Archives September 2026

Injection Mold Ownership FAQ: Tool Rights, Transfers & IP Protection

Injection Mold Ownership FAQ: Tool Rights, Transfers, and IP Protection

When you commission an injection mold, you are not just buying a piece of steel. You are making one of the largest capital commitments in your product development budget, and you are handing a supplier the physical means to reproduce your part. Yet the questions that matter most commercially — who legally owns the tool, whether your supplier can run it for someone else, and whether you can take it elsewhere — are often the ones buyers forget to ask until there is a problem.

This FAQ answers the five questions we get most often from overseas buyers before, during, and after tooling. If you are evaluating a molding partner or reviewing an existing agreement, these are the points worth getting in writing.

1. Who owns the injection mold after I pay for it?

In the overwhelming majority of commercial arrangements, the customer owns the mold. You paid for the design, the steel, and the machining, so the tool is your property. This is standard practice across the industry and is how we operate at RCH Plastic.

The confusion arises because ownership and possession are two different things. You own the tool, but it physically sits in your supplier’s facility, and they need to hold it to run production. Some suppliers blur this line deliberately, implying that because the mold lives in their factory they control it. They do not — not if your paperwork is correct.

To protect yourself, make sure three things appear in writing before tooling begins. First, the quotation or contract should state explicitly that ownership of the mold transfers to the buyer upon final payment (or upon completion, depending on your payment schedule). Second, the mold should be identified unambiguously — part number, cavity count, and a unique tool ID — so there is no dispute about which asset is yours. Third, there should be a clause confirming the supplier holds the tool as custodian only and will not use it for any party other than you.

One practical note: if your payment terms involve a final balance due after T1 sample approval, clarify whether ownership transfers at completion or at final payment. A supplier who has not been paid in full may legally retain the tool, and this is a common source of friction. Get the trigger event specified.

2. Can my supplier use my mold to produce parts for other customers?

Not without your written permission — and any reputable supplier will say so without hesitation. Your mold embodies your geometry, your tolerances, and often your intellectual property. Running it for another customer would mean producing your part for someone else, which is a straightforward breach of trust and, in most jurisdictions, a breach of contract.

This is worth dwelling on because it is the single most common fear overseas buyers have when sourcing tooling in Asia, and it is a fear worth taking seriously. The protection is procedural rather than technical. An NDA signed before you share your 3D model is the first layer. A tooling agreement with an explicit exclusivity and non-use clause is the second. Many buyers also include a provision requiring the supplier to store the tool in a designated, segregated area and to keep production records available for audit.

There is one legitimate exception worth understanding: if you have not ordered parts for an extended period and have outstanding invoices, some suppliers reserve a right to repurpose idle tooling after written notice. This is rare and should be disclosed upfront, not buried in terms. If your agreement is silent on it, assume it does not apply — but ask anyway.

At RCH Plastic, every customer tool is logged with a unique tool ID and is never run for any party other than its owner. If your program is sensitive, we are comfortable signing an NDA before you send a single file, and we will confirm non-use terms in the tooling contract itself rather than in a separate document nobody reads.

3. Can I move my mold to another supplier, and what does a transfer involve?

Yes. If you own the mold, you have the right to have it released. This is called a tool transfer, and it is a normal part of doing business — programs move for cost reasons, capacity reasons, or because a buyer is consolidating suppliers. A professional supplier will cooperate.

The mechanics matter more than the principle. A proper transfer should include the mold itself, a full set of component drawings if available, the spare parts inventory (ejector pins, springs, seals, heater bands), a maintenance and repair history, and the processing parameter sheet from the last qualified production run. That last item is the one buyers forget and later regret — without the parameter sheet, your new supplier has to redevelop the process from scratch, which costs weeks and money.

Be aware of two practical friction points. The first is outstanding balances: if you owe money on the tool or on parts, expect the supplier to hold the mold until settled. This is reasonable and legal in most cases, so close your account before requesting release. The second is packing and shipping. An injection mold is heavy, precision-ground, and vulnerable to rust and impact. It needs proper crating, rust inhibitor, and often a wooden case built to the mold’s dimensions. Budget for this — it typically runs into several hundred to a few thousand dollars depending on size and destination, and it is your cost as the owner.

We recommend agreeing the transfer process before you need it. A one-line clause stating that the supplier will release the tool within a defined number of business days after written request and account settlement removes all ambiguity later.

4. What legal and IP protection should I have in place before tooling starts?

Three documents, in this order. An NDA before you share geometry. A tooling agreement covering ownership, non-use, and transfer rights. And a supply or quality agreement if you plan to run production volumes with the same partner.

The NDA is the cheapest and most important. It should be mutual or one-way in your favour, governed by a jurisdiction you are comfortable with, and — critically — it must survive the end of the commercial relationship. A two-year NDA on a product with a ten-year lifecycle is close to useless.

The tooling agreement is where ownership lives. Beyond the ownership and exclusivity clauses already discussed, it should cover what happens to the tool if the supplier ceases trading, who pays for storage after a defined idle period, and who is liable for damage caused by negligence versus normal wear. Note that normal wear is generally the owner’s cost — molds are consumable assets with finite life — while damage from a supplier’s mishandling is theirs.

