Blow Moulding

Blow Molding Process

Process of inflating a hot, hollow, thermoplastic preform or parison inside a closed mold so its shape conforms to that of the mold cavity. A wide variety of hollow parts, including plastic bottles, can be produced from many different plastics using this process.

Main steps:

• parison is formed between mold halves
• mold closes around the parison
• sealing one end of the parison
• closing the parison around a mandrel at the other end
• parison is inflated by air blown through hollow mandrel or needle in side of parison
• cooling and solidification of the part
• mold opening and part ejection

During this process the resin raw material is melted in the machine barrel, forced over a spreader and through the die head into the mold. The mold halves are held on platens which ride on tie rods or tie bars.

Raw Materials
Most commodity grade and engineering grade resins may be blow molded,. but the most common is polyethylene, which is used for food or chemical or detergent bottles. PET or polyester is used for clear beverage bottles such as water bottles or the familiar 2-liter beverage bottles. EVA is a rubber-like material used for blow molded elastomer parts. Generally the list includes: HDPE, PET, Polypropylene, LDPE, PVC, Polycarbonate, ABS, EVOH, LLDPE, TPO, PBT,Nylon, TPE, ABS/PC Blend, Polystyrene, K-Resin®, MDPE, PUR, PETG and PPO. The "melt index", or viscosity, of the plastic must be high to keep the parison from stretching too much prior to mold closure. The resin is in the form of pellets before processing.

Tooling
Machined or cast aluminum is traditional.

Cost
Part prices are generally higher than injection molded parts, but lower than rotationally molded parts. Tooling costs are moderately expensive.

Advantages
This process lends itself to any designs involving hollow shapes. Equipment availability is good in most geographical locations. Can save tooling dollars over injection molding.

Disadvantages
Cycle times are slower than injection molding. Piece prices are higher than injection molding.

Applications
All types of bottles, toys, air ducts for automobiles, chemical & gasoline tanks, household goods.

There are three general types of blow molding: extrusion blow molding, injection blow molding, and stretch blow molding. Extrusion blow molding is usually used to make items of weight greater than 12 oz. such as containers for food, laundry, or waste. Injection blow molding is used to achieve very accurate wall thickness, high-quality neck finish, and to process polymers that cannot be extruded. Usual applications include pharmaceutical, cosmetic, single serving liquor bottles that weighs less than 12 oz. Stretch blow molding is only used for difficult to blow crystalline and crystallizable polymers such as polypropylene and polyethylene terephthalate.