If the plastic product surface yellow when you processing, it should be the injection temperature is too high, plastic product surface turn yellow. The other reason is the plastic recycled material haven’t filter, add to directly have many plastic power particle through decomposition to turned yellow, this plastic power particle is too small, normal process temperature will cause this power particle decomposition, so when you add recycled material must filter to keep the plastic surface white..
If the plastic product put a long time turned yellow, it is exposure under the sun or for a long time inside the high temperature caused it turned yellow.
In generally, plastic product surface yellowed have two reason, ultraviolet often irradiation or temperature too high.