A popular question these days that many Corporations and Entrepreneurs have been asking is "Why is this plastic part so EXPENSIVE"? Plastic Machined Parts quote do not list the driving factors of cost.
One important instance to ask about or review is materials, some materials are more expensive than others. There are standard or favorite plastics, Engineered and High-Performance plastic materials to choose from. The cost can be drastic even if the materials compared have the same properties or specs. Some materials in plastic are hard to machine or achieve dimensional accuracy due to stability issues. Keep in mind, material plays one factor into the cost of your part. Set-up Time (Plastic is soft and deforms if not held correctly), Surface Finish (Course or Smooth), Part per hour Quantity and final or in-process part inspections (COC, FAI).
For those who Engineered or designed and placed the order for a CNC Machined Part and thought to yourself this same question...
Diversified Designs might be able to help you reduce machining time or material costs through a parts main step, the design. Ask yourself and review your part for the following suggestions that might not effect your parts performance and efficiency, it should however reduce the cost of the machined plastic parts.
Material Selection: As discussed above, avoid if possible over engineered or High-Performance materials for your project or application. Use a standard material that still meets the requirement or specs that your part is used for or in.
Tight Tolerances: A Standard of +/- 0.005 and a Angular of +/- 0.5 of a degree is a good start depending on part size, anything under 0.005" requires more machining time and the time to inspect those areas. We use the following part sizes to determine a reasonable part tolerance: For very small parts that range from 0.02 - 0.236" (+/- 0.005). Next part range from 0.236 - 1.181" (+/- 0.007). For small parts that range from 1.181 - 4.724" (+/- 0.011). For mid-sized parts that range from 4.724 - 15.747" (+/- 0.020) and large parts that range from 15.747 - 39.369" (+/- 0.031).
Bores/Hole Locations: Holes with blind intersections and require multiple GD&T location features take time to setup, machine and inspect. Many engineers over use GD&T when designing parts, make sure you review and use this correctly.
Hole Depth: Limit hole depth to as shallow as possible, and even eliminate unneeded holes.
Thin Walls/Ribs: The thinner and taller the wall the more time it will take to machine. Thin plastic tends to deflect and resists cutting sometimes producing a drag. A delicate part can break at anytime during machining, handling and inspection. Walls thinner than 0.020" normally do not survive the machining process and there is no guarantee that the wall will behave properly. Limit rib depth to no deeper than 2 inches.
Undercuts: Remove any unneeded undercuts in the design
Radii: Design radii into sharp inside vertical corners to reduce additional milling time that may add to cost.
Deep Small Features: Very small features, particularly if they are deep, are more difficult to reach and mill.
Coring: Adding material into part geometries that are not critical to the design can potentially reduce mill time and save money.
Text: If you don't need text on a prototype or production parts, remove it from the design. If text is needed to show how the product will look to the end users, opt for raised text versus recessed text. Machining small text is time-consuming and expensive or use engraved text instead.
Finishes: 125 Ra surface finish is a general tolerance used, the lower the finish the higher the costs. Keep in mind this is material dependant, many plastics are not capable of being machined to the finish that you may want.
Quantity: Higher volumes almost always reduce the cost of the machined parts. Small quantities still require the same programming time as higher volumes. Also lower machine time, minimum buys or purchases on materials, outside processing and shipping increase part costs.
NOTE: This Insight Blog is for Plastic Machined Components, all materials, research, conclusions and or advise is the result of actual experience with plastics and should be used at your own discretion. Using these tips to save on cost is not required and should be avoided if the integrity of your part or project is compromised. If threaded holes, text on parts or any other features are critical to the design, we can accommodate at a higher cost...
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All statements, technical information and recommendations contained in this publication are presented in good faith and are, as a rule, based upon tests and such tests are believed to be reliable and practical field experience. The reader, however, is cautioned, that Diversified Designs does not guarantee the accuracy or completeness of this information and it is the customer’s or user's responsibility to determine the suitability of Diversified Designs products in any given application.