On patents and design rights, understand that tooling contracts do not transfer IP in your product. Your patent or registered design is what protects the part itself; the tooling agreement protects the physical tool. Many buyers assume one covers the other. It does not. If your part has protectable IP, file before you start talking to overseas suppliers, or at least before you share production drawings.

Finally, keep your own records. Save the 3D model revisions you sent, the DFM process report you received, the T1 sample report, and every approval email. If a dispute ever arises, the party with the better paper trail wins.

5. What happens to my mold if the supplier closes or the relationship ends?

This is the scenario that keeps procurement managers awake, and it is more manageable than it feels — provided you planned for it. Your protection rests on the ownership and release clauses described above. If the supplier ceases trading, the mold is your asset, not part of their estate, and you have a claim to recover it. Recovering it in practice, though, is far easier if you already know where it is and have a current contact.

Two habits reduce this risk dramatically. The first is periodic confirmation: ask your supplier once or twice a year to confirm the tool is in their facility, in good condition, and available. A one-line email is enough, and the reply is your evidence. The second is keeping an up-to-date mold drawing and component list on your side. If you ever need to have the tool replicated because it cannot be recovered, a complete drawing set means a new mold can be built in weeks rather than reverse-engineered from parts.

If a relationship ends amicably, treat the transfer like any other logistics job: settle the account, issue a written release request, arrange crating and freight, and inspect the tool on arrival before signing for it. Photograph the parting lines and cavity surfaces before it leaves the origin facility. Most disputes about tool condition come down to who has photographs.

We store every customer tool in a dedicated, labelled rack with rust protection applied between runs, and we provide condition confirmation on request. If you are consolidating suppliers or moving a program, we will crate and release your tool promptly once the account is settled — no friction, no delays.

Getting This Right Before You Cut Steel

None of these questions are difficult to answer, and none of them require an adversarial relationship with your supplier. They simply need to be asked early, while everyone is still enthusiastic about the project and before there is anything to argue about. The buyers who get burned are almost never the ones who asked too many questions.

If you are planning a new tooling program and want to understand how we handle injection mold ownership, non-use commitments, and tool release, our team is happy to walk you through the actual contract language before any money changes hands. Send us your 3D model for a free DFM review and a tooling quotation that states your rights plainly — or contact us to discuss an existing program you are thinking of moving.

Injection Molding Tolerances Guide: How Tight Can You Go?

Different industries have different tolerance expectations. Here are typical ranges you will see in purchase specifications:

  • Consumer electronics housings: ±0.1 to ±0.2 mm (MT2). Surface appearance matters, but internal snap-fits have some give.
  • Automotive connectors: ±0.05 mm (MT1). Electrical contact geometry must be precise to ensure signal integrity and vibration resistance.
  • Medical device components: ±0.02 to ±0.05 mm (MT1, sometimes tighter). Biocompatibility and sterilization repeatability require tight dimensional control.
  • Industrial enclosures: ±0.2 to ±0.5 mm (MT3). Large parts with non-critical mating surfaces.
  • Packaging caps and closures: ±0.1 mm (MT2). Thread engagement must be consistent for torque and sealing performance.

If you are sourcing from China and your spec was written for a European supplier, the tolerance class may not transfer directly. Always clarify the ISO 20457 class explicitly — do not assume that “±0.1 mm” means the same thing across all suppliers.

Avoiding Common Tolerance Mistakes

Even experienced engineers make tolerance specification errors. Here are the most frequent ones and how to avoid them:

  • Over-specifying tolerances. Tightening every dimension to ±0.05 mm is the fastest way to blow your mold budget. Ask which dimensions are truly functional before tightening.
  • Omitting reference conditions. A dimension measured hot after ejection is not comparable to one measured after 24-hour conditioning. Always state the measurement condition.
  • Specifying tolerances on non-machinable features. Wire-cut EDM can hold ±0.01 mm; sandblasted texture cannot. Do not specify a tolerance on a surface feature that cannot be measured consistently.
  • Ignoring stack-up analysis. A single dimension may look fine, but when five parts assemble, the tolerance stack may exceed your assembly clearance. Run a tolerance stack analysis on critical assemblies.
  • Assuming all cavities are equal. In a multi-cavity mold, cavity 1 may produce a slightly different dimension than cavity 8 due to flow imbalance. Specify that tolerances apply across all cavities, not just the first one.

When to Request a Tolerance Audit

If you are new to injection molding or working with a new supplier, request a tolerance audit before committing to the tooling budget. A tolerance audit is a review of your drawings where the mold maker identifies every dimension that may be difficult to hold and suggests alternatives. The audit typically takes 1–2 days and can save thousands of dollars in mold rework or production scrap.

At RCH Plastic, we include a tolerance capability matrix in every DFM report. Each critical dimension is tagged with the expected process tolerance, the required tooling grade, and any special process controls needed. This transparency lets you make informed decisions before the mold is cut — not after the first shot is rejected.

Bottom Line

Injection molding tolerances are achievable — but only when they are specified clearly, matched to the right material and process, and backed by a mold maker who documents the tolerance capability before the steel is cut. The most expensive tolerance is the one you discover is impossible to hold after the mold is already built. Share your drawing with your supplier during the DFM phase, and let them flag every tolerance that needs special attention. That conversation will save you time, money, and disputes on every program you run.

