SPI Mold Finish: Surface Finish For Injection Molded Parts

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Surface finish is a critical aspect of injection molded parts. The mold finish, in particular, directly impacts the surface quality and appearance of the molded parts. SPI (Society of the Plastics Industry) has standardized plastic part surface finishes to help define and specify surface texture requirements. This article provides a comprehensive guide to SPI mold finishes for injection molded components.

What is SPI Mold Finish?

SPI mold finish refers to standardized plastic part surface finishes defined by the Society of the Plastics Industry (SPI). The SPI standards help to define plastic part surface texture in terms of roughness average (Ra) values measured in microinches (μin).

There are several SPI mold finish standards designated with alphabet codes A-1, A-2, A-3, B-1, B-2, B-3 and C-1, C-2, C-3. The SPI finish becomes finer in texture from A-1 through C-3.

Key Aspects of SPI Mold Finish

  • Provides a standard method to define and specify surface finish for plastic injection molded parts
  • Uses roughness average (Ra) values measured in microinches to quantify surface texture
  • Ranges from roughest (A-1) to finest (C-3) texture
  • Allows standardization of mold polishing and plastic part appearance expectations

SPI mold finish standards help injection molders and part designers align on surface finish requirements for molded plastic components.

SPI Mold Finish Standards

The SPI mold finish standards are categorized into three main groups – A, B and C. Within each group, there are three classes numbered 1, 2 and 3. The classes become finer in texture progression from 1 through 3.

Here is an overview of the key SPI mold finish standards:

SPI Finish A

The SPI A finishes are the roughest surface textures. They are suitable for parts that do not require an aesthetic appearance.

  • A-1 – General purpose mold finish with 250-125 μin Ra roughness. Provides a dull matte texture.
  • A-2 – Slightly smoother than A-1 with 125-63 μin Ra roughness. Matte finish.
  • A-3 – Fine matte texture with 63-32 μin Ra roughness. Low gloss finish.

SPI Finish B

The SPI B finishes provide a medium surface texture. B finishes are commonly used for parts that require low gloss or matte finishes.

  • B-1 – 250-125 μin Ra roughness. Low luster satin finish.
  • B-2 – 125-63 μin Ra roughness. Dull satin finish.
  • B-3 – 63-32 μin Ra roughness. Smooth satin finish.

SPI Finish C

The C finishes are the finest SPI mold finishes. They produce glossy, reflective surface qualities on molded plastic parts.

  • C-1 – 250-125 μin Ra roughness. Medium reflective gloss.
  • C-2 – 125-63 μin Ra roughness. Bright reflective gloss.
  • C-3 – 63-32 μin Ra roughness. High reflective gloss finish.

This table summarizes the SPI mold finish standards and surface texture levels:

SPI FinishRoughness Average Ra (μin)Typical Surface Texture
A-1250-125Dull matte
A-363-32Fine matte, low gloss
B-1250-125Low luster satin
B-2125-63Dull satin
B-363-32Smooth satin
C-1250-125Medium gloss
C-2125-63Bright gloss
C-363-32High gloss

How SPI Mold Finishes are Achieved

The SPI mold finishes are obtained by polishing the cavity and core surfaces of the injection mold to particular roughness levels.

Here are the typical techniques used to achieve the SPI mold surface finishes:

  • A Finishes – Mold cavity/core surfaces are machined by conventional milling or Electrical Discharge Machining (EDM). No polishing is done. The machined mold finish directly produces the A-level surface texture on molded parts.
  • B Finishes – After machining the cavity/core via milling or EDM, the surfaces are hand polished or machine polished to a satin finish. This yields the B-level SPI mold finishes.
  • C Finishes – The mold surfaces are machined, then undergoes progressive hand polishing and buffing stages. Multi-step polishing with increasingly finer abrasives is done to achieve the high gloss C-level finishes.

Mold polishing requires expertise to consistently achieve the desired SPI surface finish standards. The mold builder must carefully control the polishing steps and techniques. Factors like polishing abrasive grades, pressure applied, equipment used impact the final mold surface roughness.

SPI Mold Finish Callouts

On a mold design drawing or plastic part drawing, the SPI finish levels are specified using callouts.

Here are some examples of how SPI finishes may be specified on drawings:

  • SPI-A2
  • A-2 Finish
  • Mold Finish – SPI #A-2
  • SPI B1, Max Ra 125 μin

Including both the SPI alphabet code and the Ra max value provides the clearest specification. This helps remove any ambiguity.

For very critical applications like plastic optics, more detailed surface texture requirements may be specified through area Ra limits, power spectral density (PSD) limits, or 3D roughness parameters. But for most general purpose parts, the SPI finish callouts provide sufficient mold texture control.

How to Measure SPI Mold Finish

Measuring the surface roughness average (Ra) is necessary to validate that the specified SPI finish has been achieved on the injection mold. This requires surface profilometry using instruments designed to measure micro-scale surface textures.

