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.
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
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
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 finish||SPI standard||Finishing method||Cost of surface finish|
|Super High Glossy finish||A-1||Grade #3, 6000 Grit Diamond Buff||+++++++++++|
|High Glossy finish||A-2||Grade #6, 3000 Grit Diamond Buff||+++++++++++|
|Normal Glossy finish||A-3||Grade #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 recommended||Average||Average|
|Polyurethane (TPU)||Not recommended||Not recommended||Not 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.
|Finish||SPI standard||Finishing Method||Cost|
|Fine Semi-glossy finish||B-1||600 Grit sandpaper paper||+++++++++|
|Medium Semi-glossy finish||B-2||400 Grit sandpaper paper||++++++++|
|Normal Semi-glossy finish||B-3||320 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
|Polyurethane (TPU)||Not recommended||Average||Average|
Matte finish – Grade C
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.
|Finish||SPI standard||Finishing Method||Cost|
|Fine Matte finish||C-1||600 Grit sanding stones||++++++|
|Medium Matte finish||C-2||400 Grit sanding stones||+++++|
|Normal Matte finish||C-3||320 Grit sanding stones||++++|
Material suitability for Grade C surface finish
|Polycarbonate (PC)||Average||Not recommended||Not recommended|
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.
|Satin Textured finish||D-1||Dry Blast Glass Bead #11|
|Dull Textured finish||D-2||Dry Blast #240 Oxide|
|Rough Textured finish||D-3||Dry Blast #24 Oxide|
Material suitability for Grade D surface finish
|Polycarbonate (PC)||Excellent||Not recommended||Not recommended|