Super-Ingenuity (SPI)

CNC Machining & Injection Molding — DFM/Moldflow Support, CMM Inspection, Prototype to Production Solutions.

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Industrial Finishing Standards

SPI vs VDI 3400 Mold Finish: Ra Range, Draft Rules, and Acceptance Criteria

Decoding the critical differences between SPI (US) and VDI 3400 (EU) standards to optimize optical clarity and texture consistency. Use our injection molding service (cosmetic & textured parts) to avoid texture drag, scuff marks, and cosmetic disputes through standardized inspection zones and draft feasibility audits.

Download SPI↔VDI Chart (Ra + Draft Rule) Request Finish Callout Review
We review finish callouts + draft feasibility—no sales follow-up unless requested.
Kevin Liu - VP of Mold Division at Super Ingenuity
Kevin Liu | VP, Mold Division Leads Class-A cosmetic & medical tooling programs
Focus: Finish feasibility and inspection-ready acceptance
Before and after comparison of SPI polish vs VDI 3400 texture on an ABS injection molded part, showing gloss level and surface texture difference

60-Second Selection Guide

Choose SPI standards for high-gloss or transparent parts (Ra-driven polish). Choose VDI 3400 for matte or tactile textures (EDM depth-driven). Use Optical-grade requirements when haze, bubbles, and defect visibility are critical—always define inspection lighting and cosmetic zones before steel cut to prevent disputes.
SPI A-2 high gloss polish on black ABS part

When SPI is the Right Tool (Gloss & Clarity)

  • Use for transparent optics (PC/PMMA lenses).
  • Required for Class-A high gloss (Piano Black).
  • Target Ra values below ~0.05 μm (polish-driven).
  • Risk: Weld lines become highly visible; define gating early.
Optical-grade: haze & defect control →
VDI 3400 matte texture on injection molded part

When VDI 3400 is the Right Tool (Texture)

  • Best for anti-glare matte finishes & scratch hiding.
  • Common in automotive interiors & tool housings.
  • Texture depth matters more than simple Ra values.
  • Rule: Increase draft to avoid texture drag/scuff marks.
Draft angle guide for deep textures →
Optical grade clear PC lens under D65 light

When "Optical" Grade is Not Optional

  • Essential for Class-A headlamps & light guides.
  • Zero-tolerance for haze, flow lines, or micro-bubbles.
  • Requires specialized S136 or 420ESR mold steel.
  • SOP: 100% visual audit under D65, fixed distance/angle.
Acceptance criteria: lighting & zones →

Real-World Production Impact

Finish choice changes ejection force, defect visibility, and texture replication—so draft, gating, and acceptance must be defined before steel cut.

Ejection & Scuff Risk

Surface texture dictates release force. SPI A-1 polish is prone to vacuum sticking, while heavy VDI textures increase mechanical friction. Failure to adjust draft leads to permanent scuff marks during ejection.

DFM CHECK: Verify draft on every textured wall • Add venting at EOF to reduce force.

Defect Visibility Multiplier

A mirror finish (SPI A-grade) acts as a visual amplifier. Minor weld lines, flow marks, or micro-sink marks that are hidden by VDI matte textures become glaring rejection-level defects on high-gloss surfaces.

REJECT RISK: Weld lines • Flow marks • Micro-sinks (Visible under grazing light).

Surface Replication Limits

Achieving VDI grades depends on material physics. High-viscosity resins (e.g., PEEK, glass-filled) may fail to replicate fine texture peaks if melt temp and packing pressure aren't precisely balanced.

VALIDATION: Run DOE on speed/pack to confirm replication before final etch depth.
Comparison of defect visibility on SPI high-gloss vs VDI matte textured injection molded parts, showing weld line visibility

SPI Mold Finish Standard (A/B/C/D) — Practical Meaning by Part Type

Use the SPI A/B/C/D series to decide (1) cosmetic risk level, (2) tooling steel & polishing effort, (3) cost/lead impact, and (4) inspection method—before you lock the drawing callout.

