I am looking for a new die lift design for a thermoforming machine

[abanders]

I work for a manufacturing company that makes thermoforming machines for the food industry and we are in the process of creating a new machine. The way our current machine works is it starts with a roll of film that gets pulled by a gripper chain into a form station which consists of a die set, this set has a stationary top die or head that has heater rods in it to heat the film, the lower die or bucket has the pockets that the hot film gets formed into. The lower die moves up and down. When the die is open the chain indexes the film in, then the lower die closes, at this point the film gets heated by the upper die then the top die forces air down onto the hot film forcing it into the lower dies pockets, at the same time the lower die pulls vacuum down on the film to help pull the film into the pockets to create a good looking and strong package. Once the pockets are formed the die set vents to atmosphere and opens the chain indexes and the formed pockets move along the machine where they are loaded with product, then they enter the seal station where the top film is introduced under the die top. In this die set the top die is also stationary but it has a heater plate inside it that moves up and down. So once the loaded pockets index into the seal station the lower die closes and the vacuum process begins evacuating the oxygen from the package, after that the heater plate inside the die top is forced down by air bladders and it seals the top film to the bottom film then the die set vents to atmosphere the die opens the chain indexes and the process starts all over again. Our current die lift uses pneumatic cylinders to raise and lower the bottom dies. The problem with our current system is that in the form station the cylinders are not strong enough to hold the die in place when the form air is introduced it simply forces the lower die open and leaks the air out instead of forcing the film down hard and fast. In our seal station when the seal plate is forced down to seal the package the lower die is forced down and the die vents to atmosphere before the package can be sealed completely.
We have looked at the competition, most use a scissor lift of some sort, either pneumatic or servo. I am just looking for new fresh ideas.
What I need is a die lift that will stroke 230mm, cycle 35 times a minute and hold a die closed at 35,000 pounds of down force, made out of stainless steel that will last at least 1 million cycles before any major maintenance, and of course be reliable and inexpensive.

 

[MikeHalloran]

First thought: Monster air cylinders.
Probably not even enough room for the diameter you need.

Second thought: Hydraulic cylinders.
I.e., a setup topologically like a car brake system.
Slave cylinders push die up.
Solenoid hydraulic valves lock slave cylinders up during blow cycle.
Master cylinders push on slave cylinders.
Check valves refill master cylinders from common reservoir.
Existing air cylinders push on master cylinders.

Yes, 'air over oil' would be simpler, but you'll be exhausting oily air on every cycle, which is a housekeeping problem, but worse, a fire hazard at your duty cycles, even if you never get a Diesel explosion in the air/oil space.

You need someone with decent hydraulic engineering skills, on site, to work out the details and especially to find the gotchas that we can't see from here.

Third thought:
Mechanical toggle locks, like big De-Sta-Co clamps, holding the molds closed during the blow cycle. You will probably need to put lubricators in the air supply.

Fourth thought:
Scissor lifts used at maximum stroke almost do a toggle lock for free while moving the mold, which is probably why your competitors use them.




Mike Halloran
Pembroke Pines, FL, USA

 

[abanders]

Thank you for your ideas. As you suspect there isn't enough room for larger cylinders, and the hydraulic cylinders are frowned upon because of possible contamination with the food from the oil.
We do have a locking device that rotates into place when the die is in the closed position, the problem with it is due to the variance in height from when the die has vacuum on it and when it doesn't due to a gasket between the two dies. The lock is adjustable in height where it contacts the die bottom but if you set it too high to keep the die from leaking when it is under vacuum the lock will get stuck and not be able to release when the die vents to atmosphere, so we have experience with trying to lock the die in place but not great success. Also I need to keep my cycles per minute as fast as possible so what ever lock I have needs to be fast.

 

[Pud]

MikeHalloran's suggestion re: toggle lock is a good option.

Try and have a look at at the way toggle lock injection moulding machines work - once locked over, the toggles need hydraulics to unlock. They tend to be faster cycling than hydraulic lock types too. The toggle bushings need to be robust though, as wear in those leads to erratic locking. (cf. hydraulic lock machines).

The modern trend for energy efficient all-electric moulding machines is to use recirculating ball screws driven by servo motors to clamp . These are used a lot for medical work as no oil around. (although there is no reason for hydraulics to leak... )





www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[dvd]

[ol 1]
[li]What about the process requires 35,000 lb of downward force?[/li]
[li]Can you back off on that?[/li]
[li]How can you successfully produce the downward force, but not the upward force?[/li]
[li]How much does the bottom die weigh?[/li]
[li]Can you lift the lower die and then shot pin it during pressurized upper die operation?[/li]
[/ol]

 

[abanders]

Thank you Pud for steering me to the injection molding machines. I can probably get some good ideas from them.

On our thermoforming machines it takes heat, pressure, and time to seal the top film to the bottom film. You can run longer seal times at lower pressure but time is critical we want everything to move as fast as possible. Increasing the temperature can only be done up to a certain point and then it will start burning the film.
We have die tooling of many sizes so we want one die lift that will cover all of what we do which is where we get the 35,000 lbs. On our largest dies we have several air bladders that push our heated seal plate down forcing the 2 films together which has a force of 35,000 lbs. So we cant back off on that.
The down force is easy to get because the bladders are captured in the seal head which is a solid block of aluminum machined with pockets in it for the air bladders. The air bladders push against a plate that has several return springs in it. The pusher plate is connected to the heater seal plate so when the bladders inflate the plates push down until they make contact with the lower die, at this point the two films are pressed together. The seal die top is bolted to the machine frame so it cant move so all of the force is exerted on the lower die lift.
The lower die can weigh as little as 65 lbs. up to 700 lbs.
Any pin devise that we have tried always fails for a few reasons. The closing height varies because of gasket thickness, the weight of the die changing and the pins haven't been strong enough for 35,000 lbs.