Publish Time: 2026-03-07 Origin: Site
Thermoforming packaging is one of the most important forming processes in the food packaging industry, and its technological level directly determines the performance ceiling of packaging machinery. This section will provide an in-depth analysis from three dimensions: process principles, key parameters, and materials science, to facilitate a better understanding of thermoforming packaging.
Thermoforming is a process in which a thermoplastic sheet is heated to a softening temperature and then shaped by external force (vacuum, air pressure, or mechanical force) to conform to the surface of a mold. According to the definition in 《Food and Beverage Packaging Technology 》(Wiley, 2011), the thermoforming process is essentially a three-dimensional deformation process of a polymer in a viscoelastic state.
The Essence of the Process: Thermoforming packaging is the process of transforming two-dimensional sheets into three-dimensional containers, involving the thermo-mechanical coupling behavior of polymers. Successful thermoforming requires precise control of the temperature field near the material's softening point, ensuring sufficient ductility while maintaining necessary structural strength, and avoiding perforation or sagging caused by overheating.
The three stages of thermoforming packaging：Heating→ Forming → Cooling & Demolding
Stages | Process Description | Key control points | Typical parameters |
Heating | The sheet is heated above its glass transition temperature (Tg) to enter a highly elastic or viscous flow state. | Temperature uniformity Heating time Energy density distribution | PP:150-170℃ PS:130-150℃ PET:140-160℃ PE:115-135℃ |
Forming | The sheet is softened and bonded to the mold surface using vacuum/pneumatic pressure/mechanical force. | Molding pressure Pre-stretching degree Die temperature | Vacuum：-0.8~0.95 bar Pressure：2-6 bar Molding temperature：20-60℃ |
Cooling & Demolding | The sheet is cooled below its Tg to solidify and set its shape, then demolded. | Cooling rate Demolding temperature Demolding force | Cooling rate：05-3S Demolding temperature：50-80℃ |
Depending on the source of the forming force, thermoforming can be divided into the following main methods:
Forming method | principle | Forming force | Applicable Scenarios | Advantages and disadvantages |
Vacuum Forming | Air is removed between the mold and the sheet, and forming is achieved using atmospheric pressure difference. | ~1 bar | Shallow drawing, simple shapes | Simple equipment: uneven wall thickness in deep drawing |
Pressure Forming | Positive pressure air pushes the sheet from above to fit the mold. | 2~6 bar | Medium-deep drawing, fine features | High forming precision: higher equipment cost |
Plug Assist | The sheet is pre-stretched by a mechanical plug, then vacuum/pneumatic forming is applied. | Mechanical+vacuum, pressure | Deep drawing (depth-to-width ratio > 0.5) | Controllable wall thickness distribution: high process complexity |
Matched Mold | The upper and lower molds close, and mechanical pressure directly forms the shape. | High-pressure mechanical force | High precision, complex shapes | Highest precision: high mold cost |
Wall thickness uniformity is the most critical quality indicator in thermoforming. According to research published in the journal *Polymer Engineering & Science*, the wall thickness at the bottom corners of deeply drawn containers (such as yogurt cups and meat trays) can be 50-70% thinner than the original sheet, a major cause of packaging failure (damage, reduced barrier properties).
【Core Challenge】During thermoforming, the amount of deformation varies greatly at different locations when the material is stretched from a two-dimensional sheet to a three-dimensional container. The bottom corners of the tray experience the greatest biaxial stretching, with wall thickness reduced to 30-50% of the original thickness. Meanwhile, the flange edges show almost no deformation, maintaining their original thickness.
