Multi Shot Molding

Have a look on drawing is essential for any final say, but in general it is being done in the industry for years, material used for second shot should have lower melting temp than of material used in first shot.

Thanks Rakesh

Pretty common stuff. I don't think retention will be a problem.

From your message, I understand that the retaining groove need not be anything special. It could be the rectangular profile. Thanks

You might consider some blind holes in non-critical areas, but as far as undercuts in the parts to retain the overmolding, that means slides in your mold and added cost.

It may depend on the second shot material, too.

EDIT:

Remember that the blind holes will affect hydraulic pressure in the mold and may result in reduced radius of the sealing area in those areas. Maybe a slot would be better, if needed at all.

Your molder will know all of this.

What are the possible material for second shot here. Epdm is suitable? Does any one have a source to refer? Pl share.

Hello,

A few notes of caution. You say it is time consuming to put the o-ring into the groove. If you are using a standard off the shelf o-ring, can you make a cheap loading fixture to reduce your time. Here is the reasoning. You only buy the amount of o-rings needed and you do not have to outlay cash to design and build what will be a fairly expensive mold. It will have continuous demand for cash to maintain and to run the tool every time you need new product.

It is possible to mold a seal into the acrylic casing that you produce. This can be done in two different ways. 1. Build a two shot mold which rotates back and forth to make your 2 shot part. These molds are expensive --probably in the $60-$80K on shore and the $30-$40 k range off shore. They have to have perfect stack heights and are very troublesome if they are not perfectly accurate.

2. Build a separate mold to load the acrylic casing into, to overmold the seal. With this method you have to inventory or have WIP to feed into the seal mold as opposed to the 2-shot which creates the acrylic case which then rotates to the seal side of the machine while a new case is being molded in the other side of the molding machine.

Either method can be successful but TPU Thermoplastic urethane and TPE (elastomer) are subject to flashing through a gap .0005" and therefore the shrinkage factor of the molded acrylic part is very important to know and the processing parameters must be perfectly consistent to have flash free product.

The cycle time is where you pay for the cost of molding in a seal. You can pay to have EOAT, end of arm tooling for a robot to load the part into the press but most molders will charge for an operator to do this. Figure your Case molding charge and double it to add the o-ring. If you are making thousands of parts it may be worth the cost. It is not a trouble free process even when you outlay the sizeable amount of cash up-front for tooling. That is why most mortals opt to install an off the shelf o-ring and leave the fancy 2-shot to the deep pockets to do.

Hope this is not to negative and that it helps somehow.

Thanks very much. Its a very nice headsup information for me. I will surely consider these points.

2tinker,

In your 2nd suggestion, I have a question.

Since I have a good clear finish on the acrylic part; when loaded in the second mold, do you think it get damaged due to the hot cavity surface of 2nd mold?

Is there a speacial name for this kind of 2 stage molding?

I'll give you the search page as no 'single link' appears to cover it.

Hello CADone,

There are two specific approaches to molding in your seal. One is called 2-shot molding , as I explained, it has a high entry price but is efficient on cycle time. In that situation you rotate the finished substrate to the second process and as there is no handling it is rare to sustain damage to the appearance surfaces with well constructed molds.

The second method is overmolding and there is some handling involved but again with well constructed tooling you should not have a problem with the appearance surfaces from heat or other in the second mold.

Handling WIP(work in process) is always a challenge and is best done within a day of production as post molding shrinkage will play a role in your over-mold being easy or difficult to do. Carts with multiple shelves with foam, clean plastic dividers etc are a good start for wip.

So to conclude you should have an equally good surface the second mold where it is in contact with the substrate. This can be close to the pinch point on the second mold but do not skimp on the polish over mold of the mold surface as 'Murphy' plays a big part in molding.

Hope this helps

Thanks for your support.

Guys,

Both, multishot and Over molding operations would need the first component (Arcylic Cover) to be placed in the cavity of a secondary molding. My vendors responded that there will be surface damage to the first component. This is not acceptable to me because this is a Lens for a LED light source.

Therefore, I am looking into molding both parts (Lens cover and Gasket) separately. Later join them by some means. What do you guys think about sonic welding? Is is a good application for acrylic and silicone.

Thanks

C1

Might be time to post the proposed design detail of what fits how and why.

Don't forget to indicate sizes - it can make a huge difference on the ways to do things

https://picasaweb.google.com/lh/photo/W1lk_vt5GYJMKreMHCtYkg?feat=directlink HERE IS THE LINK FOR THE IMAGE