Acrylic thermoforming is a widely adopted processing method for manufacturing custom acrylic parts. However, bubble formation is one of the most frequent defects that ruin finished products, raising scrap rates and bringing extra costs for fabricators. Bubbles can appear on the surface or inside the sheet, and they are mainly triggered by raw material conditions, heating parameters and operating errors. Below is a detailed breakdown of the key causes.

1. Moisture trapped inside the acrylic sheet

  Most acrylic sheets absorb moisture from the surrounding air over time.

  Even cast and extruded PMMA materials have tiny molecular gaps that capture water vapour. When the sheet is heated to thermoforming temperature, the water turns into steam. The steam expands rapidly and cannot escape in time, forming visible bubbles inside the material.

  Recycled PMMA sheets usually absorb more moisture than virgin acrylic, so this issue is more common for recycled acrylic products.

2. Improper heating temperature and heating duration

  Temperature control is critical in acrylic thermoforming.

  If the heating temperature is too high, the acrylic resin will degrade locally. Overheating breaks the molecular chain of PMMA and releases small volatile gas, which gathers into bubbles.

  Uneven heating is another typical problem. Some areas reach the softening point much earlier than others. The overheated zones produce gas while the surrounding softened material seals the gas inside. Overly long heating time also increases the risk of gas generation.

3. Surface contamination and unclean raw materials

  Oil stains, dust, release agent residues and adhesive marks on the acrylic surface will decompose under high heat.
  These impurities contain organic components that volatilize when heated. The generated gas stays between the mould and the acrylic sheet, creating bubbles on the product surface.
  If the recycled acrylic feedstock is mixed with other plastic impurities, cross-contamination will also lead to bubble defects during heating.

4. Air entrapment during vacuum forming

  In vacuum thermoforming, mould vent blockage or mismatched vacuum pumping speed stops air from fully discharging between the sheet and mould.
  The trapped air will be pressed and wrapped by the softened acrylic sheet, presenting as bubble-like defects.
  Poor sheet positioning and wrinkles before suction also contribute to air trapping.

Brief suggestions for prevention

  Manufacturers can pre-dry acrylic sheets before heating to remove moisture.
  They also need to calibrate heating equipment for uniform temperature, clean sheets and moulds thoroughly, and check mould vents regularly.
  These simple operations can greatly reduce bubble defects in thermoformed acrylic parts.