MIM Parts 101: A Guide to Shrinkage and Tolerances?

Over my 30 years in this industry, I've seen countless projects get delayed or go over budget because of the wrong manufacturing choice. As a buyer or engineer, you're constantly looking for a process that delivers complex, small metal parts without the high costs of CNC machining¹. This is where Metal Injection Molding² (MIM) becomes a powerful tool in your supply chain.

MIM parts shrink significantly, about 15-20%, during the sintering process³. This is not a defect; it's a planned, highly controlled part of the manufacturing cycle. As an experienced MIM supplier, we leverage this phenomenon to achieve tight tolerances⁴, often as fine as ±0.3% of the dimension, making MIM ideal for intricate, high-volume components.

Understanding and mastering this shrinkage is the key to unlocking the power of MIM. It might seem like a risk, but it's a precise science that our team at Prime has perfected over three decades. Let's break down exactly how our ISO 9001:2015 certified process ensures we deliver perfect, repeatable parts to you every single time.

What causes shrinkage in MIM parts⁵?

Alarmed by how much MIM parts shrink? If you're used to subtractive manufacturing, the idea of a part shrinking 20% can sound unpredictable and risky. But the process is highly controlled, and the shrinkage is intentional.

Shrinkage happens during sintering. The plastic and wax binder, which allows the metal powder to be molded, is completely burned away. This leaves microscopic gaps between the metal particles. High heat in our controlled-atmosphere furnaces fuses these particles, causing the part to shrink uniformly as it becomes fully dense metal.

Let's dive deeper from a factory perspective. It all starts with the feedstock—a precise mixture of [fine metal powder](https://primefabworks.com/surface-treatment/)6 (typically < 20 microns) and our proprietary binder system. The entire process is governed by principles outlined in industry standards like the [MPIF Standard 35](https://www.3deo.co/manufacturing/new-powder-metal-alloys-added-to-mpif-standard-35/)7 for MIM Materials. This ensures that a part made today will have the exact same mechanical properties⁸ as a part made a year from now, a critical factor for any serious production program.

Process Stage	Description	Key Change
1. Green Part	The part as it comes out of the injection molding machine.	Shape is formed, contains binder.
2. Brown Part	After the multi-stage debinding process.	Most binder is removed, part is porous and fragile.
3. Sintered Part	After heating in our controlled atmosphere sintering furnace.	Pores are eliminated, part shrinks and densifies to 97-99%.

How tight can tolerances be with MIM?

Are you concerned that MIM can't meet your strict tolerance requirements? You need precision, but the idea of shrinkage sounds like a gamble. It is not.

Typically, MIM can achieve as-sintered tolerances of ±0.3% to ±0.5% of a dimension. For a 10mm feature, that’s just ±0.03mm. Tighter tolerances are possible with secondary operations. Our commitment to these tolerances is validated through a rigorous [Production Part Approval Process (PPAP)](https://quality-one.com/ppap/)9 for every new part.

This is where experience truly matters. From a buyer's perspective, the last thing you want are delays from parts that don't fit. That's why our process starts with a collaborative Design for Manufacturability (DFM)¹⁰ review. My team works directly with your engineers to suggest minor tweaks that improve part consistency. This DFM is the first step in our formal PPAP submission. We build the mold using precise calculations based on the specific material's shrink rate, which we've validated over millions of parts. This proactive approach de-risks the entire project, ensuring the final parts meet spec without costly redesigns.

Tolerance Type	Standard MIM Capability (As-Sintered)	With Secondary Operations
Linear Dimension	±0.3%	< ±0.1% possible (Machining/Coining)
Hole Diameter	±0.05 mm	±0.01 mm possible (Drilling/Reaming)
Flatness	0.005 mm per cm	0.001 mm per cm possible (Lapping)

When is MIM the best choice over other methods?

Are you confused about when to use MIM, CNC machining¹, or casting? Choosing the wrong process can waste a lot of your time and budget. MIM has a clear sweet spot.

Choose MIM for high-volume production¹¹, usually over 10,000 pieces per year, of small, complex metal parts. It's perfect when a part is too intricate for CNC to be cost-effective¹² and needs better strength and surface finish than investment casting can provide. Explore our [Metal Injection Molding² services to see more examples.

