Enhancing Cosmetic Appearance on Moulded Parts
See how part geometry, material selection, and mould design can minimise or eliminate cosmetic flaws that can arise during moulding
Content in this Article
The success of the products we design can often be affected by their visual appearance. In addition to the visual appearance, the cosmetics of a plastic part impact the tactile feel. Quality judgments are often made by how a product feels when we hold it or touch it.
In product design, cosmetics must be traded off against other factors like strength, durability, function, and cost. For internal parts, cosmetics should be weighed less than other, more operational characteristics. For external components, cosmetics may be a primary consideration.
The cosmetics of a part can contribute to both function and aesthetics. For example, the colour of a part might be chosen for safety reasons as well as for its attractiveness. The texture on a handle might be chosen both for its looks as well as how it improves the feel and the grip.
There are four major drivers to the cosmetic appearance of your part: the part's geometry, the choice of material, the design of the mould (or tool), and processing parameters during the injection process. These drivers are cross‑coupled with each other and additional design factors such as strength and durability. While the part designer has complete control over geometry and resin selection, they typically have only a small input into the design of the mould.
Part Geometry
Part geometry can affect many aspects of cosmetics. There are so many possible geometries it is impossible to provide a comprehensive recipe for good cosmetic results. Following the usual rules for good injection‑moulded part design will usually improve the cosmetics of the part, and violating those rules often leads to cosmetic issues. It is simple physics: there are basic physical laws to the melting, flow, filling of the mould cavity, and solidification of resin; the geometry of the part governs how those physical laws manifest in your part’s appearance.
For detailed guidance on wall thickness, draft, and other geometry‑related design rules, see our Wall Thickness Guidelines and Draft Angle Guidelines .
Choice of Material
The resin you choose will have a dramatic effect on appearance. The most obvious effect is colour, but beyond colour there are many aspects affected by material selection. Material selection includes the base resin (the choice of which is often constrained by the part’s functions), colourant (the choice of which is constrained by the base resin and the desired appearance), and the filler, if any (the choice of which is constrained by the base resin and the part’s function).
Among resins, the most cosmetic‑friendly entrants include ABS, polycarbonate, and ABS/polycarbonate blends. Some less‑friendly resins include acetal (e.g., POM) and PBT. Highly‑cosmetically‑challenging resins include any glass‑filled resin and TPE (thermoplastic elastomer, e.g., Santoprene) .
Depending on which aspects of cosmetics are important to you (e.g., sink, uniform texture, flash), you may be able to find a substitute for your first choice of resin that also satisfies your other design constraints. Since you are the only person who knows enough about your requirements to make the decision, advisors will help with general advice and resin parameters but leave the final choice up to you.
If the resin choice is constrained, you may have to compromise on cosmetics. For example, if you must use a glass‑filled resin, do not design a part to have a highly‑polished surface. A filled resin will never achieve a highly‑polished look, and the money you spend having the mould polished will be wasted. If you use a TPE such as Santoprene, you will be disappointed if you spend money on class A or textured finishes since TPEs generally are insensitive to nuanced mould finishes.
Bear in mind that mould polish levels (e.g., SPI‑C1, SPI‑B1, SPI‑A2, bead‑blast texture) are specified and standardised for the surface of the mould and not the surface of the resin. The appearance of a part made in a mould with an A2 finish may be quite different for different resins. If you are thinking of trying a new resin, ask your moulder for a sample plaque made from your resin or a similar resin (be sure to specify fillers if you are thinking of using them). The plaque should display different mould finishes so you can judge what cosmetic level your part might be able to achieve.
For more information on material selection and surface finish compatibility, see our Materials Comparison Guide and Surface Finishes for Injection Moulding .
Mould Design (Gating, Venting, and Flow)
Many cosmetic problems arise from the design of the mould. Some of these, such as parting lines and ejector pin marks, result from the practical requirement that the part has to somehow get out of the mould.
Most mould‑related cosmetic problems have to do with resin entry and flow. The path or paths resin takes in filling the mould—from entering at the gate to flowing to the farthest ends of the cavity (as well as letting trapped air out ahead of the resin through mould vents)—greatly affect the cosmetics of the resulting part.
In most cases, the mould designer proposes gate and vent locations and, thereby, resin flow. The goal is to choose gate and vent locations that will produce the best part, but like other aspects of part design, gating can entail compromises. The mould designer cannot know your goals without your input, which is why you will be asked to approve a gate and ejector layout prior to manufacturing a mould. If necessary, the mould designer may simulate the flow of solidification of resin using an injection moulding simulation program.
