CD Storage Container Part Analysis Part 2: Post
We’re back with part 2 of a 3-part assembly analysis - the CD storage container! The post assembles with the base to complete what we will call the base subassembly. The post will pass through the center of the CD’s to keep them in line with the base, and safe from damage. While this may not be the most exciting subject matter, this assembly and it’s parts provide us with multiple opportunities to learn more about how these items are manufactured.
General Design Elements
Ideally, the base and post would be combined into one part to eliminate the need for multiple injection molds and post processing assembly operations. After all, one of the main benefits of the injection molding process is the ability to combine parts such as these to make one large complex part.
The post has four gussets that run at 0, 90, 180, and 270° along the part lengthwise for rigidity. Without the gussets, this part would either a.) have a large wall thickness, or b.) be too thin to reasonable hold the CD’s in place.
At the bottom of the gussets, there are slots molded into the part which interface with the base for assembly (pictured above).
As shown in the first picture in this post, there is a spherical depression molded into the top of the post. This depression allows for a more consistent thickness at the top of the part to combat sink marks on the tip of the post.
Given the gate location, the depression at the top of the post creates and undercut, thus increasing mold cost and complexity. This will be covered in further detail in the ejection section.
Manufacturing Method
The post is injection molded. Notable design elements only feasible via injection molding are listed above.
Parting Line Location
To make the best use of clamp tonnage and platen area, the parting line (orange, above) runs along the center of the part in the orientation that positions the largest surface area normal to the parting line, also allowing for the molding of the gussets without creating undercuts.
Gate Location
Because this gate sits off center from the parting line, we usually would rule out a two plate cold runner mold. In this scenario however, the gate vestige appears to be close enough to the parting line where the use of a submarine (aka “sub”) gate could be used. The main benefit of this type of gate is automatic degating. As the mold opens, the runner will be sheared off the part. This automatic degating design comes at the expense of mold cost and complexity. More info on sub gates here: https://www.basilius.com/blog/types-of-gating-for-injection-molding/#:~:text=Cashew%20Gate,-Much%20like%20a%20tunnel%20gate
Another aspect worth noting is that this part is gated from a thin section into the thicker walled section of the part. While the molder was able to get away with this, this decreases the odds of having a seamless production process. It is always best to gate at the thickest wall section of the part to minimize the pressure needed to fill the cavity, and to improve odds of filling the cavity completely and packing the part uniformly.
The decision to gate at this location was likely made with the aesthetics of the part in mind. Keep in mind that when the post is assembled with the base, the gate vestige will be completely hidden from the consumer. It is common the “best” way to gate a part may not always be the most attractive. These are important design considerations to discuss when considering the development and end use of a part.
Ejection
The ejection of this part would be relatively simple if the molded in depression at the top of the post was not included. The post requires 4 ejector pins, and unfortunately some sort of core pull to release the undercut created by the depression.
Material Selection
Based upon the post’s mating component (base) being molded in polystyrene (PS), it would make sense for this part to also be produced in PS. This is a good practice, as the design and design process is simplified in assemblies by utilizing materials with similar properties and shrinkage rates. While PS is typically clear, this part is black. This part contains carbon black as a colorant, and adds another layer of complexity to the supply chain aspect of getting this part produced. Finding a reputable and reliable material supplier should be a front-burner item when designing any part or manufacturing process.
This concludes the CD storage container post part analysis blog. We hope this was informative, and gives insight on how even a simple part such as this requires some complex design considerations. The next part analysis blog will cover the third and final part of the CD storage container assembly which mates to the base, the cover!