Decision Checklist

  • Cosmetic zone: Zone A (visible/Class-A) or hidden/internal?
  • Material: PC/PMMA/ABS/filled resin (replication differs).
  • Acceptance: D65 lighting + distance/angle + master plaque defined?

SPI A Series A1, A2, A3

  • Typical Use: High-gloss “piano black” cosmetics, optical lenses (PMMA/PC), medical clear-view ports.
  • Tool Steel: Premium steels like S136, 420ESR, or NAK80 to reduce inclusions/pitting.
  • Process: Diamond buff polishing to mirror-level reflectivity.
Critical defect risk: A-series amplifies flow lines, weld lines, and gas marks; gate placement and flow balance are decisive.
Inspection: Visual under D65 (defined lux/distance/angle) + master plaque for reflective comparison.

SPI B Series B1, B2, B3

  • Typical Use: Consumer electronics housings and automotive interior components needing smooth, semi-gloss feel.
  • Process: Paper polish using ~400–600 grit.
  • Benefit: Balanced cosmetic appearance with moderate tooling cost.
Typical pitfall: B-grade can still reveal flow/weld lines on large flat panels—gate location and packing matter.

SPI C Series C1, C2, C3

  • Typical Use: Structural parts, internal enclosures, industrial components where glare reduction is preferred.
  • Process: Stone-polished finish.
  • Cosmetic sensitivity: Low; hides minor scratches and molding variation.
Not recommended for: brand-facing exterior panels requiring tight gloss uniformity across batches.

SPI D Series D1, D2, D3

  • Typical Use: Textured handles, battery covers, and non-cosmetic functional parts.
  • Process: Dry blasting using glass beads or sand.
  • Function: Hides sink marks and improves tactile grip.
Draft rule: start at ≥1.5° and increase with texture depth/part height to prevent drag marks during ejection.
SPI GradeTypical Tool ProcessBest-fit Part SurfacesDefect SensitivityCost/Lead ImpactRecommended Inspection
SPI A-1#3 Diamond BuffOptical lenses, mirrorsUltra-High (flow/weld)High (manual bench time)Visual under D65 + master plaque
SPI A-2#6 Diamond BuffHigh-gloss housingsHigh (scratches)Premium (depends on Zone A size)Reflective comparison + defined setup
SPI B-1600 grit paperConsumer goodsMediumStandardStandard visual (defined distance/angle)
SPI C-1600 grit stoneIndustrial bracketsLowBaseRa spot check (process control)
SPI D-2Dry bead blastTactile gripsMinimal (hides sink)BaseVisual + texture plaque (if available)

Cost/lead impact varies by part size, cavity count, steel grade (e.g., S136/420ESR), and the defined cosmetic zone area. Use values as typical planning ranges, not fixed quotes.

Engineering Resources:How SPI finish level changes tooling cost & lead timeHow we inspect SPI finishes (D65, plaque, Ra checks)

VDI 3400 Texture Standard: Engineering Decision Controls

VDI grade primarily correlates with texture depth and replication difficulty—so draft angle and venting strategy must scale linearly with the number.

VDI Ranges (12–45) & Tactile Intent

Unlike SPI’s polish system, VDI 3400 is an EDM-based scale. Grades 12–18 provide a subtle sparkle for electronics, while 24–30 is the industry baseline for matte automotive interiors.

QUICK RULE: VDI 24–30 is the safest "default matte"; go above VDI 33 only if you can afford higher draft.

Texture Failure Zones: Ribs & Deep Cores

In deep features, EDM sparks cannot maintain uniform density, leading to "bald spots." Laser texturing is required for these geometries to maintain consistency across complex $3D$ profiles.

DFM CHECK: Flag deep ribs early and specify "No Texture" or "Laser Only" zones to prevent drag marks.
Acceptance SOP: For Class-A panels, specify texture direction + inspection lighting (D65) + 60cm viewing distance. Without these, VDI textures often look patchy due to EDM path interactions.

