Delamination on a 2 shot application

[Valentin Leung]

Hello,
I have a 2 shot application, I am shooting a PP substrate on a first machine, then a TPE overmold is shot over the PP.
I noticed that the bonding around the gate area of the TPE is very poor, if I pull the TPE off, I can distinctly separate the TPE from the PP, passed 10-15mm from the gate, the bonding force abruptly change to a real bonding, the PP/TPE bonding will break as I pull on it to delaminate it. I tried to process the TPE at higher/lower speed without affecting this issue.
Material are processed within their TDS windows.
Another extra information is that PP gate is close to the TPE gate, they are next to each other.

Anybody would give a guess at why?

My guess so far is that it needs to reach a certain speed hence shear, so that the plastic heats up during the fill and as it comes out of the center melt front, it can bond correctly with the PP. Raising temperature in the hot runner system would lead to black marks around gates, so I can't go that route.
It's a bit pulled by the hair though and only valid if the bonding is a temperature dependant phenomenon.

if you have any input.
Thanks

[rickbatey]

What's the time between first resin injection and second resin injection? That timing can be critical as you're seeing. The poor cooling around the gate could be another root cause. What does the interface area between the two resins look like? That too is critical.
Rick.

[Valentin Leung]

There are 2 shots of substrate between both machines (so 40s). I'll check the cooling.
For the interface, let's say that the substrate have an opening, and around the perimeter of this opening, the interface has the same profile. The TPE is shot to cover the substrate opening, leaving the end user to reach through the opening made out of TPE instead of PP (which gives them a better user experience). So, the way the TPE is bonded to the PP is the same all around the perimeter, just areas around the gate is not bonded correctly.
I'll definitely give the gate area another look Thanks.

[rickbatey]

I asked about the interface area because I worked for a company that specialized in 2K molding. The used a small projection around the edge of the interface. The cross section was much like a rib that was melted into the TPE/TPO over mold. They found that tge rib melting into the soft resin flow front and left a freshly melted bond area for the next resin.
Rick.

[Valentin Leung]

I think the design of the interface is fine, the tool has run for years, I am just surprised of the delamination issue as I did not encounter it in the past.
Once the part has completely cool down, the bond is stronger, but it's still in the weak side to my taste.
In the future, I'll give the design team a try at your method.

[rickbatey]

Remember that much of the bond is accomplished chemically so it will improve over time. So you can test the bond until a cure period has expired.
Rick.