Tape Dispenser Design Analysis Part 1: Housing

Welcome to the first Ashbrook Engineering blog! The purpose of these blogs is educate both the public and our customers on how the items we use every day are designed and manufactured. Hopefully this provides both information on how specific parts are designed, and the general considerations that go into all part designs.

For our first blog, we’ll be breaking down the details of a tape dispenser. This particular blog will discuss only the design and manufacture of the tape dispenser housing itself. We rely on these household items quite a bit, especially around the holidays. Have you ever wondered how the transparent tape dispenser is manufactured? Let’s dig into this design in this two-part Ashbrook part analysis blog.

General Design Elements

The first part of this two part blog will cover the tape dispenser housing. The second part of this blog will cover the tape roll itself. Overall, the housing is designed well. There are no obvious defects such as sink marks, air traps, or obvious shrinkage. The teeth which are used to cut the tape are all fully formed, and the material choice is reasonable (see: inexpensive, clear) for the application. This part maintains a uniform wall section throughout the entire part, which is essential to its manufacturability.

Manufacturing Method

The tape dispenser housing is injection molded. This is clear when considering the circled witness lines (lines formed on the part from contact with mating surfaces in an injection mold), and the obvious gate vestige circled on the right side of the part (circled in blue above) in the recycling code emblem.

The geometry of this part would render it nearly impossible to manufacture using any method outside of injection molding; One of the main benefits of injection molding is the ability to manufacture complex parts such as this housing in a transparent, cost effective material.

Parting Line Location

The parting line on the dispenser housing follows the thin, flat, continuous edge opposite the face shown in the picture above. The parting line steps to accommodate the geometry highlighted in orange. This parting line location allows for the part to molded with the largest surface area perpendicular to the clamp force of the mold, making best use of the machines clamp force.

Gate Location

The gate location of this part lies directly on the front of the part, hidden in the recycling code. This is a nice touch. Because the pinpoint style gate pictured above is not located on the parting line of the tape dispenser, this gives a hint that the part was likely manufactured on a three plate cold runner (https://www.plasticmold.net/injection-moulding/three-plate-cold-runner-mold/#), or hot runner (https://en.wikipedia.org/wiki/Hot_runner) injection mold.

The location of the gate here leads us to believe this part was manufactured with either a three plate cold runner, or hot runner (thermal gate) mold. The main benefit of a three-plate cold runner mold over a traditional two plate cold runner is that the parts will be automatically de-gated upon injection, and you enjoy gating flexibility similar to that of a hot runner at a fraction of the cost. A hot runner gives you ultimate gate location flexibility, and a very low scrap rate - This comes at the expense of a very high mold cost.

Ejection

This part is likely ejected using a combination of lifters (witness line for top retaining arm lifter shown above outlining the orange circle) and a stripper plate to evenly distribute ejection force around this thin fragile part.

The arms of the housing that hold the tape roll on will be referred to as the “tape roll retaining arms”. The geometry of the retaining arms create a scenario in the molded part called an “undercut” (undercut circled in orange above). An undercut is a geometry that cannot be released from the mold without additional moving mold parts to free the trapped part from the mold.

This part could integrate what is known as a “pass through core”, and would be integrated by designing two holes in the front of the part (circled below) to allow mold steel to pass through, and create the tape roll retaining arms. The mold could then open normally, with the retaining arms free to be ejected from the mold. A negative aspect of using a pass through core is that it would leave two holes permanently in the front of the part. This part already shows two large witness lines (circled in orange below), so holes may not be much more of a cosmetic issue. To form the tape roll retaining arms as the part sits now, a more complex (see: expensive) injection mold is required.

Material Selection

As indicated by the recycling code “6” on the dispenser, the tape dispenser is made from polystyrene (PS). This is also evident in the transparent nature of the part, and its low price per part. It is also worth noting that this part shattered when it was bent. If made of another clear amorphous material such as polycarbonate, this part would have significantly improved mechanical properties, but at a much higher cost.

Because this part is totally transparent, it is highly unlikely that there is any filler or additives used in this material

Thanks for reading! Stay tuned for the analysis of the second part of this assembly, the tape roll!