Avoiding Machining Ghosting

Visible tool paths (ghosting) occur when the pre-texture steel finish is too rough. Ensuring a clean SPI C-1 stone finish before EDM texturing is the only way to prevent machining marks from reflecting through the grain.

DRAWING NOTE: Specify grain orientation and require a master plaque to approve uniformity on large surfaces.
Real mold steel surface showing polished finish vs VDI 3400 texture side by side, highlighting texture depth
VDI Grade Depth ($ \mu m $) Rec. Draft Range Best-Fit Resins (Replication) Typical Cosmetic Risks
VDI 12 $\sim 1.5$ $0.5^{\circ} - 1.0^{\circ}$ PC, PMMA, ABS (High Fluidity) Ejection scratches, scuffing
VDI 24 $\sim 6.3$ $1.5^{\circ} - 2.0^{\circ}$ ABS, PP, PA66 (Medium Flow) Uneven gloss, flow line visibility
VDI 30 $\sim 12.5$ $2.5^{\circ} - 3.0^{\circ}$ PP, TPE, Glass-Reinforced "Bald" spots in deep ribs
VDI 36 $\sim 25.0$ $4.0^{\circ} - 5.0^{\circ}$ Structural PA, POM (High Viscosity) White stress marks during ejection
VDI 45 $\sim 70.0$ $7.0^{\circ}+$ Grips, Handles (Low MFR) Trapped gas, poor peak replication
* Draft ranges depend on part height, resin shrink rate, and venting. Validate via ejection trials during T1.

Draft Angle Rules for Textures (#1 Design Gate)

Deeper textures require exponentially more draft because the surface “hooks” onto the tool during shrinkage. Without sufficient draft, parts exhibit vertical drag marks or burnished glossy streaks. Rule of record: A texture specification is incomplete unless the drawing also states the minimum draft angle and cosmetic zone priority.

Required: $1^{\circ}$ per $0.02\text{ mm}$ of texture depth

Why Texture Increases Release Friction

Unlike smooth SPI finishes, VDI textures create thousands of microscopic undercuts. As the part shrinks onto the core, these peaks lock into the tool valleys. Design Action: Avoid textures on shut-offs and deep ribs unless draft and end-of-fill venting are validated via DFM.

Consequences of Insufficient Draft

Failure manifests as directional vertical drag marks (aligned with ejection) or glossy streaks where the grain has been burnished. Common misread: These marks are often blamed on "wrong material" but are almost always mechanical draft or ejection force spikes.

Practical Strategy by Feature Type

Sidewalls: Apply $\sim 1.5^{\circ} - 3.0^{\circ}$ for standard VDI 27-30 (Increases with part height).
Internal Ribs/Bosses: Add $+0.5^{\circ}$ safety margin to compensate for inward shrinkage.
Trade-off: Excessive draft can shift the parting line; confirm with Cosmetic Zones.
Copy-Paste Drawing Call-out: FINISH: VDI 33 texture on Zone-A only; min. draft $\ge 3^{\circ}$ on textured walls; inspect under D65 @ 300mm per master plaque.
Textured injection molded part showing drag marks and glossy streaks caused by insufficient draft angle

Roughness Metrics ($R_a / R_z$): Useful Baseline, Not the Whole Story

When $R_a$ Helps Communication

Use $R_a$ (Roughness Average) as a quantitative measurement floor for supplier alignment and repeatability checks on matte or functional surfaces. It provides the industry-standard "math" to confirm a tool has reached the baseline finish requirement before first-article trials.

ENGINEERING TRUTH: Same $R_a$ value $\neq$ Same visual gloss or light scattering.

When $R_a$ Misleads: The Visual Paradox

Ra averages all peaks and valleys, which can hide extreme $R_z$ heights. Two surfaces can share the same $R_a$ yet have completely different light scattering properties. Rule of Thumb: Use $R_z$ (Peak-to-Valley) to identify sharp asperities that cause ejection snags, even if the $R_a$ appears compliant.

