The antonyms for the phrase "crumple up" are smooth out, flatten, straighten, stretch out, unfold or unroll. These words signify the opposite meaning of crumpling up, which is to create a rough or wrinkled texture on something. Crumpling up means to crush, crease, crinkle or fold something, causing it to look unkempt or disorganized. On the other hand, the antonyms of crumple up suggest an action of pressing or pulling something to create a smooth or uniform surface. For example, when we iron a shirt, we are flattening out the wrinkles and avoiding it from being crumpled up.