Plastic Selection Tips for Machining

For low-volume and close tolerance applications, machining of plastics can be a cost-effective and reliable method for your projects. It is typically simpler to machine and less expensive than metal. Highly engineered plastics perform quite well under a variety of conditions, while inexpensive lower-performance versions are available for projects with less stringent requirements.

The typical engineered part requires three characteristics to function. Your search for plastics that will supply all three will come up with a list of potential materials. The information below lists the most common characteristics any engineer will need to consider when looking for a plastic that will function as required. The suggested materials serve as a starting point, and are not comprehensive.

Some plastics are available with fillers that will change the properties of the plastic to suit your needs without having to upgrade materials. However, these fillers, such as glass, may also reduce machinability.

Biocompatible

For medical devices. These plastics are able to come into contact with the human body without causing problems. Include a USP class VI designation (Biological Reactivity Testing), also regulated by ISO 10993 (Biological Evaluation of Medical Devices).

Biocompatible Plastics:

• Radel

• UHMW

• Polycarbonate

• PEEK

Chemical Resistance

Plastics in general have a wide variety of chemical resistance. Though some seem almost universally resistant, others are highly sensitive and crack when exposed to almost any chemical. Before choosing a plastic, consult a good reference guide to learn about specific interactions with chemicals.

High Chemical Resistance Plastics:

• UHMW

• Teflon

• PEEK

• CTFE

Low Chemical Resistance Plastics:

• Acrylic

• ABS

• Noryl

• Polysulfone

Clearness

Stock plastic begins clear but machining generally changes them from translucent to opaque. If clearness is a requirement, polishing is necessary. Polishing methods include buffing, direct machine polish, flame polish and vapor polish.

Clear Plastics:

• Acrylic

• Polycarbonate

• Clear PVC

• Polysulfone

• Ultem

Cost

When choosing material for your project, consider the machining and finishing costs in addition to the price of the raw material. Though usually more inexpensive than metals, higher performance engineered plastics cost more.

Inexpensive Plastics:

• Delrin

• UHMW

• Polypropylene

• HDPE

Expensive Plastics:

• PEEK

• Vespel

• PPS

• Radel

• HDPE

Dimensional Stability

Unlike metals, machined plastics have less structural stability. They tend to absorb water and expand when exposed to heat. More stable plastics have low water absorption and a low CTFE.

Dimensionally Stable Plastics:

• Ultem

• PEEK

• PPS

• PET

Less Dimensionally Stable Plastics:

• Nylon

• UHMW

• HDPE

• LDPE

FDA

Plastics that come into contact with food, especially packaging, require approval for use by the Food and Drug Administration.

FDA Approved Plastics:

• Delrin

• PET

• Polycarbonate

• PEEK

Limiting PV

For bearing applications, plastics need a combination of pressure and velocity to give it the thermal and structural ability to withstand rotational wear.

High Limiting PV Plastics:

• PEEK

• Nylatron

• Delrin AF

• Torlon 4301

Low Limiting PV Plastics:

• UHMW

• PBT

• PET

• Nylon

Steam Sterilization

Most biocompatible plastics will hold up to this process of using steam to sterilize material prior to first use.

Plastics Compatible With Steam Sterilization:

• Radel

• Ultem

• PEEK

• Teflon

Strength

For structural applications, two types of loading strength (measured in psi) are important to consider when choosing a plastic. Tensile strength, the most widely measured, indicates the amount of pressure required to pull a sample apart from the ends, as well as the amount of stretching before the break. In compression strength tests, the opposite is tested. Samples are pressed and the amount of force necessary to break them is measured.

High Strength Plastics:

• Ultem

• Peek

• PPS

• Nylon

• Delrin

Low Strength Plastics:

• Teflon

• UHMW

• LDPE

• HDPE

• Polypropylene

Temperature Resistance

Two types of temperature resistance are important to machined plastic parts: the optimal air temperature, or the amount of heat deflection if under a load.

High Temperature Resistance:

• PTFE

• PEEK

• Ultem

• Torlon

• PPS

Low Temperature Resistance:

• Acrylic

• UHMW

• ABS

• PVC

Toughness/Impact resistance

This index measures the ability of a plastic to endure a blow or sudden impact. This is not the same as continual pressure or load. Some plastics can withstand an impact but will not do well under a constant load.

High Impact Resistance Plastics:

• Nylon

• Polycarbonate

• UHMW

• PEEK

Low Impact Resistance Plastics:

• PET

• Acrylic

• Noryl

• Polysulfone

UV Resistance

Outdoor applications require a resistance to UV light. Without it, material will age and become brittle. Black plastics usually have some resistance because of color.

High UV Resistance Plastics:

• Ultem

• Polycarbonate (UV stabilized only)

• PBT

Low UV Resistance:

• Polycarbonate (FDA)

• Nylon

• Acetal