Blow mold
Blow molding (also known as extrusion blow molding, Extrusion Blow Molding (EBM)) is a manufacturing process used to produce plastic bottles (HDPE bottles, shampoo bottles, shower gel,…), boxes containers and custom shapes.
Specifically :
- There are 2 types of extrusion blow method. Including continuous extrusion blow molding and intermittent extrusion blow molding, in which:
- Continuous blow molding: Plastic is extruded continuously while the machine runs. This method can only be used with materials that have good melt strength and can only be used for short cycle times..
- Intermittent extrusion: During a specified period of time the plastic is extruded and fills the tank. Once the tank has been filled, a pusher is activated and pushes material from the tank through the extruder.
- When the mold opens, the reservoir pushes the material out of the mold. This method is used for parts with long cycles and materials with low melt strength.
- The blow molding process begins with the formation of a hollow plastic tube, called a duct. The duct is extruded down until it extends across the bottom of the die. The mold then closes on PARISON. Air is introduced into the mold through a blow pin at the top of the part or a needle in the middle of the part, inflating the plastic and pushing it against the mold sides. After that, the product is cooled in the mold. Finally, the product is pushed out of the mold by a pusher or robot arm.
Material:
- Materials with high melt strength are needed in extrusion blow molding to reduce the amount of Sag (and excessive thinning) in the Parison..
- Olefin (Polyethylene, Polypropylen)
- Thermoplastic elastomers (TPEs)
- Polycarbonate Poly vinyl clorua (PVC)
HDPE
- Main parameters that need to be processed during the blow molding process:
- Melting temperature of the material – Changes the viscosity of the plastic granules.
- Extruder Speed – Determines the speed required to produce the product.
- Blow Time – Although blow molding forms the product, keeping pressure on the product keeps the product in contact with the mold and increases the cooling process.
- Blowing pressure – Helps to get surface details in the mold.
- Parison release time – The tank hangs and how long it lasts before molding.
- Parison Programming – Create local thickness changes in Parison.
- If cut off-center, it will cause thickness variation around the circumference of the part.