Shrinkage and warping are among the most common quality challenges in vacuum forming. A part that looks correct when it releases from the tool may distort as it cools, ending up smaller than intended and with surfaces that are no longer flat or true. Understanding why these phenomena occur and how they are controlled is an important part of producing consistent, high-quality vacuum formed components. At Jabro Games, managing shrinkage and warping is a core element of our process expertise.
What Is Shrinkage?
All thermoplastic materials shrink as they cool from the forming temperature to room temperature. The amount of shrinkage depends on the material — typically expressed as a percentage of linear dimension — and must be accounted for in the design of the mould tool. If a part is designed to be 200mm long but the material shrinks by 0.5 percent on cooling, the tool must be designed to produce a part that is 201mm at forming temperature to achieve the correct 200mm dimension in the cooled part. Failure to account for shrinkage in tooling design results in parts that are consistently undersized.
What Is Warping?
Warping occurs when different areas of a formed part cool at different rates, creating differential stresses within the plastic that cause the part to distort as it cools. Flat parts are particularly susceptible to warping — a large flat tray that exits the tool perfectly flat may bow or twist as it completes its cooling cycle. Warping is more pronounced in parts with varying wall thickness and in materials with higher thermal expansion coefficients.
How Warping Is Controlled
The primary control mechanism for warping is controlled, even cooling — ensuring that all areas of the part cool at approximately the same rate. This can be achieved through careful management of cooling time on the tool, the use of cooling fixtures that support the part as it cools, and in some cases the incorporation of cooling channels in the tool itself. Part design also plays a role: adding ribs or gussets to flat areas increases stiffness and reduces the tendency to warp.
Material Selection and Warping Tendency
Different thermoplastics have different warping tendencies. Semi-crystalline materials such as polypropylene have higher shrinkage rates and greater warping tendencies than amorphous materials such as ABS, PETG, and polycarbonate. Where tight dimensional tolerances and minimal warping are critical, material selection should take warping tendency into account alongside other performance requirements.
Our process expertise at Jabro Games minimises shrinkage and warping issues on every project. Contact us at jabrogames.com.