Moulded Polymer Article and Manufacture Thereof

A method of forming a moulded polymer article, the method comprising:

• (a) providing a mould having a first mould part and a second mould part, the first and second mould parts having respective first and second cavity-forming surfaces;
• (b) closing the mould thereby defining a cavity between the first and second cavity-forming surfaces;
• (c) injecting into the cavity a molten plastic composition comprising a polymer and a blowing agent system which is dispersed within the polymer, wherein the blowing agent system comprises a physical blowing agent wherein the physical blowing agent comprises a first gas dissolved in the polymer, and optionally in combination with the physical blowing agent a chemical blowing agent, the chemical blowing agent comprising a chemical precursor which can decompose to form a second gas, the molten plastic composition being injected at an injection pressure,
• wherein during or after the injecting step (c), the injected plastic composition in contact with the first and second cavity-forming surfaces is cooled to form first and second solid skins respectively adjacent to and in contact with the first and second cavity-forming surfaces, whereby in at least one region of the cavity is respectively located at least one portion of the plastic composition in which portion at least some of the plastic composition between the first and second solid skins remains molten;
• (d) opening the mould before the molten plastic composition between the first and second solid skins has solidified in the at least one portion, so as to expose the molten plastic composition of the respective portion to an external pressure lower than the injection pressure thereby allowing the molten plastic composition between the first and second solid skins of the respective portion to expand by foaming to form an expanded cellular foam as a result of the molten plastic composition beneath the first solid skin expanding outwardly away from the second solid skin, wherein the opening step comprises removing the first mould part so that the first solid skin is no longer in contact with the first cavity-forming surface,
• wherein the molten plastic composition between the first and second solid skins of the respective portion expands by foaming to form the expanded cellular foam in a first foam forming phase and in a subsequent second foam forming phase,
• wherein in the first foam forming phase the physical blowing agent comes out of solution in the polymer to form bubbles of the first gas which form first and second layers of the expanded cellular foam, the first layer being adjacent to the first solid skin and the second layer being adjacent to the second solid skin, thereby to increase a separation distance between the first and second solid skins, and
• in the subsequent second foam forming phase the physical blowing agent continues to form the first gas, and if the chemical blowing agent is present in the blowing agent system in combination with the physical blowing agent, the chemical precursor of the chemical blowing agent decomposes to form the second gas, and the first gas, and the optional second gas, form further gas bubbles which form an intermediate layer of the expanded cellular foam, the intermediate layer being between, and adjacent to, the first and second layers, thereby further to increase the separation distance between the first and second solid skins, wherein the intermediate layer comprises cells which have an average aspect ratio between a maximum cell dimension and a minimum cell dimension which is greater than 2:1 and less than 5:1 and the maximum cell dimension of the cells is oriented in a direction extending between the first and second solid skins;
• and
• (e) cooling the expanded cellular foam to cause the molten plastic composition between the first and second solid skins of the respective portion to solidify and to form in the moulded polymer article a monolithic wall part comprising a core layer of the expanded cellular foam disposed between, and integral with, the first and second solid skins, wherein the core layer is multilaminar and comprises the first layer, the intermediate layer and the second layer of the expanded cellular foam, wherein the first and second layers comprise a first cellular microstructure and the intermediate layer comprises a second cellular microstructure.