What plastic to use.

I'm not real clear on the type of balance you are designing, so my comments may be more general than I would like.

It seems like you have decided to go with plastic over metal, even though it doens't seem you've looked at what is required from the design. This may work, but it is generally preferred to determine the requirements for the part first, and then find what can meet those needs.

Metals will not be degraded by solvents and many other chemicals. Metals can also handle hot crucibles (although massing a hot sample may not be the best idea). PC and other plastics may have real challenges meeting those needs.

John Aspen Research -

Opinions expressed herein are my own and may not represent those of my employer.

Nope I havn't decided on a particular material yet (metal or plastic). I'm just trying to see if plastics can infact take the place of a metal in my application.

As per some of the requirements :

- It should not deform over time with a load (I'm expecting approximately a 30 kg load on the plates) - or another way to put it would be, the it should have properties similar to metals in terms of strength, when used in a scale application.

- the ambient temparatures are room temp so no problems with heat.

- should be flame retardant (eventually UL certified)

- it will be exposed to standard industrial cleaning solutions (however the specific type of solution can be controlled)

I guess I'm asking for both, tips on refining my requirements (so that a plastics person would understand), and also tips on types of plastics that could be useful in this situation.

You need to be more specific. A bathroom scale will have different requirements than a laboratory balance. I have a small bullet scale with a plastic top and pan that gives bad results in low humidity due to static. Some plastics and composites have moisture regain problems (weigh more in high humidity) although electronic taring could compensate.

it will be a scale to do part counting in a lab. (so not a highly precise laboratory balance per say) the size of the top pan will be around 400 by 400 mm. Humidity and ambient temparatures would be fairly stable. whats a bullet scale?

Bullet or powder weighing scale for reloading ammunition. Electronic balances which can weigh up to ~100 grams +/- 10 mg are available for less than $100. For your need, something as cheap as polyethylene may do. Frank

Hello,

I'm doing a masters in materials science at them moment and we just learned about composits. These can be crosslinked thermosets like epoxy which are re-enforced with oriented fibres to provide rigidity in a specific direction. FIbres made from carbon, aramid and glass can be used. I just thought this might be a suggestion for your applications if you require the plates to be extra stiff - although it might be too expensive for production in large numbers.

Bobby

UL94 V-0 or V-2? Likewise most suppliers of plastics will give you a UL94 rating based on teh thickness f the part. e.g. grade 1 may be V-0 at an 1/8 inch, but grade 2 is v-0 at 1/32 of an inch.

Thermosets (epoxy, baklite, etc.) are really cheap and especially useful if your shape is simple. Thermoplstics are good when you want to injection mold a part. If a stable shape is important look to find a glass fiber reinforced plastic (carbon reinforced is even better, but much more expensive). The olefins are very chemcal resistant, but can deform with enough weight over time. Nylon and PET are the workhorses of the engineering resins, but may not give you as pretty a part as polycarbonate or acrylics can give you if appearance is a factor.