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Whether you're a seasoned engineer or just starting out, this section will walk you through the essential considerations for creating efficient and reliable plastic manifolds. We'll focus on the two main critical aspects: temperature resistance and mechanical properties. We will cover electrical characteristics and outgassing in the next section.
Designers understand the importance of temperature resistance and how factors like continuous service temperature, heat deflection temperature, and brittle transition temperature play a part in a product's success, overall performance, and longevity. Different applications demand specific temperature requirements, and choosing the right plastic material and designing therein stand as some of the most key factors to consider.
IDENTIFY PRODUCT'S OPERATING TEMPERATURE RANGE
Designers will need to determine firstly the temperature extremes their manifold will encounter during operation. Consider both normal operating conditions and potential extremes to choose a plastic material that can withstand these fluctuations.
SELECT THE RIGHT MATERIAL
Designers will also need to explore different plastic materials that have a high-temperature resistance, such as PEEK, PPS, or polyimides. Evaluate their temperature ranges and ensure they align with your application's needs.
INVESTIGATE MATERIAL'S THERMAL CONDUCTIVITY
Evaluate the anticipated material's thermal conductivity. This is essential to manage heat dissipation effectively in manifold assemblies and systems.
THERMAL STRESS CRACKING
Manifolds can also stress-crack from thermal shock where a hot plastic is exposed to very cold temperatures (or vice versa). The uneven cooling from outside to inside creates mechanical stresses degrading the plastic. Avoid this faulty process.
In the following, we will define a few terms that are integral to understanding the following sections when it comes to temperatures. This will also give ground-level knowledge of the various terms used later on when delving further into our materials pages and exploring those in greater detail.
Industry professionals define continuous service temperature as the temperature at which a plastic component can tolerate indefinitely without degradation. Continuous service temperature is determined when no other stresses are applied to the material. This excludes the influence of other forces outside temperature.
The chart here displays several different types of plastic materials and their continuous service temperature ranges. Vespel ranks at the top with 572°F (300°C) continuous service temperature. This means that Vespel can operate without compromising its structural integrity up to 300°C.
ABS comes in last with a continuous service temperature of 86°F (30°C). In this case, if a product needs to withstand some extreme temperatures, ABS would not be fit for the job. We would recommend it for other purposes, depending on the project's needs.
Heat deflection temperature is the temperature at which the part will start to deform under a testing load. This load is often measured as either 66.7 psi or 261 psi. Heat deflection temperature can be a better measure of plastic stability at a working temperature.
The chart here details out the heat deflection temperature ranges of several different types of plastics. The difference between continuous heat temperature and heat deflection is their time lapse. With heat deflection, the term caters more to at what point does the material soften and lose shape under a load. Continuous service temperature caters more to consistent performance despite hot conditions without load-bearing.
Data courtesy of Omnexus
Brittle Transition Temperatureis the temperature below which plastics exhibit brittle failure under impact. This is used for evaluating how a plastic will act under cold temperatures.
Did You Know?
Brittle transition temperature is only listed for semi crystalline plastics as amorphous plastics are considered brittle at room temperature.
Data courtesy of Omnexus
Manifolds need to meet the needs of their intended purpose. Typically, this means that they need to withstand pressure, stress, and other mechanical forces to ensure system reliability. Without that reliability, the whole system may result in failure.
THREE REQUIREMENTS CONCERNING MECHANICAL STRENGTH
KNOW PRODUCT'S PRESSURE REQUIREMENTS
Identify the maximum pressure your manifold will experience. Choose materials with sufficient tensile strength, compressive strength, and burst pressure resistance.
UNDERSTAND ITS NEEDED IMPACT RESISTANCE
Consider the potential for impact in your application. Will it take repeated hits? Will it be under a lot of pressure? Some plastics, like ABS or polycarbonate, offer outstanding impact resistance.
ENSURE FLEXURAL STRENGTH
Manifolds need to accommodate bending and torsional forces in your manifold system. Assure the product's needed flexural strength before deciding on materials.
MECHANICAL STRESS CRACKING
There are many ways to apply mechanical stress to a manifold that will cause it to stress crack. One common way is through over torqued fittings, fasteners, and components. Also, amorphous plastics are not considered good structural loading materials. A manifold subject to a bending moment or sheer stress can fail over time.
COMPRESSIVE MODULUS OF ELASTICITY
When talking about pressure concerns, compressive modulus of elasticity (CME) signifies a plastic's ability to "bounce back". How much consistent pressure can the manifold take before its structure is compromised? Some materials can't take it. Others can and excel in such environments.
