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Over Molding of PPS with PA66 (Ferrite Shield — Busbar Integration)

08/25/2021 6:26 AM

Hello All,

Is possible to over-mold part made of PPS GF40 with PA 66 GF30 resin?

I have a situation wherein Ferrite shields are to be over molded close to the busbar which carries HV DC current. But there should not be any voids or cracks between the busbar and sensors otherwise part will be rejected.

Therefore, the plan is to over mold the ferrite shield first to ensure Complete isolation, then integrate the pre-molded ferrite shield with busbar over molding. ferrite shield Pre-molding material is not yet finalized.

Suggestions would be much appreciated

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#1

Re: Over molding of PPS with PA66 (Ferrite shield - busbar integration)

08/25/2021 7:10 AM

12 years since I was in that industry, so take the following in that context.

Having a brief look at the data sheets, that combination would seem feasible to be done, but with some logical constraints and concerns. Check the actual heat grade of the resins being used. Try to obtain resins with the same HDT.

The dwell time of a lower melting point material in the higher melt temperature will need to be controlled. That is you will need to ensure sufficient cooling has happened before the lower melt temperature material deforms.

You will also need to be VERY careful about the material flow as it encapsulates the overmounded component. There will be vortex flow as the material encapsulates the ferrite.

Is there any reason that you are using two different materials? If you know the substrate is PPS GF40, then why not overmould with that same material grade?

Wondering also whether this is a "job lot" or whether the production runs will be significant? Tooling for overmoulding can be very simple, but consistency in cycle time is necessary to get repeatable outcomes for flow and voids.

When you are talking busbars, what coating thickness are you to achieve? The busbars will probably act as heatsinks, so you might need to consider having them pre-heated so they don't "shock chill" the material as it contacts the surfaces.

One final question, why are you selecting such high glass content materials? The described usage does not seem to need strength from such high glass content materials.

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#2
In reply to #1

Re: Over molding of PPS with PA66 (Ferrite shield - busbar integration)

08/25/2021 8:07 AM

Thank you for the insight.

Below is the HDL data of the materials being considered,

PPS GF40 – 260°C / PA 66 GF30 – 254°C

I was under the impression that the material of the substrate to be over-molded should have high HDL than the material injected in the 2nd shot. Therefore, I chose PPS for substrate. GF40 is due to the availability, as we use this grade already in the plant. PPS can’t be used for the whole project as the CTI of the PPS is not sufficient for our requirement.

Is it feasible to use same grade material for substrate and 2 shot? If so, we can just use PA66-GF30 for both parts and it makes our plant work easy!

The vortex that you mentioned is a new topic to us! What is the cause and how it can be reduced or eliminated!

Busbars dont have any coating; we are planning to preheat the bar before molding. Does pre-heating the busbar help to achieve the leak tight joint (Better adhesion)?, We are planning for potting between busbar and plastic as our experience with the terminal over molding is not good!

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#4
In reply to #2

Re: Over molding of PPS with PA66 (Ferrite shield - busbar integration)

08/26/2021 7:03 AM

You have reflected some desire for clarification, so I will attempt to answer them without writing a complete thesis.

I re-iterate, that I moved from that industry around 12 years ago.

Ideally the HDT of the second shot material should be "not greater than" the substrate, but provided the dwell time and flow rates are not sufficient to actually melt the substrate before it cools should be OK. There is sometimes benefit in having the second shot material at hotter HDT to encourage bonding at the surface interface.

There may actually be benefit in using the same material for substrate and overmould. They will have the same shrinkage rates in exposed environment, whether that be thermal expansion or humidity absorbtion and thus lower internal stresses during lifecycle. This also means less material complexity, with same drying parameters and so on.

Remember that both nominated materials require drying to process. How will you maintain the "dryness" of the substrate until it is overmoulded?

We used to manufacture automotive lighting. Some lenses were done as three shots, three different colours in the same material.

Regarding the voids/vortexes. Imagine water flowing past a "square" rock, perpendicular to flow in a waterway, but as three dimensional. These happen as the shot flows around the insert, whether that is a tooling core or the substrate that you are overmoulding. The air has to escape the mould cavity AND the material needs to flow fully around the insert. The keys to this include good venting beyond the insert, flow/fill rate slowed, the flow shape of the insert, the viscosity of the melt as it is flowing (temperature). We used a program called "moldflow" or something similar to analyse this situation. Some tricks used a vented ejector pin in the turbulent zone to assist air escape. I did hear that there was progress on use of vacuum in some tooling.

There can also be other voids created simply by material contraction on cooling. These usually relate to thick wall sections and hot material.

I would go for potting in the immediate gap to the busbar. This will assure that surface irregularities on both surfaces are consistently accommodated.

I still question your material selection. The glass fibre filled materials provide structural strength that does not seem warranted in this situation and introduces humidity tracking features that worry me in an electrical power environment. You might be inadvertently compromising the electrical separation requirements.

Preheating large inserts like busbars means that the overmold material at the surface doesn't chill and become stiff. There would be a level where if a busbar was peeled, the overmould material surface would look like "stretch marks" as it sets, then tears, sets again and then tears as the melt front progresses along the item. Careful though, typically metal contracts at a greater rate than the plastic, so too hot could lead to complete separation.

hope this helps.

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#5
In reply to #4

Re: Over molding of PPS with PA66 (Ferrite shield - busbar integration)

09/09/2021 6:00 AM

Thanks a lot for the explanation. I appreciate your effort.

Due to circumstances couldn’t respond to your comment. since apologies!

You have summarized the whole thing that I had been in discussing with the resin manufacturer for few days :).

As you mentioned the PPS is overkill, and the resin manufacturer mentioned that it doesn’t offer better adhesion with PA when it’s come to 2 shot molding. I was suggested to look from same resin family. Since I have strong die-electric strength requirement decided to use material from same family with different additives.

As I have a lot of metal inserts along with pre-molded shields, the melt will lose a lot of heat energy as it flows, hence I have a picked the lower Tg resin for pre-molding of shields. Preheating of metal inserts is being discussed, hope it shouldn’t be a concern.

As I have no restriction over gate location based on MF simulation, I will place the gate to minimize the vortex.

Potting is fixed, epoxy has been chosen as its has to resist ATF.

Thanks again.

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#3

Re: Over Molding of PPS with PA66 (Ferrite Shield — Busbar Integration)

08/25/2021 10:45 PM

Please be cautious what grade of PA66 GF30 resin you choose. Some compounders use phosphorus flame retardants, which can release phosphine, later converted to phosphoric acid, with harmful effects on the copper busbar.
Further, PA66 GF resins in general can see long term reductions in flame retardancy, due to wicking along the glass fiber axis.

Here's one research paper on this subject:

https://www.researchgate.net/publication/335124441_Failure_behavior_of_nylon_products_for_red_phosphorus_flame_retardant_electrical_connectors
Mark Bingham
fluoroplastics.com

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