Injection moulding surface finish design guide – Design for Manufacture

What are the Injection moulding surface finishes?

Injection moulding surface finish is critical 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.

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

Many product designers would agree that the product’s aesthetic appeal heavily relies on the product’s look and how it feels in the consumer’s hands. As the image above shows, Apple has used the surface finish cleverly to place its smooth surface logo with the textured surrounding to good effect.

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Why use surface finishes in injection moulding?

To increase part aesthetics

Part designers may use textures for a variety of aesthetic purposes. A smooth or matte surface texture improves its appearance and gives it a polished aspect. It also covers faults generated by injection mouldings, such as tool machining marks, sinks marks, weld lines, flow lines, and shadow markings. Parts with an excellent surface quality are more appealing to customers from a business standpoint.

To improve part functionality

Aside from the aesthetic considerations that go into selecting an injection moulding surface finish, there are also important practical considerations.

The design may necessitate a firm grip for optimal functioning. Textured plastic finishes improve grip quality. Hence injection moulding surface treatments are frequently used on slip-resistant products. A textured mould can also aid in the escape of trapped gases.

A smooth SPI surface finish may cause the paint to peel off. However, a rough surface can ensure that paint adheres better to the moulded item. A textured SPI surface treatment also increases the part’s strength and safety.

Texture has several advantages, including:

Plastic flow creases—These creases may be removed by adding textured thickness while increasing strength and non-slip properties.

Improved grip—Adding texture to the component makes handling easier, increasing usefulness and safety in specific applications.

Paint adhesion—Paint adheres firmly to a textured object during subsequent moulding.

Making undercuts—If you have a portion that will not consistently come over to the moving half of a mould, texturing on any surface may provide the necessary pull.

Injection mould tool surface finish specifications

The most common way to specify injection moulding surfaces is by using PIA (or SPI)VDI and Mold-tech standards. Injection mould toolmakers, manufacturers and design engineers worldwide recognise these three standards and PIA standards are marginally more common and widely known as “SPI grades“.

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SPI Injection moulding surface finish

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.

As per the PIA classification system, there are four major grades of surface finish, 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 minor deviation and higher numbers allowing for more deviation.

Gloss finish – Grade A – Diamond finish

SPI-A-B-injection-moulding-surface-finish
SPI-A-B Injection-moulding surface finish

These grade “A” finishes are smooth, glossy, and the most expensive. 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 methodSurface Roughness - Ra um
(mil)
Cost of surface finish
Super High Glossy finishA-1Grade #3, 6000 Grit Diamond Buff0.012 - 0.025
( 0.5 - 1)
$ $ $ $ $ $ $ $ $ $ $
High Glossy finishA-2Grade #6, 3000 Grit Diamond Buff0.025 - 0.05
(1 - 2)
$ $ $ $ $ $ $ $ $ $
Normal Glossy finishA-3Grade #15, 1200 Grit Diamond Buff0.05 - 0.1
(2 - 4)
$ $ $ $ $ $ $ $ $

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

Semi-gloss finish – Grade B

SPI-A-B-injection-moulding-surface-finish
SPI-A-B Injection-moulding surface finish

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 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.

Matte finish – Grade C

These are the most economical and popular surface finishes, polished using fine stone powder. Sometimes called stone finish, it provides good release and 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++++

Textured finish – Grade D

It gives the part a reasonable aesthetic visual appearance and is widely used in industrial parts and consumer goods. These are suitable for parts with no specific visual requirements.

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 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.

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These textured surface finishes are used on thermoset industrial parts and are suitable for parts with a dull or satin textured finish, which helps grip such as the handle for consumer products.

VDI Injection moulding surface finish

VDI surface finish refers to the Society of German Engineers’ mould texture standard and is widely utilised by tool producers worldwide, including North America, Europe, and Asia.

The VDI 3400 surface finish is mainly treated using Electrical Discharge Machining (EDM) when mould machining. However, it might also be accomplished using classic texturing techniques such as grit, stone and sandpaper.