Here, we have a chart that details that CME per plastic material in PSI. PBT stands at the top with around 928,000 PSI. It can bounce back easily before it can't bounce back. Unlike Nylon 6/6 at the bottom, the material can only withstand 15,000 PSI. However, that's not a small amount. Nylon 6/6 can still withstand intense pressure likened to that at the bottom of Mariana's Trench.
While CME explains a component's ability to retain shape and function after applying compression, compressive strength applies more towards a component's ability to "keep up" without breaking. How much can the component hold before it loses its ability to retain its shape permanently?
We have posted a bar chart here further elaborating on the compressive strength of several different types of plastics. PPS and ULTEM take the top spots with over 20,000 PSI compressive strength, displaying their durability in this regard. If you need something that can hold up against intense compression, selecting a material higher in these rankings might be in your project's best interest.
Flexural modulus of elasticity (FME) and CME differ in the way that a manifold "bounces back". FME deals more with bending and flexing of something like a sheet or rod, while CME deals more with compression and pressing without bending or flexing forces applied. FME measures how much that component or material can bow when perpendicular force is administered. Can that component stay stiff enough and not bend while performing its primary functions?
Another bar chart we have here shows the FME for different types of plastics in PSI. PEEK and Nylon 6/6 take the top spots here. When we consider materials like Nylon 6/6, we see that while it does not have the most CME (i.e. it cannot take much applied flattening force), the material has a great amount of FME, able to push back an incredible amount before it starts to bend under that perpendicular force.
Flexural strength tests how much a component or material can bend before it breaks. It's about finding out the limits of their bending power without it actually breaking and snapping. This differs from FME in that the goal is not to break the component but rather how it bends and flexes under that pressure. When it comes to flexural strength, the goal is to see how much it can handle before it breaks.
Another bar chart we have details this out for several types of plastics in PSI. PEEK once again takes the top spot with Torlon taking second place. Most engineers know PEEK's incredible ability to stand against flexural forces while still maintaining function. For that reason, we often see engineers in aerospace and military operations choose PEEK for their products.
Tensile elongation at break (TEAB) applies to how much "pull-apart" force one can apply to the material before it snaps. It measures a material's stretchiness in that regard. While the other aforementioned terms address either flattening, bending, or flexing of a material, TEAB caters more to outward forces than inwards.
This bar chart demonstrates at what percentage materials can handle those pulls. HDPE take first place with UHMW taking second. PPS comes in last.
Tensile strength applies to the maximum amount of pull-apart force that a material can handle and at what point it breaks. It differs from TEAB in that TEAB measures the pull force right before that breaking point, while tensile strength denotes that maximum point.
In this bar chart, we measured the tensile strength of several plastics in Pascals (Pa). Torlon and ULTEM take the top positions while UMWH comes in last. Using different measuring units here can create some confusion between TEAB and tensile strength, however, with differing applicable forces and units of measurement, we can think of these two qualities as being related yet different in the ways engineers use them for creating components.
A material’s tendency to stress crack can be influenced by many things. For cracks due to mechanical stress these include Tensile Elongation at Break (TEAB) and Tensile Strength at Break. We've already covered TEAB in the previous section, but to give more detail, a material with a lower TEAB will stress crack under less deformation than a material with a higher TEAB. A material with a lower Tensile Strength at Break will stress crack under less pressure than a material with a higher strength at break. Here we share a chart of common crack-sensitive plastics along with their strengths at break.
As COC/COP and Acrylic have the lowest values for both elongation and strength, they are more likely to crack under mechanical load.
Ease of machining measures a combination of three important factors when machining a plastic component: stability, abrasiveness, and burr formation. The lower the number, the easier the plastic is to machine and correspondingly lower cost to produce.
Stability correlates with how likely it is for a part to change size during or after machining, e.g. warping in large flat parts.
Abrasiveness correlates with how quickly the cutting tools must be changed from abrasive wear.
Burr formation correlates with the level of difficulty due to the formation of burrs.
We can see in the adjoining graph that Torlon comes in first place for having the highest difficulty and Delrin AF for having the lowest (i.e. easiest) ease of machining.
Rockwell Hardness is a measure of how much a material will deform under a specific load from a pointed tool. While this does not directly correlate to scratch resistance, a material with a higher hardness will often be less prone to scratching.
Here is the Rockwell M Hardness Scale for many materials. The M is based on the type of tip and pressures used for the test.
Data courtesy of Omnexus