Key Takeaways
- Plastic CNC machining cost depends on the material, part geometry, machining time, setup, tolerances, finishing, and order quantity.
- Deep cavities, thin walls, small features, and difficult tool access can require more machining operations and careful workholding.
- Plastic type affects the machining approach because different materials respond differently to cutting heat, tool pressure, and chip removal.
- Tight tolerances can increase machining and inspection requirements, especially on thin or flexible sections.
- A simpler part design can reduce machining time without changing its required function.
- Reviewing the CAD model and drawing before production can identify features that are adding machining cost unnecessarily.
Introduction
The price of a CNC-machined plastic part depends largely on how the part needs to be made, not just how much plastic it contains. Two parts made from the same material can have very different prices because their geometry, tolerances, setups, and finishing requirements are different.
For example, a part with deep pockets and thin walls may need more controlled machining and workholding than a solid part with simple drilled features. A tight-tolerance hole may also require additional machining and inspection compared with a standard hole.
This article explains the main factors behind plastic CNC machining cost, including material selection, part geometry, machining time, tolerances, workholding, tooling, finishing, and production quantity. It also covers practical design changes that can reduce unnecessary machining costs while keeping the required function of the part.
What Determines the Cost of Plastic CNC Machining?
A plastic CNC machining quote is based on the work required to turn the supplied stock into the finished part. You need to know the starting cost; the part design is going to dictate what amount of machining, setup, inspection, and finishing will be required.
For instance, two components can have the same plastic, be of comparable overall size, and quote different prices. One might require a few simple features and a single setup; the other might need several operations, careful workholding, and a closer look.
The primary cost factors include material, machining time, setup, tolerances, finish, and the order quantity. These all impact the quote differently.
Materials and Materials Accessibility
The plastic type influences the material cost and the part’s ability to be machined. Some plastics are more expensive and more difficult to cut than others, such as POM, nylon, ABS, PEEK, PTFE, etc.
Availability of stocks must also be taken into account. The size, form, and stock of the required grade should be the same as in the drawing. If a less common grade or stock dimension is required specifically for the order, then this may impact the quotation.
The selection of materials should thus begin with the working requirements of the part. The higher the grade of plastic used than that called for in the application, the more the material will cost without any benefit to the part.
Part Machining Requirements
The cost will be a lot more accurately known to the machinist by the CAD model than by just the overall dimensions of the part. The amount and kind of features dictate the manner in which the material is to be removed, and the number of machining operations needed.
An open face, easy-to-reach part could be easy to machine. A similar but different-sized piece might have a large hole, a cavity, an irregular surface, or the need for additional cutting or other work.
The critical point is that the cost of machining is dependent on the amount of work performed by the geometry, not on the amount of plastic in the part.
Setup and Production Requirements
The part needs to be correctly placed and secured in place before it is possible to machine. Some designs may be finished in one orientation, and some require the part to be turned around to capture various faces or features.
Preparing the program and setting up the equipment are also part of the first production. This preparation is spread about amongst more parts in larger orders, so that it has the effect of decreasing the cost per part. The same preparation is a greater proportion of the total quote for prototypes and small quantities.
The work required is also influenced by the requirements for drawings. Tolerances, inspection requirements, and specified finishing should be taken into account at the time of making the quotation and should not be regarded as separate items once the machining has commenced.
Which Plastic Materials Affect Machining Cost?
The type of plastic is going to play a role in the quote in two ways: the material cost and the type of machining that it requires. The material cost of a plastic can be higher, but the manufacturing may also vary due to its hardness, stiffness, heat response, and chip characteristics.
In the case of a CNC quote, it is important that the material grade is specified. Just the word “plastic” will not suffice, as there are a variety of grades that are used, each with different properties and machining capabilities.
Common Machining Plastics
The most typical materials for CNC-machined parts are ABS, POM, Nylon, and PC. The material cost and machinability of their products depend upon the grade and stock size.
The machinist still has to take into account chip removal, heat buildup, type of tool, and shape of the part for these materials. Only a material that can be easily cut in one design might require more care in another (particularly when cutting thin sections and/or small features).
Engineering-Grade Plastics
The material costs and requirements for machining can be higher for some engineering plastics like PEEK, PTFE, PPS, or reinforced grades.
