@Faith-Fellowship If you want to parboil you should use Natural Casings as collagen casings generally wont stand up to being boiled. You might also want to think about using a cellulose casing that is inedible, it will give you a skinless product at the end but it is good for cooking.
If you vacuum pack them first and then par boil them (Sous Vide style cooking) then you can use Collagen, Natural or Cellulose.
we par boil store brats, and then grill them to crisp up the outside before we eat them…what casing should i use to make some brats with
One method I have been using lately, in addition to poking holes, is to fire up my smoker at 225 and cooking them (with or without smoke) until they hit an internal temp of around 150-155 (about an hour) and then finishing them on the grill to add some color until they reach 165.