At Controlled Fluidics, we have best practices and standards regarding a product's finishing touches. We have other pages and guides about polishing and finishes available throughout our website that detail our methods further. Here, we go more into the finer details about polishing plastic and other available finishes.
What does it mean to leave a machined plastic product "as machined"?
When customers ask for products to remain "as machined", this implies that it will have no further post-manufacturing process done, such as polishing. This also means that the manufacturer will keep to a standard surface finish unless the customer gives further direction on their desired finish thickness. We have defined those terms further below.
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Standard Surface Finish
Manufacturers will have different surface finish standards and thickness, which you might need to discuss to ensure that nothing is left to assumption or chance. Our standard surface finish is a maximum of 32 microinch RA (.8 micrometer RA). We will finish all surfaces to that roughness unless otherwise stated.
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Valve Mount Surface Finish
Our maximum here is 32 microinch RA (0.8 micrometer RA). If specified, it can be produced to 16 microinch RA (0.4 micrometer RA).
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Ports
Upchurch ¼-28 flat bottom fittings are the most common for our manifolds. We manufacture the bottom of the ports to 20 to 32 microinch RA (0.8 micrometer RA) with a circular lay.
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Pockets
When considering such features as bubble traps, reservoirs, and accumulators, we suggest a maximum of 40 microinch RA for best results.
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We recommend that for channel considerations within a manifold that the maximum be capped at a maximum of 32 microinch RA. Channels used for imaging may need to be square with a better finish. If the project has specific requirements about their channels, we advise that you reach out to the manufacturer directly.
For more information about channel design, click on the title link above.
When it comes to polishing plastic, several methods exist. We advise using one (or more if suitable) of the methods listed below. Remember that some plastic materials take to different polishing methods better than others. Some materials don't take polishing at all. It is important to strike that balance of a product's needs and what might need to be negotiated when it comes to a product's intended purpose versus aesthetics.
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Buffing
Buffing uses cotton cloth to smooth the surface. Buffing is not flawless as it leaves thousands of multi-directional scratches on the surface. Buffing is only suitable for large flat pieces with limited holes and projections. It will round corners and edges.
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Flame Polishing
Flame polishing uses a flame to melt the surface to create a smooth, visually clear surface. As this method is done by hand, one can expect some waviness in the surface's finish. Manufacturers limit flame polishing to larger features at or near the surface to curb the possibility of any imperfections. This method will work on round corners, straight edges, and hole edges. We recommend this approach for acrylic products only.
Please note that Controlled Fluidics does not offer this service.
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Vapor Polishing
Vapor polishing uses a solvent to reflow the surface and make it smooth. Manufacturers typically use vapor polishing on internal features like threads and bores. Most know this as an economical solution for polishing an entire manifold, however this results often as being not as clear as flame polishing for acrylic products. We recommend this method for polycarbonate and ULTEMⓇ products as it tends not to distort or change feature size.
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The best mix of functionality and price is to polish the large faces where components mount and leave the edges as machined. The clear faces provide good sealing for the valves and visualization of the channels. The edges are left translucent as they are typically a non-working surface.
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Glass Bead Blasting
Glass bead blast produces a uniform smooth frosted finish. Colored parts especially black can look whiteish.
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Tumbling
Tumbling produces a matte finish. Specialized machines tumble products for a length of time to create that kind of finish. Material choice will affect how the finish looks. This type of finishing technique is useful for large, heavy-duty parts.
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As Machined
As we covered in the first chapter of the guide here, "as machined" means that the product has no additional polishing after machining. The surface will be 32 microinch Ra (.8 micrometer Ra) or better. Clarity for a machine finish can vary from translucent to opaque depending on cutting efficiency of the tool. The overall presentation will vary slightly depending on milling or turning operations.
A common working location of a fluidic subassembly is within an instrument and outside the view of normal operation. Customers wanting to save money will consider removing the polishing step. Things to consider:
- Polishing allows for visual confirmation of fluid flow
- Has a high degree of aesthetic value
- Polishing can be done in a cost-efficient manner
- For manifolds, request polish only on the large flat areas to save money
- Leave manifold edges as machined
- Specify as machined only for highly cost competitive manifolds
- Grade 2 (vapor) is the most cost effective polishing
- Flame polishing is not a precision process. Expect corners to be rolled
- Apply grade 3 polishing sparingly
- Angular or curved surfaces are harder to polish than flat
Manufacturing plastic parts from clear material is a common request for manifolds and machined components. Clear parts allow a user to monitor the internal activity of a process. Many amorphous materials in their raw state are clear. However, after machining they become translucent to opaque depending on tool wear and cutting efficiency.
To return the machined part to clear, a secondary polishing operation is often required. Specifying the proper clarity to match the application can be a challenge. Controlled Fluidics has defined 4 levels of clarity to aid design efforts.
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Grade 1: As Machined
This finish is the standard machined plastic finish. Surfaces range from translucent to opaque depending on the feature geometry and cutting conditions. This is appropriate for non-visual applications.
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Grade 2: Transparent Finish
This finish allows for activities like visual fluid flow and bubble detection. It is the most common request and meets a broad range of design requirements. Processes such as vapor polishing are appropriate for this clarity. Polishing the entire part is typical. Parts can vary in haze and clarity. Tool marks are present and do not affect functionality. Some scratches. This is the most cost-efficient method with limited quantitative measurements for quality.
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Grade 3: Optical Window Clear Finish
This finish is appropriate for imaging thru the manifold or plastic part for detection sensors. To the unaided eye, a Grade 3 finish looks similar to window glass. Tool marks are not readily apparent. Few scratches. To achieve this clarity, specifying a surface finish of less than 2 microinch Ra (.05 micron) is appropriate. This clarity is specified in a localized area beneath the optical sensor and not used for entire parts.
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Grade 4: Top Optical Grade Finish
This finish is a true optical finish specified in both surface finish and surface form. Surface finish for optics is measured with a white light interferometer and specified in angstroms. Surface form represents waviness of the surface and its adherence to the ideal profile. Form is also measured with an interferometer and specified in fringes. This level of clarity is appropriate for lenses and doesn’t find use in manifold applications.
Allowable Debris
One of the pitfalls of polished components is the ease in which any defect can be seen. Customers can raise concerns about debris in the material.
According to the ASTM standard D4802-16 (the standard regarding cast acrylic sheets) there is an allowable level of particulate debris in acrylic sheets. This standard states that any amount of particles under 0.8 mm (0.031 in) are acceptable provided they do not affect the serviceability of the sheet. Particles between 3.2 mm (0.125 in) and 0.8 mm are allowed given no more than one particle of this size is found in 0.4 m2 (4.3 ft2) of the sheet. As the sheets of plastic, we purchase conform to this standard, we also hold our products to this standard (including parts in other clear materials).
While debris can be aesthetically not pleasing, it usually does not have an impact on functionality including those directly above a channel. A common debris, carbon specs are fully encapsulated in the material (though they might not look that way). For those applications requiring debris free material, please define the requirements during the quoting process. It is possible to cull material based on contamination levels with an additional cost impact.