The cost is not just in the cost of the material itself. The cutting action may influence the type of tools to be used, the cutting conditions, and the degree of control required during the machining process. Reinforced plastics can be different from their unfilled counterparts.
Material Selection for the Application
The cheapest material does not always perform the best, and the most expensive plastic does not always have to be used.
Begin by considering what the part must do: Temperature, wear, chemical exposure, electrical requirements, load, and dimensional requirements. Then choose a grade that meets those requirements.
Quoting the correct grade at the initial stage will prevent changes to the material after the machine shop plan has been made, which can impact both cost and the production schedule.
Which Part Features Can Increase Machining Cost?
There are some features that make the CNC job slower, just because the cutter is difficult to reach. Others require less power, more passes, or slower cutting parameters. These features should be checked before completing a drawing, and the question posed is: Are size and shape really necessary?
Deep Pockets and Cavities
A long-reach cutter is usually necessary for a deep pocket. The more the cutter is extended from the holder, the more care will have to be taken to ensure that it does not deflect or vibrate as it is being run.
A simple tool may be used for a 20 mm deep pocket. If a 60 mm deep pocket has the same width, a longer tool might be necessary, multiple roughing passes, and a separate finishing pass.
When the depth isn’t needed for assembly or performance, then it can be shortened, and the part will be quicker to machine.
Thin Walls and Ribs
The thin sections may slide as the cutter presses on them. The extra material that the machinist leaves in during the roughing pass can be removed in a lighter finishing pass.
For instance, a 2mm wall is harder to hold when it is being milled as compared to a 5mm wall of a certain height. Increase wall thickness to reduce finishing work and facilitate part holding in machining processes, if possible.
Small Holes and Narrow Slots
Small features must be cut and drilled with small cutters and drills. These tools tend to be slower and necessitate more attention to feed, speed, and chip removal.
For instance, a slot in the nose as small as 2 mm cannot be cut by a large end mill. The machine needs to have a small cutter, which can lengthen the machining time.
Only maintain small holes and slots where the function needs them. Don’t just add in narrow features for appearance.
Complex Curved Features
It generally takes more tool movement to make a curved surface in complex parts compared to flat or stepped surfaces. Also, smaller ball-nose cutters and extra finishing passes may be required in tight areas.
Generally, a large radius is easier to machine than a surface with a large number of small changes in radius.
When the contour of a component does not need to be very detailed, simplify the surface. With a minimized number of profile changes, there is less toolpath and less finishing work.
How Do Tolerances Affect Plastic CNC Machining Cost?
The more control is required during the machining and inspection process, the more costly the tolerances will be. The practical way to do this is to specify only close tolerances on those dimensions which influence the fit and/or function of the part.
General Tolerances and Critical Dimensions
Apply general tolerances for normal features. Introduce a particular tolerance when the dimension has an impact on assembly or function.
For instance, an overall length specification could be ±0.1 mm while a bore for the bearings might be Ø25.00 ±0.02 mm. A more careful control of the bore is required, as the bearing should fit properly.
Tight Fits and Mating Features
Check both mating dimensions when two plastic parts are required to fit together.
For instance, a 20 mm shaft and 20 mm bore may not be considered a 20 mm bore and shaft. Drawing should show clearance/fit required.
This is to inform the machinist which surfaces must be held closer than others and not create close tolerances on the other surfaces.
Dimensional Stability During Inspection
Plastic has the property of expanding and contracting in response to temperature. Even thin sections and large flat parts can shift slightly after they are cut.
For critical dimensions, check at a controlled temperature and use the same measuring technique during production.
Maintain tight tolerances on function, general tolerances on the balance. This prevents the work from getting out of hand during the machining and inspection processes.
How Does Machining Time Affect the Final Price?
The cost of machine time is a direct cost of the machining. The longer the part requires the number of passes, tool changes, or finishing operations, the more time it will take to produce.
Material Removal
The amount of material the machine has to remove is influenced by the starting amount of material.
If you want to produce a part with stock 50 x 50 x 20 mm, it will take less time than if the part is of the same size but the stock is 80 x 80 x 30 mm. There’s more material to trim on the bigger blank leaves.