Best Practice: The Hybrid Spec Pattern

Specify the standard grade (SPI/VDI) as the primary visual target and $R_a$ as a supporting reference for process control. Always define the cosmetic zone and inspection setup alongside these values to ensure the "look" and the "math" are perfectly aligned.

Surface roughness tester measuring Ra and Rz on a mold steel insert for SPI and VDI verification
Technical Specification Template (Standard SOP) 
Finish Standard: SPI A2 on Cosmetic Zone A; 
Ra Reference: 0.05 - 0.10 µm (Supportive Only); 
Light Source: D65, 1000 Lux Minimum; 
Inspection: 300 mm Distance, 45° Angle; 
Master Plaque: Approved limit sample required;
Acceptance: No visible weld lines or pits under defined conditions.

SPI vs VDI Conversion: Equivalence vs. Reality

Practical Rule: SPI↔VDI charts are $R_a$ references only—visual match requires physical sample approval.

The Limits of Physical Conversion

Technically, SPI polishing and VDI texturing cannot be 1:1 converted. VDI 3400 is an EDM scale, while SPI A-D is a polishing scale. Any conversion chart provides a theoretical $R_a$ approximation. The final gloss depends on resin shrinkage and tool steel grade.

Engineering Disclaimer:
SPI and VDI are not visually interchangeable. "Equivalence" is for reference only unless validated by a master plaque under D65 lighting conditions.

"Finish Equivalence" Spec Template

Primary Standard: VDI 3400 Ref 24 (Zone A);
Ref Equivalence: SPI B-2 (Gloss focus);
Inspection: D65, 1000 Lux, 45° View;
Approval: Master Plaque + FAI Required.
  • Cut a steel coupon from the production heat-treat lot.
  • Apply the intended texture using production-grade electrodes.
  • Mold 20 pieces with target resin to verify "Replication Capability."
  • Lock specification before full tool texturing to avoid costly rework.
Professional SPI polish vs VDI 3400 texture comparison plaque used for industrial surface finish validation

Optical-Grade Mold Finish: Beyond Mirror Polishing

Success is controlled by steel cleanliness, shear stability, and measurable haze—not $R_a$ alone.

Tooling: ESR Steel & Pitting Control

Standard tool steel contains micro-inclusions that lead to pitting. We mandate ESR (Electro-Slag Remelted) steels like $S136$ or $420ESR$, ensuring zero non-metallic contaminants. Engineering SOP: Require ESR certification + a coupon polish trial before final cavity work.

Process: Gate Shear & Pressure

Clarity is a function of molecular orientation. Without rigorous Moldflow Analysis, surface defects like shear whitening will persist regardless of the $SPI\ A-1$ polishing quality.

Critical Controls: Drying & Thermal Stability

Polishing cannot mask internal haze. These are managed by resin dew-point drying and tool temperature precision ($ \Delta T \le 2^{\circ}C $), essential for Automotive Lens production.

$R_a < 0.02\ \mu m$
S136 ESR ($HRC\ 52-54$)
$\le 1.5\%$ ($ASTM\ D1003$)
$D_{65}$ / $1000\text{ Lux}$ / $300\text{ mm}$
High-precision optical grade mold core with mirror polish on ESR stainless steel

How to Specify Finish on Drawings (Checklist)

  • Standard + Grade (e.g., SPI A2 / VDI 27)
  • Cosmetic Zones (Zone A/B/C clearly demarcated)
  • Minimum Draft (Validated for all textured walls)
  • Acceptance SOP (Setup + Master Plaque + FAI)

Engineering Mandate: Without cosmetic zones and inspection SOPs, "meeting the grade" still fails quality audits.

Zone Definition (A-Surface vs. Internal)

Clearly demarcate Zone A (Visible/Class-A), Zone B (Tactile/Semi-visible), and Zone C (Non-cosmetic). Drawing Method: Use leader notes or hatch boundaries for Zone A; anything unmarked defaults to Zone C to optimize polishing costs.