Mould material can have several effects on cosmetics. Aluminium moulds cannot be polished as highly as steel moulds (SPI‑A2 is about the highest achievable polish in aluminium, although this is sufficient for most purposes). Aluminium moulds cannot withstand as high injection pressure as steel moulds, so some processing options are not available for extreme part geometries. Aluminium moulds are more easily damaged than steel moulds, so specialised finishes such as Mold‑Tech™ finishes are not advised for aluminium moulds due to the turn‑around time and expense of repair in case of accidental damage or wear by abrasive resins.
For more on gate design and placement, see our 3 Gate Considerations to Improve Mouldability guide.
Cross‑coupled Variables
Fixing potential cosmetic problems involves choices, and problems often result from trade‑offs between two or more factors. Table 1 shows some common cosmetic problems and their causes, and some ways to address them. Sometimes addressing one problem will exacerbate another.
Table 1: Common Cosmetic Issues, Causes, and Potential Solutions
| Cosmetic Issue | Common Causes | Potential Solutions |
|---|---|---|
| Gate Vestiges | Resin needs to be injected into your part somewhere. The place where that occurs will have a gate vestige or blemish. | Change type of gate or gate location. Note that changing gate placement or type to a sub‑optimal configuration can often cause other issues. |
| Sink | All resins shrink as they cool. Thick areas tend to shrink more than thin areas around them causing noticeable dips in surfaces. | 1. Core out thicker areas. 2. Keep wall thicknesses uniform. 3. Changing base resin can sometimes improve sink if the geometry cannot change. |
| Ejector Pin Blemishes | Ejector pins always mar the surface of the part. The mould designer will usually try to orient the part in the mould to allow the ejector pins to interact with a non‑cosmetic surface of the part. | 1. Place pins on non‑cosmetic areas (not always possible). 2. When the entire part is cosmetic (e.g., a plastic lens), special features need to be designed into the part to aid ejection. 3. More complex moulds may use methods such as stripper plates, which are much more expensive and still leave a blemish. |
| Drag Marks | Usually caused by insufficient draft. The part drags along the mould wall during ejection, marring the surface. | 1. Add draft. 2. Change the finish on the affected areas to show the marks less. |
| Texture Shadowing | Any blemish (e.g., mild sink) will be accentuated by a textured surface because raised features of the texture cast shadows when light strikes the surface at an angle. | 1. Change to a lighter colour resin if possible. 2. Correct the underlying blemish issue. 3. Remove the texture. 4. Add features (e.g., text, decorative ribs) to break up the surface. 5. Eliminate thick and thin wall section transitions under the textured area. |
| Knit Lines | 1. Holes in the part will always cause knit lines. 2. Multiple gates will always cause knit lines. 3. Extreme geometries can cause knit lines. 4. Incorrectly placed single gates may cause knit lines. | 1. Remove holes if possible. 2. Change gate locations. 3. Use the fewest number of gates possible. 4. Correct geometry issues that cause pathological flows. |
| Flash | 1. Excessive pressure is required to fully fill the mould. 2. Some materials flash more easily than others. 3. Very complex parting lines may not be able to shut off properly. | 1. Correct any underlying issue requiring excessive injection pressure. 2. Change the resin to one less susceptible to flashing. 3. Simplify the design to allow simplifying the parting line. |
| Burns | Air trapped in dead‑ended sections of the mould can rapidly increase in temperature as it is compressed by high‑pressure incoming resin, causing scorching. Some resins (e.g., nylon) burn more easily. | 1. Allow the mould designer to add vents as needed. This may leave blemishes similar to ejector pin marks. 2. Change the material to one less susceptible to burning. 3. Change the design to remove dead ends that trap gas. |
| Inconsistent Colour | 1. Custom colours created by mix‑in colourants. 2. Much less often, processing issues. | 1. Arrange for pre‑compounded resins. 2. Correct any design issues driving processing issues. |
🔗 For more detailed guidance on specific defects, see our Cosmetic Defects in Injection Moulding guide.
Example: Knit Lines
The cross‑coupled nature of solving cosmetic issues is best shown by example. Refer to Figure 1 (not shown, but described). This shows a section of a cover for a consumer device. The external side of the cover is textured with a medium bead blast. The mould designer chose to use two edge gates (located on the opposite non‑cosmetic side of the part). The reasoning was the part would be hard to fill since it is fairly large and quite thin with a few thicker sections. Gate location was chosen to hide the gate vestiges.
