Removal Constraint

The removal constraint measures how much an impression must deform to be extracted from a part.

If the shape has an undercut, a thread, or any narrowing, the impression cannot slide straight out. It must compress or stretch to pass through the smaller opening.

If you choose a product that cannot tolerate the required deformation, the impression will tear or remain stuck inside the part.

When Does a Removal Constraint Exist?

A removal constraint exists whenever the impression must pass through an opening smaller than its widest section.

Common examples:

• Internal threads. The helical grooves trap the impression.
• Grooves and undercuts inside a bore. The widening after the groove creates a lock.
• Stepped bores. The impression formed in the larger diameter must pass through the smaller diameter to exit.
• Blind holes with complex bottoms. The impression wider than the entrance must compress to come out.

If the shape is a simple cylinder with straight walls and no internal features, the removal constraint is zero. Any product can be extracted.

How to Calculate It

The removal constraint is expressed as a percentage. Use the following formula:

Where:

• Dmax = the largest internal dimension of the impression
• Dmin = the smallest opening the impression must pass through to exit

Here is some examples to demonstrate in more visual situations how it works.

Example 1:

A triangle shape hole has a diameter of 10 mm at the top and a 14 mm at the bottom. The impression must exit through the same 10 mm opening it entered.

C = (1 − 10 / 14) × 100 = 29%

The impression must support at least a 29% removal constraint to exit.

Example 2:

An internal thread. Dmax = 12 mm, Dmin = 11 mm.

C = (1 − 11 / 12) × 100 = 9%

The impression must support at least a 9% removal constraint to exit.

Example 3:

A reversed triangle shaped hole with major diameter 14 mm and minor diameter 10 mm, but the minimal diameter exits through the maximal one (unlike example 1). In this case, the maximum diameter that must pass trough the 14mm exit way is itself : 14 mm.

C = (1 − 14 / 14) × 100 = 0%

There is no removal constraint, we can use any product of the range.

Multiple Constraints

If a part has several narrow sections, calculate the constraint for each and use the highest value.

A product must tolerate the worst case, not the average.

Example:

A bore has two restrictions:

• Section A: 16 mm down to 14 mm. C = (1 − 14 / 16) × 100 = 12.5%
• Section B: 15 mm down to 14 mm. C = (1 − 14 / 15) × 100 = 6.7%

The removal constraint to use is 12.5%, the higher of the two.

Product Removal Constraints

Each Plastiform product has a maximum removal constraint it can tolerate. The product is usable only if its limit is strictly greater than your calculated constraint.

Applying the filter:

If your calculated constraint is 12.5%, any product with a max constraint above 12.5% is eligible. From the table above, that includes F30 Max, F40, F20, P25, M25, P35, M60, and F30 Visual.

Products at 10% or below (F50, F65, and all rigid products) are eliminated.

Common Mistake

Do not look at Shore A hardness alone. Two products with the same hardness can have very different removal constraints.

F30 Max and F30 Visual both have 30 Shore A. Both are Flexible. But F30 Max tolerates 40% constraint while F30 Visual tolerates only 20%. For a shape requiring 30% compression, only F30 Max works.

Always check the removal constraint percentage listed on the product technical sheet or label.

You can quickly compare product removal constraint using the recap board sticked inside any Plastiform Case.

Related Pages

• -> Initial Consistency
• -> Final Hardness
• -> Select the Right Plastiform
• -> Plastiform Selector