VDI ValueDescriptionApplicationsSurface roughness (Ra)
µm µinch
12600 StoneLow polish parts0.416
15400 StoneLow polish parts0.5622
18Dry Blast Glass BeadSatin finish0.831
21Dry Blast # 240 OxideDull finish1.1244
24Dry Blast # 240 OxideDull finish1.663
27Dry Blast # 240 OxideDull finish2.2488
30Dry Blast # 24 OxideDull finish3.15124
33Dry Blast # 24 OxideDull finish4.5177
36Dry Blast # 24 OxideDull finish6.3248
39Dry Blast # 24 OxideDull finish9354
42Dry Blast # 24 OxideDull finish12.5492
45Dry Blast # 24 OxideDull finish18709

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Mold-Tech Injection moulding surface texture

These are manufactured per Mold-Tech specifications, and they are formed by chemical etching or laser-based alteration of the mould cavity. Each one is denoted by an MT- followed by a five-digit code and are classified according to the serial number and texture depth.

Mold-Tech A
Pattern #Depth (mm)Draft Angle (min)
MT-110000.01016
MT-110100.02541.5˚
MT-110200.03812.5˚
MT-110300.0508
MT-110400.07624.5˚
MT-110500.11436.5˚

Mold-Tech is divided into four series: A, B, C, and D. Mold-Tech Series A finishes are most widely utilised on products because they include a spectrum of fine to coarse matte finishes that do not need a laser, masked chemical etching, or other designed texturing procedures

For example, MT-11010 has a sand-like appearance, MT-11120 has a smooth concrete appearance, MT-11555 has a wood panel appearance, and so on.

Mold-Tech B  
Pattern #Depth (mm)Draft Angle (min)
MT-112000.07624.5˚
MT-112050.0635
MT-112100.08895.5˚
MT-112150.11436.5˚
MT-112200.1277.5˚
MT-112250.11436.5˚

There are hundreds of Mold-Tech specs, aside from the previously mentioned sand, concrete, and wood textures, checkerboards, diamonds, straight or curved lines, and other patterns.

Because mould texturing contains coarser features than SPI finishes, a more aggressive draught angle is required; it is advised to add 1.5° of draught for every 0.001″ of texture depth.

How to choose a suitable injection moulding surface finish?

Chose injection moulding surface finishes by considering part function, the material used, and visual requirement. Most of the typical plastic injection moulded material can have a variety of surface finishes.

The surface finish selection must be established in the early embodiment design stage of the product design because the surface dictates material selection and the draft angle, influencing the tooling cost. For example, a course or textured finish needs a more significant draft angle so that the part can be ejected from the mould.

So what are the prime factors to consider when choosing the surface finish for injection moulding plastics?

Gloss finish Grade A
Figure 2 Gloss finish Grade A (Source: uddeholm.com)

Tooling cost

Surface finish and the material significantly influences 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, consider the surface finish at the conceptual stages of the product design.

Many parts of the injection moulding process have been automated, but polishing is an exception. It is only the simplest of shapes that can be automatically polished. Polishers now have better equipment and materials to work with, but the process remains labour intensive.

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Draft angle

Draft Angle (Degrees)Surface Texture
DA-PADA-PCDA-ABSSPI GradeVDI Grade
010.5A12
0.510.5A15
0.510.5B18
0.510.521
0.51.5124
121.527
1.52230
232.533
2.54336
35439
46542
57645

Mould tool material

The mould tool greatly influences the surface smoothness of the injection moulding. A mould may be made from various metals, although steel and aluminium are the most popular. The effects of these two metals on moulded plastic components are vastly different.

SPI injection moulding surface finish
SPI injection moulding surface finish (Source: spifinish.com)

In general, hardened tool steel can produce smooth plastic finishes compared to aluminium alloy tools. Hence consider steel moulds if the pieces have an aesthetic function that requires a low level of surface roughness.

Moulding material

A wide range of injection moulding plastics is available to cover all sorts of parts and functions. However, not all plastics can achieve the same injection moulding surface finish. Some polymers are better suited to smooth finishes, while others are better suited to roughening up for a more textured surface.

