DeRisking Your MIM Manufacturing Operation

DeRisking Your MIM Manufacturing Operation 

Introduction 

Metal Injection Molding (MIM) and Plastic Injection Molding (PIM) are fundamentally very similar processes. Both utilize raw material (resin or feedstock) that is then molded using an injection molding machine. While plastic parts are typically finished once they separate from the mold, this is just the beginning for metal components.  

Instead of resin, metal powder is mixed with a binding agent that fills the mold cavity. The part is then removed from the mold; this stage is referred to as green stage parts, whereas it requires a catalytic of solvent debinding process. Following debinding, often referred to as the brown stage, the parts must be sintered to fully remove residual binder and densify the parts. In this final step, arguably the most complicated, the metal parts can shrink up to 20%.  

The repeatability of this process is determined by a variety of factors including feedstock & binder consistency, mold & setter design, molding, debinding, sintering, and quality assurance. Each of these considerations requires highly attentive process control and experience to optimize the process and derisk your MIM manufacturing operation.  

For those already engaged in MIM manufacturing, the following document will outline reminders and suggestions to improve operations. For those that are new to MIM or investigating the process, we are confident that this outline will serve as a guide to help avoid unnecessary challenges or problems in the future.  

Understanding MIM Feedstock  

MIM feedstock is a complex mixture of binders and metal powders made with the exact volume fractions of the different materials to ensure an exact repeatable shrinkage. Typically, feedstock for MIM is 60% metal powder by volume and shrinkage varies between 16-22% depending on the material and part. The variation in shrinkage is typically held to better than ± 0.025%. Not only must the feedstock be made to the correct shirnkage with a tight tolerance, it must also flow the same and react the same when used for injection molding. The binder in the feedstock is made up of several components. Typical binders have the following constituents: 

• A wetting agent that allows the binder to wet the metal surface and reduce the surface tension between the two. 

• A constituent that can be removed first from the mixture whereby a network of pores are opened so that the rest of the binders may come out easily. The wax in a wax-based feedstock is easily dissolved in a solvent, opening up the pores.  

• A secondary binder that holds the powder particles together until the particles begin to form bonds by diffusion.  

• A material that allows the primary binder to wet and mix with the secondary binder. 

  
  

Metal powder can be pre-alloyed, a mixture of elemental powders, or a base powder with a master alloy. For every powder, a certain amount of binder is necessary before the feedstock flows. Also, particle size, particle size distribution, powder morphology, and shape affect the flow of the feedstock. The resulting interactions are difficult and complex so the recommendation is to purchase feedstock from well-recognized and reliable commercial feedstock manufacturers.  

If you are looking for additional guidance, consider speaking with DSH Technologies, a technology agnostic sintering service provider with two decades of experience in MIM manufacturing and metallurgy operations.  

Optimizing the Design 

It’s important to holistically review the design process for the mold, individual part, setters, etc. MIM molds are 16-22% larger than the actual part so compensating for that shrinkage and designing accordingly can be challenging. For example, metal powders have higher heat conductivity than plastic, so the MIM feedstock cools faster indicating that the gate sections need to be larger. Additionally, the rapid velocity changes of the molten feedstock within the mold will cause the powder to separate from the binders and lead to dimensional and/or density variation in the sintered part. Therefore, sharp corners must be avoided as much as possible by rounding them by as much as the application will allow.  

Oftentimes, design modifications are necessary to support better feedstock flowability, gating, or part ejection. Setters or setter designs can be a useful alternative to accommodate for overhangs or non-flat surface parts susceptible of warpage in the sintering process. Here is a list of options to consider: 

•  Allow a certain amount of distortion in the sintered part and correct it with a coining operation.  

• Add appendages to the part that creates a flat part. Appendages must be easily and inexpensively removed after sintering (secondary machining). 

• Use strips as shims to support the part as the sintered part rests on the added support. The green part is larger than the support placed.  

• Use specially machined ceramic setters to support the part. Ceramic setters tend to be expensive, but the results are also better, especially when the parts have complex surfaces.  

  
  

Combining the part intention with a proactive approach to design optimization will eliminate headaches in the future. Producing a fully densified MIM part requires sintering and it’s critical to understand the science, shrinkage, and sometimes secondary machining operations.  

Molding (Size, Spec, Speed) 

Each mold for each different part will have its own quirks and problems. The feedstock is heated and injected into the mold cavity under high pressure, resulting in a green stage part. Optimizing the metal injection molding process requires thoughtful consideration surrounding the type of equipment, injection speed, temperature, and a variety of other factors.

For example, since the feedstock is predominantly metal powder, it’s relatively abrasive and requires molding equipment that is robust and equipped with hardened barrels, screws, tips, and check rings. Additionally, the mold temperatures and injection speed will vary by feedstock (powder and binder) so it’s helpful to make that determination upfront. Additional considerations: 

• Any degradation of the feedstock will cause the part to have a different shrinkage ratio, which can lead to part dimension issues.  