For a detailed DFM review of your tolerance spec, or a tolerance capability quote for your part, contact RCH Plastic with your drawing and material selection. We will return a dimension-by-dimension tolerance analysis within 48 hours.

A well-written tolerance spec prevents disputes and reduces the cost of a mold program. Follow these rules:

  • Specify only critical dimensions. Non-critical dimensions should use ISO 20457 MT2 or MT3 by default. Every tightened tolerance adds cost.
  • Use ISO 20457 classes. Write “MT2” next to the dimension, not just ±0.1 mm. This removes ambiguity.
  • Define the reference condition. State whether dimensions are measured at 23°C / 50% RH, after 24-hour conditioning, or as-ejected.
  • Separate functional from cosmetic dimensions. A mating surface may need ±0.05 mm; a visible A-surface may only need ±0.2 mm for appearance.
  • Include datum references. GD&T (Geometric Dimensioning and Tolerancing) with datums is the professional standard for critical assemblies.

When in doubt, ask your mold maker for a tolerance capability matrix before you finalize the drawing. A good supplier will show you what each dimension can realistically achieve with the selected material and process — and flag any that require special tooling or process controls.

Tolerance vs. Cost: The Real Trade-off

Tightening a tolerance from ±0.2 mm to ±0.05 mm can increase mold cost by 30–50 percent and part cost by 15–25 percent. The reasons are straightforward: harder tool steel, more machining time, tighter fit tolerances, longer trial cycles, and stricter process control. For most commercial parts, MT2 (±0.1–0.2 mm) is the sweet spot — tight enough for assembly, affordable for volume production.

If your design requires MT1 tolerances on multiple features, discuss this with your mold maker early. There may be a cheaper alternative: a different material with lower shrinkage, a revised part geometry that reduces the number of tight features, or a post-molding machining step on a single critical dimension instead of holding it in the mold.

When to Use Coordinate Measuring Machine (CMM) vs. Go/No-Go

CMM inspection is the standard for production validation — it measures X, Y, Z coordinates with micron-level accuracy and generates a full dimensional report. It is the right choice for first-article inspection, PPAP submissions, and ongoing process validation.

Go/No-Go gauges are faster and cheaper for high-volume production checking. They are best suited for single features (hole diameter, pin length, thickness) rather than complex geometries. For a part with 20 dimensions, CMM is more efficient; for a part with one critical hole diameter, a custom go/no-go gauge is faster and sufficient.

Real-World Tolerance Examples by Industry

Different industries have different tolerance expectations. Here are typical ranges you will see in purchase specifications:

  • Consumer electronics housings: ±0.1 to ±0.2 mm (MT2). Surface appearance matters, but internal snap-fits have some give.
  • Automotive connectors: ±0.05 mm (MT1). Electrical contact geometry must be precise to ensure signal integrity and vibration resistance.
  • Medical device components: ±0.02 to ±0.05 mm (MT1, sometimes tighter). Biocompatibility and sterilization repeatability require tight dimensional control.
  • Industrial enclosures: ±0.2 to ±0.5 mm (MT3). Large parts with non-critical mating surfaces.
  • Packaging caps and closures: ±0.1 mm (MT2). Thread engagement must be consistent for torque and sealing performance.

If you are sourcing from China and your spec was written for a European supplier, the tolerance class may not transfer directly. Always clarify the ISO 20457 class explicitly — do not assume that “±0.1 mm” means the same thing across all suppliers.

Avoiding Common Tolerance Mistakes

Even experienced engineers make tolerance specification errors. Here are the most frequent ones and how to avoid them:

  • Over-specifying tolerances. Tightening every dimension to ±0.05 mm is the fastest way to blow your mold budget. Ask which dimensions are truly functional before tightening.
  • Omitting reference conditions. A dimension measured hot after ejection is not comparable to one measured after 24-hour conditioning. Always state the measurement condition.
  • Specifying tolerances on non-machinable features. Wire-cut EDM can hold ±0.01 mm; sandblasted texture cannot. Do not specify a tolerance on a surface feature that cannot be measured consistently.
  • Ignoring stack-up analysis. A single dimension may look fine, but when five parts assemble, the tolerance stack may exceed your assembly clearance. Run a tolerance stack analysis on critical assemblies.
  • Assuming all cavities are equal. In a multi-cavity mold, cavity 1 may produce a slightly different dimension than cavity 8 due to flow imbalance. Specify that tolerances apply across all cavities, not just the first one.

When to Request a Tolerance Audit

If you are new to injection molding or working with a new supplier, request a tolerance audit before committing to the tooling budget. A tolerance audit is a review of your drawings where the mold maker identifies every dimension that may be difficult to hold and suggests alternatives. The audit typically takes 1–2 days and can save thousands of dollars in mold rework or production scrap.

At RCH Plastic, we include a tolerance capability matrix in every DFM report. Each critical dimension is tagged with the expected process tolerance, the required tooling grade, and any special process controls needed. This transparency lets you make informed decisions before the mold is cut — not after the first shot is rejected.

Bottom Line

Injection molding tolerances are achievable — but only when they are specified clearly, matched to the right material and process, and backed by a mold maker who documents the tolerance capability before the steel is cut. The most expensive tolerance is the one you discover is impossible to hold after the mold is already built. Share your drawing with your supplier during the DFM phase, and let them flag every tolerance that needs special attention. That conversation will save you time, money, and disputes on every program you run.

