Starting Thickness vs. Wall Thickness in Thermoforming: Why Material Thickness Does Not Equal Wall Thickness

Starting Thickness and Wall Thickness in Thermoforming – Key Facts at a Glance

• The starting thickness is the thickness of the material before thermoforming.
• The wall thickness is the thickness of the material in the finished thermoformed part.
• During thermoforming, the material is stretched. As a result, the wall thickness is usually lower than the starting thickness.
• Wall thickness is not constant. Depending on the geometry, depth, forming direction and process conditions, thinner and thicker areas are created.
• The resulting wall thickness can be estimated using the formula d2 = (F1 / F2) × d1.
• Local variations are possible in practice. Minimum wall thicknesses and safety allowances should therefore be taken into account.

→ Not sure whether your wall thicknesses are suitable for thermoforming? formary’s DfM analysis checks your CAD model for critical wall thicknesses, radii, draft angles and stretch ratios.

What Is the Difference Between Starting Thickness and Wall Thickness?

The starting thickness describes the thickness of the plastic sheet or film before the thermoforming process. At this stage, the thickness is uniform. Wall thickness in plastic thermoforming, by contrast, describes the thickness of the material in the finished thermoformed part. It is created during the forming process and depends on how much the material is stretched in each individual area.

In other words, the starting thickness is the initial value. The wall thickness is the result of the forming process.

Thermoforming vs. Injection Molding: Why Wall Thickness Is Created Differently

In injection molding, wall thickness is a direct design parameter. The material is injected into a closed, two-sided mold. The cavity therefore defines very precisely what wall thickness is produced in each area.

Thermoforming works differently. Here, a sheet with a defined starting thickness is heated and formed. The final wall thickness is not created by a completely closed mold cavity, but by the stretching of the material during the forming process.

Depending on the part geometry, material, temperature, forming speed, mold design and forming direction, the resulting wall thickness can therefore vary considerably. Thermoformed parts should consequently not be designed around exact wall thicknesses, but around functional minimum wall thicknesses with appropriate tolerances.

Why Does Wall Thickness Decrease During Thermoforming?

The physical principle is simple: material cannot appear from nowhere. When a sheet or film is drawn into depth, the available material must be distributed over a larger surface area. This reduces the wall thickness.

In shallow areas, the wall thickness often remains closer to the starting thickness. In deep areas, tight radii or heavily drawn sections, the material is stretched more and therefore becomes thinner.

Where Do Thin Spots Occur During Thermoforming?

Thin spots occur where the material is stretched particularly strongly. Their location depends heavily on whether the thermoformed part is produced using male or female forming.

Thin Spots in Female-Formed Parts

In female forming, the material is drawn into a mold cavity. Material is stretched from the edge toward the bottom of the cavity. The weakest area is therefore often located at the bottom or in the transitions between the bottom and the side wall.

The deeper the cavity and the less favorable the forming ratio, the more strongly the material thins in these areas.

Thin Spots in Male-Formed Parts

In male forming, the material is drawn over a raised mold core. The material first contacts the male mold in what will later become the bottom area. The edge area is then drawn farther downward and is therefore thinned more strongly.

In male-formed parts, the weakest area is consequently often located closer to the edge or along the side walls.

Why the Forming Direction Is Crucial

The decision to use male or female forming should not be based solely on visual or dimensional considerations. The desired wall thickness distribution also plays an important role.

If certain areas of the part are subject to mechanical loads or must maintain a minimum wall thickness, the choice of forming direction can be decisive. Depending on the wall thickness requirements in specific areas, it must therefore be assessed whether male or female forming is more suitable.

Forming Ratio, Draw Ratio and Area Ratio Explained Simply

In thermoforming, different parameters describe how strongly the material is formed. The forming ratio, draw ratio and area ratio are particularly relevant to the resulting wall thickness.

Forming Ratio

The forming ratio describes the relationship between the height and width of the area to be formed. It therefore indicates how far a limited area of material must be drawn into depth. The greater the depth in relation to the opening width, the more the material has to stretch. This reduces the resulting wall thickness.

The following are particularly critical:

• deep, narrow geometries
• steep side walls
• tight radii
• large differences in depth within a part
• starting thicknesses that are too low

Draw Ratio

The draw ratio describes how strongly the material is stretched during the forming process. A high draw ratio means that the material is drawn deeply and is therefore stretched more strongly.

Area Ratio

The area ratio describes the relationship between the area of the material blank and the surface area of the finished thermoformed part. The larger the surface area of the finished part compared with the original material area, the more strongly the material is stretched.

How Do You Calculate Wall Thickness in Thermoforming?

