Issues and related information about hot isostatic pressing (HIP) process
Issues Related to Hot Isostatic Pressing Process
Hot isostatic pressing brings many beneficial improvements to material properties, but people have expressed concerns about the potential negative effects of this process, including part deformation, potential surface contamination, and deformation differences between different part batches. For enterprises seeking reliable metal injection molding parts supplier or MIM hot isostatic pressing service provider, understanding these variables is critical to achieving consistent high-performance components.
As mentioned earlier, hot isostatic pressing can bring the density of metal injection molded parts close to 100%. However, in the injection molding process, previous process steps may cause density gradients in the parts. For example, inclusions at the gate location may lead to different densities at different positions in the part. The powder particle content is higher near the gate, while in areas far from the gate, the powder particle content is lower due to lower molding pressure. If such density gradients exist during the sintering process, the area near the gate will shrink more than the area far from the gate, and this difference in shrinkage will be further amplified during the hot isostatic pressing process, with low-density areas shrinking more than high-density areas, leading to part deformation and anisotropic shrinkage. Professional metal injection molding factory with hot isostatic pressing capability can effectively control gate design and filling parameters to minimize such risks from the source.
Due to the cooling gradient inside the hot isostatic pressing furnace, parts may also deform. If the wall thickness of the part is uneven, the thinner portion will cool faster than the thicker portion, which may cause deformation. This phenomenon can also be observed during heat treatment.
Metal injection molded parts respond very well to the hot isostatic pressing process because the surface porosity of the parts is very low, and the pores generated inside the parts are usually very small. Unlike die casting, parts manufactured by MIM show almost no dents after hot isostatic pressing treatment, although slight deformation can sometimes be observed near the pores. If defects appear in the parts after hot isostatic pressing treatment, the pores causing the defects are usually not from the small holes present in the injected parts, but are more likely caused by poor hot isostatic pressing conditions. Therefore, the forming conditions should be checked, and the parts before hot isostatic pressing should be sectioned to determine whether there are defects caused by forming conditions. Choosing an experienced metal injection molding and HIP integrated solution provider can significantly reduce post-process defect rate.
Another practical issue is that during the hot isostatic pressing process, the surface of the part may be contaminated. Since suppliers process a variety of alloys in hot isostatic pressing equipment, surface contamination may occur during processing. The author observed titanium and hafnium contamination on the surface of parts after hot isostatic pressing treatment, and analysis showed that this is difficult to avoid completely. Coating the parts with a layer of tool steel or entrusting a dedicated medical-grade metal injection molding HIP service supplier can prevent such contamination or minimize it to meet aerospace and medical device certification requirements.
Some people claim that reheating parts under atmospheric pressure or vacuum may produce open pores. Although such descriptions occasionally appear in the literature, the author has not observed this phenomenon.
Examples of Hot Isostatic Pressing Process Parameters
Typical hot isostatic pressing process conditions are limited. Most suppliers have standard process flows, in which the variables are temperature, pressure, and time. When executing the process flow, the same pressure is specially set along with different times and temperatures. Table 9.3 lists the standard hot isostatic pressing process parameters that can be used for metal injection molded parts. Leading precision metal injection molding turnkey supplier usually provide customers with material-specific optimized HIP parameter packages to ensure batch stability.
Table 9.3 Process parameters for static hot isostatic pressing (HIP) of common metal injection-molded parts
| Material | Heating temperature / °C | Pressure / MPa | Holding time / h |
|---|---|---|---|
| Aluminum alloy (e.g., A335, A357, A201) | 510 | 100 | 2 |
| Soft magnetic iron alloy | 900 | 100 | 2 |
| Ordinary carbon steel and low alloy steel | 1 065 | 100 | 4 |
| Series I martensitic precipitation-hardening stainless steel (1N – 718 Rene77) | 1 185 | 100 | 4 |
| Cobalt-base alloy (F75) | 1 220 | 100 | 4 |
| Series II martensitic precipitation-hardening alloy (Mar – M247 Rene 125) | 1 185 | 175 | 4 |
Summary
Since metal injection molded parts do not contain open pores, hot isostatic pressing technology can be used to treat them. The performance of metal injection molded parts treated by hot isostatic pressing is improved, the density is close to 100%, the grains grow significantly, the dimensional consistency between different batches of metal injection molded parts is improved, and the polishability and weldability of the parts are improved. For companies looking for high-performance small complex metal parts, cooperating with a professional full-process metal injection molding and hot isostatic pressing manufacturer has become the preferred solution to achieve the ultimate performance and cost balance.