Standardized Inspection SOP

Specify the environment to prevent disputes: Inspection SOP: $D_{65}$ lighting, $1000\text{ lux}$ minimum, $300\text{ mm}$ viewing distance, and $45^{\circ}$ viewing angle. Apply defect thresholds (e.g., bubbles/pits) only to Zone A unless functional.

Tool-side vs. Part-side Approval Gate

Never rely solely on the first part. Gate Requirement: Approve the tool-side steel finish (High-res photos + physical plaque) before tool hardening or texture etching. Post-etch rework is often irreversible and costly.

Supplier-Proof Callout Templates

// OPTICAL GRADE CALLOUT (CLASS-A) 
FINISH: SPI A-1 (Mirror Polish) on Zone A;
STEEL: S136 ESR (52-54 HRC);
INSPECTION: D65, 1000 lux, 300 mm, 45° View;
APPROVAL: Master Plaque + FAI Required.
// TEXTURED COSMETIC CALLOUT 
FINISH: VDI 3400 REF 27 (Zone A only);
MIN DRAFT: ≥ 3.0° on all textured walls;
INSPECTION: D65, 1000 lux, 300 mm, 45° View;
APPROVAL: Master Plaque ID #____ + FAI Required.

Inspection & Measurement Plan

Engineering Rule: Matte/textured surfaces are verified by $R_a/R_z$ metrics; Class-A gloss MUST be verified by controlled visual comparison (D65 booth + Master Plaque).

Quantitative Measurement (Ra/Rz)

We utilize precision contact profilometers to establish a compliance baseline for VDI textures. While $R_a$ provides the floor for repeatability, we prioritize Reflective Comparison for SPI A-series to detect defects that averaging metrics often miss.

  • $R_a/R_z$ Profilometry (Matte Only)
  • Gloss Meter (60°/85°) for Semi-Gloss
  • CMM 3D Dimensional Scan
  • Interferometry for Optical Path

Visual SOP for Cosmetic Zones

Inspection is strictly governed by the viewing environment to ensure batch-to-batch consistency. Standardized SOP: Zone A surfaces are audited under $D_{65}$ lighting (1000–1200 Lux), at a distance of $300\text{ mm}$ and a $45^{\circ}$ viewing angle.

FAI Package Deliverables

Every project concludes with a comprehensive FAI (First Article Inspection) package that establishes the production baseline:

  • High-Res Tool Surface Macro Photos
  • Signed Golden Sample (with Plaque ID)
  • Inspection Condition Logs (Lux/Distance)
  • Zone-A Defect Mapping (if applicable)

Maintenance & Finish Consistency

Mold finishes degrade over time—mirror polishes can "fade" after ~50,000 cycles. We implement a proactive maintenance plan to schedule periodic re-polishing based on cycle counts and cosmetic drift, ensuring long-term CPK stability.

Cost, Lead Time & Risk: The Finish-Grade Jump Rule

Rule: One finish-grade jump often changes labor hours, acceptance risk, and ECN probability more than it changes "appearance."

Polishing Time is Exponential Near Mirror Levels

Moving from SPI B-1 to SPI A-2 is not a linear cost increase. For high-gloss surfaces, manual labor hours grow exponentially as every micro-scratch must be removed under $D_{65}$ inspection lighting. Impact depends heavily on Zone A area size and steel cleanliness (ESR vs. standard grades).

Texture Adds Hidden Tooling Requirements

Specifying a VDI texture dictates mold core architecture. Beyond the etching cost, it mandates significant design constraints to ensure production stability:

  • Draft Scaling: Angles must increase with texture depth (Ejection Force & Drag Risk).
  • Venting Criticality: Trapped gas in textured valleys causes visible "bald" or burnt spots.
  • Rework Visibility: Texture matching after steel welding/modification is extremely difficult to hide.
Project Management Standard:
Set Zone C (hidden ribs/internal walls) to standard stone finish by default. Reserve SPI A or heavy VDI textures for Zone A only to save $15-20\%$ of tooling budget and reduce ejection risks.

