Dents are caused by "differential shrinkage" ..when the thicker, hot core of the bait cools slower than the skin and as the core cools (and contracts) it pulls from the weakest point in a mold.
You can see the same thing in a lot of plastic parts.
It can be minimized by texturing the mold (thus hiding the imperfection), injecting cooler, and holding pressure.
In hard plastic parts it is controlled in a lot of ways, none of which we are willing to pay for in a bait mold!
I inject Sinko type baits at the coolest temperature I can and still get enough flow to fill the entire mold. Heat your plastic to 350 F so that it completely changes state (or you will get cloudy, stinky, sticky baits) and let it cool to 320 F and see if that helps you. You need to use a good Digital Probe thermometer (like Do-it sells) which are designed to take the temperature of a liquid. It is also great for stirring the plastic to make sure you have all the plastic the same temperature. I also like to hold a slight pressure on the injector for a count of 20 which seems to help.