Two mold quotes for the same plastic part can arrive with very different prices, and the first instinct is to assume one supplier is overcharging. Often the real reason is simpler: they are quoting different molds. One may be pricing a hardened-steel tool built for a million shots. The other may be pricing a pre-hardened tool built for fifty thousand. Neither quote says so.
The SPI mold class system exists to remove exactly that ambiguity. This guide explains what each class means, which steels and features usually sit behind it, how it relates to the “Level 1–4” wording you may see in quotes from Asian mold shops, and how to choose the class that fits your project instead of paying for a tool you do not need.
The short answer
| SPI class | Expected life | Typical use | Typical construction |
|---|---|---|---|
| 101 | 1,000,000+ cycles | Extremely high volume | Hardened tool steel, cavities and cores at 48 Rc minimum, guided ejection |
| 102 | Up to 1,000,000 cycles | Medium to high volume | Quality steel, relaxed requirements compared with 101 |
| 103 | Under 500,000 cycles | Medium volume | Cores and cavities at 280 BHN or higher, base at 165 BHN minimum |
| 104 | Under 100,000 cycles | Low volume | Mild steel or aluminum allowed |
| 105 | Up to 500 cycles | Prototype and development | Soft aluminum, built fast and cheap |
Cycle counts are the expected life under normal conditions. Actual life depends on resin, part geometry and maintenance, as explained below.
What an SPI mold class is, and what it is not
SPI stands for the Society of the Plastics Industry, now the Plastics Industry Association. Its mold classification gives buyers and mold makers a shared language for how long a mold is expected to last and how robustly it is built.
It defines expected cycle life and certain construction requirements. For example, a Class 101 mold calls for cavities and cores hardened to at least 48 Rc and a mold base of at least 280 BHN, while Class 103 allows a base from 165 BHN.
What the class does not do is equally important:
- It does not specify the exact steel brand. Two Class 102 molds may use different steels.
- It does not set your part tolerances.
- It does not define the surface finish. SPI surface finishes (the A, B, C and D families, from mirror polish to blasted textures) are a separate standard.
So a class on a quote is a starting point for a conversation, not a complete specification. Always ask for the steel grade, hardness and finish in writing as well.
The five classes in plain language
Class 101: the million-shot tool
Built for the highest volumes and the longest programs: consumer electronics components, closures, medical disposables and automotive parts. Expect hardened steel, strong cooling design, wear plates and guided ejection. It has the highest upfront cost and usually the lowest cost per part over a long program.
Choose it when annual volume is high, the program will run for years, and downtime for mold repair would be expensive.
Class 102: the workhorse for serious production
Similar thinking to Class 101, with some of the heavy-duty requirements relaxed. A sensible choice for many products that will run in the hundreds of thousands.
Choose it when you expect solid, sustained production but not a million-plus shots.
Class 103: the practical middle
Often the best value for products with moderate volume. Pre-hardened steel is common here, which keeps cost and lead time under control.
Choose it when volumes are in the tens to low hundreds of thousands, or when the product’s market is still unproven.
Class 104: low-volume production
Mild steel or aluminum is acceptable. These tools are faster and cheaper to make, but wear sooner and are less forgiving with abrasive resins.
Choose it when you need a few thousand to tens of thousands of parts, or bridge production before committing to a production tool.
Class 105: prototype tooling
Intended for a few hundred shots at most, usually in soft aluminum. Its purpose is to validate the design, fit and function with real molded material.
Choose it when you need production-intent samples quickly and have not frozen the design.
How this maps to "Level 1–4" in Asian mold quotes
If you source tooling from China, you may see molds described as “Level 1” to “Level 4” rather than SPI classes. This wording is common shop shorthand, and it is not a single formal standard, so the numbers can differ slightly between suppliers. As a rough guide:
| Common quote wording | Closest SPI class | Typical steels mentioned |
|---|---|---|
| Level 1 (highest) | 101 | H13, S136, hardened to roughly 48–52 HRC |
| Level 2 | 102 | 718H, NAK80 |
| Level 3 | 103 | P20 pre-hardened |
| Level 4 | 104 or 105 | 45# steel, aluminum |
Treat this as a translation aid, not a guarantee. When a supplier writes “Level 2”, ask which SPI class they mean, how many shots they expect, and which steel they will use for the cavity and core.