The key factors are complexity and quantity. The high upfront cost of a hardened steel MIM mold is quickly offset by the extremely low per-part price at high volumes. I once worked with a client, Kevin from the U.S., who was sourcing electronic components. His previous supplier's inconsistent quality caused major shipment delays. By switching to our controlled MIM process, he not only got better parts but also a reliable supply chain, allowing him to hit his sales targets. This is the real value of a trusted manufacturing partner.

Cost Factor	Metal Injection Molding2 (MIM)	CNC Machining
Tooling Cost	High (one-time)	None
Material Usage	Very efficient (>98% utilization)	High scrap rate (material is cut away)
Labor Cost / Part	Very low (automated process13)	High (machine & operator time per part)
Ideal Quantity	10,000+ per year	1 - 1,000

How does material choice¹⁴ affect MIM shrinkage and tolerances?

Does your material choice¹⁴ complicate the MIM process? You might fear that picking a different metal will ruin your tolerances. It's a factor we manage every day.

Different metal powders create unique feedstocks, each with a specific shrink rate. For example, stainless steels like 17-4PH are highly predictable. We use our extensive material database to adjust mold designs and furnace profiles for each alloy, ensuring consistent, precise results.

This is where our expertise shines. We verify the chemistry and particle size distribution of every batch of incoming metal powder. From a buyer's standpoint, this means you can be confident that the material in your parts provides the exact properties your application demands, batch after batch¹⁵. We handle the science so you can focus on your product.

Common MIM Material	Key Properties (Based on MPIF Standard 357)	Typical Applications
[17-4 PH Stainless Steel](https://www.upmet.com/sites/default/files/datasheets/17-4-ph.pdf)16	High strength, good corrosion resistance.	Firearm components, medical tools, aerospace parts.
316L Stainless Steel17	Excellent corrosion resistance, biocompatible.	Watch cases, jewelry, surgical instruments.
Fe-2%Ni (FN02)	Good magnetic properties, moderate strength.	Automotive sensors, electronic housings, soft magnetic cores.
Titanium Alloys (Ti-6Al-4V)18	High strength-to-weight ratio, biocompatible.	Medical implants, high-performance sporting goods.

Conclusion

MIM is a powerful process for complex, high-volume metal parts. By mastering the science of shrinkage through deep experience and rigorous process control, we deliver precision and value, turning a manufacturing challenge into your competitive advantage.

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FAQs about Metal Injection Molding²

What are the most common materials used in MIM?

The most common materials are stainless steels (17-4PH, 316L), low-alloy steels (FN02, FN08), and titanium alloys. As a specialized factory, we can also process materials like tungsten and copper alloys for specific applications.

How strong are MIM parts?

MIM parts are very strong, reaching 97-99% of wrought metal density. Their mechanical properties⁸ are far superior to die casting and are specified to meet industry standards like MPIF Standard 35⁷. We can also perform post-sinter heat treatments to further increase strength.

What is the typical lead time for a MIM project?

A new project has two phases. Phase 1: Tooling & PPAP typically takes 6-8 weeks. This includes DFM, mold fabrication, and submitting initial samples with a full PPAP report for your approval. Phase 2: Mass Production begins only after your approval, with a typical lead time of 3-5 weeks.

What is the main difference between MIM and metal 3D printing?

The main differences are volume and cost. Metal 3D printing is ideal for prototypes and very low volumes, as it has no tooling cost. MIM is vastly more cost-effective¹² for high-volume production¹¹ (10,000+ pieces) where the low per-part cost justifies the initial tool investment.

Can you apply surface finishes¹⁹ to MIM parts?

Yes. MIM parts are fully dense metal and can be treated like machined parts. We frequently provide services like polishing, passivation, black oxide coating, and Physical Vapor Deposition (PVD) coatings, which add durability and specific aesthetic or functional properties.

What information do you need for a MIM quote?

For an accurate quote, please provide: a 3D CAD file (STEP/IGES), a 2D drawing showing dimensions, critical tolerances, and material callouts, and your estimated annual usage (EAU). This volume information is crucial for confirming that MIM is the right choice for you.

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