DDS and FDM: know the difference


Schematic of FDM (Fused Deposition Modeling)

When selecting a 3D printing process an important consideration how the process itself can affect the part. If the correct process is not selected, a part could turn out to be weak in certain areas, or in certain directions. This is especially a consideration if the process of choice is 3D printing. This article highlights the main difference between two 3D printing methods (FDM and DDS), and describes how a part can be structurally different depending on which is used.

In the world of plastic printing, FDM (Fused Deposition Modeling), lays down the material in a linear pattern. Therefore, defining the build process before the 3D Printing begins is crucial because the printer head will always move in the same pattern, but you can orient the part in unlimited positions. This allows the user to pick and choose the material path in relation to the part’s geometry. This defines the part structure and allows control over points of weakness in the part.

A process like DDS (Droplet Deposition Stereolithography) works differently: It cures the material whilst laying it down in tiny “dots” which gives a smoother appearance. The strength of the part will depend on how many different durometers (materials of different hardnesses) are being used in the part. When using multiple durometers the printer will actually oscillate between the two materials during the transition to the new durometer/material. The weakest point of the part is therefore at the transition between two different durometers.

DDS can print in extreme detail, but depending on the application for the part this detail could go to waste. During printing it is necessary to create a support structure, and when this is removed delicate parts have a tendency to break. It must also be noted that FDM and DDS utilise different methods of removing the support structure from the part. FDM uses a chemical bath, which is too harsh for finer details and would require a less harsh vinegar soak. Mechanical methods are used for DDS: the operator will actually peel the support material away and then use a water jet. For a more delicate part this requires a soft touch and experienced hands. Overall, the extent of the part strength and detail required for the part plays a huge role in the 3D Printing method chosen.

Image courtesy of Zureks, Based on drawing in: D. T. Pham, S. S. Dimov, Rapid manufacturing, Springer-Verlag, 2001, ISBN 1-85233-360-X

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