Plastic Injection Mold For Sunglasses (lens slot)

[RMG143]

I apologize if this is a juvenile question but I was thinking about how sunglasses could be made in a plastic injeciton mold setting and began to wonder if the main frame (not the legs) could be made in a single process. What troubles me in the creation of the slot where the lens sits.

The slot creates a full loop so I cannot figure how a mold could create this type of geometry. It seems that the mold would need to have a shutoff for the lens and then a secondary process would involve cutting the slot.

Here are links to photos which might clear up what I am trying to get at.
Link 1
Link 2

If there is a method I am unaware of I'd love to be educated.


Thanks in advance all

 

[MikeHalloran]

I've tried molding lenses with integral frames. In my case, the lenses were much smaller than eyeglass lenses, and rotationally symmetrical. It was a great idea on paper. In practice, the lenses came out distorted.

In the case of making an eyeglass frame with an undercut aperture for the lens, there are at least two ways to do it:
- A collapsing core, where the mold core that makes the undercut disassembles itself as the mold opens, sort of like a block puzzle.
- A sturdy ejector just strips the finished part off the core. Remember that the molded object is a thermoplastic, and at cycle end is still quite hot. It's also much more stretchy than it eventually will be. Stretching a freshly molded object to get it off a core is a fairly common practice. Some objects then need to to be supported, e.g. in a simple fixture, so they will shrink and cool into the desired shape.


Mike Halloran
Pembroke Pines, FL, USA

 

[cowski]

It may be possible to overmold the frame around the lenses.

 

[Pud]

A sturdy ejector just strips the finished part off the core

Usually a person in China operating an unguarded machine (I've seen a video!) peels them off when hot. The undercuts are not too severe - material is usually a semi-crystalline nylon 12. Split lines are manually polished off as well on the samples I have seen.

Cheers
H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[steveapathy]

I was going to suggest precisely what cowski mentioned. Molding the lenses first, then overmolding them with the frame would work quite well. Of course, this option will often increase capital and piece price, so if you're looking for low cost, stripping it off the mold as MikeHalloran mentioned is probably the way to go.

As Pud mentioned, you don't need a deep undercut, and it's probably ideal to NOT have a deep undercut for this frame style, so the lenses can easily be inserted.