At Controlled Fluidics, achieving the appropriate surface finish is crucial in plastic products to ensure functionality, reliability, and ease of maintenance. We have other guides about polishing available throughout our website that detail our methods further. Here, we go more into the finer details about polishing plastic and its effects on the final product. 

When clients ask for their products to remain "as machined" or "as is", this implies that those products won't have further post-manufacturing process done, such as polishing. This also means that the manufacturer will keep to a standard surface finish, figured as roughness average (RA), unless the client gives further direction on their product's desired finish thickness. Taking one of our custom plastic manifolds as an example, certain features like valve mounts and channels will need to meet a certain RA to function effectively. We have elaborated on those further below.

STANDARD SURFACE FINISH

We maintain a maximum RA of 32 µin (0.8 μm), which is still considered a high-grade finish and suitable for stress concentration and loading. We will finish all surfaces to that roughness unless otherwise stated. 

VALVE MOUNT SURFACE FINISH

Our maximum here is 32 µin RA (0.8 μm). If specified, it can be produced to 16 µin RA (0.4 μm).

PORTS

Upchurch ¼-28 flat bottom fittings are the most common for our manifolds' ports. We manufacture the bottom of the ports to 20 µin (0.5 μm) to 32 µin RA (0.8 μm) with a circular lay to minimize leakage risks. 

POCKETS

When considering features as bubble traps, reservoirs, and accumulators, keep a maximum RA of 40 μin (1 μm) for best results.

CHANNELS

We recommend for channels a maximum RA of 32 μin (0.8 μm). Channels used for imaging may need to be square with a finer RA finish, which could mean including some specialized plastic polishing at the end of the production cycle. If the project has specific requirements about their channels, we advise that you reach out to the manufacturer directly.

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.

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.

Read More

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.

Read More

VAPOR POLISHING

Known as an advanced polishing technique, vapor polishing uses a vaporized chemical 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.

Read More

GLASS BEAD BLASTING

Glass bead blast produces a uniform smooth frosted finish. Colored parts especially black can look whiteish.

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.

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.

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.

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.

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.

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 μin RA (0.05 μm) is appropriate. This clarity is specified in a localized area beneath the optical sensor and not used for entire parts.

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.

One issue with polished components is that any flaws can be easily noticed, leading customers to worry about debris in the material. 

According to the ASTM standard D4802-16, which covers cast acrylic sheets, there is an acceptable level of small particles in these sheets. This standard allows for particles smaller than 0.8 mm (0.031 in) as long as they don’t affect how the sheet is machined for the end product's final operation. For particles between 3.2 mm (0.125 in) and 0.8 mm, only one particle of this size can be present in an area of 0.4 m² (4.3 ft²). Since the plastic sheets we purchase meet this standard, we also apply it to our products, including parts made from other clear materials.

While debris might not look nice, it usually doesn’t affect an end-product's performance, even above channels. Common types of debris (like carbon specks) are fully contained within the material, even if it doesn't appear that way. If you need completely debris-free material, please let us know your requirements when you request a quote. We can sort the material based on contamination levels, but this may come with additional costs.