Which is better, plastic based or wax based, for MIM adhesive?

 

Date：[2024/3/12]
 

Differences between plastic based and wax based binders in mixing, injection molding, degreasing, and sintering processes
1、 Mixing stage
Wax based adhesive
1) When the mixing temperature is below 120 ℃, the mixing time is shorter;
2) Sealed stirring mixing and airtight design to reduce wax heating loss;
3) Plastic based mixing equipment can be used, but temperature control should be taken into account when the shear force is high.
Plastic based adhesive
1) Mixing temperature greater than 150 ℃, long softening time;
2) Adopting open mixing and kneading;
3) Do not use a wax based mixing machine due to insufficient shear force;
4) Special powder mixing should be protected by adding nitrogen gas;
5) High shear force can fully soften the plastic based adhesive material.
Wax based binders are mainly composed of paraffin, with low viscosity and high volatility; Therefore, it is easy to mix, but it is also important to note that the temperature should not be too high (below 120 ℃), and it should have an airtight design to reduce volatilization loss.
The plastic based binder is mainly POM, with high viscosity and low volatility; It is necessary to use a mixer with high shear force and set the temperature higher (greater than 150 ℃) to reduce the viscosity of the binder; Due to the high temperature and often being open, nitrogen protection is required in special cases to prevent oxidation of some special metal powders.
2、 Injection molding stage
Wax based adhesive
1) Good lubrication, slow firing rate can also be achieved, and hydraulic press can also inject;
2) The temperature of the mold and injection tube is low;
3) Fast firing rate can easily lead to significant shell effect in powder adhesive separation, which is due to the low viscosity of wax. Therefore, parts with high polishing brightness are not easy to salvage their sand hole defects.
Plastic based adhesive
1) Only high-speed motors can lay a good plastic base for feeding;
2) The injection molding machine has high injection rate, high injection pressure, and long holding time;
3) The compression ratio of the screw is very important, and the length of the screw should be short;
4) The mold temperature should be high (>120 ℃) to reduce residual stress in the injection blank;
5) The temperature control of the material pipe should be stricter, and the temperature should be immediately lowered during non injection operations to avoid POM cracking!
Similar to mixing, the high viscosity of plastic based binders also poses difficulties for injection molding. Wax based binders, due to their low viscosity and good lubricity, can meet the requirements of low pressure, low injection rate, and low mold temperature injection molding machines. On the contrary, fast injection rate can easily lead to powder rubber separation, resulting in sand hole defects in the final product.
Plastic based adhesives require high-speed electric injection molding machines, with short screws, high injection rate, injection pressure, mold temperature, and long holding time; In addition, the temperature control of the material pipe should be stricter, and the temperature should be immediately lowered during non injection operations to avoid POM cracking!
3、 Degreasing stage
Wax based (dissolution/thermal desorption)
1) Dissolution causes the product to deform easily;
2) Low efficiency of solvent extraction, poor safety of solvent extraction equipment, and environmental and safety issues caused by solvent fire prevention and recovery;
3) Dissolution is suitable for large parts, and the structure should have a supporting plane.
Plastic based (acid catalyzed removal)
1) Rapid catalysis, suitable for meat thickness below 5mm, with high efficiency;
2) Simple equipment and high security;
3) The by-products are all gases (CO2, N2), which are easy to manage.
The main solvent for base binders is solvent or hot atmosphere degreasing. The solvent will deteriorate the skeleton support agent, and drying after dissolution often causes the billet to warp and deform. The corresponding solvent also needs to be fireproof. The recovery of solvent and the safety performance of degreasing furnace need to be emphasized.
Plastic based binders are mainly degreased through the catalytic action of acids, which is fast and produces safe gases such as CO2 and N2. The equipment is simple and easy to manage.
4、 Sintering stage
Wax based (dissolution/thermal desorption)
1) Long time before sintering of wax based binder;
2) Large product deformation;
3) Low labor cost.
Plastic based (acid catalyzed removal)
1) The sintering time of the wax based binder is short before sintering;
2) Small product deformation;
3) The labor cost is relatively high.
The sintering time of the wax based binder before sintering (<600 ℃) is relatively long, resulting in nasal mucus like and yellow wax residue; Due to the deterioration of the skeleton agent during degreasing, the product undergoes significant deformation; Although the price of hot melt wax based binder is high, it is popular in Japan due to its ability to use a degreasing sintering furnace with low labor costs.
The pre sintering time of the plastic based binder (<600 ℃) is relatively short, resulting in asphalt like and dark green yellow wax residue, which is difficult to clean. In addition, plastic based adhesives have relatively small deformation.
By summarizing the differences between the four processes mentioned above, it can be seen that the wax based binder mixing and injection molding processes require simple requirements, but the sintered parts have significant deformation; The mixing and injection molding processes of plastic based binders require high requirements, and it is precisely due to their relatively high viscosity that the deformation of sintered parts is small. Of course, the specific details and parameters of different enterprises may vary, and there may also be some differences.
5、 What are the global market trends for adhesives?
Acid de esterification is most commonly used in Europe, followed by domestic applications; The North American region mainly uses solvent degreasing/wax based binders, while Japan mainly uses thermal degreasing/wax based binders. However, these two regions have also gradually accepted plastic based binders since 2013. Plastic based adhesives are gradually gaining recognition from the global market due to their small product deformation and relatively less susceptibility to defects such as sand holes!