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Injection molding of an AirTag Holder – plastic case with overmolded bushing inserts.

Updated: Aug 16, 2022

SUSHI Bikes GmbH

It is not the sushi you may instantly think of. It is an e-bike that was developed and designed to cut an average e-bike cost by half - to just 999EUR. SUSHI bikes is a young growing start-up company disrupting e-bikes industry by taking not only a green approach to the mobility and the future but also to the manufacturing and sourcing of the bike’s components. How otherwise they could cut those costs so considerably without losing bikes’ quality?

Since here at Micromolds not only do we cut molding costs twice too but we also help companies to fill the market gaps caused by demand fluctuations and bridge manufacturing from low to medium size production. We believe that our cooperation with SUSHI bikes very much relates with their overall objectives. This time we helped this company to develop and manufacture an AirTag holder which can be easily attached underneath the saddle.

Design for manufacturing (DFM)

As every molding project begins with DFM this was no different and we immediately started moldability analysis and part optimization for injection molding. Mainly we did these changes:

1. Draft angles have been added to the parts.

2. Some areas have been hollowed out to make walls of uniform thickness. This was done to prevent certain areas from sink marks, voids and distortions. Nevertheless, there were still remaining some places where we could not make a uniform wall thickness without changing the part’s geometry. So, we ran a moldability analysis and images were provided to see which places might be at risk of having those defects.

3. Logo has been extruded out instead of cut out to enable surface finish on flat surfaces.

4. Some walls have been pushed inside 0.5mm because of impossible mold construction.

5. Some fillets were removed around the round surface because of impossible mold machining.

6. Visuals were provided to depict: parting line, injection points and locations of ejectors.

Injection Molding DFM report

Overcoming the sink marks

Thanks to our smooth communication with the SUSHI bikes’ mechanical engineer Max, we had an opportunity to exchange our know-how and thus we came up with a slight design changes to reduce those sink marks marked in red.

Sink marks shown by CAD simulation

Sink marks shown by CAD simulation

The hollowing of thick wall region was a great balance for not losing a contact surface area in the assembly but also decreasing a sink region considerably.

Hollowed spaces to reduce sink marks

Mold making

It took us exactly 8 days to machine the molds after the DFM was confirmed. We used aluminium molds for this low-volume production batches. Polishing was required of outer plastic casing surfaces, however, machining marks were left for the inside. Both sides of the plastic AirTag holder fit in two separate micro molds having single cavity each.

aluminum molds with single cavities

Overmolding (insert molding) reduced assembly time (for pressing bushings)

When we were asked to also do the assembly of the plastic housing we were thinking of using a press to tight-fit the bushings. However, only after several days, still in the initial stages of the project we agreed that overmolding will be more effective solution. We had to do slight modifications to the mold during the DFM but therefore we could save assembly costs and time considerably.

Injection molding, the assembly and packing

We were happy that we could provide end-to-end service for our client. After samples were confirmed we finished the first batch of 1000 units in a few days. We did the manual assembly of the whole batch. We also stuck EAN code labels to the packing boxes that were provided by the client and successfully delivered project on time.

In the end,

What does this really have to do with the Japanese speciality - sushi?

“One rolls, the other are Rolls. So who doesn't immediately think of SUSHI when they think of e-bikes?” - SUSHI Bikes - this is how we roll.


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