• Slower injection speed will result in better part uniformity and density. 

• Prevent binder separation of the feedstock inside the barrel using a slow screw reversal process.  

• Voids in the part will result in lower density and can be avoided by using proper venting technique or vacuum evacuated molds.  

• Avoid sink marks in the part by adding a cushion to the end of the molding cycle due to temperature drops. 

  
  

Once the part is completed within the mold, it is necessary to have the identical density as the starting material. Feedstock density measurements may be used to ensure that regrinds are still within the tolerance limits that provide acceptable dimensions of the parts. A mold flow analysis that uses a MIM feedstock as the starting material can be a great asset. Check out this article by Autodesk, How to Simulate Injection Molding Metals in MoldFlow. 

 Primary Debinding 

The purpose of primary debinding is to open up passageways for the secondary debinder to escape. Using either physical or chemical methods, the part is transformed from a mixture of powder metal and binder to a green stage metal component that maintains its shape and structure. The time required for primary debinding is dependent on the debinding process, thickness of the part, and powder particle size. The different types of debinding processes are categorized as either catalytic or solvent.  

Catalytic debinding is a chemical decomposition using a gaseous catalyst (e.g., nitric acid) and requires specialized equipment and strict safety considerations. While it’s generally a faster process with less distortion risks, it has higher operational costs and hazardous material handling.  

Solvent debinding is the physical dissolution of a soluble binder using a liquid solvent (e.g. acetone, heptane, or water) and utilizes a multi-component binder system. It’s cost-effective, low environmental risks, and ideal for delicate, thin-walled parts. Alternatively, it’s a slow process with a higher risk of distortion due to temperature sensitivity.  

Once primary debinding is completed, parts are considered in the brown stage and prepared for secondary debinding and sintering.  

Learn more about Elnik’s catalytic and solvent-based primary debinding processes.  

Secondary Debinding  

The purpose of the secondary binder is to hold the particles together until bonds form between the metal powder particles, which cause the shape of the part to be retained after the polymeric binders are removed. The technology currently employed heats up the part in a protective atmosphere, allowing the binder to evaporate slowly.  

A slow ramp and a hold at a temperature where the binder evaporates are necessary to achieve this – if the binders evaporate too rapidly, the part will distort or disintegrate since the bond between metal particles is tiny and rupture easily. The parts at this stage are extremely fragile. If the debinding time is insufficient and the part moves on to the sintering stage, the heating becomes too rapid for the binder to come out slowly as a vapor from the part and could cause additional problems that include part cracking, blistering, soot build-up, and chemical composition distortion.  

More recently and unique to Elnik Systems is the MIM3000 batch furnace that is considered the most versatile, one-stop debind and sinter furnace on the market. This furnace is built on an all-metal hot zone and retort designing enabled customers to process metal with the highest density and best mechanical properties. Equipped with a gas plenum retort, the MIM3000 has super gas distribution, pre-heat gas for low temperature convection, and high efficiency binder removal. This takes the guess work out debinding and automatically preps parts for sintering in the same system.  

Learn more about the Elnik Systems MIM3000 Batch Furnace.  

Sintering Science & Expertise  

Sintering is a critical step of the metal injection molding (MIM) process where debinding is complete and parts are ready for final densification. At the atom level, all materials move in a solid state and after a certain temperature is reached, the movement of the atoms crosses particle boundaries, and bonds are formed between particles (atomic fusion).

At high temperatures, in most cases below the melting point of the alloy, pores between the particles are eliminated, and one solid body is formed. Understandably, this results in part shrinkage, up to 20%, which must be considered early in the design phase. Historically, second stage debinding and sintering existed in separate processes, but is now capable within a singular platform.  

Sintering is the science of combining and controlling temperatures to eliminate pores and bond particles to increase density. While this is a relatively straight forward concept, managing gas flow, purity, consistency, gravity, friction, and temperature is not as simple. Now, developing a custom recipe per material, per part that encompasses these factors, can make the process much more complicated.  

That’s why Elnik Systems introduced AccuTemp™ temperature control allowing each temperature zone inside the furnace to be controlled within +/- 1° of its set point. It’s this type of sintering innovation that has positioned Elnik Systems as the global authority for debinding and sintering excellence.  

Uncompromising Quality 

In actuality, quality is the standard the dictates each stage of the process from beginning to end. From design to sintering and everything in between, having a thorough approach can reduce bottlenecks and improve efficiency. Margins in manufacturing are slim, so it’s imperative to be intentional about your current or new metal injection molding operation. When it comes to quality, we define it as the foundation of our organization. We refuse to take short cuts or use low value substitutes. Our level of care shows in the craftsmanship and functionality of our equipment. We also cultivate long lasting, high value relationships based on trust and experiences in all areas of our lives. Elnik Systems is synonymous with Quality, and we stand by that.