Presses used for molding plastic plaques and sheets from pellets typically have heating/cooling platens and a programmable controller.
HunterLab manufacturers electro-optical instruments for the measurement of color and appearance of pellets, sheets and plaques. A molding presses suitable for molding plastic plaques and sheets from pellets would come from a third-party supplier such as:.
Carver, Inc., Wabash, IN 46992 USA +260-563-7577 carverpress.com
Carver is a manufacturer of manual and automatic laboratory presses for molding plastic plaques and sheets. It is advisable to discuss your needs with a technical representative of the company, who can further advise. You could send them a couple of pellet samples to press as examples to make sure it meets your needs.
Typically thermoplastics where plaque or sheet pressing might be used are PET (polyethylene terephthalate), PMMA (polymethyl methacrylate), Polyalkylene terephthalates, Polyamide (PA); Polycarbonate (PC), Acetal plastics, Polypropylene (PP); Reciprocating screw injection molding machines and thermoplastic elastomers (TPE).
Key Points for Color and Haze Measurement
The diameter of the plaque must be greater than the port diameter of the instrument. For the LabScan XE, the standard port diameter is 50 mm which is large for a plaque. To read on a LabScan XE using a smaller port diameter, the SAI (Small Area Illumination) option is needed. For most other models, a minimum plaque diameter of 35 mm is sufficient.
The thickness or path length of a plastic sheet or plaque must be standardized or fixed as a condition of measurement as color, transparency and opacity are dependent on the plaque thickness. For a near-opaque polypropylene plaque, the thicker the plaque, the higher the opacity. For a near clear, transparent polystyrene, the thinner the plaque or sheet, the higher the transmission.
A single reading per plaque is usually sufficient for measurement, particularly if the port is 25-mm or larger. However, a more robust measurement technique is to average 2 readings per plaque (one on each side), with replacement at the port between each reading.
References for Molding Plastic Plaques and Sheets
ASTM D4703 Standard Practice for Compression Molding Thermoplastic Materials into Test Specimens, Plaques, or Sheets
ISO 293 Plastics – Compression Molding Test Specimens of Thermoplastic Materials
FAQ: “Will the Yellowness Index values be the same for pellets and pressed plaque?”
The measured YI value will not be the same as translucent opaque pellets are typically measured in reflectance while transparent plastic sheets or plaques are measured in transmission. Also, as heat tends to degrade plastic color, the molded plaque typically is slightly more yellow than in the pellet or cut granule form.