June 15, 2010 The Basics of Plastic Selection for Machined Components

Machining of plastics can be advantageous for applications requiring low volume production, close tolerance dimensions and/or difficult to mold configurations. There is an array of plastics to choose from; highly engineered, high performance products to low cost, low performance.  Determining which is appropriate for a particular application can be challenging.

Typically an engineer will require 3 or less characteristics that are critical to the functionality of the component. A basic search will prioritize and look for a material that can supply all three. The below list presents only the most common machined plastics. Many plastics are available with fillers to enhance particular characteristics. For example, glass fill improves tensile strength while reducing machinability. Molybdenum disulphide improves wear while reducing dimensional stability.

For a thorough search of plastics, please contact the Connecticut Plastics’ engineering department at 203-265-3299 or on the web at www.ConnecticutPlastics.com

Clear Plastics: Stock shapes are typically clear but once machined they become translucent to opaque. Polishing is required to return the component to clear. Polishing methods are direct machine polish, vapor polish, flame polish, and buffing

Good Choices: Acrylic (clearest), Polycarbonate, Clear PVC, Polysulfone, Ultem

Loading Strength (psi): Tensile strength (plastic in tension) and compression strength (plastic under compression) are important values for structural applications.

Good Choices: Ultem, Peek, PPS, Nylon, Delrin

Poor Choices: Teflon, UHMW, LDPE, HDPE, Polypropylene

Temperature Resistance: Consider either continuous service temperature in air or heat deflection temperature if under load.

Good Choices: PTFE, PEEK, Ultem, Torlon, PPS

Poor Choices: Acrylic, UHMW, ABS, PVC

Limiting PV: A combination of pressure and velocity that determines whether a plastic material has enough thermal and structural ability to withstand a bearing application in rotational wear

Good Choices: Peek, Nylatron, Delrin AF, Torlon 4301

Poor Choices: UHMW, PBT, PET Nylon

Dimensional Stability: Machined plastics do not have the structural stability of metals. This is primarily caused by their tendency to absorb water and a high coefficient of thermal expansion. Stable materials have low water absorption with a low CTFE

Good Choices: Ultem, Peek, PPS, PET

Poor Choices: Nylon, UHMW, HDPE, LDPE

Chemical Resistance: Chemical resistance of plastics is variable. Some materials show an almost universal resistance to chemicals while others are very sensitive and stress crack when exposed. Best practice is to consult a reference guide for a specific chemical/plastic interaction.

Good Choices: Teflon, UHMW, CTFE, PEEK

Poor Choices: Acrylic, ABS, Noryl, Polysulfone

Toughness/Impact resistance: A measurement of a plastics ability to withstand a sudden shock or a blow. Do not confuse this with resistance to steady state stress where failure can occur from chemical attack or configuration stress risers. Polycarbonate can tolerate high impact but stress cracks from steady loading.

Good Choices: Nylon, Polycarbonate, UHMW, Peek

Poor Choices: PET, Acrylic, Noryl, Polysulfone

Cost: General measure of cost of the material. Higher performance engineered plastics are more expensive.

Inexpensive Choices: Delrin, UHMW, Polypropylene, HDPE

Expensive Choices: Peek, Vespel, PPS, Radel, HDPEH

Biocompatible: Suitability for use in medical device applications. Plastics carry the USP class VI designation.

Good Choices: Polycarbonate, Peek, UHMW, Radel

FDA Approved:  The Food and Drug administrations’ classification for plastics which contact food.

Good Choices: Delrin, PET, Polycarbonate, Peek

Steam Sterilization – Same as autoclaving, a process where steam is used to sterilize plastics for medical device use.

Good Choices: Radel, Ultem, Peek, Teflon

UV Resistance: Resistance to UV typically seen in outdoor use. Plastics without resistance will age and embrittle. Because of the coloring, most black plastics have at least a small amount of resistance.

Good Choices: Ultem, Polycarbonate (UV stabilized grade only, non FDA), PBT

Poor Choices: Polycarbonate (FDA), Nylon, Acetal

Various Precision Machined Plastic Components

Various Precision Machined Plastic Components

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9 Comments Posted by Thomas Rohlfs in Machined Component
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June 7, 2010 Tips for Machined Plastic Components Design

The following are guidelines for good machined plastic component design. These are not hard rules but suggestions to maintain reasonable pricing. Exceeding the guidelines will usually result in higher effort and cost for the component.

Each part configuration and application is unique. For specific design support, contact Connecticut Plastics’ engineering at 203-265-3299 or on the web at Connecticut Plastics.

