Blow Molding 101

Blow Pin

After the parison leaves the extrusion head and reaches the required length, the mold closes and the parison is cut off inside the mold. The parison now has to be formed into the final container shape, which is the shape of the cavity in the mold. To do this, compressed air is blown into the parison so that the still molten plastic material expands to fill the mold, and thus take shape. This is what puts the “blow” in extrusion blow molding.

This is typically done via a blow pin, a nozzle or mandrel that enters the parison inside of the mold and blows the compressed air. Typically, this blow pin is located in a station adjacent to the extrusion head, so that the mold is not in the way during the blowing and cooling process. The amount of air and time required during blowing is dependent on many factors, including the volume of the container, the wall thickness, and the type of materials used. Denser materials require greater air pressure in order to properly take the shape of the final container.

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Another application of this technology involves inflating portions of the container disconnected from the main cavity, such as a center handle with sealed edges. This handle will be inflated separately by “needle-blowing”.

Discontinuous Extrusion

Early blow molding had a dilemma; plastic would continue to be extruded even when the mold was closed for blowing. As the industry and technology have evolved, three solutions to this problem have been invented.

The first is an extrusion head that “bobs”, or raises up after the parison has been cut by the mold to ensure there is enough clearance, and then lowers again for the next cycle. The second is mold that moves out from under the nozzle after it cuts the parison, and blows the mold in an adjacent blow station. In these two configurations, the parison is continuously being extruded, so we call it Continuous Extrusion.

The third method involves accumulating the plastic material and ejecting it from the extrusion head in one burst, which we call discontinuous extrusion. The molten plastic is stored in an accumulator head and ejected using a piston, with both the amount of material accumulated and time spent in the accumulator being dependent on the properties of the container and the material.

For example, very large containers require a higher amount of material, and as that material hangs in its parison form, it can begin to stretch, sag, or even break, which would negatively impact the quality of the final container. In these instances, accumulating the material reduces the time it spends in parison form, and thus gives it no opportunity for unwanted stretching. Operations that extrude heavy materials which require greater pressure often use this solution as well.

W. Müller accumulator heads work on a “first-in first-out” principle, where the material that enters the accumulator first is the material that is extruded first.

1. Ensure the parison length/Speed is appropriate for the container's cycle time. If the container is small, you don't Need a Long parison.

2. If the container is large, the parison needs to be longer to fill the mold.

3. The accumulator head will gather all of the plasticized material at once and outputs it with a higher speed than a continous extrusion head. This process limits the time the parison spends in suspension.

Filling

Often times, a bottle is not a product until it is filled. It can be food, drink, cosmetics, toiletries, chemicals, or even medicine. What type of product the bottle will hold has a huge impact on its design and materials!

For example, chemicals must often be bottled in special materials that do not react with the chemical. This allows the chemical to stay pure in the bottle, and prevents hazardous chemicals from being released.

Foods, especially dairy products, also need a special bottle design because they react with light and quickly spoil. Other foods can also last longer thanks to light or oxygen impermeable packaging.

Bottles filled with hot liquids must also be specially designed, as not to deform. This design can often be very different than the designs for bottles that are filled cold, and the materials must be able to withstand the increased temperature.

A thin-walled PET beverage bottle shrinks when you fill it with hot liquids.

Bottle designs can also allow for pleasing aesthetic and eye-catching marketing.

Flash

The flash is the "waste product" of the blow molding process, created by excess plastic being pressed to the edges of the mold. When the two halves of the mold press onto the suspended plastic, the excess plastic is squeezed to the top and bottom of the mold, where it stays during blowing. After the bottle is blown, the flash stays attached to the bottle during the cooling process, and must be removed.

This isn’t really “waste” though! After the flash is removed, it can be reground and introduced back into the extrusion process. This can help provide a cost-effective and environmentally sustainable alternative to the “virgin,” or new material, which comprises parts of the bottle.

Plastic Extruded with an Open Mold
The plastic, pictured in green, is first extruded from the head with the mold open.

Mold Closed and Plastic Blown
Then, when the mold closes around the suspended plastic, the plastic is blown to the edges of the mold.

Bottle with Neck and Bottom Flash
Here you can see the excess plastic at the bottom and neck of the bottle.

Screw

The screw is the main component of an extruder. It transports the plastic, which is poured into the extruder as granules through a hopper. Due to the heat that results from the friction of mixing of the granules, and from the heating bands, which are located around the extruder, the granules are plasticized.

The screw ensures the best possible mixing (homogenization) and transports the plastic compound to the extruder outlet. In some cases, new plastic granules and recycled material are mixed together, so the screw helps to ensure a uniform distribution of materials. If color is added in the form of a masterbatch, the screw ensures that the color is homogeneously mixed with the neutral plastic.