Colour matching and colour theory

Colour matchingColour matching andand

colour theorycolour theoryAn introduction to ComputerAn introduction to ComputerAided Colour ManagementFor the Injection Moulder

Siddhartha Roy 1

•Special thanks to Mr. V. C. Gupte for p pelaborating and educating us on ‘Colour Measurement’ and ‘Colour Management’.g

• A major part of this presentation covers the concepts and theories taught by him to ourconcepts and theories taught by him to our team at V.I.P. Industries.

• I would like to record my appreciation to his• I would like to record my appreciation to his contribution.

Importance of Colours inImportance of Colours in Mouldings

• Aesthetics are as important as Physical properties in mouldings.Th bilit t i t ll l Pl ti M ld d• The ability to integrally colour Plastic Moulded articles is an important edge over other materials like metals and even Rubber.

• Matching of colours, especially across assembled mouldings is crucial in many applications.

• The task becomes more difficult when colours need to be exactly matched with different Polymers.

• Understanding Colour theory and its applications• Understanding Colour theory and its applications has become a must for the Injection Moulder. 3

Trends in Colouring• More and more Auto Interiors, Appliances

and other Plastic assemblies will be requiring exact colour matched components

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Components must have the same colour even whenwhen

• Moulded at different Vendors/ Locations• Moulded from different Plastics• Moulded with addition of regrind.• Moulded from different lots of Resin/ Masterbatch

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Perception of colour

• The Light source

•The objectj

Th h•The human eye

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The Human Eye and Brain

• Light receptors– Rods - Perceives light and darknessRods Perceives light and darkness– Cones-Perceive Colours

• Rods• Rods– The black & white world. (Night vision)

C• Cones– Red, Green & Blue sensitive receptors

• The optic nerves transmits stimuli to the brain which interprets colour. 7

Optical Overload

• There is a mismatch between the large number of rods and cones in the retina and the transmitters to the brain.

• This results in false perception of colour i ll h h i d b i hespecially when the eye is exposed to bright

images for a length of time.I th t lid t t th t l hit• In the next slide concentrate on the central white dot for 30 seconds.Ob h t h h th t• Observe what happens when the screen turns white.

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Even Black and white images can give strange resultsgive strange results

Optical Illusions

• The optical illusions can be resolved byThe optical illusions can be resolved by actual measurements by simple instruments like a measuring scale, Compass etc.like a measuring scale, Compass etc.

Need for Quantifying Colours• Like in the previous examples we need aLike in the previous examples we need a

system for measuring and quantifying colourscolours..

• Subjectivity in Colour perceptions is commoncommon– All brains do not process Colour information

identicallyidentically.– Prime example is Colour blindness- Inability to

distinguish between Red and Greendistinguish between Red and Green.15

Colour Theory

• A Standardised scientific method is required to record Colour Information.

• Two of the major Colour quantification systems aresystems are

–Munsell Colour Theory–The CIE Theory

The Munsell Colour Theory

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Definitions

• Hue- An attribute by which a sample appears to be similar to one or a mixture of two of the perceived colours –red, yellow, green and blue.

• Chroma- Is the colourfulness of a given illuminance level.

• Value- The lightness or darkness, i.e. the attribute by which a sample appears to

fl t t ll f ti f threflect a greater or smaller fraction of the incident light.

The CIE TheoryThe CIE TheoryStandard Illuminants

RedStandard Observer

MagentaY ll

Green Blue

MagentaYellow

White

Green BlueCyan

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CIE Colour Specifications:• XYZ Tristimulus Values• L a b C h• L a b C h

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Computerised ColourComputerised Colour Measurement

• Spectrophotometers are very effective in measuring and recording Colours

• The CIE System is commonly used • Software is crucial in recording analysing• Software is crucial in recording, analysing,

comparing and matching colour samples.• Samples should have the same surface finish• Samples should have the same surface finish

and shape as far as possible.h S h l f• The Spectrophotometer scans samples for

comparison and the Software takes over. 21

Colour DifferencedE (L bCh f l ) (L bCh f St d d)• dE = (LabCh of sample)-(LabCh of Standard)

– dL- When positive -- lighter negative-- darkerdL- When positive -- lighter, negative-- darker.

– da- When positive-- redder, negative-- greener

– db- When positive-- yellower, negative-- bluer

– dC-When positive-- brighter, negative-- duller

dh di d h dd– dh- Indicated hue address22

Colour difference-Colour on ScreenColour on Screen

Batch 1

Standard

Batch 2

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Colour Difference-L a b PlotL a b Plot

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Colour Strength

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Pass - Fail

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Formulation

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Batch Correction

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Recap

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Pigment Database• The effectiveness of the Spectrophotometer• The effectiveness of the Spectrophotometer

– Colour Matching software is very dependant on the Pigment Database loadeddependant on the Pigment Database loaded.

• The LabCh values for each pigment being d h b i ki l dused has to be painstakingly generated

– In full strength and white and black reduction.– On the different Polymer bases (PO, Styrenic,

PVC etc.)– Some Pigment manufacturers provide such

data. 30

Important Aspects for ColouringImportant Aspects for Colouring of Plastics

• Pigments should be thoroughly dispersed– Very difficult with powder pigments– Much easier with Masterbatches

• Colour of base plastic– Colourability of different plastics vary widely– Masterbatch selection should be appropriate to base

plasticplastic• Polyolefin base• Styrenic base• PVC base• Universal base 31

Major Causes of ColourMajor Causes of Colour Variation

• Weighing and mixing of colourants.– Cleanliness of Blending Equipment– Adequate mixing time

• Interference from regrind. Contamination.• Degradation during processing.

– Machine stoppages and inadequate purging.

• Improper selection of Colourant/ Masterbatch.• Interference with processing additives

– E.G. With PVC chrome pigments containing lead will discolour if Tin Stabilisers are present. 32

How to Face the Challenge?• Accent on cleanliness

– Poor housekeeping = colour contamination• Selecting correct Colourant

– Clear understanding of colour behaviour in Polymer involved. Masterbatch supplier plays crucial role.

• Use of Precoloured Materials (esp ABS PC/ABS)• Use of Precoloured Materials (esp ABS, PC/ABS)– Best for replicating same colours at different locations– Cost is more– Inventory and logistics issues

• Use of Pigment colouring for small quantities h M t b t h d l t i t f iblwhere Masterbatch development is not feasible

• Making use of Colour matching facilities. 33

For more Info on Colour Theory:

• http://www.premiercolorscan.com/color_theory/color theory.htm_ y

• http://www hunterlab com/ColorEducation/ColorTh• http://www.hunterlab.com/ColorEducation/ColorTheory

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35Email: royplastech@rediffmail.com Mobile: 9890366632