If your clothes have visible impurities on them, then it would be highly desirable to scrap them off before dumping them into the washing machine with other clothes. If, however, you did not do so, they would still be considered as pure once washed in the machine so long as there are no visible traces of impurities.
Machine washing is hardly different from hand-washing. The water runs in the machine in cycles: the entire process has been described thus: “After the machine fills the tub with water, and then stirs the clothes around using an agitator; after some time agitating, the washer drains the water and then spins the clothes to remove most of the water. Then, it refills, and agitates the clothes some more to rinse out the soap. Then it drains and spins again.”
It is clear from the above, that machine washing after all is not at all different from hand washing; hence there is no reason to consider the clothes thus washed as other than pure.
It would save us lot of trouble to remember that the Shari’ah is based on ease and comfort; it saves us from hardship and inconvenience. The Prophet, peace be upon him, has warned us against hair splitting and rigidity in matters of religion when he said, “Whosoever acts rigidly in applying religion will eventually be defeated by it.”