Some of you know me as one who has been trying to expose people to the design options available to them with blow molding. In workshops, often the most-interesting discussions start with the terms used to describe the blow molding process. Some terms people seem to be unfamiliar with are:
Blow ratio has to do with the polymer stretching over or into a detail.
Part line is where the mold halves come together.
Pinch-off detail is the area I refer to as the ``cookie cutter'' or edge of the part as it is defined in the mold block.
Parison is the cylindrical shape of plastic that is presented to the open mold halves before molding.
Blow pin or blow needle is the apparatus used to inflate the parison inside the mold cavity.
Venting allows the air that is between the mold cavity and the parison wall before the mold closes to escape, thus allowing the plastic to take the shape of the mold.
Tack-off detail or spot welds make blow molded parts strong by touching one wall to the other.
Compression molding is where the two parison walls are compressed to one wall that is about two parison walls' thickness thick.
Why has the blow molding process had such localized and/ or limited success? It certainly seems to have had a lot going for it since the early 1980s when GE Plastics and Borg Warner pushed to use blow molding for more than bottles. They were instrumental in developing a plastic resin that would meet Underwriters Laboratories Inc. flammability and heat requirements.
Most designers and engineers think in terms of a single-wall process. With blow molding, however, the designer/engineer must change that thought process to include planning a double wall.
Most part designers forget to include all the features and functions that are possible with this process. It can, far and away, provide the best opportunity to move air, absorb shock, duct wire, hold liquid, support a load, and still have cosmetic appeal.
Perhaps another reason is the smaller number of blow molders compared to the injection molders. Also, in the early to mid-80's, too many new programs were pushed into this process for a variety of the wrong reasons because of inexperience.
Lack of knowledge of the differences in molding flat panels instead of bottles; the new resin drying requirements; and the need to manage regrind material were some of the pitfalls.
Today, with more education and experience, blow molding is seeing a renaissance. The Society of the Plastics Industry Inc.'s Structural Plastics Division has started a materials and process sharing seminar series with the Industrial Design Society of America.
The next time you design a plastic part or a total product, see if blow molding would fit. I'll bet you will be surprised how and where.
Bank is owner of Papago Plastics Inc. in Rochester, N.Y. He is co-chairing today's Session II conference, ``When Should Big Parts Be Blow Molded?'' to be held 8-11:30 a.m. in Room S402AB.