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Month: November 2023

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27 Nov
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WHAT IS CREEP?

Creep in polymers, also known as cold flow, it is the tendency of a material to slowly undergo deformation when constant mechanical stresses are applied. These stresses include tensile, compressive, shear or flexural loading. If a plastic is under constant stress, a constant change in strain will occur.1 Thus the stressed polymer will continue to deform over time. It will occur even if the stress is below the yield strength of the material.

Materials will experience greater creep if they are exposed to heat for long periods of time. So the rate of creep depends on the stress applied, temperature and the material involved. Glass, metals, polymers, ice etc. all experience creep but at different rates and temperatures. The ice in glaciers flows slowly down a slope because of creep. There is slippage between and within the ice crystals. It is important to note that high temperatures are not required. Materials also creep under ambient temperatures.

VISCOELASTICITY

Polymers are viscoelastic. That is they have the properties of both fluids and elastic solids. This is because plastics are made up of long molecules that get entangled with each other. Also the polymer chains are mobile and can slide past each other.

PLASTIC FLOW

Creep is not the same thing as plastic flow. It is the deformation of a polymer under a small constant load. Plastic flow occurs when there is fast deformation of a polymer under a high constant load.

POLYMERS THAT RESIST CREEP

Crystalline polymers are more creep resistant when compared to amorphous polymers. This is because they have a higher degree of secondary bonding. Polymers with more chain entanglement are also more resistant. Additionally glass fibers added to a polymer will increase entanglement and this will lower creep.

EXAMPLES OF POLYMERS THAT EXHIBIT LOW CREEP

Vespel®: Excellent strength and impact resistance. It has low rates of wear and friction. Vespel® is thermally resistant and maintains its mechanical properties in temperatures as high as 260°C (500°F.) It has exceptional creep resistance.

PAI: (Polyamide-imide) Also known as Torlon® it is an extremely strong, stiff and dimensionally stable polymer. It is often used in high temperature applications as it can perform in continuous temperatures up to 260°C (500°F.) It is used when a tight tolerance plastic is needed as its has extremely low thermal expansion and excellent creep resistance.

PEEK: (Polyetheretherketone) An engineering plastic known for its mechanical strength and dimensional stability. It performs well in harsh chemicals. PEEK has a continuous service temperature of 170°C (338°F.)

ECTFE: (Halar®) has greater strength, abrasion resistance, and creep resistance than other fluoropolymers that are softer such as PTFE (Teflon®.) Additionally it has excellent mechanical properties, dimensional stability and is fire resistant. The surface of ECTFE is exceptionally smooth making it ideal for high purity applications.

1 Stress is the force applied to a material, divided by the material’s cross-sectional area. Strain is the deformation or displacement of material that results from an applied stress.

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Categories: Fasteners, Material, Plastic Material
tpu Isoplast®-plastic
15 Nov
0 49 Post by

by Barbara Gerard

Thermoplastic Polyurethane (TPU)

Thermoplastic Polyurethane plastic is used to make parts including TPU Isoplast® screws, hex bolts, hex nuts and many custom components. Because of its high strength and impact resistance thermoplastic polyurethane TPU Isoplast® is used when a rigid polymer is needed due to its high strength and impact resistance. Technically thermoplastic polyurethane is a thermoplastic elastomer made up of block copolymers which have hard and soft segments. Basically a TPU resin consists of linear polymeric chains in block-structures. Such chains contain rather long (called soft segment) chains alternating with shorter ones (called hard segments). Thus TPU is a bridge between rubber and thermoplastic resins.

Isoplast® 

TPU parts were originally used for medical purposes. It is sold under the brand name Isoplast® among others. The resin with 40% glass filled compares well with some metals. This product adds together the toughness and dimensional stability of amorphous resins with the chemical resistance of crystalline materials. Thermoplastic polyurethane (TPU) properties include elasticity and resistance to oil, grease, and wear. Additionally, it is resistant to sea water and ultra violet rays making it an excellent plastic for use in the oceans.

Strength

With a yield strength of 27,000 psi (186 MPa), TPU Isoplast® 40% added glass fiber has the strength of aluminum.  Also it has water and chemical resistance, low creep and UV resistance. Thus TPU 40% glass fiber is an ideal material for plastic parts such as custom parts, hex bolts, hex nuts and TPU Isoplast® screws. Additionally it is used in applications such as internal scaffolding products such as beam clamps, pipe hangers, post bases, pipe clamps, clevis hangers, channel nuts, U-bolts, and more.

Uses

TPU 40% glass filled Isoplast® has a wide variety of uses. For example, it is used to make medical devices such as healthcare mattresses, dental materials, medical device cables, and wound dressings. Also it is used in power tools, sporting goods, parts for cars, fire hoses, and drive belts as well as inflatable rafts and footwear. Additionally TPU Isoplast® is also a popular material for use in keyboard protectors and in the cases of mobile phones. TPU phone cases are flexible because the material has a high elasticity rate and absorbs shock. It is also an excellent material for this purpose because it can be dyed to a wide variety of colors.

