Investment Casting Cluster Assembly: 12 Practical Tips to Prevent Shrinkage, Bridging & Defects

Investment Casting Cluster Assembly: 12 Practical Tips to Prevent Shrinkage, Bridging & Defects

As the saying goes, "Practice makes perfect." If you are observant and attentive, you can certainly discover a wealth of practical tips. Today, let’s discuss some key insights related to the cluster assembly process.

The assembly should not be too dense. Overcrowding can lead to blocked areas during shell building, which not only increases the risk of bridging but also hinders heat dissipation. Bridging may cause shell leakage, while poor heat dissipation often results in shrinkage porosity. Is there a minimum spacing requirement? Yes, generally at least 1 cm—approximately equivalent to four to four-and-a-half coating layers—is sufficient for small hardware components to avoid bridging and blockage. Sometimes, blindly pursuing high process yield isn’t always beneficial.

When assembling two components on the same tree, try to align them in the same direction. This makes slurry dipping and draining much easier during shell building. While many may already assemble parts this way, they might not fully realize how much it streamlines the coating process.

During tree design, position thin-walled or sharp-cornered areas closer to the main runner, while large flat sections with uniform wall thickness should be placed farther away. The former arrangement helps utilize heat for better filling, while the latter avoids excessive heat to prevent shrinkage defects. Thick and heavy castings generally benefit from top-gating systems to enhance feeding, whereas small to medium parts can use side-gating to improve yield.

When assembling parts with holes or grooves, tilt them at an angle to prevent the accumulation of slurry and moisture inside cavities. This helps ensure proper shell drying and heat dissipation. Poor drying may lead to steel leakage, while inadequate cooling can cause shrinkage issues.

Leave some space at the root of the runner to serve as a buffer zone. This helps reduce turbulence, promote smoother metal flow, and facilitate slag flotation and gas venting.

For thick-walled castings, gates can be designed thicker, but for thin-walled parts, gates should be wider rather than thicker. A thick gate in thin sections can create a contact hot spot, leading to shrinkage defects instead of improving feeding.

For parts prone to deformation or with low rigidity, position them during assembly to enhance overall stiffness—for example, placing frame-like parts vertically. When using a thick sprue with dual gates, adding a thin connector strip between the gates can reduce sprue shrinkage and minimize casting distortion.

A single bottom gate may overheat locally. Using multiple gates distributes metal flow and reduces defects like shrinkage, porosity, and hot tearing caused by concentrated heating.

If two gates on a casting are too close, consider merging them into one. Otherwise, the blocked shell between separate gates can negatively affect casting quality.

In reality, there are many such practical tips, yet when it comes to writing them down, one truly feels that "knowledge is most needed when it's to be used." Many methods from sand casting, such as orienting machined surfaces vertically, also apply to investment casting. We welcome everyone to share more hands-on insights from their own experience!