Discussion on the Deep and Fine Hole Investment Casting Shell Process

I've been working in the investment casting industry for over 20 years. I'd like to share the operational details and process design of the deep and fine hole investment casting shell process for your reference.

As shown in the figure above, the product has abnormal casting nodules and fins. This problem is a typical problem caused by insufficient strength after shell making in the hole. The insufficient strength comes from the following reasons and corresponding measures are as follows:

1. The mold shell is not dense and the strength is low.

Reasons for the mold shell not being dense:

(1) The slurry viscosity is high and cannot flow fully into the hole;

Measures: Reduce the viscosity

(2) The sand material is coarse, and the sand on the mold shell is bridged and not dense;

Measures: According to the size of the hole, select the appropriate sand material particle size to meet the principle of shell making density.

(3) The hole itself is very small (the diameter-to-depth ratio is not appropriate), the coating and sanding cannot be dense, and the hole forming by shell making is not feasible;

Measures: Before designing the shell making process, evaluate the diameter-to-depth ratio. If it is too large, the hole can be formed by machining or by ceramic core, glass tube, self-hardening core, inserting ceramic rod or wooden stick, etc. Generally speaking, if the diameter-to-depth ratio is greater than 1.2, the difficulty of shell making will increase.

(4) In terms of sizing and slurry control, the sizing and slurry control were not sufficient, and the coating was not dense;

Measures: The sizing and slurry control angle and time should be sufficient to ensure the effective coating of the slurry; grouting is also a common method.

(5) In terms of sanding operation, the sanding was not sufficient;

Measures: The sanding angle and time should be sufficient to facilitate sanding; grouting is a common method.

(6) In the first three layers of shell making, the bridging was not handled, and the bridging existed, resulting in the subsequent shell making being not dense;

Measures: After the shell was sanded, check whether there was a bridging phenomenon, and if so, deal with it;

(7) There were sand lumps in the sand material, which was not conducive to sanding the hole.

Measures: The sand material should be screened regularly.

2. The paint deteriorated and its own strength was insufficient;

Measures: Daily maintenance of the slurry should be done.

3. Insufficient drying and insufficient strength;

Measures: The drying temperature, humidity, wind speed and time should be controlled. Consider using special drying tooling when necessary. New technologies: Quick-drying adhesives, infrared drying, and vacuum drying can be used.

4. Surface tension prevents the coating from being fully applied.

Measures: Add sufficient wetting agent and defoaming agent to the slurry mix; consider using compressed air to actuate the slurry during application; pre-wetting should be used to remove microbubbles from the slurry and improve its fluidity; ensure sufficient mixing time during slurry preparation to allow the slurry to mature and maintain good fluidity.

5. Unreasonable sharp corners on the hole edges can lead to stress concentration.

Measures: Design rounded edges.

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