If the stock is appropriate to the size of the part and workholding, then using stock closer to its finished size can lead to a reduction of roughing time.
Number of Machining Operations
Machine time is added for each operation. Generally, a part that can be produced in one setup is quicker to produce than a part that requires more setups.
Some additional operations may include flipping the part, changing work holding, drilling, tapping, boring, and second-side machining.
The machining sequence should be designed with consideration of the actual components, so as to prevent unnecessary settings, which is suitable for production use.
Tool Changes and Finishing Passes
All the tool changes introduce non-cutting time. It is normal to take longer to manufacture a part that requires 8 tools than a similar part that can be manufactured with 4 appropriate tools.
Roughing is not complete until the finishing passes are finished. If a surface doesn’t require a fancy finish, do not apply additional finishing passes unless there is a functional purpose.
The cost link of the basic machine is straightforward: the more machine time, the higher the machining cost. The fewer the unnecessary material removal, the operations, tool changes, and finishing passes, the lower the final price
How Do Tooling and Cutting Conditions Affect Cost?
Plastic can be machined quickly, but the wrong cutter or cutting condition can turn a short job into a slow one. A sharp tool, a clear chip path, and suitable cutting data help keep the cut clean and avoid unnecessary tool changes.
Tool Selection
Select a cutter appropriate for the plastic and the operation. The cutting edges have to be sharp, and suitable flute geometry is used to shear the material rather than rub it.
For softer plastics, a sharp single- or two-flute cutter can provide more space for chips and help prevent chips from packing into the flutes.
Heat and Chip Evacuation
Unlike metal, plastic is not a conductor of heat. The temperature may rise at the cutting edge and melt the material.
Use an air blast or appropriate coolant to remove chips from the cut. Never recut chips within a pocket. When chips begin to adhere to the cutter, inspect the chip clearance and cutting conditions before increasing cutting speed.
Tool Wear
An outdated cutter might cause rough edges, burrs, melted regions, or poor surface finishing. It could also need more conservative cutting parameters.
For reproduction, monitor tool life in terms of the number of parts or cutting time. It’s easier to change the cutter at a scheduled time than to do so when the cutter is failing or the dimensions are not being met.
How Do Finishing Requirements Change the Cost?
Finishing is often a separate step after CNC machining. There are a number of factors that influence the cost, such as the number of surfaces to be treated, the desired finish, and the amount of manual work required.
Deburring
Not all of the edges of a machined part require the same amount of work. If the part has only outside edges to be deburred, the process can be carried out quickly. A part that has a lot of holes, slots, pockets, and intersecting edges takes longer.
A drawing with just sharp edges to remove is not the same as one that defines chamfering on certain edges of 0.5 mm. The second one must be done in a controlled manner, which requires machining and inspection.
Surface Finish
The final machining work will be done depending on the specified finish.
The surface may only require a finishing pass on a normal CNC-machined surface. For instance, a surface with an “Ra 1.6 µm” finish may need a finer finishing process than a surface with a more “coarse” finish.
If plastic parts, also look for surfaces that require a good appearance. Surface-mounted housing may require more maintenance than face-mounted housing.
Secondary Finishing
Polishing, vapour polishing, painting, coating, printing, and bonding preparation are processes added to the machining.
Handling between processes also contributes to the cost. A part may require cleaning, masking, finishing, and re-inspection prior to delivery.
In costing, indicate the surfaces that must be finished and the desired outcome. This provides enough data for the CNC supplier to do the work on the finished part instead of treating the entire part.
Does Order Quantity Affect Plastic CNC Machining Cost?
Quantity mainly changes how the setup and production costs are spread across the parts. A one-off part carries the full setup cost, while a repeated order spreads it over more pieces.
Prototype Parts
For a prototype, the cost includes:
- CNC programming
- Workholding setup
- Tool setup
- First-piece inspection
- Material preparation
These costs are applied to a very small number of parts.
Small Production Orders
For a small batch, the same setup is used for several parts.
The cost per part can drop because programming, setup, and first-piece inspection are shared across the order.
Material purchasing can also affect the quote, especially when the required plastic stock is purchased specifically for the job.
Repeated Production
For repeat orders, the machining process can be kept consistent.
