Injection moulding surface finish design and application guide

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  4. Injection moulding surface finish design and application guide

Injection moulding surface finish is key to a successful part design and used for aesthetic and functional reasons in injection moulded parts for engineering products. The surface finish improves the look and the feel of a product as the perceived value and quality of the product increases with a suitable surface finish. Many product designers would agree that the aesthetic appeal of the product heavily relies on the look of the product and how it feels in the consumer’s hands.

As the image below shows, Apple has used the surface finish cleverly to place its smooth surface logo with the textured surrounding to good effect.

Apple TV 4K case
Apple TV 4K case (Source: Apple)

Chose injection moulding surface finishes by considering part function, the material used, and visual requirement as most of the common plastic injection moulded material can have a variety of surface finishes. The most common way to specify injection moulding surfaces is by using PIA (or SPI) and VDI standards. Injection mould toolmakers, manufacturers and design engineers worldwide recognise these two standards and PIA standards are marginally more common and widely known as “SPI grades”.

Surface finish and the material will significantly influence the tool design and cost, so consider and evaluate the functionality in terms of surface early on the embodiment design. If the surface finish is critical to its functionality, then consider the surface finish at the conceptual stages of the product design.

The selection of the surface finish must be established in the early embodiment design stage of the product design. Because the surface will dictate the material and the draft angle which would influence the tooling cost. Course or textured finish will need a bigger draft angle so that the part can be ejected from the mould.

The Plastics Industry Association (PIA) is an independent US trade organisation formally known as SPI. Their surface finish standard for injection moulding tools is accepted across the industry, hence the article is based on PIA standards.

Types of Injection moulding surface finish

SPI injection moulding surface finish (

There are four major grades of surface finish as per the PIA classification system, ranging from polished gloss (A1) to textured rough (D3) finish. Each category is then subdivided into 3 sets and has different requirements for allowable deviation from perfect, with lower numbers allowing for less deviation and higher numbers allowing for more deviation.

Gloss finish – Grade A – Diamond finish

Gloss finish Grade A
Gloss finish Grade A (

These grade “A” finishes are smooth, glossy, and the most expensive of all the finishes. These grades would need hardened tool steel moulds, which are buffed using various grades of diamond buff. Because of the fine-grain buffing paste and random directional rotary polishing method, it will not have a clear texture and scatter light rays, giving a very glossy finish. These are also called “Diamond finish” or “buff finish” or “A finish”.

surface finishSPI standardFinishing methodCost of surface finish
Super High Glossy finishA-1Grade #3, 6000 Grit Diamond Buff+++++++++++
High Glossy finishA-2Grade #6, 3000 Grit Diamond Buff+++++++++++
Normal Glossy finishA-3Grade #15, 1200 Grit Diamond Buff++++++++++

SPI gloss grades are suitable for products with a smooth surface finish for both cosmetic and functional reasons. A2 is the most common diamond finish used in the industry, resulting in good visually pleasing parts with a good part release. Grade “A” surface finishes are used on optical parts such as lenses, mirrors, and visors.

Material suitability for Grade A surface finish

Polypropylene (PP)Not recommendedAverageAverage
Polystyrene (PS)AverageAverageGood
HDPENot recommendedAverageAverage
Polycarbonate (PC)AverageGoodExcellent
Polyurethane (TPU)Not recommendedNot recommendedNot recommended

Semi-gloss finish – Grade B

These semi-gloss finishes are great for removing machining, moulding, and tooling marks with a reasonable tooling cost. These surface finishes are produced using different grades of sandpapers applied with linear motion, giving a linear pattern as shown in figure 2.

FinishSPI standardFinishing MethodCost
Fine Semi-glossy finishB-1600 Grit sandpaper paper+++++++++
Medium Semi-glossy finishB-2400 Grit sandpaper paper++++++++
Normal Semi-glossy finishB-3320 Grit sandpaper paper+++++++

SPI semi-gloss surface finishes would give a reasonably good visual appearance and remove mould tool marks. These are often used in parts that aren’t a decorative or visual important part of the product.

Material suitability for Grade B surface finish

Polypropylene (PP)GoodGoodExcellent
Polystyrene (PS)ExcellentExcellentExcellent
Polycarbonate (PC)GoodGoodAverage
Polyurethane (TPU)Not recommendedAverageAverage

Matte finish – Grade C

Semi-gloss and Matt surface finish

These are the most economical and popular surface finishes, which are polished using fine stone powder. Sometimes called stone finish, it provides good release and also helps hide machining marks. Grade C is also the first step of grades A and B surface finishes.

FinishSPI standardFinishing MethodCost
Fine Matte finishC-1600 Grit sanding stones++++++
Medium Matte finishC-2400 Grit sanding stones+++++
Normal Matte finishC-3320 Grit sanding stones++++
It gives a good aesthetic visual appearance to the part and is widely used in industrial parts and consumer goods. These are suitable for parts with no specific visual requirements.

Material suitability for Grade C surface finish

Polypropylene (PP)ExcellentExcellentExcellent
Polystyrene (PS)ExcellentExcellentExcellent
Polycarbonate (PC)AverageNot recommendedNot recommended
Polyurethane (TPU)GoodGoodGood

Textured finish – Grade D

These textured finishes are first smoothened using fine stone powder and then dry blasted using aluminium oxide or glass beads. The random nature of the dry blasting results in a smooth and non-directional surface finish. A textured finish gives a good grip quality on the surface, hides deformations and marks, and even helps mask fingerprint smudges.

FinishSPI standardMediumCost
Satin Textured finishD-1Dry Blast Glass Bead #11
pressure blasting
Dull Textured finishD-2Dry Blast #240 Oxide
pressure blasting
Rough Textured finishD-3Dry Blast #24 Oxide
pressure blasting
These textured surface finishes are used on thermoset industrial parts and suitable for parts with a dull or satin textured finish which aid gripping such as the handle for consumer products.
Material suitability for Grade D surface finish
Polypropylene (PP)ExcellentExcellentExcellent
Polystyrene (PS)ExcellentExcellentGood
Polycarbonate (PC)ExcellentNot recommendedNot recommended
Polyurethane (TPU)ExcellentExcellentGood
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