Polishing vs Texture/EDM vs Coating for Mold Cavities: What the Evidence Can Decide

Rule out any finish callout you cannot measure

A drawing that names a gloss level or an SPI grade without a measurable Ra or an acceptance method gives you nothing to inspect against. And the sources in this set provide no grade-to-Ra or gloss-unit mapping.

What they do show is that finish specification is a cost driver in its own right: over-specifying tolerances and Ra values is named as a leading cause of inflated part costs, with one vendor estimating increases above 30%².

Treat that figure as one supplier's observation rather than a market benchmark, but the direction is consistent with the rest of the evidence. The mechanism is scope, not just machining.

Finishing, heat treatment, marking and inspection can add days of lead time and significantly change total cost, and they often go unnoticed when only the unit price is compared¹⁵.

Finishing and inspection are also the line items most often excluded or estimated separately, so an unmeasurable callout tends to reappear later as a re-price or a schedule slip²⁴. Where a number does exist, it is application-specific.

For stainless steel product-contact surfaces in pharmaceutical plant, Ra ≤0.8 µm is the usual roughness requirement, verified by profilometer⁶. That is a cleanability requirement for one service, not a default for a mold cavity.

If your part is plastic, measurement itself is harder: contact profilometry risks scratching the surface, so statistical sampling or non-contact methods such as white-light scanning may be used⁶. Before you write a finish callout, decide which of those you are actually asking the supplier to prove.

Polishing or texture/EDM: which route reaches your finish level

The two routes are not interchangeable at the fine end. Polishing spans mirror to satin, while bead blasting produces a matte grey, uniform grain⁴.

EDM produces smooth surfaces, but the finish is not as fine as grinding or polishing, and some applications need additional post-machining to reach the desired surface quality¹⁸. That ordering decides the sequence rather than the choice.

If the cavity needs a fine finish and the geometry requires EDM, you are likely buying both processes: EDM for the form, polishing afterward for the surface. If a matte, uniform cosmetic is acceptable, bead blasting is the cheaper and faster texture route in the same comparison table⁴.

What the sources do not give you is the number that would settle a borderline case. There is no SPI A-1 to D-3 grade table with Ra or gloss values, so a callout like "SPI B-1" cannot be converted into a measurable target from this evidence.

Ask the supplier to state the Ra they will deliver and how they will measure it, and compare quotes on that basis rather than on the grade label.

When the cavity needs wear or corrosion resistance, not just appearance

Polishing removes material but leaves hardness unchanged and gives only slight salt-spray improvement, so it is an appearance and friction step rather than a wear or corrosion solution⁴. If the resin is abrasive or the cavity sees corrosive conditions, polishing alone does not address that requirement.

Coatings are the family the sources associate with those properties. Electroless nickel reaches 500–700 HV and 500–1,000+ h salt spray in the same comparison table, at a relative cost of 2.5× and 4–7 days added lead time⁴.

A second source describes ENP as offering the best overall combination of corrosion and wear resistance on steel, often exceeding 1,000 hours salt spray at 25 µm⁵. Mechanical, chemical and electrochemical finishing families are not interchangeable; each addresses a different surface requirement¹⁴.

One boundary matters here. That coating evidence is general metal finishing, not mold-cavity data: no source in this set addresses Ni-PTFE, DLC, chrome or PVD on a mold and what any of them does to release.

If you are specifying a coating for release rather than for cavity wear, the sources do not support the claim, and you should ask the coating supplier for mold-specific references instead.

Set draft and release expectations before the finish

Finish choice affects release, but it does not replace draft. Draft angles help parts release without surface damage, and poor draft makes parts stick, which lengthens processing time¹¹.

For materials with strong friction, draft angles between 1° and 3° are cited, and regular materials are said to work well with those angles too¹¹. Texture, polishing and EDM are named as the finishing processes that affect how a part looks and performs¹¹.

What the sources do not provide is any quantified ejection friction or release data by finish or by polish direction, so you cannot trade a polish direction against a draft angle on this evidence. Fix the draft first, then choose the finish for appearance, cleanability or wear.

If release is the problem you are trying to solve, ask the toolmaker what draft and what surface they recommend for your resin, and treat the answer as their position rather than a sourced rule.

Relative cost and added lead time by finish option

The figures below come from a single peer comparison table and are relative multipliers against a baseline, not currency amounts⁴. Read them as an ordering: polishing is the labor-heavy, multi-day option, bead blasting is the cheap and fast one, and coatings add both cost and days.

Absolute polishing hours, texture/acid-etch fees and vapor-polishing batch costs are not quantified in this set.

Write the finish and its inspection into the RFQ

Whether finishing is done in-house or subcontracted, and how inspection is scoped, are among the largest sources of quote-to-quote variation²⁴. These checks keep the finish from being re-priced or re-scheduled after you have chosen it.

• ✓State the finish as a measurable Ra with the measurement method, not a grade or gloss label.
• ✓Ask whether polishing is performed in-house or subcontracted, and whether the quote includes it.
• ✓Confirm how much inspection is included, by which method, and with which report.
• ✓For plastic parts, agree whether roughness is verified by sampling or by non-contact scanning, since contact probes risk scratching.
• ✓Ask the supplier to state the Ra they will deliver and how they will measure it before you compare prices.

Where the sources disagree or leave a number open

Most of the finish figures in this article trace to one peer comparison table, so there is little genuine cross-source disagreement to report. The rows below separate what is a single-source figure from what is simply absent, so you know which numbers to verify rather than rely on.

Before you request a quote

• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.