Molding of expandable polystyrene
Methods of making molded polystyrene articles are described. The methods include introducing steam into a mold simultaneous to filling the mold with foam particles, and cooling the mold without the use of cooling water. The method can be performed faster and with a lower temperature differential than conventional molding processes, permitting faster turn-around to the production of subsequent molded articles. The molded articles produced by the method can have thicker side walls, improved foam particle fusion, and improved compression properties as compared to conventional EPS molding processes.
1. A method of making a molded foam article, comprising:
(i) introducing foam particles comprising polystyrene into a mold for an article while simultaneously introducing steam into the mold;
(ii) subjecting the mold to direct steaming; and then
(iii) subjecting the mold to a negative pressure cycle, in the absence of cooling water, in which the foam particles are expanded and fused together to form the article; and then
(iv) removing the molded foam article from the mold.
2. The method of claim 1 , wherein step (ii) comprises direct steaming from 10%-50% shorter as compared to a conventional EPS molding process.
3. The method of claim 1 , wherein step (i) further comprises subjecting the mold to slight negative pressure simultaneously with the introduction of the foam particles and the steam.
4. The method of claim 1 , wherein step (i) comprises (a) introducing the foam particles into the mold, and then (b) introducing steam into the mold while venting the mold.
5. The method of claim 1 , wherein:
steam heats the mold to a first temperature, and then
the mold, following filling of the foam particles, is cooled, without the use of cooling water, to a second temperature during the negative pressure cycle, and
the first temperature is from 5 to 25° C. higher than the second temperature.
6. The method of claim 5 , wherein the first temperature is 10° C., 15° C., or 20° C. higher than the second temperature.
7. The method of claim 1 , wherein the molded foam article has a thickness from 0.3 inches (7.62 mm) to 72 inches (1.83 m) in at least one dimension.
8. The method of claim 1 , wherein the molded foam article has a thickness of greater than 3 inches (76.2 mm).
9. The method of claim 1 , wherein the method is effective to yield a molded foam article having a thickness of greater than 3 inches without any machining step.
10. The method of claim 1 , wherein the method is effective to yield a molded foam article having 99.999% lower microbial loading without any additional sterilization step, as compared to a conventional EPS molding process.
11. The method of claim 1 , wherein the method is effective to yield a molded foam article which has a greater flexural strength than a foam article molded using a conventional process, as measured by ASTM C203.
12. The method of claim 1 , wherein the method is effective to yield a molded foam article which has greater compressive resistance than a foam article molded using a conventional EPS molding process, as measured by ASTM C1621.
13. The method of claim 1 , wherein the method is effective to yield a molded foam article which has a ratio of horizontal compressive resistance to vertical compressive resistance of between about 0.95 and 1.05.
14. The method of claim 1 , wherein the method is effective to yield a molded foam article which passes the standard defined by ASTM D6817 without any additional densification step or trimming step.
15. The method of claim 1 , wherein the method is effective to yield a molded foam article which has greater fusion uniformity than a foam article molded using a conventional process.
16. The method of claim 1 , wherein the method is effective to yield a molded foam article with reduced density variation than a foam article molded using a conventional process.
17. The method of claim 1 , wherein the method is effective to yield a molded foam article with a density variation of less than 2%.
18. The method of claim 1 , which has an overall molding time that is at least 10% less than an overall molding time of a conventional expandable polystyrene shape molding process.
19. The method of claim 1 , wherein both the foam particles and the steam are introduced into the mold using one or more fill injectors, wherein both the foam particles and the steam are introduced into at least one fill injector at an upstream location effective to mix the foam particles and steam within the at least one fill injector.
20. The method of claim 1 , wherein the foam particles are pre-treated with steam in a hopper or filling tank before introduction into the mold.
21. The method of claim 1 , wherein steam is introduced into the mold at an angle effective to agitate the foam particles within the mold.
22. The method of claim 1 , wherein the foam particles comprise homopolymers, graft polymers, or copolymers of polystyrene.
23. The method of claim 22 , wherein the foam particles further comprise one or more additives and/or fillers.