Preventing ECN Churn: The Validation Gate

Mismatched visual expectations after T1 are the #1 driver of finish-related Engineering Change Orders (ECNs). Gate Requirement: No final texture release or mirror-polish commit until the master plaque and inspection setup are approved by the client.

Manual mirror polishing and D65 inspection of a mold insert showing labor intensity

Common Spec Mistakes (The Root Cause of Tool Rework)

Correcting these five mistakes prevents T1 rejections and costly modification loops.

Surface finish is a manufacturing requirement, not just a cosmetic choice. Misdefining these standards is the leading cause of "T1 Rejections" and irreversible steel damage. Use the checklist below to audit your drawing callouts before steel cut to ensure the mold is optimized for both budget and quality.

Injection mold surface finish rework showing patchy texture mismatch

“SPI A2 Everywhere”

Applying mirror polish to non-cosmetic areas increases tooling costs by 30% and reveals internal flow lines or sinks that a matte finish would hide. Fix: Limit SPI A-surfaces to Zone-A only; set internal/rib walls to SPI C-1 standard.

VDI Texture without Draft

Textures create mechanical undercuts. Without scaling draft (min. 1.5°–3.0°), parts suffer from permanent vertical drag marks and glossy streaks during ejection. Fix: Specify VDI grade + minimum draft angle per zone in the drawing notes.

No Zone Definition

Without A/B/C zoning, suppliers choose the easiest surfaces to polish. Critical cosmetic regions may be overlooked, leading to total FAI failure. Fix: Mark Zone A/B/C on the 2D drawing; unmarked surfaces default to Zone C.

Mixed Goals without Test Methods

Requesting "High Gloss Matte" is a subjective contradiction that leads to disputes. Visual results remain unmeasurable without defined lighting and distance. Fix: Define the approval method: Master Plaque + D65 setup (or specific Gloss Meter target).

Ignoring Resin Capability

Glass-filled materials or high-shrink resins (like PP) cannot replicate a mirror SPI A-1 finish perfectly, rendering expensive polishing invisible. Fix: Validate resin replication with a plaque coupon before committing to mirror polish.

Recommended Finish Matrix (By Part Category)

Category Rec. Grade Typical Resin Approval / Inspection Method Risk if Mis-Spec'd Draft
Optical / Lens SPI A-1 / A-2 PC, PMMA, PETG $D_{65}$ + Haze measurement + Master Plaque Internal haze / Birefringence visible $0.5^{\circ} - 1.0^{\circ}$
Class-A Visible SPI A-3 / B-1 ABS, PC/ABS $D_{65}$ + Reflective comparison + FAI Weld & flow lines highly amplified $1.0^{\circ} - 1.5^{\circ}$
Medical Housing SPI B-1 / B-2 PEEK, Med-PP Visual + Cleanability criteria (Zone A) Pitting traps / Bio-film buildup $1.0^{\circ} - 2.0^{\circ}$
Auto Interior VDI 24 - 30 PP, PA66 + GF Approved texture plaque + Zone audit Texture drag / Patchy light scattering $2.5^{\circ} - 3.5^{\circ}$
Internal Functional SPI C-1 / D-2 POM, Nylon, PBT $R_a$ Spot check + Ejection release test High friction / Ejection scuff marks $1.5^{\circ}+$
* Draft ranges depend on texture depth, part height, and resin shrink rate. Validate for Zone-A via Master Plaque FAI.
Optical clear PC lens under D65 inspection

Transparent & Optical Parts

Focus on SPI A-series diamond buffing. Essential for light guides where internal scattering must be minimized via mirror-level tool surfaces.

PITFALL: A-1 polish on PC reveals gate shear whitening; requires Moldflow prior to gating lock.
Medical device housing with SPI B1 satin finish

Medical Device Surfaces

Focus on SPI B-series paper finishes. Provides a professional "satin" look that is easy to sterilize without the extreme maintenance of mirror grades.

