Last month, I shared a post about a maker project where I built three cedar flower boxes. My wife had purchased six plastic planter boxes from a hardware store, and I used them as inserts inside the cedar boxes to contain the potting soil and flowers. To design and model the cedar box assemblies, I first needed to create a digital representation of the plastic planter so I could build the new design around it. Initially, I created a quick model that captured the overall dimensions without spending much time on the details. Later, I decided it would be a good opportunity to create a more accurate model of the purchased planter in xDesign and demonstrate the workflow required to build this type of part.
The planter is a single injection-molded part that is formed in one direction and has a relatively uniform thin wall throughout. Many plastic consumer products are injection molded because the tooling can be more economical than other molding processes, particularly for larger parts produced in volume. Because this part includes a return wall, or flange, around the entire top perimeter while maintaining a thin-walled construction, there is really only one practical workflow I can think of that will work reliably in most parametric CAD systems. The primary form needs to be constructed using surfaces and then Thickened to create the final solid part. A conventional Shell workflow would not easily produce the return wall around the top perimeter while maintaining the required geometry.
Because of this thin-walled construction and the return flange, the inner surfaces of the planter transition around to the outside, while the outer surfaces transition back toward the inside underneath the flange.
Here is the workflow I used to model this injection-molded plastic part in xDesign.
I started by measuring one of the physical planter boxes. I needed the height and width of the opening at the top of the box, excluding the flange, as well as the height and width of the bottom. I also measured the overall depth of the box. I then created rectangular sketches representing the top and bottom sections and positioned them at the correct distance apart by creating the top sketch on a new reference plane. From these two profiles, I created a Surface Loft representing the tapered outer walls of the planter while initially leaving the corners sharp. Because I did not have a practical way to accurately measure the draft angle of the physical part, using a loft between the measured top and bottom profiles provided an effective way to reproduce the tapered form. I made sure to select Surface mode in the Loft dialog.
I added a large Fillet to the four vertical tapered edges. Because these fillets were applied after the tapered shape was created, the fillet radius itself is not tapered, but the resulting geometry is still moldable. The top of the planter surface now has a continuous edge loop that can be used as the Path for a Surface Sweep to create the flange, or return wall. I sketched the cross section of the flange on the ZX plane and used that sketch as the Profile for the sweep.
I then created a Fill Surface to close the bottom of the planter and used the Knit command to combine the three surface bodies. Next, I added a Fillet along the bottom edge to more closely match the physical planter. At this point, I could have Thickened the surface and produced a part that would be suitable for injection-mold tooling. However, the original planter includes several additional features, so any modifications to the primary shell geometry needed to be completed before applying the Thicken command.
This type of planter is designed to sit on top of a fence or railing using common dimensional lumber sizes. The bottom includes recessed areas that accommodate different board widths. To recreate this geometry, I extruded a surface representing each dimensional substrate along the length of the bottom of the planter. I then trimmed the individual surface bodies until I was left with a single surface having a cruciform cross section. I capped each end with Fill Surfaces and Knitted the surfaces together to create one closed surface body that remained separate from the tapered planter body.
With the two surface bodies complete, I used the Trim Surface tool to remove the outer portions of the cruciform body, leaving only the stair-stepped faces required along the bottom of the planter. I then Knitted those surfaces into the main surface body and added Fillets along the stair-stepped edges. I also added fillets along the inner perimeter of the flange where it transitions into the tapered side walls.
The majority of the planter is now complete as a single surface body that remains open along the lower edge of the flange and is ready to be Thickened. I used the Thicken command, selected the complete surface body, and applied a 1/8-inch wall thickness toward the inside of the surface, which naturally wraps around to the outside of the return flange. Unlike a Shell operation, the open edge of a thickened surface can be somewhat irregular, so I added a cut to flatten and clean up this area for better moldability. This edge effectively represents the parting edge of the injection-mold tooling.
At this point, the model is manufacturable as an injection-molded part, but a component this large with broad, relatively flat surfaces could be susceptible to warping. To reinforce the flange and upper portion of the planter, I used the Rib tool. I started by sketching a single line segment spanning the underside of the flange opening and used it to create one rib. Once I had a rib positioned on each side, I created a pattern to distribute the ribs along the underside of the flange around the planter. These ribs significantly increase the stiffness of the upper portion of the box and help reduce the potential for warping.
The final step was to create crush ribs within the rectangular recessed areas along the bottom of the planter. As noted earlier, large molded surfaces can warp, so some clearance is necessary between the molded surfaces and the dimensional lumber that fits into these openings, such as 2x4, 4x4, or 2x6 members. To both reinforce these areas and provide a snug fit around the wood, I added crush ribs along the inside surfaces of the rectangular cutouts. I started by sketching a single line spanning the opening and used it to create one continuous rib. I then patterned that rib along the length of the opening and cut away most of the rib geometry, leaving only the smaller crush-rib features needed to securely contact the wood beam.
