1

Don’t just tear off

the top of the bag

Simple In House Powder Testing

ARMA Rotomould Gold Coast, Australia conferencePresented by Ian Hansen

www.ianhansenconsult.com

2

You will check the quantity delivered is correct. Grade number. Colour. What else?

In this presentation we will review three simple tests that the rotational moulder is capable of undertaking. While your supplier should have undertaken quality checks before they delivered the powder to you, how do you know if a problem is caused by the powder or your process.Your ability to test your incoming powder raw material provides you with information on one of the key components in your process.

What can be measured, can be controlled.

www.ianhansenconsult.com

3

Powder quality, your area of

interest

Source : Prof Roy Crawford

www.ianhansenconsult.comSlide 4

Particle Size Distribution (Coarse & Fines)

0

5

10

15

20

25

30

35

40

Per

cen

tag

e R

etai

ned

Fines 500+

Test 1 Particle Size Distribution. We have a range of powder batches randomly tested during our incoming quality test programme. We try and simplify our initial evaluations by looking at the coarse (powder 500microns or larger) and at the fine particles (powder smaller than 150microns)The fine particles tend to stick to the mould first, and the coarse particles tend to be the last to stick. As a result we look for variations in these two extremes as causes of potential processing differences when we mould a part.

In the above example we can seeCoarse, (larger than 500microns) lowest 9.5%, highest 36%, average 23.2% Fines, (smaller than 150microns) lowest 3.6%, highest 13.6%, average 8.1%

Coarse Average %

Slide 5

Coarse, 500-600 microns

Fine, smaller than 150microns

Coarse and Fine particlesWhat do they look like?

What is the effect?Too coarse particles causes increased oven cycle time and may create bridgingToo fine particles may reduce powder flow and create improper filling of the mould

www.ianhansenconsult.com

www.ianhansenconsult.com www.ianhansenconsult.com

6

TEST ONE - PARTICLE SIZE DISTRIBUTIONPowder has been characterised into three groups:

Sieve SizeFine % Average % Coarse %

600 um 0 0 3

500 um 0 3 15

425 um 1 15 20

300 um 15 25 22

212 um 27 22 20

150 um 32 20 15

Pan (fines) 25 15 5

(Source : R.J.Crawford – Rotational Moulding of Plastics, 2nd Edition).

Finely ground powders tend to be more susceptible to thermal deterioration due to their greater surface area to volume ratio. Finer powders don’t flow as well and fluidize more rapidly so heating cycles can be extended.Coarse powders tend to require extended heating cycles and often an irregular, matt outer surface with pin holes is produced.

Coarse Sieve,Largest particles

Fine SieveSmallest particles

A good particle size distribution reduces voids, air bubbles and minimises porosity.

Particle Size Distribution shall be agreed between the moulder and the powder supplier. It also needs to be defined in writing, so that you can measure compliance.

www.ianhansenconsult.com

ww

w.ian

hansenconsult.com

7

Test 2 Dry Flow. We compared 45 consecutive batches of powder supplied, from the same powder company, and saw fastest dry flow at 22secs and slowest dry flow at 32secs.

Dry Flow results for 45 batches

20

22

24

26

28

30

32

34

0 10 20 30 40 50

Se

co

nd

s

www.ianhansenconsult.com

The target dry flow was 26sec, and the average dry flow over the 45 batches was 27secs. A good result, on average. But, remember that the difference between the slowest batch and the fastest batch was actually 10 seconds, or 45% These two batches are not part of your uniform result.This means your powder will not behave consistently in your moulding process.

8

TEST TWO – DRY FLOWDry flow is the measure of how a powder will tumble and flow in the mould during the rotational moulding process. The dry flow affects the heat transfer in the powder pool and how the powder distributes itself over the mould surface.Dry flow depends mostly on the particle shape, and to a lesser extent particle size. Particles that have been ground poorly will have ‘tails’ and tend to be ‘fluffy’. These two characteristics will give the powder poor flow properties - possible leading to uneven wall thickness, bridging across narrow recesses and a high void content within the moulded product. Dry Flow is probably the single most used result to reflect overall ‘quality’ of a powder

DRY FLOW TEST METHODDry flow is tested in accordance with ASTM D1895-89, and is the time taken for 100g of powder to flow through a standard funnel. (copies of the standard are available from your local standards organisation, contacts are listed in the appendix. A recommended powder flow rate should lie between 25 and 32 seconds (Paul Nugent – Rotational Moulding – A practical guide).

