MBB's are sealed using a vacuum sealer, if you have a look on YouTube there are plenty of video clips demonstrating the process. Heat sealed foil bags are sealed as the name suggests with a heat sealer and/or hair straighteners. Mylar is just a brand name for a certain type of packaging, if you want to know more I suggest you do some research on the net, For those serious interested in the technical details re selecting MBB see info below: BARRIER TECHNOLOGIES There are 3 types of barrier bag technology currently in use:  Foil/Polymer: The oldest and highest barrier technology. A thin sheet of aluminum foil (usually about .00035 thick) is laminated to nylon or Tyvek for support and protection.  Aluminized Polymer: This newer technology reduces material costs. Aluminum is vapour deposited onto polyester. As the metal is so thin multiple layers of aluminized polyester are laminated together this way any voids in a layer are covered by the next.  Engineered Polymer: Clear plastics that do not use metallic layers provide limited moisture protection and are primarily used for food packaging. In the electronics industry their use tend to be very short-term for dry storage and clean room situations. Thes bags do not meet test standard requirements for MIL-B-81705C. MOISTURE BARRIER BAG STRUCTURES Nylon/Foil/Poly Typically, this structure consists of a 60 gauge nylon laminated to 0.00035 aluminum foil, which is laminated to heat sealable polyethylene. This is the most common foil/polymer laminate. MVTR for this structure, when properly converted into bags is very low at about 0.0005 g/100in2/24hrs(1). Tyvek/Foil/Poly A structure consisting of Tyvek, laminated to 0.00035 aluminum foil, which is laminated to heat sealable polyethylene. Tyvek Foil is the oldest barrier structure, however has gradually been replaced by nylon/foil or metallized polyester structures. MVTR for this structure, when properly converted into bags is very low at about 0.0005 g/100in2/24hrs(1). Aluminized Polyester/Poly This is the newest technology for barrier materials with a structure consisting of two layers of 48 gauge-aluminized polyester laminated to sealable polyethylene. These bags are generally lower in cost and have become are a big success with medium to short term dry packaging users. For 3.6 mil materials, MVTR is about 0.02 g/100in2/24hrs*. Structures that are 7.0 mils thick can achieve 0.005 g/100in2/24hrs(1).