Shrink Space Size: How to Calculate It

This calculator works out the size of the shrink space (also called head shrink or shrinkage) for a plastic injection molding part. You enter the mold shrinkage rate (as a percentage, from the material datasheet) and the part dimension, and you get back the amount the part shrinks and the resulting final dimension. The result is given in the same unit you entered, so you can use millimeters, inches, or any other length unit.

Formula

The shrink space is the difference between the mold cavity dimension and the final part dimension. It is calculated with:

Shrink amount = Part dimension × (Shrinkage rate / 100)

Where:

• Part dimension (d) – the intended final dimension of the plastic part, in mm, inches, or any unit.
• Shrinkage rate (t) – the material’s mold shrinkage percentage, usually given on the datasheet (e.g., 1.5% for many unfilled thermoplastics).
• Shrink amount (s) – how much the part shrinks from the cavity size, in the same unit as d.
• Final dimension (f) – the part dimension after cooling, equal to d minus the shrink amount.

To find the required mold cavity size, add the shrink amount to the desired final part dimension. This calculator gives you the shrink amount for a given final dimension, which is the “shrink space” you need to account for in the mold.

Worked Example

Using the default values: shrinkage rate = 1.5%, part dimension = 100 mm.

1. Convert the percentage to a decimal: 1.5 / 100 = 0.015
2. Shrink amount = 100 × 0.015 = 1.5 mm
3. Final dimension = 100 − 1.5 = 98.5 mm

So a 100 mm part made from a material with 1.5% shrinkage will come out of the mold at 98.5 mm. To get a final 100 mm part, the mold cavity must be 101.5 mm.

What Is Shrink Space in Injection Molding?

Shrink space (or mold shrinkage) is the allowance built into a mold to compensate for the volume reduction that happens when molten plastic cools and solidifies. All thermoplastics shrink as they cool; the amount depends on the material, the part geometry, wall thickness, and processing conditions. The mold cavity is therefore made slightly larger than the final part, and the difference is the shrink space.

For example, unfilled polypropylene might shrink 1.5–2.5%, while glass-filled nylon might shrink only 0.3–0.5%. The datasheet from the resin supplier gives a typical shrinkage range, but the actual value can vary with molding parameters.

How to Use the Calculator

Enter the shrinkage rate from your material datasheet and the desired final dimension of the part. The calculator shows the shrink amount and the final dimension. To design the mold, add the shrink amount to the final part dimension to get the cavity size.

For example, if you need a 50 mm part and the material shrinks 2%, the shrink amount is 1 mm, so the cavity should be 51 mm. The calculator gives the shrink amount directly, so you can add it to your part dimension.

Common Mistakes and Misconceptions

• Using the wrong shrinkage value – Datasheets often list a range (e.g., 1.0–2.0%). Use the average or the value recommended for the wall thickness you are molding.
• Forgetting that shrinkage is per unit length – A 1.5% shrinkage means 1.5 mm per 100 mm, not 1.5 mm total. This calculator does that conversion automatically.
• Ignoring anisotropic shrinkage – Many materials shrink differently in the flow direction vs. across flow. For critical parts, use separate values for each direction.
• Confusing shrink space with sink marks – Sink marks are local surface depressions, not the overall part shrinkage.

When to Use This Calculation

Use this calculator when you are designing an injection mold and need to determine the cavity size for a given part dimension. It is also useful for estimating the final size of an existing part if you know the mold dimensions and shrinkage rate. For multi-cavity molds, apply the same calculation to each cavity.

Keep in mind that shrinkage is not perfectly uniform. Ribs, bosses, and thick sections can shrink differently. For high-precision parts, mold flow analysis software is recommended.

Limitations and Real-World Considerations

The simple formula assumes uniform, isotropic shrinkage, which is rarely true in practice. Actual shrinkage depends on:

• Wall thickness – thicker walls shrink more.
• Melt and mold temperatures – higher temperatures generally increase shrinkage.
• Injection pressure and hold time – higher pressure reduces shrinkage.
• Glass fiber content – fibers reduce shrinkage, but often make it anisotropic.

Therefore, the calculated shrink space is an estimate. Mold makers usually start with the datasheet value, then adjust the mold after trial shots. This calculator gives a solid starting point, but it is not a substitute for empirical mold trials.

Frequently Asked Questions

What is the typical shrinkage rate for common plastics?

Unfilled polypropylene is about 1.5–2.5%, ABS about 0.4–0.7%, nylon 6/6 about 1.2–1.5%, and glass-filled materials can be as low as 0.2–0.5%. Always check the specific datasheet for the exact grade you are using.

Should I use the shrinkage rate as a percentage or a decimal?

The calculator accepts the percentage value as it appears on the datasheet, like 1.5 for 1.5%. The formula internally divides by 100, so you do not need to convert it yourself.

How do I calculate the mold cavity size from the final part dimension?

Add the shrink amount to the final part dimension. For example, if the final part is 100 mm and the shrink is 1.5 mm, the cavity should be 101.5 mm. This calculator gives you the shrink amount, so just add it.

Why does my part shrink more than the datasheet says?

Possible reasons include low injection pressure, short hold time, high melt temperature, or a thick wall section. The datasheet value is a typical value under standard conditions; actual processing can shift it.

Does the calculator work for any unit?

Yes. Since the shrinkage rate is dimensionless (a percentage), the shrink amount and final dimension will be in whatever unit you enter for the part dimension. Just be consistent.

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in t | Shrinkage rate | % | 1.5 | 0
in d | Part dimension | mm | 100 | 0
out s | Shrink amount | mm | d * (t / 100) | 3
out f | Final dimension | mm | d - (d * (t / 100)) | 3