What is a good Cpk value?

Cpk 1.33 is the common benchmark, meaning the process spread fits within the tolerance with a safety margin. Cpk 1.0 is borderline, and values above 1.33 indicate a well-centered, capable process. The target depends on the industry and the risk of the dimension.

How is Cpk calculated?

Cpk compares the distance from the process mean to each specification limit against the process spread (three standard deviations on each side). It takes the smaller of the two, so it reflects both variation and how centered the process is.

Why is Cpk harder for silicone parts?

Silicone is flexible, so dimensions shift with temperature, durometer and how the part is measured. This makes process variation harder to control than rigid materials, which is why capability must be measured on a defined dimension with a defined method.

Do you report Cpk honestly even when it is below 1.33?

Yes. We report the measured result as it is. For example, one of our studies (LSR-MED-007) measured Cpk 1.24, and we state that rather than presenting it as better than the 1.33 target.

Silicone Cpk & Process Capability Explained

Cpk is the number buyers and quality engineers ask for — but a single headline figure without context says little. Here is what Cpk actually measures, how it applies to flexible silicone parts, and why we report our results honestly instead of rounding them up.

The Basics

What Cpk Measures

Variation

Cpk compares the process spread (three standard deviations each side of the mean) against the tolerance width — how consistently the process stays inside spec.

Centering

It also checks whether the process is centered in the tolerance. A shifted process scores lower even if the spread is tight.

One Number, Both Signals

Because it takes the worst of the two sides, a single Cpk value captures both spread and centering in one figure.

Reading the Number

What Different Cpk Values Mean

Cpk value	Interpretation
< 1.0	Process cannot reliably hold the tolerance — high reject risk
1.0	Borderline — the process just fits the tolerance with no safety margin
1.33	Common benchmark — capable with a reasonable safety margin
1.67+	Highly capable — typical target for critical or high-risk dimensions

Why Silicone Is Different

Capability Is Harder on Flexible Parts

Silicone is elastic, so a measured dimension shifts with temperature, durometer and even how firmly the part is fixtured during measurement. That is why capability must always be tied to a defined dimension and a defined measurement method — quoting a Cpk without stating what was measured and how is meaningless.

What We Specify

Part, dimension, tolerance, sample size, measuring instrument and lot — so the study is reproducible, not just a number on a slide.

What We Report Honestly

We state the measured result as it is. Our study LSR-MED-007 measured Cpk 1.24 — we report that figure rather than presenting it as better than the 1.33 target. A separate automotive FVMQ seal study (FVMQ-SEAL-010) measured Cp 1.39 · Cpk 1.32 on seal lip thickness.

See the full capability and measurement approach in engineering evidence and quality inspection.

Go Deeper Into Quality

PPAP & Quality Docs

First-article inspection, Gage R&R, SPC, control plan and PFMEA.

See the document set

Engineering Evidence

Cleanroom report, Cpk data and desensitized quality records.

See the evidence

Quality Inspection

Three-stage IQC/IPQC/OQC and calibrated measuring equipment.

See quality control

Need a Capability Study for Your Part?

Tell us your critical dimensions and tolerances — we'll confirm what we can measure and hold, and quote the project.

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