For a detailed DFM review of your tolerance spec, or a tolerance capability quote for your part, contact RCH Plastic with your drawing and material selection. We will return a dimension-by-dimension tolerance analysis within 48 hours.

Every thermoplastic shrinks as it cools from melt to solid. The shrinkage rate is material-specific and direction-dependent — flow direction shrinks less than the transverse direction. Common shrinkage ranges:

  • PP: 1.5–2.5%
  • PE: 1.5–4.0%
  • ABS: 0.4–0.9%
  • PA66: 0.7–1.5% (moisture-dependent)
  • POM: 1.5–2.5%
  • PC: 0.5–0.7%
  • PTFE: 3.0–4.0%

A 100 mm feature in PP with 2% shrinkage will target 98 mm in the mold cavity. If the shrinkage varies between lots — and it will, unless the material is pre-dried and processed consistently — the finished part will drift. This is why material certification and batch control are part of tolerance management.

2. Part Geometry

Thin walls cool faster and shrink less, but they also require higher injection pressure, which can increase flash risk. Long, slender features flex during ejection and may spring back unpredictably. Undercuts and ribs create local cooling variation that warps the part. Simple, symmetric geometries are easier to hold to tight tolerances than complex, asymmetrical ones.

3. Mold Design and Construction

Tight tolerances require tight tooling. This means hardened steel cores and cavities (HRC 50+), precision ground parting surfaces, matched ejector pins, and adequate venting to prevent gas traps that cause local dimensional variation. A mold designed to MT1 tolerances will cost 20–40 percent more than one designed to MT3 because of the extra machining time, tighter fit tolerances, and longer trial cycles.

4. Machine and Process Control

The injection molding machine must be stable: consistent melt temperature, repeatable screw position, and adequate clamp force. Process parameters — injection speed, packing pressure, cooling time, and mold temperature — must be documented and controlled. Statistical process control (SPC) with CpK ≥ 1.33 is the minimum expectation for production parts. Processes running without documented parameters will produce variable dimensions.

5. Post-Molding Conditioning

Parts continue to shrink and distort after ejection, especially hygroscopic materials like PA66 and PC. Post-molding conditioning — drying, annealing, or humidity conditioning — stabilizes dimensions. A PA66 part measured immediately after ejection may be 0.3 mm undersize; after 24 hours at 50% relative humidity, it may be 0.1 mm oversize. Always specify the conditioning state in your drawing.

How to Specify Tolerances in Your RFQ

A well-written tolerance spec prevents disputes and reduces the cost of a mold program. Follow these rules:

  • Specify only critical dimensions. Non-critical dimensions should use ISO 20457 MT2 or MT3 by default. Every tightened tolerance adds cost.
  • Use ISO 20457 classes. Write “MT2” next to the dimension, not just ±0.1 mm. This removes ambiguity.
  • Define the reference condition. State whether dimensions are measured at 23°C / 50% RH, after 24-hour conditioning, or as-ejected.
  • Separate functional from cosmetic dimensions. A mating surface may need ±0.05 mm; a visible A-surface may only need ±0.2 mm for appearance.
  • Include datum references. GD&T (Geometric Dimensioning and Tolerancing) with datums is the professional standard for critical assemblies.

When in doubt, ask your mold maker for a tolerance capability matrix before you finalize the drawing. A good supplier will show you what each dimension can realistically achieve with the selected material and process — and flag any that require special tooling or process controls.

Tolerance vs. Cost: The Real Trade-off

Tightening a tolerance from ±0.2 mm to ±0.05 mm can increase mold cost by 30–50 percent and part cost by 15–25 percent. The reasons are straightforward: harder tool steel, more machining time, tighter fit tolerances, longer trial cycles, and stricter process control. For most commercial parts, MT2 (±0.1–0.2 mm) is the sweet spot — tight enough for assembly, affordable for volume production.

If your design requires MT1 tolerances on multiple features, discuss this with your mold maker early. There may be a cheaper alternative: a different material with lower shrinkage, a revised part geometry that reduces the number of tight features, or a post-molding machining step on a single critical dimension instead of holding it in the mold.

When to Use Coordinate Measuring Machine (CMM) vs. Go/No-Go

CMM inspection is the standard for production validation — it measures X, Y, Z coordinates with micron-level accuracy and generates a full dimensional report. It is the right choice for first-article inspection, PPAP submissions, and ongoing process validation.

Go/No-Go gauges are faster and cheaper for high-volume production checking. They are best suited for single features (hole diameter, pin length, thickness) rather than complex geometries. For a part with 20 dimensions, CMM is more efficient; for a part with one critical hole diameter, a custom go/no-go gauge is faster and sufficient.

Real-World Tolerance Examples by Industry

Different industries have different tolerance expectations. Here are typical ranges you will see in purchase specifications:

  • Consumer electronics housings: ±0.1 to ±0.2 mm (MT2). Surface appearance matters, but internal snap-fits have some give.
  • Automotive connectors: ±0.05 mm (MT1). Electrical contact geometry must be precise to ensure signal integrity and vibration resistance.
  • Medical device components: ±0.02 to ±0.05 mm (MT1, sometimes tighter). Biocompatibility and sterilization repeatability require tight dimensional control.
  • Industrial enclosures: ±0.2 to ±0.5 mm (MT3). Large parts with non-critical mating surfaces.
  • Packaging caps and closures: ±0.1 mm (MT2). Thread engagement must be consistent for torque and sealing performance.