TEST PROCEDURE1. Weigh 100g of powder into a container (beaker).2. Pour the 100g of powder into the dry flow test funnel while blocking the outlet.

Remove the blockage from the funnel and record the time taken for all the powder to flow from the funnel.

3. The form below may be used to record the dry flow.

Supplier Product Code Batch no. Sample Weight (grams)

Dry Flow Time (Seconds)

Powder temperature C

Martogg M22 3456 100 28 28

Micropellets 3500 5678 100 28 45

ww

w.ian

hansenconsult.com

9

Test 3. Bulk Density. We compared the same 45 batches of powder, and saw the best density at 0.384g/cc and the lowest density at 0.334g/cc. A spread of 15%Poorly ground materials, with many tails will have lower bulk densities, as the powder does not pack very well. Typically the bulk density increases (packs better) as dry flow improves (gets faster)

Apparent Bulk Powder Density for 45 batches

30

32

34

36

38

40

B/D

gra

ms/

100m

l

Dry flow showed a 45% difference between slowest and fastest result, while Bulk Density only shows a 15% difference for the sample powder batches.

10

1. Weigh the empty cylinder and record the weight2. Fill a container (beaker) with powder (typically 100g, as per Dry Flow test)3. Block the outlet of the funnel and pour the powder into the funnel4. Place the cylinder under the funnel with the opening facing up5. Remove the blockage and allow the powder to flow and fill the cylinder6. Gently remove the powder extending above the cylinder by scraping a level

object across the cylinder opening, careful not to cause any settling of powder.7. The powder should now fill the cylinder but not protrude above the cylinder walls8. Carefully weigh the cylinder filled with powder and record the result.

TEST THREE – APPARENT BULK DENSITYBulk density is the measure of how well the powder particles will pack together. A good quality powder with few ‘tails’ or ‘fluff’ will pack together very well and have a high bulk density. Powders with a very high number of ‘tails’ and that tend to be ‘fluffy’ will not pack together well and have a lower bulk density.

BULK DENSITY TEST METHODBulk density is tested in accordance with ASTM D1895. Here, the bulk density is calculated by measuring the mass of powder needed to fill a known volume. In this case a cylinder of known volume (100ml) is used. Typically you would undertake this test in conjunction with the Dry Flow test (copies of the standard are available from your local standards organisation, contacts are listed in the appendix).

Density Cylinder

More accurate measurements can be taken if you decide to use a larger container with a larger calibrated cylinder. However, for simplicity and speed, most powder processors would measure using 100ml cylinder.

www.ianhansenconsult.com

ww

w.ian

hansenconsult.com

11

Good ideas, but too expensive?• At the very least, get the dry flow

funnel and have the ability to measure the pourability rates.

• Your investment is around US$500• This is best price/performance

quality tool available to you.

www.ianhansenconsult.com

ww

w.ian

hansenconsult.com

12

Contact : ianhansenconsult@gmail.com

www.ianhansenconsult.com

Ian Hansen ExperienceA rotational moulding industry veteran with over 30 years experience in the plastics industry.His background includes :• 15 years in colour extrusion and size reduction of powders for rotational moulding• 15+ years in rotational moulding operationsIan has previously served as President of ARMA (Association of Rotational Moulders Australasia), and on the board of ARMO (Affiliation of Rotational Moulding Organisations). I have given technical presentations to rotomoulding audiences around the world and have written a number of technical articles for Rotoworld.Ian was involved in the preparation and formulation of the Australian tank standard AS/NZS4766, and have promoted the benefits of this globally recognised industry standard around the world.

Now available as an independent consultant to the global rotational moulding industry.

Remember: What can be measured can be controlled