The resulting wall thickness can be estimated using the following formula:

d2 = (F1 / F2) × d1

Where:

• F1: area of the material blank excluding the clamping edge, i.e. length × width
• F2: surface area of the thermoformed part after forming
• d1: starting thickness of the sheet material
• d2: estimated resulting wall thickness

Example: If the surface area of the finished thermoformed part is approximately three times the usable original area, the material is distributed over three times the area. With a starting thickness of 3 mm, this results in a calculated average wall thickness of approximately 1 mm.

Important: This formula only provides an approximate value. The actual wall thickness is not uniform throughout the part. The wall area may exhibit variations between the thinnest and thickest points compared with the estimated d2 value. The design should therefore not target an exactly constant wall thickness, but a reliable minimum wall thickness in the most critical area.

Why Can Wall Thickness Not Be Predicted with 100% Accuracy?

Even when calculations, empirical values and pre-stretching are used, a degree of uncertainty remains. Thermoplastics behave viscoelastically. This means that deformation depends on temperature, time, speed and the material’s behavior.

The resulting wall thickness is influenced by factors including:

• Material tolerances: The starting thickness of sheets or films is subject to manufacturing tolerances.
• Temperature distribution: Different areas of the heated material may have slightly different temperatures.
• Heating time: Heating times that are too short can cause uneven forming.
• Forming speed: A high forming speed can cause greater local stretching.
• Forming direction: Male and female forming produce thin spots in different areas.
• Mold temperature: Cold areas of the mold cool the material more quickly and influence material distribution.
• Material type: Not every thermoplastic can be stretched to the same extent or with the same uniformity.

As a general rule: The colder the sheet material is during stretching, the greater the force required for forming. The faster the forming speed, the greater the required force as well. If the material is stretched too strongly or too unevenly, thin spots, tears or structurally unstable areas may occur.

How Can Differences in Wall Thickness Be Reduced?

A completely uniform wall thickness is not realistic in plastic thermoforming. However, designers can positively influence the wall thickness distribution.

Important measures include:

• Use generous radii: Tight radii concentrate stretching and increase the risk of thin spots.
• Avoid extreme draw ratios: Deep, narrow areas are particularly critical.
• Prefer uniform depths: Large differences in depth make uniform material distribution more difficult.
• Include draft angles: Draft angles of approximately 2–5° make demolding easier and reduce stress.
• Choose the forming direction deliberately: Male and female forming result in different wall thickness distributions.
• Do not select the starting thickness too tightly: A starting thickness that is too low increases the risk of critical thin spots.
• Test prototypes: Samples reveal the actual wall thickness distribution and allow adjustments before series production.

Pre-Stretching with a Plug Assist: Influencing Wall Thickness Distribution

For demanding geometries, a plug assist can help improve material distribution. During pre-stretching, an additional plug preforms the heated material before it is drawn completely over the mold or into the cavity.

The objective: The material is distributed more selectively. This can prevent excessive material from remaining in non-critical areas while critical areas become too thin.

A plug assist is particularly useful for:

• deep cavities
• high draw ratios
• female molds
• demanding geometries
• parts with defined minimum wall thicknesses

Nevertheless: Pre-stretching does not replace a design suitable for thermoforming. If the draw ratio is too high for the material, starting thickness and geometry, thin spots or tears may still occur even when a plug assist is used.

What Starting Thickness Is Appropriate?

An appropriate starting thickness depends on more than the desired wall thickness. The main factors are the part’s function, handling, application, material type, geometry and cost sensitivity.

Important design questions include:

• Does the thermoformed part need to protect, support, guide, cover or merely position something?
• Will the part be moved manually, gripped automatically, stacked or transported?
• Are there mechanical, thermal, chemical or hygiene requirements?
• Are there critical functional surfaces that require a minimum wall thickness?
• How deep is the part?
• What radii, side walls and contours are planned?
• Which material is to be used?

Common Mistake: Selecting a Starting Thickness That Is Too Low

A common mistake in practice is calculating the starting thickness too tightly in order to save material costs. As a result, critical areas become too thin and the part either fails mechanically or cannot withstand the required loads.

Consequences:

• cracks or holes during the thermoforming process
• parts breaking during handling or use
• rework or mold modifications being required
• project delays and higher overall costs

A better approach is to select the starting thickness with a safety allowance, manufacture prototypes and measure the actual wall thicknesses. The additional cost of using slightly thicker starting material is often lower than the cost of scrap, mold modifications or failed production runs.

Conclusion: Starting Thickness Is the Initial Value, Wall Thickness Is the Result

The starting thickness of the material is not the same as the wall thickness of the finished thermoformed part. During plastic thermoforming, the material is stretched. This reduces the wall thickness, which varies depending on the geometry, forming direction, material and process conditions.

The forming ratio, draw ratio and area ratio help estimate the subsequent wall thickness more accurately. The formula d2 = (F1 / F2) × d1 provides an approximate value. In practice, however, local variations must be expected.

Anyone designing thermoformed parts should therefore define minimum wall thicknesses, include safety allowances and have feasibility assessed before mold production.

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