Pros of TPU Isoplast®40% Glass-filled

  • high abrasion resistance
  • high mechanical properties
  • high chemical resistance
  • high shear strength
  • high elasticity
  • oil and grease resistance
  • good injectability
  • good adhesion strength
  • good low-temperature flexibility
  • TPU scrap can be ground and reused.
  • lighter in weight than metals
  • minimal creep
  • good in UV radiation
  • non-conductive

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Categories: Fasteners, Material, Plastic Material
TPU-Isoplast-plastics
09 Nov
0 70 Post by

Plastic parts are generally not known for their strength. Consumers often choose plastic parts for their good chemical and acid resistance. And also the fact they do not conduct electricity, and can perform well in the presence of water. However, despite this reputation, there are plastic materials that have all of the better known characteristics of plastics and yet rival the strength of metals. Plastics such as TPU Isoplast®, FR4/G10, and Polyarylamide or PARA are exceptionally strong plastics.

Does your application require strength as well as non-corrosive and non-conductive properties?   Check out the top three strong plastics for fasteners and custom parts.

1.      Thermoplastics Polyurethane TPU Isoplast®

Thermoplastic Polyurethane is an impact modified amorphous polymer*** with the chemical resistance of crystalline resins.  TPU has a low uniform shrink rate which allows minimal creep*.  TPU 40% glass filled plastic fasteners are significantly lighter than the same metal parts.  With an ultimate tensile strength of 27,000 psi (186 MPa), TPU 40% glass-filled is like aluminum.   Also it has strength, water and chemical resistance, low creep and UV resistance. Thus TPU 40% glass fiber is an ideal material for plastic fasteners such as hex bolts, hex nuts and TPU screws. It is also used in applications such as internal scaffolding products, beam clamps, pipe hangers, post bases, pipe clamps, clevis hangers, channel nuts, U-bolts, and more.

TPU 40% glass filled is sold under the brand name Isoplast®, among others.

2.       FR-4/G10

FR-4/G10  is a composite material made up of woven fiberglass cloth and an epoxy resin.  This combination gives the laminate good strength to weight ratios.  Typical physical and electrical properties of FR-4 are expressed by LW (length wise, wrap yarn direction), and CW (cross wise, fill yarn direction).   LW and CW refer to fiber orientations that are perpendicular to each other. With a tensile strength of 45,000 psi  (310MPa), FR-4 plastic parts compare well to brass and aluminum.  Aluminum (Grade 6463-T6) has an ultimate tensile strength of 35,000 psi (241 MPA). While the number for free machining brass (Grade C3600)  is 50,000 psi (345 MPA). 

Also FR-4 plastic parts are flame resistant.  Flame resistant means that the parts are self-extinguishing according to the UL 94 test.  G10 plastic parts are not flame resistant. Thus they are no longer commonly used, although they are still used in applications where thermal destruction of the material is desired.  FR-4 epoxy resin often uses bromine, a halogen, to give the material a UL 94 V-0 flammability rating.  With flame retardant and near zero water absorption, FR-4 plastic fasteners retain their high mechanical strength and electrical insulating qualities in both dry and humid conditions.  FR-4 is not available in an injection molding grade but rather in sheets, plates and rods.

FR-4 is the insulating backbone of printed circuit boards (PCBs). Also it is also used for screws, bolts, nuts, switches, standoffs, threaded rod, transformers other parts that require strong plastics.

Ultimate Tensile Strength

MaterialMpaPSI
FR-4/G-1031044,960
PARA (50% glass-filled)23033,358
TPU (40% glass-filled)18629,977
Magnesium (Grade HK31A-T6)22532,633
Brass (Grade C3600)34550,000
Zinc Alloy28040,610
Aluminum (Grade 6463-T6)24135,000
Steel33047,862

3 .     Polyarylamide (PARA)

Polyarylamide (PARA) contains a base resin that has lower and slower water absorption than typical polyamides.  This resistance to water absorption is found in PARA’s semi-aromtic structure.  Polyarylamide compounds containing glass fiber result in plastic fasteners that have very high rigidity, excellent surface quality, extremely low creep* and high dimensionally stability.  Also PARA material containing 50% glass can stand up to a variety of chemicals including those used in the automotive industry.

PARA 50% glass-filled plastic parts have an ultimate tensile strength of 34,000 psi (230MPa). These parts made of strong plastics are often used to replace metal. It is available in a wide variety of colors. Thus PARA is often used in covers for cell phones and other electronic devices. Also it is used in aircraft and automotive applications as well as in furniture, small appliances, power tools, camera parts, gears, handles, hinges and knobs.

PARA 50% glass-filled is sold under the brand name of IXEF®, among others. 

*Creep is the tendency of a solid material to move slowly or deform permanently under the influence of stresses.

**An impact modified polymer is one whose impact resistance and toughness has been increased by the incorporation of the phase microdomains of a rubbery material.

*** An amorphous polymer has a molecular structure that lacks a definite repeating form.

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