Injection moulding surface finish-SPI-VDI
Injection moulding surface finish-SPI-VDI

Chemical and physical qualities differ between injection moulding materials. Melting temperature, for example, is a crucial factor in a material’s capacity to give a certain surface quality. Additives also have an impact on the outcome of a completed product. As a result, it is critical to evaluate the various materials before deciding on a surface texture.

Furthermore, material additives like filler and pigments might impact the surface finish of a moulded object. The tables in the next section illustrate the applicability of several injection moulding materials for various SPI finish designations.

Material suitability for Grade SPI-A surface finish

MaterialA-1A-2A-3
ABSAverageAverageGood
Polypropylene (PP)Not recommendedAverageAverage
Polystyrene (PS)AverageAverageGood
HDPENot recommendedAverageAverage
NylonAverageAverageGood
Polycarbonate (PC)AverageGoodExcellent
Polyurethane (TPU)Not recommendedNot recommendedNot recommended
AcrylicExcellentExcellentExcellent

Material suitability for Grade SPI-B surface finish

MaterialB-1B-2B-3
ABSGoodGoodExcellent
Polypropylene (PP)GoodGoodExcellent
Polystyrene (PS)ExcellentExcellentExcellent
HDPEGoodGoodExcellent
NylonGoodExcellentExcellent
Polycarbonate (PC)GoodGoodAverage
Polyurethane (TPU)Not recommendedAverageAverage
AcrylicGoodGoodGood

Material suitability for Grade SPI-C surface finish

MaterialC-1C-2C-3
ABSExcellentExcellentExcellent
Polypropylene (PP)ExcellentExcellentExcellent
Polystyrene (PS)ExcellentExcellentExcellent
HDPEExcellentExcellentExcellent
NylonExcellentExcellentExcellent
Polycarbonate (PC)AverageNot recommendedNot recommended
Polyurethane (TPU)GoodGoodGood
AcrylicGoodGoodGood

Material suitability for Grade SPI-D surface finish

MaterialD-1D-2D-3
ABSExcellentExcellentGood
Polypropylene (PP)ExcellentExcellentExcellent
Polystyrene (PS)ExcellentExcellentGood
HDPEExcellentExcellentExcellent
NylonExcellentExcellentGood
Polycarbonate (PC)ExcellentNot recommendedNot recommended
Polyurethane (TPU)ExcellentExcellentGood
AcrylicAverageAverageAverage

Moulding parameters

Surface finishes for injection moulded objects can be affected by injection speed, pressure and temperature. For example, a high melt temperature and quick injection speed can assist generate glossier injection moulding surface surfaces with fewer weld lines.

A quicker filling rate through mould cavities can also minimise weld line visibility. As a result, it enhances the overall look of plastic coatings. A smoother SPI surface finish is achieved by combining high mould and melt temperatures with fast injection speed.

Surface finish comparison

VDI is not the same as Mold-Tech, which is used in various ways, including hand-finishing, laser etching, and chemical etching with patterns. Some textures, however, may have near equivalencies, such as SPI D-3, VDI 33, and MT-11020, which all appear highly similar. In addition, VDI 12 and SPI C-1 are equal in many ways.

VDI 3400ISO, VDI,ASAISO/TC 213SPI
CharmillesRa=CLA=AARz
CHµmµinchµm
-0.0251N1A1
-0. 052N2A2
00. l4N3A3
10. 114.48
20. 125.04
30. 145.6
40. 166.4
50. 187.2
60. 28Bl
70. 228.8N4B2
80. 2510
90. 2811.2B3
100. 3212.8
110. 35141.5Cl
120.416N5
130. 4518C2
140. 520
150. 5622.42.4
160.6325.4C3
170.728
180.832N6Dl
190.936
20140D2
211.1244.84. 7
221.2650.4
231.456
241.62646.5N7
251.872
26280
272.28810 .5
282.5100
292.8112
303.212812.5N8D3
313.5140
324160
334.518017.5
345200
355.6224
366.325224N9
377280
388320
39936034
4010400
4111.2448
4212.650448N10
4314560
4416640
451876069

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