Injection Molding Surface Finish Standards: SPI

How to Create and Measure Surface Textures for Injection Molding?

Whether a plastic part has a textured or polished surface depends mainly on the specific application requirements and design aesthetics. Polished surfaces usually have a smooth, shiny appearance, which can highlight the texture and high-end feel of the product, so they are suitable for products with high appearance requirements. However, polished surfaces may also be more likely to show defects such as scratches and fingerprints. Textured surfaces can add some unique visual effects and touch to the product, while also hiding some surface defects. The choice and design of textures can also be adjusted according to specific needs and aesthetics.

To get the surface finish and texture that you want, it’s important for you to understand how we make and measure such finishes in our inspection department.

The VDI 3400 and SPI Finish standards are mainly used to evaluate the cleanliness and roughness of surfaces.

What is SPI Finish?

SPI Finish (SPI Finishes) is also known as “SPI Mold Finish (SPI Mold Finishes)” or “SPI Surface Finishes“. It refers to the American surface finish standard set by SPI (Society of the Plastics Industry). The standard has twelve SPI levels of polish finishes: SPI A1 to #SPI D3 (RA 0.012 μm to RA 18.00μm).

It is generally acknowledged that SPI surface finish standards are an essential component of designer-manufacturer collaboration. They offer an easy-to-use method for defining the required texture and look of injection-molded plastic components.

They are separated into four categories by the SPI polish grades, ranging from the brightest to the faintest. There is a permissible deviation for each level, where a lower number permits less deviation and a higher number permits more.

SPI Finish Categories and Levels

There are four main categories in the PIA classification system: A, B, C, and D. Finishes in Group A are achieved by diamond buffing, in Group B by grit sandpaper, in Group C by grit sanding stones, and in Group D by pressure blasting with aluminum oxide grains or glass beads.

Twelve classes, from SPI A1 to D3, are covered under the whole SPI finish standard.

The surface roughness Ra (μm), finishing techniques, and suggested steels are used as references to indicate the finish of each grade in the SPI finish chart below.

Utilize the SPI Finish card to examine the actual appearance of the textures.

SPI Finish Standard Surface Roughness Ra (μm) Finishing Method Fnishing Look Steels Recommended
SPI A1 0.012 to 0.025 6000 Grit Diamond Super High Glossy finish S136 (54HRC) or 8407(52HRC)
SPI A2 0.012 to 0.025 3000 Grit Diamond High Glossy finish S136 (54HRC) or 8407(52HRC)
SPI A3 0.05 to 0.10 1200 Grit Diamond Normal Glossy finish S136 (300HB) or 718H
SPI B1 0.05 to 0.10 600 Grit Paper Fine Semi-glossy finish 718H
SPI B2 0.10 to 0.15 400 Grit Paper Medium Semi-glossy finish 718H
SPI B3 0.28 to 0.32 320 Grit Paper Normal Semi-glossy finish 718H
SPI C1 0.35 to 0.40 600 Grit Stone Fine Matte finish 718H
SPI C2 0.45 to 0.55 400 Grit Stone Medium Matte finish 718H
SPI C3 0.63 to 0.70 320 Grit Stone Normal Matte finish 718H
SPI D1 0.80 to 1.00 Dry Blast Glass Bead Satin Textured finish 718H
SPI D2 1.00 to 2.80​ Dry Blast Dull Textured finish 718H
SPI D3 3.20 to 18.0 Dry Blast Rough Textured finish 718H

How Do Different Finishes Affect Product Cost?

The finish requires more time and work to achieve the higher you go in the classification system. Actually, finishing the step before refining anything is a prerequisite for each subsequent stage. Here, there are no quick cuts, thus the expenses mount up. Let’s examine each finish’s execution.

SPI Finish Card

Plastic SPI Finish cards have acurate hand touch or visual polish finish for your plastic parts and mold. The card has twelve SPI finish levels from A1 to D3.

SPI Finish Card

VDI 3400

While SPI Finish is mostly used for mold polish, the VDI 3400 standard is primarily used as a reference for surface roughness.

Despite being designed for distinct surface treatments, they still have some similar finishes due to differences in grade. If you want a rough match, the SPI to VDI 3400 form is provided below.

