Design and Material Considerations

When starting your design for your plastic part or assembly you need to consider

a few things to help reduce cost and improve quality.

Types of thermoplastic polymers

What type of plastic you choose can greatly affect the cost and quality of the

finished project. Thermoplastic polymers are vastly different in their mechanical

properties and chemical makeup. This makes for a wide variety of choices including

chemical resistance, U.V. resistance, weight and durability.

Thermal expansion, warping and tolerance.

Every kind of thermoplastic (all materials) has a different coefficient of thermal

expansion (CTE), meaning as the material is heated it expands and as it cools it

shrinks. We must take those values into consideration as we design and select our

materials for manufacturing. Imagine if you designed a tank with a material that has a

high rate of thermal expansion. Without thinking about this the tank is then wrapped in a

stainless-steel frame for support leaving no gap for tolerance. If this tank is left out in the

sun the tank will expand at a greater rate than stainless steel. Most likely causing

warpage and possibly a complete failure of the tank wall. Understanding the materials

you are working with is essential to a successful design.

Warping can be a consideration as well. When most thermoplastic sheets are

extruded and made into sheets, they are squeezed through a set of rollers to create a

uniform thickness. Why does that matter, you may ask? It matters when you decide to

heat the plastic up, in thinner materials its easier to see how the plastic will release

internal stresses as the material expands causing a once straight edge to become

twisted.

The ability for a thermos plastic material to maintain a tight tolerance is directly

corelated to the temperature at which the material was machined, or fabricated and the

temperature at which it is going to be used. If a part was machined at 70F inside in the

machine shop, but then deployed to the oil field where it will see 110F or -40F that part

is going to grow and shrink accordingly. Manufacturer tables are available for all

thermoplastic materials that will state the expected coefficient of thermal expansion.

Polyethylene for example expands 18 times more than carbon steel, making it critical to

take into account, especially when mating materials together, or calling out tolerances

that are appropriate for steel, but not plastic.

Machining

This is less of a factor but still something to keep in mind when selecting

materials. Whether the plastic is soft or hard or somewhere in between, you will need to

consider the tooling necessary to cut the material. Not only the tooling but also the

travel speeds and plunge rates. All these factors can greatly affect cost and time over

the course of a large project.

Weldability or Solvent Bonding

Weldability is a huge factor when it comes to producing a quote for the customer.

If we are dealing with a material that is difficult to weld it will cost more to manufacture.

Not only material but thickness is a factor in the weldability as well. If we accompany a

thin material along side a material that is harder to weld, we will have to take a great

deal of time to fixture and maintain quality in the finished product. HDPE,

Polypropylene and PVC are examples of weldable thermoplastics.

Some materials are not able to be welded, but instead should be solvent bonded.

These materials are often more reactive to other chemicals as well. PVC,

Polycarbonate & Acrylic are examples of materials that can be solvent bonded.

Durability

Durability cannot be overlooked. Density and stiffness are two factors that I

would consider to be the greatest factors in the durability of a thermoplastic polymer.

First, you must look at what environment the plastic will be in, is it stationary on a

concrete floor indoors? Or perhaps in the back of a work truck in the sun on a bumpy

road. These two scenarios would greatly change what plastic material would be the best

long-term solution for the design.

List of common thermoplastics in order from easiest to work with to more

difficult:

1)HDPE (Marine and pipe grade included)

2)Polypropylene

3)LLDPE

4) PVC (unplasticized)

5)PVDF

Thinner does not always mean cheaper

Many customers have it in their minds that designing a part out of thinner

material will make it cheaper, this is not always the case. As we’ve previously spoken

about thermal expansion and warping thinner materials can react to heat much faster

and much greater than a thicker piece of material. This means that keeping a tighter

tolerance is not only harder but the machines and process for welding can change to a

slower more involved process. Any material under 3/16” is much more difficult to work

with if welding. Our preferred range of materials is from 3/16”- ¾” anything under or over

these values takes special design and prep work in the welding and fabrication process.

Smaller doesn’t always mean cheaper

This situation comes up from time to time. Not all designs are created equal.

Large simple tank builds are often pound for pound cheaper than builds half their size, it

all comes down to complexity and how many hours we are going to spend making your

project come alive while achieving our quality standards. If we take a few hours

producing dxf files and programming our machines to cut a set of parts, it will cost us

time and money to make changes to those. Time and money we don’t get back. We

include in all our quotes the time it takes to review, produce dxf files, and quote your

project. If changes are made, we must decide whether to eat that cost or pass it along.

Design considerations

When designing with plastic welding in mind you need to think about the welder’s

access to all the joints that need to be welded. Not only do we need to think about the

welder but also the finished look of the build. Exterior welds are simple enough to

access with a large extruder, but the interior welding may need to take place with a

different or smaller size of welding equipment to reach all the seams. Many of the

exterior seams will need to be prepped before welding with v grooves or gaps before

the welding process can even begin. Watertight seams will usually require the exterior

and interior seams to be welded to insure a watertight seal.

Surface finish

The surfaces finish of the final part needs to be considered during material

selection. Harder plastics by their very nature are harder to scratch. Polycarbonate will

avoid scratching much more than acrylic.

Some materials are available with different surface finishes to help hide defects.

For example, HDPE sheets are soft and do not come with protective wrapping from the

manufacturer. Transportation and handling naturally scratch the surface of these sheets

before the fabrication process even starts. Often soft materials such as HDPE,

Polypropylene & ABS can be purchased with a texture such as an orange peel that will

hide scratches.

Chemical Resistance

Each plastic manufacturer lists the chemical resistance of their plastic against

other common chemicals that may encounter the material. These interactions are

usually rated from A to D. A being very compatible with negligible effects on

mechanical properties. D suggests that the material will decompose or dissolve when in

contact with the chemical in a short period of time. B & C are varying degrees in

between. Often the temperature of exposure or concentration of the chemical if in an

aqueous solution will also need to be considered when determining chemical

resistance. It is important to select a plastic that is compatible with your environment.

Working Temperature Range

Each plastic also has a working range of temperatures that are recommended.

When you get outside this temperature range it will either start to deform or become

brittle. Some plastics have big working ranges, and others very small. Check with the

plastic manufacturer for your specific brand of plastic to see the recommended working

range. In general the following plastics have the following working ranges.

PTFE ( Teflon ) has a working range of -200C to + 260C (-328 F to 500F )

HDPE ( High Density Polyethylene ) has working range of -50C to + 80C (-58F to 176F)

PP ( Polypropylene ) has a working range of 0c to 100c ( 32F to 212F )

PVC ( Polyvinyl chloride) only has a working range of 0C to 60C (32F to 160F)

Note that the plastic deformation point and melting point for all of these plastics is higher

than the working range and should not be confused as the high end of the working

range.