In the extrusion blow molding (EBM) process a hollow tube of molten material, called a parison, is extruded out of a head and through a male and female part, which are commonly referred to as pin and bushing or mandrel and die. A number of factors must come together to let this parison inflate into a part of even wall thickness in both the axial and hoop direction:

  1. The melt coming from the extrusion screw must be homogeneous
  2. The heat in the head must be even
  3. The parison must be straight and have even wall thickness
  4. The parison wall thickness must be programmed to reflect the container geometry
  5. All parisons in a multi-cavity system must have the same length
  6. If the container is oblong the parison wall thickness should reflect that
  7. Venting must be sufficient to avoid any air entrapment

To 1.: EBM extruders should have a L/D ratio of 24:1, 22:1 for heat-sensitive materials like PVC. EBM materials have a very low MI compared to injection molding and generally need longer residence time to properly melt. Barrier screws with or without end mixers have become very popular and are doing a fine job. Parisons are easier to control when they run at the lower end of the temperature range and modern screws allow this to happen.

To 2.: If the parison has a warmer and colder side, the warmer side will always blow into a thinner wall. Even parison heat is therefore paramount. Heater bands should not be aligned along the head. This looks pretty but leaves a slightly cooler area always in the same parison part. All thermocouples must have good contact with the metal of the head, distributor, and etruder.

To 3: While the pin is always stationary, the die is moveable and must be adjusted so the parison runs straight. If it ‘hooks’ to one side the parison is thin on the side it is hooking to and the die must be moved in that direction. Ideally, a straight running parison guarantees even parison wall thickness. However, because of slight temperature and pressure differences in the head the parison may run straight with uneven wall thickness. This can be tested by squashing the end of the parison as it comes out of the head and letting head support inflate it. The warmer and/or thin side will inflate more and this is easily observed by the development of the inflating bubble. In this case a compromise must be struck between a straight and even parison if there is no time to investigate the reason for this discrepancy.

Manually closing the extruding parison allows observing wall thickness and/or temperature variations.
Manually closing the extruding parison allows observing wall thickness and/or temperature variations.

To 4:  Practically all EBM machines now use a wall thickness controller or programmer that changes the die gap during extrusion to reflect changing container geometry and/or counters the propensity of the parison to sag under its own weight. On shuttle EBM machines it can be a challenge to correlate particular points on the container with the parison programming points. This is because there is a distance between the top of the parison and the die bottom depending on the distance between head and knife. Operators should use the ‘marking’ feature of the programmer that increases or decreases wall thickness dramatically for suspected points and lets them see the result in the container. This helps greatly in optimizing overall wall thickness.

To 5: When there is more than one cavity parisons must have the same length or else the programmed parison points do not end up in the same spot on the container. Parison length can be adjusted with either temperature or chokes in the distributor between extruder and head. The former method must be used with materials like PVC that do not allow for chokes but otherwise is not recommended as it introduces variation in parison temperature that will affect shrinkage.

To 6: Oblong containers will have thinner walls in the far container sides as the parison thins out during inflation. To counter this parison wall thickness can be increased in those areas that will form these far sides. This is mostly done by cutting pockets into the die after the parison has been marked to find the proper parison locations. Because of weight swell up to 200% these cuts must be done in increments as they may have a large effect on the container. Often two or three iterations are necessary to get it just right. Companies that sell die sets often have experience for common bottles that allows them to get it right the first time.

To 7: The mold halves contain air when they are closing and this air must have a way out. Otherwise, entrapped air prevents proper distribution and cooling of the plastic. All molds have a surface texture that is tailored to the material being molded. For polyolefins molds are sand-blasted for example. The texture allows the air to move behind the inflating parison towards the mold faces where vents with a depth of approximately 0.05 mm (0.002”) allow escape. Special geometries like handles may also require hole or corner vents that are increasingly cut with lasers instead of drill bits.

As you can see, many parameters have to come together to yield perfect containers. Operators need training and experience to master them.