SPI Finish Standard VDI 3400 Standard Roughness
(RA/μm)
Typical Finishes
SPI A1 Not Available 0.012 to 0.025 Lens / Mirror
SPI A2 Not Available 0.025 to 0.05 High Polish
SPI A3 VDI #0 ~ #5 0.05 to 0.10 High Polish
SPI B1 VDI #6 0.05 to 0.10 Medium Polish
SPI B2 VDI #7 ~ #8 0.10 to 0.15 Medium Polish
SPI B3 VDI #9 ~ #10 0.28 to 0.32 Medium- Low Polish
SPI C1 VDI #11 ~ #12 0.35 to 0.40 Low Polish
SPI C2 VDI #13 ~ #15 0.45 to 0.55 Low Polish
SPI C3 VDI #16 ~ #17 0.63 to 0.70 Low Polish
SPI D1 VDI #18 ~ #19 0.80 to 1.00 Satin Finish
SPI D2 VDI #20 ~ #29 1.00 to 2.80 Dull Finish
SPI D3 VDI #30 ~ #45 3.20 to 18.0 Dull Finish

Mold polishing

Mold polishing is a useful technique for increasing the die’s surface finish, which in turn produces a smoother, more attractive product surface. Mold polishing may also significantly raise the effectiveness and caliber of items produced at the same time. This is due to the fact that in the injection molding process, a poorly smoothed mold surface makes it easier for the product to become stuck and prevent it from being demoulded, which lowers both the efficiency of production and the quality of the final product.
The standard procedure for polishing a mold involves using instruments like sandpaper, polishing paste, and whetstone to create a mirror-like shine on the mold cavity’s surface. The material, shape, and surface condition of the mold all affect the particular polishing technique.

How to judge the surface quality of the mold?

When assessing the level of polishing, the mold surface’s geometry must be precise and free of lengthy, wavy patterns that undulate.

The imperfections left behind by early grinding with whetstones or grinding wheels are typically the origin of this condition. The product’s quality and appearance may be impacted by these flaws.

Second, the mold surface needs to be devoid of flaws such as tiny holes, orange peel lines, micro-pits, pinholes, and scratches following the mirror treatment. Usually, the damage to the mold that occurs during the mirror treatment procedure is what causes these flaws.

Sometimes it’s challenging to determine the mold’s surface quality with the unaided eye. In geometry, the seemingly smooth plane is not quite flat. To check the quality and level of flatness of the mold surface, we employ devices. Take optical interference technology, for instance.

Finishes Created by Polishing

Diamond buffing is used to create Category A, which is the highest standard grade and the glossiest. This is caused in part by the diamond buffing paste’s fine grain and in part by application technique. The surface is polished in an erratic, non-linear pattern using a rotary tool. This usually disperses or reflects light without revealing a distinct texture. The surface’s waviness has no “direction,” therefore regardless of the viewing or illumination angle, it seems the same.

PIA-A-Grade-surface-quality

PIA A-Grade surface quality

The A category has three levels, each of which represents a particular level of polish and has a unique Ra value. For instance, A-3 and Ra 2^3 are equivalent.Although surface flatness is a topic in and of itself, this basically indicates that it is exceptionally flat.

Finishes Made By Sandpaper

Now contrast this with Category B, which is sandpaper-made. This type of surface finish creates a particularly noticeable pattern since it is applied in a back-and-forth motion, leaving behind distinctive linear scratches. For B-1, an SPI surface finish that additionally gauges Ra 2^3 flatness is feasible. However, neither this finish nor the component created with such a tool would appear the same as A3.

PIA-B-Grade-surface-finish

PIA B-Grade surface finish

Semi-gloss finishes are categorized as B-grade. It is less costly to make than A-Grade and fine enough to conceal tooling and machining marks, mold errors, and other flaws. Sanding grit, 600 grit for B-1, 400 grit for B-2, and 320 grit for B-3, is used to apply these finishes.

Finishes Made with Grit Stone

Sanding stones with grit are used to create C-Grade finishes. The grits of the stones vary from 600 to 320, just as B-grade finishes. However, because the stones cut more aggressively, they leave behind a less smooth and rough surface. This method leaves plastic items with a matte appearance and is used to swiftly remove tooling and mold markings.

Grade-C surface finish comparators

Grade-C surface finish comparators

Finishes Made By Blasting

Sand or glass beads are used for abrasive blasting to create rougher finishes. SPI class D is corresponding to these. The spraying process is random, which results in a consistent and non-directional finish. Flat or drab finishes are produced using this categorization.

D-Grade surface finish comparators for plastic injection molding

Finishes Made with EDM

In addition to these, spark erosion and electronic discharge machining (EDM) can yield outcomes that are comparable. Depending on the characteristics and kind of metal being treated, we employ both strategies. The PIA standards are used to measure both procedures.

D-Grade surface finish comparators for plastic injection molding

Though they are all homogeneous and aimless, the top row of four instances nearly matches A3–B4 in terms of flatness and finish. The bottom row is sufficiently rough to possibly harm the extremely sensitive testing apparatus, thus we do not assess its flatness.

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