The photo shows a knit line on the textured area. The biggest issue is with the texture. The cooler flow fronts of the resin are not forced into the mould surface texture as well as the hotter resin behind the flow front. Since the part is thin and cools quickly, the process tech cannot fill the part to achieve uniform texture appearance. This leaves a noticeable change in the appearance of the texture near the knit line.
There were several possibilities to mitigate the problem. A single edge gate would eliminate the knit line in this area, but the part would likely have had problems filling. The part could have been made thicker to make a single edge gate practical, though this adds weight and cost. The gates could have been moved closer to the affected area, but this would have put edge gate vestiges in cosmetic areas.
In this case, working with our applications engineering team, the customer chose to use a hot tip gate in the centre of the part. A hot tip gate injects resin through the A‑side of the mould directly into the part (without sprue or runners), leaving a small vestige. In addition, there is a slight blushing around the hot tip. Moving the gate was a trade‑off between having a hot‑tip vestige and eliminating the knit line.
Example: Sink
Another example of a problem with several potential solutions is shown in Figures 4 and 5 (described). This shows a bad sink mark on a cosmetic part. Flipping the part over, we find a thin‑walled part with a large solid boss. As the resin in the boss froze from the outside in, the core contracted and pulled from both ends of the boss. This left a significant cosmetic blemish that was not able to be processed out.
The material exacerbated the problem; the customer needed polycarbonate, which is extremely stiff and very prone to sink. The design guidelines for polycarbonate call for wall thickness of at least 1 mm in small parts (and this is not a small part) and no more than 3.8 mm in larger parts. The base of the boss exceeds the maximum thickness recommended for polycarbonate.
The ideal solution would be to core out the boss to its bottom, leaving only a thin tube standing on the back side of the cosmetic surface. This was not possible in this case due to draft requirements on the inside and outside of the tube. Without draft on the outside, the boss would stick in the mould. Without draft on the inside, the boss grabs onto the core pin, either breaking off the pin during ejection or distorting the part.
Substituting a different material (polycarbonate/ABS blend) helped quite a bit. The sink issues were almost completely eliminated. Material selection can have a huge effect on cosmetic appearance, and not just for sink.
Material Samples Comparison
Figures 7 through 12 (described) show several different materials shot in exactly the same mould (a material sample plaque) and photographed under lighting designed to bring out surface contrasts. The different finishes on the plaque include SPI‑A2, SPI‑B1, SPI‑C1, PM‑F0, PM‑F1, PM‑T1, and PM‑T2.
Key observations:
Black ABS (Figure 7): Faithfully reproduces the finishes in the mould. This resin makes good‑looking parts.
Light Gray ABS (Figure 8): A lighter‑coloured resin can often be used to cover surface defects such as uneven texture or sink.
Black Nylon 66 (Figure 9): Like ABS, unfilled nylon makes good‑looking parts.
Black Nylon 66, 33% Glass‑Filled (Figure 10): Compared to unfilled nylon, the difference between texture, stone, and paper finishes is much less apparent. The SPI‑A2 area shows a slightly textured appearance.
Black Santoprene TPE (55 durometer) (Figure 11): Note almost no difference in appearance between a highly‑polished mould surface (SPI‑A2) and as‑machined (PM‑F0).
Black Delrin 500 (Acetal) (Figure 12): Note the flow lines and the uneven sink in the SPI‑A2 area. It is possible to make a good‑looking acetal part, but the design rules for acetal must be followed carefully.
Example: Changing Part Geometry
Occasionally, the best solution to a cosmetic problem is to modify the geometry of the part. Figure 13 (described) shows a part intended to illustrate the cross‑coupled nature of all aspects of the design—from geometry to material to processing parameters. This part is made from acetal, which is considered difficult to process for highly cosmetic results.
The original part has thick sections in violation of good plastic design practices. The process tech varied one aspect of the process (injection speed) to find the best compromise. At slow speed, gate blush was minimised but produced an orange peel effect. At higher speed, orange peel was eliminated but resulted in gate blush and exacerbated sink in thick areas. After optimising the process for the best cosmetics without compromising material properties, the appearance was still substandard, with noticeable sink marks and some gate blush.
To improve cosmetics, this part was redesigned following basic plastics design rules. In this case, the walls were designed with uniform thickness in the dimensional range recommended for acetal. The end result was definitely better.