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Buffered Pressure Limit Settings

This question came in last month. Although I could not get any specifics about the machine such as age or model, I believe the question still makes a good discussion…

E-Mailer

Today I tried to adjust limit of injection pressure on Demag machine. I adjust maximum of machine 210 bar actual injection pressure was 160 bar, so I decided to decrease limit to 190 bar, but actual pressure decreased to 150 bar. I tried to decrease limit to 170 bar, but actual pressure decrease again to 130 bar. 
I would say this is not a good thing.  It seems like that this is built into the programming of the Demag machine.  Might be automatically trying to create a pressure differential, but by doing it this way it moves the process back to pressure controlled rather than velocity controlled.
My Response
Although I cannot speak specifically about that particular machine, there are some general reasons for this behavior.

Most machines do have a buffer to ensure the machine does not overcompensate when approaching the maximum setting. Sometimes this buffer compensates for line pressure or pressure losses in the system. 

Often machines set the limit near the pump and take peak pressure measurements at the cylinders… which does not compensate for losses in the system. Certain machine types even use a pressure differential to control velocity, which could cause this difference.

Another cause for this could be dynamic breaking… to compete with the accurate controls of electric molding machines, machines are applying a small amount of hydraulic pressure to the front of the injection cylinders to better control the velocity transitions during injection. The displayed injection pressure will most likely be calculated as a differential between the front and back of the cylinders.

In any of these cases, to test if you are actually hitting this peak limit, increase your max pressure and see if the peak pressure increases.

You may also want to perform a Dynamic Load Sensitivity Test on the machine to ensure the machine controls are functioning properly and there is not a problem with the hydraulics or controls.

Additional Thoughts
As the molding machine technology increases, the specific functionality of molding machines is becoming less obvious. This is why it is very important to good relationship with the technical representatives at your machine supplier. If the machine is an older model, a machine re-builder can also be a great resource as they tend to have great knowledge of your machine’s shortcommings.

-Andy

‘If I Train My Employees… They Will Leave’

I very often hear people justify not training their employees with the age old argument ‘If I Train My Employees… They Might Leave.’ One of the funniest responses to this I have heard is from Zig Ziglar who says ‘If you don’t train your employees… they might stay!’.
All humor aside, ignorance is never a good employee retention policy. For example, one company we worked with had a 75% turnover. This meant that 3 out of every 4 employees leave each year. When we discussed employee trianing, it turns out that they never train, nor do they promote from within. They went silent when we asked them… ‘So, am I correct to understand that everyone at your company is in a dead-end job’.
Unless your company has a true culture of learning and advancement, sending only one or two people to specialized training provides them credentials not applicable to your production environment. For example, ‘5S’ training provides great information… but sending one employee out for a couple days of training will not transform the attitude and actions of your entire workforce.
Ask yourself two questions… (1) do you have enough ‘key’ people on your production floor…? and (2) are any of your production employees in ‘dead end jobs’…?
The goal here is not to sell…  but to help people understand that you need a plant-wide focus on improving the skills of the entire workforce so ultimately, everyone shares the load.
-Andy

Boss Cracking on a Polysulfone Part

A recent question was received via e-mail…

RZ
We are encountering cracking at the bosses on a polysulfone part. We press fit inserts on the bosses but we don’t see cracking during insertion. It is not until a day or two later that we see the cracking. Any thoughts?
My Response
The occurrence of cracking and crazing in this case is most likely due to the factors of molded-in stresses and strain.
Since polysulfone has a high degree of semi-crystallinity and rigidity, the resulting parts have many molded-in stresses… especially in the case of a boss. Many injection molders will anneal the parts with a heat cycle to reduce these molded-in stresses. Doing this before and after the insertion would significantly reduce these stresses.
Reducing the strain during insertion will also reduce the likelihood of cracking and crazing. Using a barbed, threaded, or trilobal insert may reduce this strain. The use of ultrasonic insertion or in-mold insertion will also reduce this.
Additional Thoughts
It may also be a good idea to mold the part in a translucent material so that you can see where the stresses are present under polarized light.
-Andy

Troubleshooting Voids… In The Sprue

I recently received an email from a friend of mine regarding voids…

Ron
How do you troubleshoot voids in the sprue?
My Response
The presence of voids in the sprue is very common, but the need to troubleshoot this is unique to sprue-gated parts. Voids in a sprue gate are a little different than a typical sink or void at the gate.
There are three ways to approach this… Cooling, Pressure, and Temperature.
The first way is to improve cooling at the sprue with additional cooling in the sprue area or mold cavity at the gate region. Beryllium copper inserts or a sprue made from an alternative material would improve the cooling in this region.
The second way to reduce this would be to increase the pressure applied during second stage time. As this pressure is increased, the time required to seal the gate will actually increase. Since the increased pressure causes a higher tendency for the polymer to flow back through the sprue.
The third way to improve the situation would be to increase the mold temperature… assuming sinks in the region are acceptable. Since higher mold temperatures increase the tendency to for sinks over voids.
Additional Thoughts
Such a situation is difficult to correct… and typically requires much trial and error to determine a correct process. Again, as with any other process, you really need to document the process thoroughly once the issue is resolved.
-Andy

Venting Issues

I received this general venting question the other day…

LG
Many of our molds have venting issues as evidenced by either burns or filling problems. I realize that a “flow analysis” done previous to cutting the steel would answer most of these questions.  The gases have to get out… What do you recommend regarding placement, size and depth?

My Response
With I am asked how many vents should be added, I always say more. Each material has design guidelines for vent depth which are typically very helpful.
Ultimately, the more vents you can have on the part, the better. Obviously… strategic venting, such as a porous steel in a boss, can be very important; but getting as much gas out of the mold prior to that boss will minimize the amount of localized venting that is required. Also consider venting the runner, sprue, and cold slug wells.
Since the polymer only flashes as a result of thickness, the width of the vent has virtually no effect. As a result, you can make the vents relatively wide. I have seen large vents taking up high percentages of the parting line with great success.
One commonly overlooked aspect to venting is the vent drop. Behind any well designed vent is a substantive vent drop.  These drops are deep grooves, which channel the gas from behind the vent to the outside the mold. Think of the vent as a gate designed to transfer the gas from the mold to the vent drop.
Lastly, high clamp pressures may reduce the effectiveness of your vents… and may close them completely.
Additional Thoughts
Many mold designers put extensive thought into the getting the polymer into the mold… but pay little attention to how the gas will get out of the way. Most of the molds I see with vent clogging problems often have thin vents, too few vents, no vent drop, or combinations of all three. Give the gas trapped within the mold many wide vents with a big vent drop to help it escape to the atmosphere.
-Andy

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