Thermally expandable sheet and method of manufacturing thermally expandable sheet

A thermally expandable sheet comprising:

• a stress buffer layer provided on one surface of a base material and having an elastic property;
• a thermal expansion layer provided on the stress buffer layer and containing a first thermally expandable material that expands according to heat and a first binder;
• and
• a cover layer provided on the thermal expansion layer and having an elastic property,
• wherein:
• the stress buffer layer, the thermal expansion layer, and the cover layer are stacked in this order on the base material,
• the stress buffer layer contains a second binder and a second thermally expandable material,
• the thermal expansion layer contains the first thermally expandable material at a first ratio to the first binder,
• the stress buffer layer contains the second thermally expandable material at a second ratio to the second binder,
• the second ratio is smaller than the first ratio,
• the cover layer contains a third binder and a third thermally expandable material,
• the cover layer contains the third thermally expandable material at a third ratio to the third binder,
• the third ratio is smaller than the first ratio, and
• the first ratio, the second ratio, and the third ratio are one of a weight ratio and a volume ratio.