If you are sourcing from China and your spec was written for a European supplier, the tolerance class may not transfer directly. Always clarify the ISO 20457 class explicitly — do not assume that “±0.1 mm” means the same thing across all suppliers.

Avoiding Common Tolerance Mistakes

Even experienced engineers make tolerance specification errors. Here are the most frequent ones and how to avoid them:

  • Over-specifying tolerances. Tightening every dimension to ±0.05 mm is the fastest way to blow your mold budget. Ask which dimensions are truly functional before tightening.
  • Omitting reference conditions. A dimension measured hot after ejection is not comparable to one measured after 24-hour conditioning. Always state the measurement condition.
  • Specifying tolerances on non-machinable features. Wire-cut EDM can hold ±0.01 mm; sandblasted texture cannot. Do not specify a tolerance on a surface feature that cannot be measured consistently.
  • Ignoring stack-up analysis. A single dimension may look fine, but when five parts assemble, the tolerance stack may exceed your assembly clearance. Run a tolerance stack analysis on critical assemblies.
  • Assuming all cavities are equal. In a multi-cavity mold, cavity 1 may produce a slightly different dimension than cavity 8 due to flow imbalance. Specify that tolerances apply across all cavities, not just the first one.

When to Request a Tolerance Audit

If you are new to injection molding or working with a new supplier, request a tolerance audit before committing to the tooling budget. A tolerance audit is a review of your drawings where the mold maker identifies every dimension that may be difficult to hold and suggests alternatives. The audit typically takes 1–2 days and can save thousands of dollars in mold rework or production scrap.

At RCH Plastic, we include a tolerance capability matrix in every DFM report. Each critical dimension is tagged with the expected process tolerance, the required tooling grade, and any special process controls needed. This transparency lets you make informed decisions before the mold is cut — not after the first shot is rejected.

Bottom Line

Injection molding tolerances are achievable — but only when they are specified clearly, matched to the right material and process, and backed by a mold maker who documents the tolerance capability before the steel is cut. The most expensive tolerance is the one you discover is impossible to hold after the mold is already built. Share your drawing with your supplier during the DFM phase, and let them flag every tolerance that needs special attention. That conversation will save you time, money, and disputes on every program you run.

For a detailed DFM review of your tolerance spec, or a tolerance capability quote for your part, contact RCH Plastic with your drawing and material selection. We will return a dimension-by-dimension tolerance analysis within 48 hours.

The international standard for injection molding tolerances is ISO 20457:2018. It defines three tolerance classes:

  • MT1 — Highest precision. Typically ±0.05 to ±0.1 mm for small features. Used for optical, medical, and connector housings.
  • MT2 — Standard precision. Typically ±0.1 to ±0.2 mm. The default for most automotive and industrial components.
  • MT3 — General purpose. Typically ±0.2 to ±0.5 mm. Used for large structural parts, enclosures, and non-mating surfaces.

ISO 20457 does not replace supplier judgment. It provides a common language so that a tolerance written by a German engineer and a tolerance held by a Chinese mold maker describe the same requirement. When you write your RFQ, reference the ISO class and the specific dimension — not just “±0.1 mm” without context.

Five Factors That Control Your Tolerances

1. Material Shrinkage

Every thermoplastic shrinks as it cools from melt to solid. The shrinkage rate is material-specific and direction-dependent — flow direction shrinks less than the transverse direction. Common shrinkage ranges:

  • PP: 1.5–2.5%
  • PE: 1.5–4.0%
  • ABS: 0.4–0.9%
  • PA66: 0.7–1.5% (moisture-dependent)
  • POM: 1.5–2.5%
  • PC: 0.5–0.7%
  • PTFE: 3.0–4.0%

A 100 mm feature in PP with 2% shrinkage will target 98 mm in the mold cavity. If the shrinkage varies between lots — and it will, unless the material is pre-dried and processed consistently — the finished part will drift. This is why material certification and batch control are part of tolerance management.

2. Part Geometry

Thin walls cool faster and shrink less, but they also require higher injection pressure, which can increase flash risk. Long, slender features flex during ejection and may spring back unpredictably. Undercuts and ribs create local cooling variation that warps the part. Simple, symmetric geometries are easier to hold to tight tolerances than complex, asymmetrical ones.

3. Mold Design and Construction

Tight tolerances require tight tooling. This means hardened steel cores and cavities (HRC 50+), precision ground parting surfaces, matched ejector pins, and adequate venting to prevent gas traps that cause local dimensional variation. A mold designed to MT1 tolerances will cost 20–40 percent more than one designed to MT3 because of the extra machining time, tighter fit tolerances, and longer trial cycles.

4. Machine and Process Control

The injection molding machine must be stable: consistent melt temperature, repeatable screw position, and adequate clamp force. Process parameters — injection speed, packing pressure, cooling time, and mold temperature — must be documented and controlled. Statistical process control (SPC) with CpK ≥ 1.33 is the minimum expectation for production parts. Processes running without documented parameters will produce variable dimensions.

