Essentially, blow molding involves trapping a hollow tube of thermoplastic material in a mold. Air pressure applied to the inside of the heated tube blows the tube out to take the shape of the mold. There are many variations on the basic technique.
In short, the process is an economical high speed, high-production-rate method of forming thermoplastic parts of hollow shape, or parts that can be simply made from a hollow shape.
Uses include the container and toy field, where bottles and toys of many different shapes are formed in large quantities at low cost. The most commonly used material is polyethylene (PE).
Although any thermoplastic resin can be considered a candidate for blow molding, PE was the first used when blow molding started with low-density PE for blow-molded squeeze bottles. Now, low-, intermediate-, and high-density PE resins are used, as well as special ethylene copolymers designed to provide greatly improved stress cracking resistance compared with PE homopolymers, needed for detergent containers.
One of the main criteria of selection of a PE resin for blow molding is the proper balance of physical properties required for the specific use.
With the extension of blow molding into broader use in industrial products, the need for engineering properties other than those of PE has stimulated interest in other thermoplastics. The main plastics available for blow molding, other than PE, include cellulosics, polyamides (nylons), polyacetals, polycarbonates, polypropylene, and vinyls.
The cellulosic family of plastics includes acetate, butyrate, propionate, and ethyl cellulose. For blow molding, the cellulosics offer strength, stiffness, transparency, and high surface gloss. They have unlimited color possibilities. Chemical resistance and availability of nontoxic resins make them potentially suitable for medicine and food packaging. Their strength, stiffness, and transparency make them suitable for industrial parts, toys, and numerous decorative and novelty items.
Polyamides or nylons, although relatively high cost materials, offer potential benefits in industrial parts and special containers, such as aerosols. Special developments have resulted in formulations tailored to special viscosity requirements for blow molding.
Polyacetal resins for blow molding offer toughness, rigidity, abrasion resistance, high heat distortion temperatures, and excellent resistance to organic solvents. Also, they are resistant to aliphatic and aromatic hydrocarbons, alcohols, ketones, strong detergents, weak organic acids, and to some weak inorganic bases. Aerosol containers are another application for blow molding.
Polycarbonates have found their place in blow-molded industrial parts. Primarily they offer high toughness, strength, and heat resistance. They are transparent with almost unlimited colorability and are self-extinguishing.
Polypropylenes, somewhat similar to higher-density PEs, but with lower specific gravity, higher rigidity, strength and heat resistance, and lower permeability, offer interesting properties at low cost. Because of their lower permeability they are used in containers where PE is unsuited. They also have excellent stress-crack resistance.
PVC (polyvinyl chloride) for blow-molded parts offers benefits in terms of variability of engineering properties. PVCs are available with properties ranging all the way from high rigidity in the unplasticized grades to highly flexible plasticized PVCs. The variability in performance resulting from the many possible formulations means that engineers must consult with the materials supplier in attempting to obtain a formulation with the proper performance and processing characteristics to meet their needs.
The scope of blow-molded design has already broadened beyond that of round hollow objects. Production of such parts as housings by blowing a unit and sawing the item along the parting line to produce two housings is already a reality. As many as ten cavities have been incorporated into a mold, using a wide tube and allowing air to pass through a hollow sprue or runner system between parts.
The future design possibilities of blow molding appear bright. The constant improvements in equipment design continue to add flexibility to the blow-molding process.
Secondary Operations
The equipment used will determine to a great extent the amount of finishing required on the parts. Parts must usually be trimmed and often decorated. Trimming may be by hand, or may involve highly automated trimming, reaming, and cutting.
Decorating of parts depends on the shape of the part and the type of material used. But generally, a variety of techniques is available, including labeling, hot stamping, silk screening, and offset printing.
By far the largest market for blow-molding applications is in the container field, ranging from containers for food, drugs, and cosmetics to household and industrial chemicals. Toys and house wares represent sizable markets.
The industrial product area represents one of the biggest potential uses for the process. Present products include controls for television sets, rollers for lawn mowers, oil dispensers, toilet floats, molds for epoxy potting, and auto ducting for air conditioning.
