What is Hypoallergenic Medical Grade Plastic and will studs that are made out of this material cause infection to the piercing?

Let's have a look at what you're asking.

Hypoallergenic is a catch phrase developed by the cosmetics industry that means actually nothing, it's like new and improved which also means nothing but instills a sense of being better just from the use of the Wordage. It's intended to mean "non-allergenic to the skin" or "it won't cause and infection of the skin".

Now when you say medical grade plastic, one needs to qualify this, there are several types of medical grade plastic in various formulations for various uses. Some designed to go into the dermis (skin) and some intended to just be in contact with the dermis or mucous tissue for an extended time.

Tygon is a medical grade plastic, "I.V. cannulas used to transport medications into the body", however as per the manufacturers instructions it should not be left in place any longer than 72 hours.

BioPlast / Bioflex are a medical grade plastics developed for the body piercing industry they is designed for extended wear with no ill effects to the body.

PTFE Polytetrafluoroethylene, you know it more commonly as Teflon, this is also a medical grade plastic which can be used in body piercing with no ill effects to the body.

There are other materials out there under various names, that are medical grade plastics, however you need to either do your research or trust your research into your piercer and know that he or she has done there homework and knows what they are doing and what they are using.

Knowing what plastics are safe for extended wear in specific locations is critical. So will they cause an infection? No. Infections are caused by pathogen entering the point of incision and setting up camp, this is what causes infections. Poor aftercare or over cleaning or misuse of "aftercare products" will cause irritation and can lead to infections. If you are cleaning the piercing with simple warm water rinse daily after bathing or showering and moving the jewellery to flush out the piercing you should have no issue with any of the properly applied medical plastics.

( the use of the phrase "plastic" is just a generalization for discussion and explanation, these materials listed, some of which are polymers and co-polymers and not actually "plastic")