5. Post-Molding Conditioning

Parts continue to shrink and distort after ejection, especially hygroscopic materials like PA66 and PC. Post-molding conditioning — drying, annealing, or humidity conditioning — stabilizes dimensions. A PA66 part measured immediately after ejection may be 0.3 mm undersize; after 24 hours at 50% relative humidity, it may be 0.1 mm oversize. Always specify the conditioning state in your drawing.

How to Specify Tolerances in Your RFQ

A well-written tolerance spec prevents disputes and reduces the cost of a mold program. Follow these rules:

  • Specify only critical dimensions. Non-critical dimensions should use ISO 20457 MT2 or MT3 by default. Every tightened tolerance adds cost.
  • Use ISO 20457 classes. Write “MT2” next to the dimension, not just ±0.1 mm. This removes ambiguity.
  • Define the reference condition. State whether dimensions are measured at 23°C / 50% RH, after 24-hour conditioning, or as-ejected.
  • Separate functional from cosmetic dimensions. A mating surface may need ±0.05 mm; a visible A-surface may only need ±0.2 mm for appearance.
  • Include datum references. GD&T (Geometric Dimensioning and Tolerancing) with datums is the professional standard for critical assemblies.

When in doubt, ask your mold maker for a tolerance capability matrix before you finalize the drawing. A good supplier will show you what each dimension can realistically achieve with the selected material and process — and flag any that require special tooling or process controls.

Tolerance vs. Cost: The Real Trade-off

Tightening a tolerance from ±0.2 mm to ±0.05 mm can increase mold cost by 30–50 percent and part cost by 15–25 percent. The reasons are straightforward: harder tool steel, more machining time, tighter fit tolerances, longer trial cycles, and stricter process control. For most commercial parts, MT2 (±0.1–0.2 mm) is the sweet spot — tight enough for assembly, affordable for volume production.

If your design requires MT1 tolerances on multiple features, discuss this with your mold maker early. There may be a cheaper alternative: a different material with lower shrinkage, a revised part geometry that reduces the number of tight features, or a post-molding machining step on a single critical dimension instead of holding it in the mold.

When to Use Coordinate Measuring Machine (CMM) vs. Go/No-Go

CMM inspection is the standard for production validation — it measures X, Y, Z coordinates with micron-level accuracy and generates a full dimensional report. It is the right choice for first-article inspection, PPAP submissions, and ongoing process validation.

Go/No-Go gauges are faster and cheaper for high-volume production checking. They are best suited for single features (hole diameter, pin length, thickness) rather than complex geometries. For a part with 20 dimensions, CMM is more efficient; for a part with one critical hole diameter, a custom go/no-go gauge is faster and sufficient.

Real-World Tolerance Examples by Industry

Different industries have different tolerance expectations. Here are typical ranges you will see in purchase specifications:

  • Consumer electronics housings: ±0.1 to ±0.2 mm (MT2). Surface appearance matters, but internal snap-fits have some give.
  • Automotive connectors: ±0.05 mm (MT1). Electrical contact geometry must be precise to ensure signal integrity and vibration resistance.
  • Medical device components: ±0.02 to ±0.05 mm (MT1, sometimes tighter). Biocompatibility and sterilization repeatability require tight dimensional control.
  • Industrial enclosures: ±0.2 to ±0.5 mm (MT3). Large parts with non-critical mating surfaces.
  • Packaging caps and closures: ±0.1 mm (MT2). Thread engagement must be consistent for torque and sealing performance.

If you are sourcing from China and your spec was written for a European supplier, the tolerance class may not transfer directly. Always clarify the ISO 20457 class explicitly — do not assume that “±0.1 mm” means the same thing across all suppliers.

Avoiding Common Tolerance Mistakes

Even experienced engineers make tolerance specification errors. Here are the most frequent ones and how to avoid them:

  • Over-specifying tolerances. Tightening every dimension to ±0.05 mm is the fastest way to blow your mold budget. Ask which dimensions are truly functional before tightening.
  • Omitting reference conditions. A dimension measured hot after ejection is not comparable to one measured after 24-hour conditioning. Always state the measurement condition.
  • Specifying tolerances on non-machinable features. Wire-cut EDM can hold ±0.01 mm; sandblasted texture cannot. Do not specify a tolerance on a surface feature that cannot be measured consistently.
  • Ignoring stack-up analysis. A single dimension may look fine, but when five parts assemble, the tolerance stack may exceed your assembly clearance. Run a tolerance stack analysis on critical assemblies.
  • Assuming all cavities are equal. In a multi-cavity mold, cavity 1 may produce a slightly different dimension than cavity 8 due to flow imbalance. Specify that tolerances apply across all cavities, not just the first one.

When to Request a Tolerance Audit

If you are new to injection molding or working with a new supplier, request a tolerance audit before committing to the tooling budget. A tolerance audit is a review of your drawings where the mold maker identifies every dimension that may be difficult to hold and suggests alternatives. The audit typically takes 1–2 days and can save thousands of dollars in mold rework or production scrap.

At RCH Plastic, we include a tolerance capability matrix in every DFM report. Each critical dimension is tagged with the expected process tolerance, the required tooling grade, and any special process controls needed. This transparency lets you make informed decisions before the mold is cut — not after the first shot is rejected.