  • Flatness: From the extrusion and cooling, plastics have stress gradients through their cross section which are not relieved by post processing at the manufacturer. Plastic’s flatness stability is substantially lower than metals. Consider the following to improve.
    • Balanced design. Dish shaped plastic machined components are notoriously difficult to avoid warpage.
    • .003” per a 4” square, unrestrained, is typical. Tighter tolerances require better fixturing and more machining.
    • Consider specifying parallelism instead of flatness. If the plastic part is attached to another (especially a metal part), unrestrained flatness is unimportant. The plastic part will follow the contour.
    • Consider restrained flatness. Again if attached to another component, the plastic part will conform to the contour.
    • Pick a stable material: Low performance materials will easily warp. For tight flatness requirements, harder plastics are better. Try Ultem, PPS, Peek and Acrylic
    • Hire a plastic machining company. Flatness can change over time while sitting on a shelf in the stock room. Advanced processing really works for flat critical components.
  • Threaded Inserts: For single time assembly, threaded inserts are not necessary given proper torque levels are utilized. Inserts do not add appreciable additional holding power. Applications requiring repeated disassembly (usually for cleaning) do benefit to avoid thread stretch. Heat staked inserts are a better solution than Helicoils by giving higher pullout power. Helicoils impart high stress levels into a threaded hole resulting in crazing at the thread root in sensitive materials. Helicoils are acceptable in materials that do not heat stake well and have good toughness like UHMW.
  • Polishing: Typically applies to clear plastics like Acrylic, Polycarbonate, Polysulfone and Ultem but is important for finish improvement in opaque materials as well. Polishing methods are direct machine polish, vapor polishing, flame polishing, and buffing. The most professional looking finishes are accomplished by machining and polishing with one vendor. Polishing only services are available but the finish usually is not as good. Heavy polishing can never make up for and overcome poor machining. Melted surfaces in the machining process will invariably result in stress cracking after polishing.
  • Finishes: There is continual demand for better finishes to improve product performance. Given the capabilities of modern CNC equipment, do not accept surface finishes above 63 micro inches from a machining vendor.
    • A 32 to 40 micro inch is consider a standard machined plastic finish for a +/- .005 toleranced component and should not incur any additional cost to produce.
    • Finishes below 32 are reasonable and to be expected on close tolerance parts (+/- .0005).
    • Finishes between 10-20 micro inches are typical for a polished plastic component especially as a turned component.
    • Inside milled pocket finishes always have higher/worse finishes. The action of the nose of the endmill does not facilitate clean cutting and chip removal.
    • Exterior flat milled areas can have a very good surface finish
    • In 3D profile milling a good finish requires a large number of passes from a ball endmill. Carefully specify the maximum allowable finish to avoid excessive machine time.
    • Finishes below 10 are difficult to achieve and may require lapping or similar with limited application to part geometry.
    • Some plastics like Teflon cannot have a surface finish below 20 via any method because of the porosity of the material.
  • Tolerances: Dimensional tolerance holding is very much configuration and material dependant. However some general rules do apply. +/- .005” is an easy to reach tolerance for all but very large dimensions in unstable material. For parts with a volume of 1” cubed and below, +/- .003 is readily held. For small components, even in soft materials like Teflon, +/- .001 is reasonable. +/- .0005” and +/- .0002” are manufacturable with small dimensions in stable materials. The corresponding surface finish must be improved as tolerances are tightened to avoid measurement repeatability errors.
  • Material Selection: There are many good references for plastic material selection by the major manufacturers of both resin and stock shapes. Select the material carefully, as plastics are capable of long term change. Field failures do occur when a plastic component reacts with a chemical in service long after the part has been approved for use by inspection. Be aware that plastic choices are not limited to unfilled materials only. Fillers such as glass fiber can enhance particular characteristics.
  • Dimensional Stability: Like flatness above, machined plastics do not have near the same stability as an identical metal component. Plastic components are affected over the long term by many factors
  • Thermal change: Plastics’ CTFE is higher than most metals. Some plastics have a measurable change simply though contact with body heat.
  • Moisture absorption: Nylon can soak up to 9% of its weight at saturation having a dramatic affect on dimensional change. Try Noryl for hot water applications.
  • Creep: Materials like Teflon and UHMW when placed under high pressure will change shape and extrude from a joint.
  • Off-gassing: Many machined plastics off gas as a reaction to the extrusion and machining process. Notably, Delrin components shrink over time from the effect.
  • Stress relaxation: Both the extrusion and machining process impart stress into a machined plastic component. Post processing can help, however part configuration determines which direction the plastic will relax and change size.
  • UV/aging: Plastics age and embrittle especially from exposure to UV or chemicals.
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5 Comments Posted by Thomas Rohlfs in plastic part design
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June 2, 2010 Connecticut Plastics Launches On-Line Learning Center

Connecticut Plastics, Inc. is a manufacturing company located in Wallingford Connecticut. We provide precision plastic machining services to OEM’s. We know that a well educated employee is key to our goal of providing good quality parts to our customers. For this reason we added an On-Line Learning Center to our website. We encourage our employees to use it, but we also realize it can be a great source of information for students of chemistry, engineering or any one with an interest in polymer science.

Our on-line learning center is separated into five sections. The first section provides an introduction to plastics. It discusses the uses of plastics in our everyday life, the history of plastic and plastic chemistry. There are links to other sites with more information.

The second section is the ultimate polymer science guide. It gives the chemical names of some of the plastics that you use every day. How polymers are made, and what is the difference between polymers. There are also links to sites that give more in depth info.

Section three is by far the most popular. It is our ultimate chemistry homework helper, with over 30 links to sites with chemistry tutorials, resources, and practice tests. These links provide information on chemistry for all age groups. This is a great resource for home schooling.

The fourth section is a page with links to chemistry resources on the web. There are links to commercial and government sites providing excellent information, including the periodic table and chemical lab safety regulations.

The fifth and final section provides information on engineering along with engineering resources. If you are considering a career in engineering, you will find links here to websites about civil engineering, mechanical engineering, aerospace, chemical, electrical and materials engineering. There are links to engineering colleges, engineering organizations and publications.

We hope that you find the Connecticut Plastics learning center helpful.   Feel free to leave a comment and let us know what you think.

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