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Plastic Blow Molding

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Blow Molding Basics
Using compressed air to form glass and plastic containers
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Plastic Blow Molding
Simple container forming from heated plastic

To blow a plastic container, take a thermoplastic tube, soften it evenly in the temperature range of its plastic, pinch one end shut inside a split metal mould and inflate it with low-pressure air until it presses against the mould wall. Stretching also thins it: a 25 mm tube blown out to an 80 mm bottle turns a 3 mm wall into roughly 0.94 mm, so start with a thicker tube than the finished wall.

Part of Pneumatic Systems

Forming simple containers from heated thermoplastic tube by inflating it inside a mould with compressed air.

Which Plastics and What Tube

Only thermoplastics, which soften when heated and harden when cooled, can be blown. The plastics article lists polyethylene (PE) melting at 120-180 °C and polypropylene (PP) at 160-220 °C, both named for containers, and a die with a centre pin to extrude tube (Plastics Manufacturing). Use PE or PP. Do not use PVC, which gives off hydrochloric acid gas when burned, per the same article.

The window between “too stiff to inflate” and “too runny to hold shape” is narrow and depends on the plastic and wall; the corpus gives no figure, so find it on offcuts. Start at the low end of the melting range, bring the tube to temperature evenly, and watch how it sags.

What You Need

  • Plastic tube, extruded with a pin die or cut from salvaged pipe of a known plastic (use the float and burn tests in the plastics article)
  • A split mould, metal if possible; wood chars on hot plastic. Cut small vent holes at the corners where air gets trapped.
  • A heater that warms the tube evenly, such as a hot air stream or a radiant heater held at a distance. Boiling water, at 100 °C, is below the lowest melting figure given above.
  • A blowing head to seal the open end of the tube and feed air, with a regulator and a relief valve.
  • A clamp or toggles to hold the mould shut; see the force arithmetic in Blow Molding Basics.

Method: Blow a Bottle

Step 1: Heat the tube Warm the tube slowly and evenly until it turns soft and limp but still holds its form. Cold spots will not stretch and hot spots will thin and burst.

Step 2: Pinch and seal the bottom Close the mould around the tube so the mould edge pinches one end shut, trimming the excess later.

Step 3: Inflate gently Open the regulator from a low setting. The tube should swell evenly and press against the mould. If it bulges at one spot, the tube was unevenly heated.

Step 4: Cool and release Hold the pressure while the plastic cools against the mould until it is stiff. Vent the air, open the mould and trim the pinch seam.

Worked Example: The Wall Thins as the Tube Stretches

If a tube is stretched radially and its length stays about the same, the volume of plastic is constant, so wall thickness falls in proportion to the diameter rise: t_final = t_start x d_start / d_final.

  1. Tube 25 mm diameter, wall 3 mm; bottle 80 mm diameter
  2. t_final = 3 x 25 / 80 = 0.94 mm

The wall is only a third of what you began with, and a bottle’s shoulder and base stretch further and thin more. This is an approximation; it ignores stretching along the length. Check the finished walls by cutting a sample.

Common Mistakes

  • Using unknown plastic. Different plastics need different temperatures; sort by float and burn tests first.
  • A mould that is too cold, which freezes the plastic before it fills, or too hot, which lets it sag.
  • No vents. Air trapped between plastic and mould leaves dimples or stops the plastic reaching the corners.
  • Assuming the bottle is food-safe. The corpus does not establish that salvaged plastic is safe for food; do not use it for food or drink unless its origin is known to be food grade.

Safety

Hot plastic burns and its fumes are harmful

Softened plastic sticks to skin and burns deeply, and a burst blank sprays it. Wear gloves, eye and face protection, work with strong ventilation, and keep the plastic below the temperature at which it smokes. Never heat PVC. Run the air through a regulator and a relief valve and vent the line before opening the mould.

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