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The use of custom blow molding is required to create uniform hollow shapes in plastic parts. Think of it as a balloon within a design that allows a hollow expansion or extension of the sides of a tube or shape.

There are three different processes that can be used to create the balloon or hollow in the part. These include extrusion, injection and injection stretch. Each uses slightly different methods to achieve the desired shape.


With the extrusion process, also known as EBM, the specific plastic is heated to the melting point. It is then forced into a hollow tube that is known as a parison. This is placed into a metal mold that is cooled. At the same time, air is blown into the parison, which then inflates in the mold to the desired shape.

After cooling, the mold is opened and the part is retrieved. The shape of the “balloon” is controlled by the sides of the mold, allowing for completely uniform component production.


Injection custom blow molding is not as commonly used outside of medical components as the other options. As with EBM, the polymer is heated to melting. It is then pushed into a manifold that injects the liquid through nozzles into a cavity that also has a core pin. The mold forms the exterior shape of the part, which is known as the pre-mold. This is cooled and then removed from the mold.

At this time the core rod is used to allow compressed air into the interior of the shape to push that against the cooled exterior. After complete cooling, the core rod is removed and the process is complete.

Injection Stretch

This is similar to the injection process. However the preforms are actually heated and then blown to form the hollow area. At the same time, the core rod stretches the interior to the desired shape. This is typically completed in either a single or a two-stage process. These can be used to create various shapes of bottles and containers, offering more options than other methods.

At MDI we offer custom blow molding to accommodate the needs of our clients. Our experience in utilizing the three different methods allows us to streamline the process for the highest efficiency and to the specification required. We can also complete fabrication on the part or component to avoid having to deal with third-party services.