Injection Mould Cavity Surface Roughness Measurement Using Silicon Replicas
Indirect silicone replica metrology resolves deep mould cavity roughness parameters non-destructively within five percent variance of direct steel scans.
Addition Polymerisation
Meaning
Monomer units containing unsaturated carbon-carbon double bonds join together to form a long chain molecule without the loss of any small molecules. Addition polymerisation represents a chemical reaction where initiation, propagation, and termination phases result in high molecular weight resins. Heat and pressure drive the conversion of monomers such as ethylene or styrene into polymers that possess identical elemental compositions to the starting material.
Molecular weight distribution governs the flow characteristics and thermal stability of the resulting plastic.
Reaction Mechanism
Radical initiators often split to provide the reactive site that opens the pi bond within the monomer. These free radicals then attack adjacent molecules to extend the polymer backbone rapidly. Chain growth continues until two radicals combine or disproportionate to stop the process.
Moulders find that the length of these chains determines the viscosity of the melt and the final mechanical strength of the moulded component. Variations in the concentration of the initiator alter the chain length distribution which shifts the melt flow index of the product.
Moulding Economics
Virgin resin properties depend upon the specific conditions held during the reaction stage. Regrind incorporates material that has undergone thermal degradation, and this introduces shorter chains that change the melt behaviour during injection. Cost calculations must account for the loss of physical properties when higher percentages of regrind are mixed with virgin pellets.
Process engineers select materials by comparing the datasheet values of the original resin against the expected deviation caused by heat history and shear.
Processing Variable
Controlling the temperature profile inside the reactor ensures consistency in the degree of polymerisation across different production batches. Precise monitoring of this parameter prevents the formation of low molecular weight oligomers that create surface defects or excessive flash on the finished part. Pressure serves as an additional control to influence the density of polyethylene or polypropylene resins during manufacturing.
Deviation in these settings results in inconsistent shrinkage rates and compromised part dimensions in mass production.