SOP: Define Zone-A cleanability; ensure no micro-pits in functional assembly areas.
Automotive interior trim with VDI 30 matte texture

Automotive Interior Textures

Standardized VDI 3400 grades. Engineered for anti-glare properties and tactile feedback in cockpit environments.

RULE: Increase draft by $0.5^{\circ}$ per VDI grade jump to prevent texture drag on deep walls.

Surface Finish Technical FAQ

Do I need a draft angle rule when specifying VDI textures?
Yes, you must scale draft angles with texture depth to prevent ejection scuff marks. A safe engineering rule is to add $1.5^{\circ}$ of draft for every $0.02\text{ mm}$ of texture depth. Failure to specify this results in vertical drag marks or texture wash-out. Refer to our minimum draft rules for textured ribs & deep walls.
Can SPI and VDI be converted using Ra? What are the limits?
SPI and VDI can be approximately correlated via Ra, but they are not visually equivalent. Polishing (SPI) creates a reflective topography, while EDM (VDI) creates a cratered surface. Even with identical $R_a$ values, the light scattering and tactile feel will diverge significantly. If appearance is critical, approve a physical plaque under D65 inspection setup.
How do you define cosmetic surface acceptance to avoid disputes?
Acceptance requires three defined variables: Visual Zones, Inspection Setup, and Master Samples. We audit Zone A surfaces under $1000\text{ Lux}$ $D_{65}$ lighting at a $300\text{ mm}$ distance and $45^{\circ}$ angle. Any defect $>0.05\text{ mm}$ in critical zones is grounds for rejection. See our standardized cosmetic zones and plaque approval SOP.
Which VDI 3400 grade is typical for matte housings vs. tactile grips?
VDI 24 to 30 is the industry baseline for standard anti-glare matte housings. For tactile handles or non-slip grips, VDI 36 to 45 is used to provide an aggressive texture that enhances friction and hides sink marks. Note that $VDI\ 36+$ requires strict adherence to increased draft angles to avoid drag marks.
What is the practical difference between SPI A2 and SPI A1?
SPI A-1 is a true mirror finish ($R_a < 0.025\ \mu m$) reserved for optical-grade parts. It requires premium ESR tool steel ($S136$) and diamond buffing. SPI A-2 ($R_a \approx 0.05\ \mu m$) is the standard for high-gloss consumer housings; it offers high reflection while being more cost-effective for mass production. Check our optical-grade requirements: haze control & steel cleanliness.
Should finish be specified on the part drawing or mold drawing?
Finish specifications must be synchronized across both the part and mold drawings. The part drawing defines the end-user cosmetic requirement (e.g., VDI 27), while the mold drawing specifies the required tool steel and polishing sequence (e.g., #600 Grit Stone) to ensure consistent batch-to-batch replication.
What inspection method is best: roughness measurement or visual?
Use a hybrid approach: Ra measurement for matte surfaces and visual comparison for gloss. Roughness metrics ($R_a/R_z$) provide measurable consistency for VDI textures, while visual comparison against a master plaque is mandatory for SPI A-series where light reflection clarity is the primary quality gate.
Why do textures look uneven near gates or ribs, and how to prevent it?
Uneven texture is typically caused by high shear heat near gates or trapped gas in deep ribs. This results in "glossy patches" or "bald spots." Prevention requires optimizing gate location and venting depth through moldflow validation of gate shear & venting before the tool steel is etched.
Engineer reviewing surface finish callouts and cosmetic zones on a technical drawing

Standardization Opportunity

Standardize Your Mold Finish Early

Send your 2D/3D files—our engineering team will return a comprehensive Finish Feasibility Pack before steel cut to lock in quality and cost:

  • Callout Audit: SPI/VDI Grade + Zone A/B/C Demarcation.
  • Draft Validation: Required angles for textured walls & deep ribs.
  • Process Risk Summary: Gate shear, venting, & replication limits.
  • Acceptance SOP: $D_{65}$ lighting, distance, angle, & Master Plaque plan.
Get Finish Feasibility Review → Send Drawing for Audit
We review finish callouts & feasibility first—no sales follow-up unless requested.