A. What are the initial binders?
At the beginning of the development of PIM, the formulation of raw materials is not very complicated. Any ingredient engineer can go to the grocery store to buy some paraffin wax and car polishing lubricant, add some peanut oil, melt certain plastics (such as sandwich bags), you can invent a secret binder; then, add small enough solid particles, powder shot was born. In fact, starting in 1940, the first patents disclosed the use of mothballs, shellac, epoxy resins and binders of cellulose, with water as the solvent for the mixture.
These homemade injection systems seemed to work well, and the injection was very easy to fill into simple shaped molds. As a result, in the late 1990s, there were nearly 300 PIM plants operating around the world. However, the real problems slowly emerged, how to solve the following 4 problems became the key to PIM!
1. Defect control: How can we eliminate fusion marks, cracks, voids, and other defects?
2. Improve process yield: When you have a molding yield of only 80%, it is difficult to be profitable.
3. Size control: As much as possible to achieve the same size and shape of the product.
4. Carbon control: bonding aids can easily contaminate the product, so it is difficult to achieve a specific carbon content of heat-treatable steel; therefore, even today, low-carbon stainless steel is difficult to PIM do.
Second, what are the criteria for choosing a binder for PIM?
Through a long period of exploration, it was found that the selection of a good bonding agent is a key factor in PIM. Powder injection molding (PIM) bonding agent should meet at least the following five conditions.
1. At the molding temperature, the additive is usually a short polymer filler, with low viscosity;
2. During the heating process, there should be a main chain polymer to maintain the shape of the raw material;
3. Smaller molecular weight to avoid residual stress deformation and to facilitate bonding with the solid powder surface;
4. The surface of solid powder can ensure good wettability with the additives, so that the combination is good and not easy to separate;
5. Not expensive, environmentally friendly, low water absorption rate.
Third, MIM how to choose the binder?
As we have said at the beginning, PIM including MIM, PIM when choosing a bonding agent required conditions MIM generally also need to meet. In addition, MIM also need to consider other issues, such as the problem of rust, etc.
1. Can water-soluble bonding additives be used?
MIM in degreasing, water-soluble bonding aids such as polyethylene glycol or polyvinyl alcohol, can be dissolved in warm water for easy removal. Although they are very attractive because of their low cost and minimal environmental impact, they cannot be used in MIM. There are two main reasons for this.
a. Water-soluble bonding additives tend to absorb moisture from the air and change from a solid to a rubbery state, affecting dimensional stability;
b. Steel powder is easy to rust.
2. Why POM can be used as a binder?
a. From the chemical structure analysis: the main chain for the alternating structure of carbon and oxygen atoms, with a certain degree of polarity, and metal powder surface full contact; side chain for the hydrogen atoms, small space obstruction, the regularity of the main chain is strong, so the POM crystallinity is high, generally above 70%, low viscosity and the ability to maintain the shape of the raw, these two characteristics are estimated to be reconciled only with crystallization.
b. Analysis from the molecular weight: POM polymerization is not high, easy to depolymerization by heat. POM molecular weight is small, viscosity is small, to ensure that as little as possible binder that can meet the injection molding conditions, in addition, the smaller molecular weight makes it easy to remove from the raw, and not easy to produce residual stress; high crystallinity, POM has a good self-lubricating properties.
c. Finally, POM is a thermoplastic material, the defective products can be recycled, from another aspect to reduce the cost.
