Technical Manual for Heat Pipe & Vapor Chamber Manufacturing Processes

01 Significance of Secondary Degassing

Secondary degassing serves two core purposes in the production process:

1. Further removal of non-condensable gases (air) in the cavityAfter the vapor chamber is filled with working fluid and vacuum-pumped (primary degassing), residual non-condensable gases such as hydrogen, nitrogen, carbon dioxide and oxygen still remain inside the cavity.Studies have shown that non-condensable gases in heat pipes will reduce their heat transfer performance and service life (e.g., through oxidation). For vapor chambers, the presence of non-condensable gases will obstruct vapor flow and slow down the reflux speed of liquid working fluid, which in turn increases the temperature difference of the vapor chamber and degrades its heat transfer performance.Since the secondary degassing process directly affects the internal vacuum degree and non-condensable gas content of the vapor chamber, special attention must be paid to this process in production.
2. Trimming of the liquid injection nozzle (rat tail)The liquid injection nozzle is often referred to as the "rat tail nozzle/rat tail tube" in the industry due to its similar shape to a rat tail. There are four common structural forms of liquid injection nozzles: concave, convex, mixed concave-convex, and flush.Dual liquid injection nozzles are also available for some products with extremely complex shapes and large sizes, which may require multiple degassing ports—though this design is rare in mass production. Separate designs for liquid injection ports and degassing ports can also be found in some patent documents.As the equipment for primary degassing is inherently complex and the designs of different liquid injection nozzles vary greatly, the sealing after primary degassing can be regarded as an initial seal (or rough seal). Secondary degassing enables trimming the liquid injection nozzle to the final desired shape with simple equipment and molds.

02 Methods of Secondary Degassing

The primary objective of secondary degassing is to further remove non-condensable gases in the cavity. There are three common methods, with the first two being the most widely adopted in industry:

Vacuum pumping degassing method

This method simply involves repeating the vacuum pumping process once or even multiple times. From the perspective of process and equipment, there are two approaches to secondary vacuum pumping:

Approach 1: Use a long degassing tube and repeat the cycle of pumping (pre-vacuum) → breaking (breaking the nozzle seal) → pumping (connecting to the cavity for continuous pumping with bottom heating) → sealing (sealing the liquid injection tube).

Approach 2: Replace the breaking and sealing of the degassing tube with a valve, following the same pumping-breaking-pumping-sealing cycle. The principle is consistent with Approach 1, with only the actions of piercing the degassing tube and using a sealing mold replaced by a vacuum valve. The key advantage of this approach is that it allows continuous secondary degassing for multiple cycles, whereas Approach 1 requires disassembly, cutting off a section of the tube, re-clamping and restarting after each cycle.

Evaporation extrusion degassing methodIt is worth noting that the third step of the first secondary degassing method also involves heating, which actually incorporates the evaporation extrusion degassing method.This method involves heating the product to raise its temperature, causing internal moisture to evaporate. The flowing vapor drives the non-condensable gases trapped in the cavity and capillary structures to move vertically upward into the degassing tube. The degassing tube is then sealed at the root, and the upper section containing non-condensable gases and vapor is cut off.This is the most commonly used method in actual production due to its simple equipment, low cost, high production efficiency and ease of automation.

Vacuum pumping first, then liquid injectionAs mentioned in previous technical documents, this method is rarely used due to its extremely complex equipment and difficult process control, and thus will not be elaborated in detail here.

03 Parameters Affecting Secondary Degassing Effect

Numerous scholars have conducted experimental research on the secondary degassing process, and manufacturers also adjust and test parameters for different products—balancing the thorough removal of non-condensable gases, control of water loss and production efficiency.The key parameters affecting the secondary degassing effect are as follows:

1. Heating temperature
2. Constant temperature holding time
3. Heating area and heating position
4. Vacuum pumping time (for the vacuum pumping degassing method)
5. Length of the degassing tube
6. Diameter of the degassing tube (variable diameter designs are adopted in some cases, e.g., a large gas storage space is intentionally designed in the middle of the degassing tube as shown in relevant diagrams)

There are no fixed formulas for designing these structures and parameters. Only the variation rules can be mastered, and the specific parameters need to be continuously adjusted and optimized in actual production.

04 Summary

In general, secondary degassing is a critical process for both heat pipes and vapor chambers. Especially for slender products and products with numerous corners and edges, primary degassing alone is insufficient to fully remove non-condensable gases, and secondary degassing is required to reduce the content of non-condensable gases.In addition, it is impossible to remove 100% of non-condensable gases (e.g., a 5% residual rate may remain). In such cases, the performance loss can be appropriately compensated by increasing the area of the condensation section (by 5% accordingly).Multiple sealing actions are involved in the above processes, making the design of sealing molds also crucial. Extrusion sealing can achieve short-term airtightness, but for passing reliability tests, more reliable sealing methods such as argon arc welding and resistance welding must be combined.

• Written by

  CoolingThermal Engineering Team

  CoolingThermal is an automation equipment manufacturer based in Kunshan, China, specializing in heat pipe and vapor chamber production equipment since 2017. Our engineering team designs, builds, and commissions complete production lines covering forming, degassing, welding, testing, and assembly processes. The technical content on this blog is written by the same team that develops the equipment — based on real production experience, not secondary research.