minimize waste and increase efficiency and value.

This article was written by Dr. Mark Cross, Commercial Sales Director, Ultraseal International Group Ltd. (Coventry, UK). For more information, visit here .