Factor Category | Specific parameters | Influence on wall thickness distribution | Optimization direction |
Temperature factor | Material heating temperature | Higher temperatures result in better material flowability, but overheating can lead to sagging and perforation. | Precisely control the temperature to 15-30°C above Tg. |
Temperature uniformity | Localized overheating areas deform first, leading to uneven wall thickness. | Multi-zone temperature control, zoned heating | |
Mold Factors | mold temperature | Low-temperature molding allows for rapid solidification of the contact area, restricting material flow. | Mold temperature 60-100°C delays curing |
Mold | The greater the depth-to-width ratio, the thinner the bottom wall thickness. | Optimize the circular U-shape design (R≥3mm) | |
Containment factors | plug temperature | Cold-plug (25°C) clamping material results in a thick bottom and thin sidewalls; hot-plug (100°C+) allows for sliding. | Select the appropriate sealing temperature strategy based on the product shape. |
plug shape | Flat-bottomed plugs retain material at the bottom; round-bottomed plugs promote material flow towards the sidewalls. | Custom plug shape matching products | |
Artistic Factors | Plug speed/vacuum delay | The speed is 0.15 - 0.27 m/s, and the vacuum delay is 0 - 0.3 seconds, which affects the pre-distribution of the material. | Optimize process timing |
Problem area：Corner
Original thickness 300μm -- after molding, it may only be 90-120μm (thinning by 60-70%).
Problem | reason | Traditional solutions | Advanced Solution (Matrix Heating) |
The bottom corner is too thin. | The maximum biaxial tensile zone | Increase the thickness of the original substrate (cost ↑) | Lower the heating temperature in the bottom region to reduce flow. |
Bottom is too thick. | The cold plug is clamped, preventing the material from flowing. | Use heat plugs or lubricating plugs | Increase the heating temperature at the bottom center to promote flow. |
Side wall not All | Heavy weight causes body parts to sag. | Reduce heating time | Zoned temperature control to compensate for gravity effect |
Traditional thermoforming uses a uniform temperature to heat the entire material, which is the root cause of uneven wall thickness. The cera2heat matrix heating technology developed by the German company Watttron uses 5×5mm independent, controllable heating pixels to set different temperatures for different areas, fundamentally solving the problem of uneven wall thickness distribution.
Thermoforming films are a key factor determining packaging performance. Modern food packaging commonly employs multi-layer co-extrusion structures to achieve the optimal combination of mechanical, barrier, and heat-sealing properties.
Layer | Material | Function | Typical thickness percentage |
Outer Layer | PA (Nylon) | Puncture resistant, abrasion resistant, good thermoformability | 15-20% |
Adhesive Layer | Tie (Modified Polyolefin) | Bonds dissimilar materials | 5% |
Barrier Layer | EVOH | Oxygen barrier (core functional layer) | 5-10% |
Adhesive Layer | Tie | Bonds dissimilar materials | 5% |
Inner Layer | PE/PP | Heat-sealing, food contact safety | 55-70% |
【Key Finding】 Thermoforming significantly increases the oxygen permeability (OTR) of thin films. According to a 2014 study by Buntinx et al. published in the journal *Polymers*, the OTR of multilayer films containing EVOH barrier layers was 0.48-1.7 cc/m²·day·atm before thermoforming, but after thermoforming, due to the reduced wall thickness, the OTR may increase by 2-3 times. The wall thickness of the barrier layer is the key factor determining the barrier performance.
Membrane structure | Original thickness | OTR(Before molding) | OTR(Deep drawing zone after molding) | Change factor |
PA/PE | 166-293μm | 21-26 cc/m²·day·atm | 40-60 cc/m²·day·atm | ~2× |
PA/EVOH/PE | 150-250μm | 0.48-1.7 cc/m²·day·atm | 1.0-3.5 cc/m²·day·atm | ~2× |
PE/EVOH/PE | 180-220μm | 0.5-1.5 cc/m²·day·atm | 1.2-3.0 cc/m²·day·atm | ~2× |
The barrier properties of thermoformed containers are determined by the thinnest part (usually the bottom). The design must ensure that the EVOH layer thickness after molding still meets the product's shelf-life requirements.
This can be achieved by:
(1) increasing the initial film thickness;
(2) optimizing molding parameters to reduce excessive stretching;
(3) using matrix heating technology to improve wall thickness distribution.