Plastic Blow Molding: A Complete Guide to the Process and Applications

Plastic blow molding is a versatile production method used to manufacture hollow plastic parts and products at high volumes. The blow molding process utilizes air pressure to inflate softened thermoplastic material against a mold cavity to form a variety of plastic containers, bottles, and other hollow components.

Understanding the blow molding process, equipment, tooling, and material options enables effective plastic part design and process optimization for manufacturing. This guide provides a detailed overview of plastic blow molding technology and applications across industries.

How Does Plastic Blow Molding Work?

Plastic blow molding involves three primary steps to produce a hollow part or container from thermoplastic material:

  1. Molten plastic is extruded into a tube-shaped parison between mold halves.
  2. The parison is clamped into a blow mold cavity and sealed.
  3. Compressed air is injected into the parison, inflating it against the mold to form the desired shape.

There are three main blow molding process types:

  • Extrusion blow molding - Continuous extruded parisons are captured and inflated in succession using interchangeable molds. High volume production of bottles and containers.
  • Injection blow molding - The parison is injection molded before blow molding into finished shape. Used for small, precise containers and parts.
  • Stretch blow molding - Parisons are stretched axially and blown radially for orientation and strength. Commonly used to produce PET bottles and jars.

The steps of the blow molding process are as follows:

  1. Plastic pellets are melted and extruded into a hollow tube-shaped parison between two open mold halves.
  2. The mold halves close around the parison and seal against each other, enclosing the parison.
  3. High-pressure air is injected into the parison through a blow pin, inflating it like a balloon against the interior mold cavity.
  4. Once fully inflated and cooled, the mold opens and the hollow molded part is ejected for post-processing.
  5. Excess plastic from the parison sprue is trimmed away and the part is inspected.

Precise process control is critical throughout each phase of blow molding to produce consistent, high-quality parts and optimal cycle times.

Blow Mold Tooling and Equipment

Producing blow molded components requires an integrated system of equipment and tooling:

  • Extrusion equipment - For extrusion blow molding, a continuous extruder and die produce parisons.
  • Mold - Two-part molds with cavity halves that close around parisons. They contain air passages for blowing.
  • Pinch off area - Seals the parison enclosed in the mold before air inflation.
  • Blow pin - Inserts into the parison to inject high-pressure blowing air into the softened plastic.
  • Clamps - Open and close mold halves and provide necessary clamping forces during blowing.
  • Ejectors - Mechanism to remove finished parts from the opened mold.
  • Air compressor - Provides compressed air for parison inflation. Pressures can surpass 400 psi.
  • Temperature controls - Careful heating and cooling of molds and plastic is required for proper shaping and part release.

Blow molding machines integrate the extrusion, mold, and air control systems. More advanced all-electric blow molding machines offer superior precision and energy efficiency.

Plastic Materials for Blow Molding

The most commonly blow molded plastics include:

  • Polyethylene (PE) – The most widely used blow molding material. Low density PE and high density PE offer flexibility. Used for bottles, containers, tanks, and household goods.
  • Polypropylene (PP) – Cost-effective polymer with high strength-to-weight ratio. Suited for containers, industrial parts, automotive components.
  • Polyvinyl chloride (PVC) – Versatile, durable, low cost material used for pipe, conduit, bottles, and packaging.
  • Polyethylene terephthalate (PET) – Stronger engineering thermoplastic used for stretch blow molded drink bottles and food containers.
  • Polycarbonate (PC) – High-performance plastic with superior impact and heat resistance. Used for specialized medical parts and transparent components.
  • Nylon (PA) – Exceptional strength and abrasion resistance. Used for automotive fuel tanks and technical components.

The plastic material is selected based on application requirements like strength, chemical compatibility, temperature capability, clarity, and cost. Material suppliers offer plastic formulations optimized for blow molding.

Key Blow Molding Design Considerations

Effective plastic part design is vital for successful blow molding production. Key design factors include:

  • Uniform wall thickness – Varying wall thickness will lead to defects. Keep walls between 0.025 to 0.188 inches thick depending on material and size.
  • Parison programming – Precisely controlling parison shape and temperature distribution ensures proper inflation.
  • Moldability – Avoid overly complex geometries. Draft angles and radii will improve fill and release.
  • Parting lines – Position parting lines on non-critical surfaces.
  • Handles and threads – These can be incorporated with slides and collapsible cores.
  • Reduction of sinks – Sinks appear as depressions or voids. Changing gate location, wall thickness, and cooling can help.
  • Ejection – Add sufficient draft to walls and use proper mold coatings to enable easy part ejection.

Prototyping and mold flow analysis helps assess moldability before tooling fabrication.

Applications of Blow Molded Plastics

Plastic blow molding produces hollow parts and products across many industries including:

  • Bottles and containers – Shampoo bottles, bleach containers, milk jugs, detergent and soap dispensers. PET and HDPE are commonly used.
  • Toys – Hollow plastic balls, dolls, figures, and water guns are efficiently blow molded.
  • Automotive – Ducts, vent tubes, washer fluid reservoirs, bumpers, and instrument panels.
  • Appliances – Interior liners, bucket spinners, detergent dispensers, covers, and panels.
  • Construction – Plastic conduit, pipe, and outdoor drainage systems like rain gutters.
  • Furniture – Blow molded plastic chairs, tables, and storage units.
  • Packaging – Bulk containers, paint cans, chemical drums and tanks.
  • Electronics – Molded cases, covers, and housings for consumer electronics.
  • Medical – Single-use components like IV bags, medical tubing, and collection containers.

Plastic blow molding will continue growing across industries as products transition from glass and metal to molded plastic designs. With its design flexibility, range of available plastics, and economical high volume production, blow molding enables innovative molded products across diverse markets and applications.

In summary, plastic blow molding is a versatile, scalable plastics molding process suitable for producing hollow parts and products. Understanding the blow molding methods, tooling, equipment, and design considerations allows product designers and manufacturers to effectively utilize this popular manufacturing technology.