Some common methods for measuring SPI mold finish include:

  • Stylus Profilometer – A diamond-tipped stylus is traced along the surface to map the microscopic peaks and valleys. Measures Ra and generates a surface profile trace.
  • Optical Profilometer – Non-contact method that uses focused light and camera to measure surface roughness.quickly measure Ra over entire mold surface.
  • Atomic Force Microscopy – Uses an extremely fine probe to map the mold surface. Can achieve nanometer-scale resolution. Very accurate but slower measurement.
  • White Light Interferometry – Also a non-contact optical technique with high accuracy to quantify surface texture.

Mold makers use these various instruments to validate mold finish and ensure compliance to specified SPI standards. The capability to accurately measure surface finish is crucial for quality control of the polishing process.

Factors Affecting SPI Mold Finish

While the mold finish directly influences the plastic part appearance, there are some other factors to consider:

Material Properties

The raw plastic material properties impact how well micro-scale mold textures replicate onto parts. Amorphous resins like ABS, PC, PMMA reproduce mold finish details better than semi-crystalline resins like PP, HDPE. Fillers and reinforcements in plastic compounds can also affect surface reproduction.


Higher shrinkage materials will better pick up mold surface details compared to low shrink resins. Shrinkage causes the fluid plastic to pack tightly against the mold wall, picking up micro-scale textures.

Mold Release

External mold releases or internally lubricated resins that easily release from mold walls may not accurately reproduce surface finish details. Materials that adhere well to the mold will replicate finish better.

Process Settings

Injection pressures, speeds, temperatures affect how the hot plastic flows into the textured mold surfaces. Optimized process settings are needed to transfer the mold finish to the molded parts.

While the SPI finish sets the baseline surface texture, realized part appearance will depend on the factors above. Testing and validation with the selected plastic material is recommended.

Benefits of SPI Mold Finishes

Specifying SPI mold finishes for injection molded parts offers the following benefits:

  • Standardization – Provides a common language to define surface texture. Avoid ambiguity.
  • Appearance Control – SPI finish levels allow tailoring parts appearance – glossy, matte, satin.
  • Benchmarking – Standard finishes simplify comparing molded parts across different molds, materials, processes.
  • DFM – Design engineers can select the optimum finish for function, aesthetics, and cost.
  • Quality Control – Objective metrics for mold makers to inspect and control polishing quality.
  • Documentation – Clear callouts on drawings to communicate finish requirements.

The SPI standards for plastic part surface finish continue to provide value for injection molding and plastic component design.

Overview of Typical SPI Finishes for Injection Molded Parts

Here is an overview of typical SPI finishes specified for different injection molded component types and applications:

A – Matte Finish

  • Industrial machine components – A-1 and A-2 finishes are commonly used for parts like gears, pulleys, housings, fittings that require no special aesthetic appearance. Provides good durability.
  • Hidden Interfaces – A-2 or A-3 finishes may be specified for attachment regions or parting line areas that are not visible in final product assembly.

B – Low Gloss Satin Finish

  • Consumer Products – B-2 and B-3 finishes provide pleasing low luster finish for products like appliances, furniture, sporting goods, toys.
  • Text/Labels – B-1 or B-2 finishes are ideal for molded parts that will have text, graphics or decals applied. Provides good surface for paint or pad printing.

C – Gloss Finish

  • Clear Plastics – Where high clarity is needed, C-1 or C-2 finish should be specified for acrylic, polycarbonate or copolyester parts.
  • Lens & Lighting – C-2/C-3 critical for optical quality, clarity and light transmission for lighting components.
  • Display Components – Clear glossy molded parts using finishes C-2 and higher are widely used in display, electronics and touchscreen applications.
  • Medical Components – Precision optical grade C-3 finish required for plastic disposable syringe barrels, vials, reaction vessels.

Consider the visual appearance needs, functionality, post-molding operations, and cost when selecting SPI finishes. Testing is highly recommended to verify performance with selected resin.

Frequently Asked Questions

Here are some common FAQs regarding SPI mold finishes for injection molded parts:

What is the main purpose of SPI mold finishes?

The SPI system provides a standardized method of specifying surface texture and appearance for injection molded plastic parts. It allows designers, mold makers and molders to align on surface finish requirements.

What plastic resins typically use SPI C-1 to C-3 finishes?

Clear amorphous plastics like acrylic, polycarbonate, copolyester are commonly molded to C-class high gloss finishes for applications where optical clarity is important.

How are SPI B finishes molded?

SPI B-class finishes are primarily achieved by polishing the steel mold cavity and core to the specified surface texture. No secondary operations on molded parts are required.

What does Ra value mean in SPI standards?

Ra stands for Roughness Average. It represents the arithmetic mean deviation of the surface roughness profile from the mean line. Ra is measured in microinches (μin) or micrometers (μm).

Can you modify an existing tool to a different SPI finish?

Yes, it is possible to re-polish a used injection mold to obtain a different SPI finish. The mold may need to be resurfaced if stained or damaged. The mold cavity will need to be re-polished to the specified texture.

If an SPI C-2 finish is required, what Ra value should be specified?

For SPI C-2 finish, the Ra range is 125 – 63 μin. To be safest, the maximum Ra value should be specified, so the callout should be: SPI C-2, Ra max 125 μin.

This provides a detailed guide to standard SPI mold finishes for injection molded plastic parts. Let me know if you need any clarification or have additional questions!