Steel is the main lever behind the class
Steel choice drives much of a mold’s life, polish quality and price. Hardness values below are typical ranges quoted by mold steel suppliers and vary by heat treatment and supplier.
| Steel | Typical hardness | Strengths | Best suited to |
|---|---|---|---|
| P20 | about 28–32 HRC | Easy to machine, economical, but does not take a high polish | Prototypes, low and medium volume, simple parts |
| 718H | about 33–37 HRC | Pre-hardened, good machinability, decent finish | Medium production, commercial parts |
| NAK80 | about 38–42 HRC | Excellent polishability, can reach mirror finish | High-gloss, cosmetic or transparent parts |
| S136 | about 48–52 HRC | Stainless, corrosion resistant, hardened | Medical, food contact, corrosive resins, high volume |
| H13 | about 46–50 HRC | Tough hot-work steel, handles heat and wear | Engineering resins, very high volume |
Commonly cited mold life ranges differ between sources: P20 from tens of thousands up to about 300,000 shots, and hardened H13 up to around one million shots under the right conditions. Treat any single number as indicative, not guaranteed.
Mold accuracy is not part tolerance
Some mold grading charts list extremely tight figures such as ±0.005–0.01 mm. Those describe how precisely the mold is machined, not the tolerance a plastic part will hold. Plastic shrinks, warps and varies with temperature and pressure, so molded part tolerances are much looser than the tool itself. Do not use a mold-accuracy figure to promise a part tolerance, and do not assume a higher class automatically gives a tighter part.
The practical rule: define your critical-to-function dimensions on the drawing, then ask the supplier how they will hold them, not just which class they are quoting.
Your resin can shorten a mold's life
The class assumes normal operating conditions. Certain materials wear steel faster:
- Glass-filled plastics are abrasive and erode gates and cavities.
- Acrylic, polycarbonate and PVC can be harder on tooling than commodity polyolefins; PVC in particular can attack unprotected steel.
- Poor setup, over-packing and inadequate cooling break down any mold early.
If your part uses glass-filled nylon, for instance, a Class 103 mold in plain P20 may not reach its expected life. Tell your supplier the resin before they choose the steel.
Four questions to choose the right class
- How many parts do you need, in total, over the life of the product? Multiply annual volume by expected years. This sets the class range.
- Which resin and additives will you use? Abrasive or corrosive materials push you toward harder or stainless steel.
- What surface do you need? A high-gloss or optical surface calls for a highly polishable steel such as NAK80 or S136. A textured or functional surface is more forgiving.
- How stable is the design? If the design may still change, a lower-class tool is cheaper to modify or replace than a fully hardened one.
The two classic mistakes
Overbuilding. Paying for a Class 101 tool when the product will make 20,000 parts. The extra steel and machining never pay back.
Underbuilding. Choosing a Class 104 tool for a product that later grows to 300,000 units. You face mold wear, rising scrap and a second tooling bill at the worst time.
A good middle path is bridge tooling: run an aluminum or P20 tool first to validate the market, then invest in a hardened production tool once demand is proven.
How to compare mold quotes properly
When two quotes differ, ask each supplier to state the following in writing:
- SPI class (or equivalent) and the expected number of shots
- Steel grade for cavity, core and mold base, and hardness after heat treatment
- Number of cavities and runner type (cold or hot runner)
- Surface finish and texture specification
- Included trials, sample reports and modification rounds (see our guide to T0, T1 and T2 mold trials)
- Tooling ownership, storage and maintenance responsibilities
A supplier who answers these clearly is easier to trust than one who only sends a price. For typical price ranges by mold type, see our injection molding cost guide.
How we approach it at RpProto
We design, machine, EDM, assemble, polish and trial molds in-house in Xiamen, China, so the class and steel decision stays with the engineers who build the tool. Every project starts with a free DFM review, because fixing wall thickness, draft, undercuts and gate location on screen costs far less than fixing them in steel. You can read more in our DFM guide for injection molding and on our injection mold making page.
Get a quote that states the class
Send us your 3D file, expected volume and resin, and we will recommend the mold class and steel, with the reasoning, as part of a free DFM review. Request a quote