Bottom Line

Injection molding tolerances are achievable — but only when they are specified clearly, matched to the right material and process, and backed by a mold maker who documents the tolerance capability before the steel is cut. The most expensive tolerance is the one you discover is impossible to hold after the mold is already built. Share your drawing with your supplier during the DFM phase, and let them flag every tolerance that needs special attention. That conversation will save you time, money, and disputes on every program you run.

For a detailed DFM review of your tolerance spec, or a tolerance capability quote for your part, contact RCH Plastic with your drawing and material selection. We will return a dimension-by-dimension tolerance analysis within 48 hours.

Injection Molding Tolerances Guide: How Tight Can You Go?

When you submit an RFQ for injection molded parts, the first line in your spec sheet should be the tolerance requirement — but “±0.1 mm” means something very different to a mold designer than it does to a procurement manager. The gap between what looks good on paper and what your supplier can reliably hold in production is where most tooling disputes start. This guide explains what injection molding tolerances actually are, how they are specified, which factors control them, and how to write a tolerance spec that gets you good parts without inflating your mold cost by 30 percent.

What Are Injection Molding Tolerances?

Injection molding tolerance is the allowable deviation from a nominal dimension. It is usually expressed as a bilateral value such as ±0.1 mm or as a unilateral stack such as +0.15 / −0.05 mm. Tolerances apply to linear dimensions, angular dimensions, wall thickness, hole position, and surface features. They are not a single number for an entire part — each critical dimension gets its own tolerance, and non-critical dimensions are left to the process window.

Standard injection molding tolerances for most thermoplastics fall in the range of ±0.1 to ±0.2 mm for dimensions up to 100 mm. Beyond 100 mm, tolerances typically widen by about 0.05 mm per additional 100 mm of travel. These are process capabilities, not guarantees — a reliable mold maker will confirm each tolerance against the material shrinkage table, the expected part geometry, and the tool steel selected.

ISO 20457 and the Three Tolerance Classes

The international standard for injection molding tolerances is ISO 20457:2018. It defines three tolerance classes:

  • MT1 — Highest precision. Typically ±0.05 to ±0.1 mm for small features. Used for optical, medical, and connector housings.
  • MT2 — Standard precision. Typically ±0.1 to ±0.2 mm. The default for most automotive and industrial components.
  • MT3 — General purpose. Typically ±0.2 to ±0.5 mm. Used for large structural parts, enclosures, and non-mating surfaces.

ISO 20457 does not replace supplier judgment. It provides a common language so that a tolerance written by a German engineer and a tolerance held by a Chinese mold maker describe the same requirement. When you write your RFQ, reference the ISO class and the specific dimension — not just “±0.1 mm” without context.

Five Factors That Control Your Tolerances

1. Material Shrinkage

Every thermoplastic shrinks as it cools from melt to solid. The shrinkage rate is material-specific and direction-dependent — flow direction shrinks less than the transverse direction. Common shrinkage ranges:

  • PP: 1.5–2.5%
  • PE: 1.5–4.0%
  • ABS: 0.4–0.9%
  • PA66: 0.7–1.5% (moisture-dependent)
  • POM: 1.5–2.5%
  • PC: 0.5–0.7%
  • PTFE: 3.0–4.0%

A 100 mm feature in PP with 2% shrinkage will target 98 mm in the mold cavity. If the shrinkage varies between lots — and it will, unless the material is pre-dried and processed consistently — the finished part will drift. This is why material certification and batch control are part of tolerance management.

2. Part Geometry

Thin walls cool faster and shrink less, but they also require higher injection pressure, which can increase flash risk. Long, slender features flex during ejection and may spring back unpredictably. Undercuts and ribs create local cooling variation that warps the part. Simple, symmetric geometries are easier to hold to tight tolerances than complex, asymmetrical ones.

3. Mold Design and Construction

Tight tolerances require tight tooling. This means hardened steel cores and cavities (HRC 50+), precision ground parting surfaces, matched ejector pins, and adequate venting to prevent gas traps that cause local dimensional variation. A mold designed to MT1 tolerances will cost 20–40 percent more than one designed to MT3 because of the extra machining time, tighter fit tolerances, and longer trial cycles.

4. Machine and Process Control

The injection molding machine must be stable: consistent melt temperature, repeatable screw position, and adequate clamp force. Process parameters — injection speed, packing pressure, cooling time, and mold temperature — must be documented and controlled. Statistical process control (SPC) with CpK ≥ 1.33 is the minimum expectation for production parts. Processes running without documented parameters will produce variable dimensions.

5. Post-Molding Conditioning

Parts continue to shrink and distort after ejection, especially hygroscopic materials like PA66 and PC. Post-molding conditioning — drying, annealing, or humidity conditioning — stabilizes dimensions. A PA66 part measured immediately after ejection may be 0.3 mm undersize; after 24 hours at 50% relative humidity, it may be 0.1 mm oversize. Always specify the conditioning state in your drawing.