The supplier can reuse:
- CNC program
- Workholding setup
- Tool list
- Inspection method
- Process settings
This reduces preparation work for future orders and makes the cost per part easier to control.
How Can You Reduce Plastic CNC Machining Cost?
The easiest cost savings usually come from the drawing rather than the machine settings. A part with fewer unnecessary features, sensible tolerances, good cutter access, and fewer setups takes less time to produce.
Simplify Non-Functional Features
Eliminate unnecessary features that are not needed for assembly or part function.
For instance, a shallow decorative pocket can be a separate toolpath but does not provide any mechanical function. Likewise, there are cases where a series of small features can be substituted with one larger feature.
Simple geometry equates to fewer cutting moves and fewer finishing operations.
Apply Tolerances Where They Are Needed
Don’t have all tolerances tight.
An external dimension might fit the general tolerance of the drawing, but a bearing bore, shaft fit, or locating hole may require a specific tolerance.
This restricts additional finishing and inspection to the dimensions that determine the part’s function.
Make Tool Access Easier
Allow sufficient space around deep pockets, holes, and inside corners for the cutter to access the feature.
A pocket with a long reach, for instance, a 100 mm deep pocket, may be harder to cut than a pocket with a shorter reach. If it is possible to decrease the pocket size without impacting the part, the shorter tool may make the task easier.
Higher internal corner radii also enable the use of higher cutters.
Avoid Unnecessary Setups
A part that is able to be formed in one setup doesn’t require the additional handling of a second setup.
Install features on the same accessible side as the design. When a hole or pocket can be made without turning the part over, this can eliminate a second re-clamping, alignment, and inspection operation.
Specify Only Required Finishing
If not required for applications, do not mention polishing, cosmetic finishing, or surface roughness on all surfaces. For instance, if a pocket is used for a mating face, the surface finish may require control, but if it is an internal non-functional pocket, then it may not.
The drawing should show the surfaces that must be deburred, polished, coated, or have a given Ra value. This only requires extra finishing where needed.
Plastic CNC Machining Services at YD Rapid
Material and DFM Review
YD Rapid checks the plastic grade, part shape, wall thickness, tolerances, and tool access before machining. We can flag features that may add unnecessary machining time or cost.
CNC Milling and Turning
We provide CNC milling and turning for plastic prototypes and production parts. The machining method is selected from the part geometry, material, quantity, and drawing requirements.
Inspection and Finishing
Parts are inspected against the drawing and required dimensions. Deburring, polishing, and other finishing can also be arranged if specified.
Get Your Plastic Part Quoted
Send us your CAD file, drawing, material, quantity, and finish requirements. YD Rapid will review the part and quote the machining work based on the actual job.
FAQ’s
What Makes Plastic CNC Machining Expensive?
The main cost usually comes from machine time, material, tooling, setups, and finishing. A part with deep pockets, many small features, several setups, or a large amount of material to remove will take longer to produce.
Does Plastic Material Affect CNC Machining Cost?
Yes, the plastic grade can change both the material cost and the machining time. Common engineering plastics can have very different prices, and some materials require different cutters or cutting conditions. Specify the exact grade rather than only stating “plastic” when requesting a quote.
Do Tight Tolerances Increase Plastic Part Cost?
They can, particularly when the tolerance is for a functional feature. Further finishing and inspection may be required on a close-tolerance bearing bore or mating surface. Do not exceed the normal drawing tolerances for general dimensions, and only draw out the dimensions that affect the fit or function.
Are Thin-Wall Plastic Parts More Expensive to Machine?
They can be. The walls may be thin and must be supported, may require lighter cuts, and may require a finishing pass during cutting. If the wall thickness does not need to be very thin, then make it thicker for easier and faster machining.
Does Part Quantity Change the CNC Machining Price?
The number of parts placed in an order can influence the setup and programming cost per part. The prototype has these costs associated with it, while a larger batch has these costs associated with a larger number of parts. To repeat the production can be done on a developed setup and machining process, too.
How Can I Reduce the Cost of a CNC-Machined Plastic Part?
Start with the drawing. Remove unnecessary features, use practical tolerances, allow standard cutter access, reduce unnecessary setups, and specify finishing only where required. Also provide the correct material, stock size, quantity, and drawing requirements so the supplier can quote the actual job accurately.
