From the GST outlook, The Essence of the Business is 3D printing services where the deliverable is on Plastic Material. It has the characteristic of a "Composite Supply" i.e 2 or more goods and/or services are Naturally bundled.
However, if there is a work of only printing 3D designs on your machinery without any designing, then it would be treated as sale of goods and the HSN/SAC code and the GST rate might differ.