Molding over inserts

[JMorgan]

I am trying to mold over inserts and I can't seem to get a consistently filled cavity. My mold consists of 4 cavities that measure 5mm diameter x 20mm long with rounded ends on each side. On one end of the sausage shaped cavity enters a jacketed 24# soft pair of wires, and on the other end is a 1mm diameter stainless steel thermocouple. The wires connect to the thermocouple and the filled cavity serves the purpose of an electric insulator and protector for the wire junctions.

I am using a small 6 ton machine that doesn't have a screw & barrel, but a 14mm diameter piston instead. The material is PEEK, which melts at 350°C [662°F], so it likes to cool quickly. The machine can inject at pressures up to 150 bar [2175 psi], but from what I've tried, I can't inject at anything higher than 40 bar [580 psi], or the plastic will push the jacketed wire right out of the cavity, breaking the joint to the thermocouple.

Most of my research says that you need high injection pressures for PEEK, is this why I can't get consistent parts? Please give suggestions on anything...

Other info:


Mold temp is 215°C [419°F]
PEEK is being dried before use
Not using secondary injection due to small size

[Suhas]

Hi JMorgan,
This is very challenging and you need a good mold design. Questions:
1. Are you holding your inserts in any way inside the mold?
2. Are you injecting along the inserts or perpendicular to them?
3. Does any part of the insert have any other plastic casing?
You have to inject at low speeds and low pressures. There is no other option to that. The tolerances on the inserts themselves will matter for the consistency.The size of the machine is perfect.
Regards,
Suhas

[JMorgan]

Hi Suhas,

1. The inserts are not being held in place, other than that the openings for each insert are very close tolerance to the size of the insert, so when the mold is closed, no plastic can leak past them.
2. The mold was designed to inject perpendicular to the insert, right at the joint of the wires (I just recently had the gate moved closer to the thermocouple end, but still perpendicular).
3. The soft lead wires have a Kapton jacket, but no other plastic.

The tolerance of the inserts are critical and they are verified with any inconsistency rejected prior to molding.

Thanks again.

[Suhas]

If you cannot hold these in place, it will be difficult to maintain the shot because they will move. Any way, you can post some info (with the permission of your customer) with drawings or hand sketches? This one will require someone to look at the drawings, insert pictures and so on. A hand made sketch that is 'similar' is best.
Also what is your approximate location? - North, south? China, Mexico, etc. You could look for a local consultant.
Suhas

[pjhall]

Try putting alternating posts to hold the wires. You will need some sort of "anchor" or the injection pressure will most likely push the wires in the direction of fill. Mold temp of 419F seems a bit high for PEEK. I've always run mold temps around 300F. But, I've never done any wire over molding with PEEK. Why PEEK? Will this application see extreme temperatures or extreme solvents? Why not an LCP? Same high temp properties, but much easier to process.

[kelley]

Suhas is correct, your going to have to fill the mold and generate low cavity pressure while doing so.

Maybe downsizing the gate is a possibility?

kelley

[JMorgan]

Update, and sorry but I cannot post any pictures at this time.

Right around the same time pjhall said:
Why not try a LCP?I got my hands on a free sample of LCP. So I tried it, and wow, what a difference from PEEK. I turned the injection pressure down to 30 bar, and made perfect shot after perfect shot hundreds of times in a row...

Unfortunately, once I performed a melt test in the our calibration lab, the decision makers tell me that the LCP melts at too low of a temperature...:mad: The melting point I found was 340°C, and our customers need at least 360°C, but above or around 400°C would be even better.

There must be something better out there than PEEK? I need something that flows easily.

Thanks!

Oh, and I live and work in Alberta, Canada if that helps.

[MoldEngineer]

I would check into mold flow anylasis. Flow3D does a nice job.

[Suhas]

I think flow analysis will help to decide the gate locations and so that is surely a good option to 'move' the gates around and see. But the moving of the inserts, etc will be difficult to predict.
Suhas

[Ashley]

I am sorry to differ since that seems what I like to do, I would suggest injecting at the highest rate you can. Plastic is non-Newtonian and gets less viscous as flow rates increase. you will pressurize the heck out of the cavity but you wont "push" the parts out of position as the material will fill around it better.

Just give it a shot you will know in two or three shots. but only if the machine is capable at injecting at a fast enough rate.