How to Specify Tolerances in Your RFQ

A well-written tolerance spec prevents disputes and reduces the cost of a mold program. Follow these rules:

  • Specify only critical dimensions. Non-critical dimensions should use ISO 20457 MT2 or MT3 by default. Every tightened tolerance adds cost.
  • Use ISO 20457 classes. Write “MT2” next to the dimension, not just ±0.1 mm. This removes ambiguity.
  • Define the reference condition. State whether dimensions are measured at 23°C / 50% RH, after 24-hour conditioning, or as-ejected.
  • Separate functional from cosmetic dimensions. A mating surface may need ±0.05 mm; a visible A-surface may only need ±0.2 mm for appearance.
  • Include datum references. GD&T (Geometric Dimensioning and Tolerancing) with datums is the professional standard for critical assemblies.

When in doubt, ask your mold maker for a tolerance capability matrix before you finalize the drawing. A good supplier will show you what each dimension can realistically achieve with the selected material and process — and flag any that require special tooling or process controls.

Tolerance vs. Cost: The Real Trade-off

Tightening a tolerance from ±0.2 mm to ±0.05 mm can increase mold cost by 30–50 percent and part cost by 15–25 percent. The reasons are straightforward: harder tool steel, more machining time, tighter fit tolerances, longer trial cycles, and stricter process control. For most commercial parts, MT2 (±0.1–0.2 mm) is the sweet spot — tight enough for assembly, affordable for volume production.

If your design requires MT1 tolerances on multiple features, discuss this with your mold maker early. There may be a cheaper alternative: a different material with lower shrinkage, a revised part geometry that reduces the number of tight features, or a post-molding machining step on a single critical dimension instead of holding it in the mold.

When to Use Coordinate Measuring Machine (CMM) vs. Go/No-Go

CMM inspection is the standard for production validation — it measures X, Y, Z coordinates with micron-level accuracy and generates a full dimensional report. It is the right choice for first-article inspection, PPAP submissions, and ongoing process validation.

Go/No-Go gauges are faster and cheaper for high-volume production checking. They are best suited for single features (hole diameter, pin length, thickness) rather than complex geometries. For a part with 20 dimensions, CMM is more efficient; for a part with one critical hole diameter, a custom go/no-go gauge is faster and sufficient.

Real-World Tolerance Examples by Industry

Different industries have different tolerance expectations. Here are typical ranges you will see in purchase specifications:

  • Consumer electronics housings: ±0.1 to ±0.2 mm (MT2). Surface appearance matters, but internal snap-fits have some give.
  • Automotive connectors: ±0.05 mm (MT1). Electrical contact geometry must be precise to ensure signal integrity and vibration resistance.
  • Medical device components: ±0.02 to ±0.05 mm (MT1, sometimes tighter). Biocompatibility and sterilization repeatability require tight dimensional control.
  • Industrial enclosures: ±0.2 to ±0.5 mm (MT3). Large parts with non-critical mating surfaces.
  • Packaging caps and closures: ±0.1 mm (MT2). Thread engagement must be consistent for torque and sealing performance.

If you are sourcing from China and your spec was written for a European supplier, the tolerance class may not transfer directly. Always clarify the ISO 20457 class explicitly — do not assume that “±0.1 mm” means the same thing across all suppliers.

Avoiding Common Tolerance Mistakes

Even experienced engineers make tolerance specification errors. Here are the most frequent ones and how to avoid them:

  • Over-specifying tolerances. Tightening every dimension to ±0.05 mm is the fastest way to blow your mold budget. Ask which dimensions are truly functional before tightening.
  • Omitting reference conditions. A dimension measured hot after ejection is not comparable to one measured after 24-hour conditioning. Always state the measurement condition.
  • Specifying tolerances on non-machinable features. Wire-cut EDM can hold ±0.01 mm; sandblasted texture cannot. Do not specify a tolerance on a surface feature that cannot be measured consistently.
  • Ignoring stack-up analysis. A single dimension may look fine, but when five parts assemble, the tolerance stack may exceed your assembly clearance. Run a tolerance stack analysis on critical assemblies.
  • Assuming all cavities are equal. In a multi-cavity mold, cavity 1 may produce a slightly different dimension than cavity 8 due to flow imbalance. Specify that tolerances apply across all cavities, not just the first one.

When to Request a Tolerance Audit

If you are new to injection molding or working with a new supplier, request a tolerance audit before committing to the tooling budget. A tolerance audit is a review of your drawings where the mold maker identifies every dimension that may be difficult to hold and suggests alternatives. The audit typically takes 1–2 days and can save thousands of dollars in mold rework or production scrap.

At RCH Plastic, we include a tolerance capability matrix in every DFM report. Each critical dimension is tagged with the expected process tolerance, the required tooling grade, and any special process controls needed. This transparency lets you make informed decisions before the mold is cut — not after the first shot is rejected.

Bottom Line

Injection molding tolerances are achievable — but only when they are specified clearly, matched to the right material and process, and backed by a mold maker who documents the tolerance capability before the steel is cut. The most expensive tolerance is the one you discover is impossible to hold after the mold is already built. Share your drawing with your supplier during the DFM phase, and let them flag every tolerance that needs special attention. That conversation will save you time, money, and disputes on every program you run.

For a detailed DFM review of your tolerance spec, or a tolerance capability quote for your part, contact RCH Plastic with your drawing and material selection. We will return a dimension-by-dimension tolerance analysis within 48 hours.