COMPANY SPOTLIGHT
Quality control for injection molding
In plastics processing injection molding is the most common method of part manufacturing . The variety of molded parts produced by injection molding extends from smallest components like cell phone buttons up to entire body panels of cars . Plastic moldings are manufactured in production batches of up to several million . In addition to dimensional stability , colour and gloss are the decisive quality criteria .
In principle injection molding consists of high pressure injection of the raw material into a mold which shapes the polymer into the desired shape . Initially , plastic material , usually supplied in granular or powder form by the raw material manufacturer , is fed by gravity from a hopper into a heated barrel with a reciprocating screw . As the raw material is moved forward in the heated barrel by the screw , the granules get mixed , homogenise and melt . Following the plasticising process , the hot molten plastic material is injected through the axial feed of the screw under high pressure into the mold . For thermoplastic materials , the melt needs to be cooled in the mold to ensure the necessary dimensional stability . Once the part is sufficiently cool , the mold opens and the part is ejected .
Influence of plasticising
Melt temperature High influence on colour & gloss :
• Higher temperature � darker , lower saturation
Dwell time Medium influence on colour & gloss :
• Longer dwell time � darker , lower saturation
Screw speed Low influence on colour & gloss :
• Parts tend to be brighter
Influence of molding parameters
Mold temperature High influence on gloss :
• Polished mold : Higher temperature � gloss �
• Eroded mold : Higher temperature � gloss �
Injection rate Medium influence on colour :
• Higher rate � amorphous thermoplastics tend to be
Solid Color & Gloss spectro2guide
Sample Holder Small Parts 11 mm
Gloss micro-gloss
Objective Visual Evaluation byko-spectra pro brighter
• Higher rate � Depending on material slightly different impact on colour change High influence on gloss :
• Polished mold : Higher rate � gloss �
• Eroded mold : Higher rate � gloss �
Influence of flow distance
• Amorphous thermoplastics � tend to be darker , brighter
• Semicrystalline thermoplastics � tend to be brighter
Influence of material :
PP / PMMA Very colour stable
Polyamide / ABS Sensitive to colour change ( especially in b-value )
• Higher melt temperature � darker
• Higher melt temperature � increased yellowness
4 FEBRUARY 2021 read online : www . surfaceworld . com