Injection mold making

[Tom S]

I am a plastic injection mold maker by trade, but I now nothing about building a mold for soft plastics. A friend of mine has asked me to build him a mold for soft grub tails to add on to the back of his jerk baits. It would be high qty. so he does not want to hand pour. So I need to start learning about soft plastic molds before I tell him I will build it. Here are some question I hope you can answer for me.

1. can soft plastics be injected with a standard injection press, or is the plastic a 2 part resin wich requires some kind of mixer nozzle?

2. What is the cycle time ( cure time) with soft plastic like used by big companies ie. Berkley

3.Does the mold need water lines for cooling.

4.How do they "eject" parts from the mold, or do they need to be hand peeled out?

5. Does the cavities need a certain finish for releasing or is it best to leave cutter finish?

Sorry to ask so many questions,please reply even if you can answer only one of the questions Thanks in advance!

Tom

[redg8r]

please reply even if you can answer only one of the questions Thanks in advance!

Tom

I can tell you the process isnt much different than typical thermal injection molds you make now.

1) The plastic is a heat cure, 1 part component, no mixing.

2-3) The cycle rate depends on how fast you can cool the molds, so yeah, water cooling might be a good consideration in the design.

4) As far as ejection, thats a good question, I wondered how its done myself.

5) A nice finish is always good, but Del, Bob & others, to my knowledge, dont do any extra steps on the cavities, once off the mill. (they have great mills so not really required)

Any imperfections, of course will be carried onto the bait, but the oils in the vinyl make it release well, even from intricately detailed molds.

Thats my take, good luck!

[Kamakazee]

If you don't have any luck with this, have your friend contact me.

I've been injection molding ( soft plastics ) for 10 years and can have the mold made & I can shoot it.

Dave

[redg8r]

I've been injection molding ( soft plastics ) for 10 years and can have the mold made & I can shoot it.

Whats your opinion on his 4th question? how does the baits expel from the mold on a high production run?

Thanks

[Delw]

A directed air blast will drop the bait out of the mold when the machine opens it.

the baits tend not to fall out by themselves.

Delw

[Tom S]

Delw,

Is that something built into the mold , like an air poppet valve in the cavity, or is someone actually blowing it out with an air gun?

Tom

[Delw]

It could be but generally not.

Depending on what type of injection machine you got and what your psi is, you might have alot of things to do differently.

Changeing to a real injection plastic, most injection machines for softplastics have 100-500 cavities in each mold, not like a traditional hard plastic mold.

Kamakazee has an awsum machine(zorn molds I believe), they are about the best for a low semi high production machine.

molds for these machines are high dollar usually 1k - 20k a mold like he uses your buddy wont be able to run unless his boss gives him about 3-6 days of machine time.

Just read your post again, Are you shooting hard plastic ie bottles and such? I dont believe your type of machines will work due to the high pressure and the time it takes to cool, unless you build in cooling lines.

One of my customers back when I was doing areospace work had of those machines and we talked about it at one time, he said many modifications needed to be done to the machine.

On a single cavity mold like most small hard plastic places shoot:

the plastic melts at about 300-400 degrees so you would need some refrigerated type cooling I believe, cycle times on just regular soft plastic would be somewhere like 30secs with cooling. you might get it down to15 but I doubt it.

since you know how the molds open and close on these machines you could put a blast of air in a stationary position just above the mold shooting down, the only problem is this is a soft plastic and unlike the hard plastics that drop when the molds open you might get one stuck on the next cycle( ie when mold closes) that woul be a tad dangerous with out some sort of safety device to detect of the last part got jammed up.

I really think your buddies best bet is to get a mold made for the machine that kamakazee has. and let him or the others that are out there shoot them. Kama, can give you the details on what is needed in order to shoot and make that mold

[Bass1cpr]

Of the injection molds I have seen it depends on the complexity of the design. Most molds have between 12 to as high as 60 cavities.

36 to 48 cavities are high side of count.

All the molds have stringgers or runners that feed the cavities.

When the mold opens the operator removes them by hand and lays them on a drying rack.

The operator hits a button and the process is repeated.

Cycles usually run about one minute and the molds are water cooled.

Plastic is mixed in 55 gallon drums with a mixer turning to keep the glitter stirred. IT is then sucked uo into a heated hose delivery system that is heated to about 340 degrees. Pressure is not usually over 40 Lbs. Usually less.

I had a complete tour of Eufalla Mfg. by the operations manager when I was there to pick up an order of Hula grubs.

Look at molds running about $6000. Number of cavities and simplicity or complexity of design will influenace price.

Also solid colors or firetails and to complexity. Molds with firetails have actuated gates for firing the colors.

[IBHUFFY1]

Injection molds are generally water cooled with a 30-90 second cycle time, depending on the amount of plastic to be cooled. Depending on the injection machine, they can range from 12"X18" to 24"X36" and be in the vacinity of 4" thick.

Check out Garst Lure Design. He designs molds for some of the biggest in the industry. Design costs and final cut of a mold can run upwards of $15,000 or more depending what you want and just how much you want to spend.

Something else to consider is when injecting salt, you are wearing away the mold, much the same as using an abbrasive.

I have a few contact that cut molds for customers.