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The Affect of Clamp Type on the Effectiveness of Venting…

I just received a question which highlights the difference between cylinder and toggle clamping systems…

JP
We have a mold in our plant with vents problems (not enough) We have problems with our plastic parts due gas residues on them when we run it in a electric machine (toggle system) and when we run it in a hydraulic machine clamping cylinder) runs without any problem. Can you explain me what is the difference between this two types of clamping systems? Can the vents be affected with the toggle system?
My Response
Your first assumption is most likely correct…
The hydraulic clamp system on your machine uses a large diameter hydraulic cylinder in the center of the movable platen to apply tonnage to the mold. Since the clamping force is centered on the platen… deflection occurs on the perimeter. This deflection reduces the tonnage appied to the perimeter of the mold resulting in improved venting. Adversely, toggle clamps tend to provide a more evenly distributed force to the platen.
Additional Thoughts
Before you make the assumption that the machine is the only cause, keep a few more aspects in mind… 1) The clamp force might be more accurate on the newer machine. 2) If the mold is too small for the electric molding machine (uses less than 2/3 of the platen) then you can cause platen concavity resulting in additional force applied to the perimeter of the mold. 3) The newer machine may also provide a faster injection speed which would also affect the mold venting.
-Andy

Adjusting Hot Runner Systems…

I received this question the other day…

Milan
When I am setting up a process with a hot runner system, I always have to adjust the hot runner nozzles to avoid unbalanced parts. Do you know of any techniques to help eliminate this problem?
My Response
Although adjusting the temperature of your hot runner drops can help balance your tool, there are a few alternative ways molders use to help balance their tool.
1) Adjustable drops – In many hot runner tools, the height of the hot runner drop can often be adjusted to increase or decrease the thickness of the specific drops
2) Use true balanced runner systems – Many hot runner manifolds are not built with balance and symmetry in mind. Additionally, most hot runner systems do not use features such as Beaumont’s Melt Flipper to balance the shear within the hot runner system.
3) Balance the clamping – All platens deflect, and many older platens will be somewhat concave. Check and measure these conditions to ensure spacing or additional bolster plates are not needed. Additionally, a review of parallelism during clamping can be very helpful.
4) Balance the cooling – In many injection molds, the cooling supply each individual cavity may not be properly balanced. This is very common when the part geometries are not symmetrical… resulting in variations the effectiveness of the cooling from part to part. You may want to measure and compare the coolant temperature and flow going to each region of the tool.
5) Balance of venting – This can be an often overlooked cause of cavity imbalance. The effectiveness of the melt entering the mold cavity is based much on the air’s ability to get out the plastic’s way. I have seem many molds with virtually no venting to interior cavities… or even caes where the inner cavities actually vent to the out cavities. causing all sorts of complications in gas removal.
Additional Thoughts
When specifying new hot runner systems or new tooling, try to incorporate systems which can be easily adjusted. Many molders see great benefits to the additional control and flexibility brought forth through the use of valve and thermal gate systems.
-Andy

The Role of Consultants…

I am often asked questions similar to the one I just received…

Ben
We are going to need some technical help… how do I determine whether I should hire another engineer or get a consultant?
My Response
We deal with technical issues within the plastics industry all the time. Although we openly advertise our expertise in creating employee development systems… we routinely visit companies around the globe to provide technical consultation for plastics, processing, and training.
In general, this question can be handled using four common approaches…
1) If you have a unique issue such as troubleshooting, equipment evaluation, or beginning a new program… a consultant is often the best way to resolve the situation. Many companies hire an engineer to help handle a specific situation… and then under use their talents performing routine tasks once the situation is resolved.
2) If you want to create a long term change in behavior such as 5S or process documentation… a consultant can often help you determine the best approach to determining the best behavior, educating your employees, and maintaining the behavior. Once this is established, it will be very easy to determine whether your current employees can implement the strategy… or if additional staff is necessary to ensure the success of the initiative.
3) If there are only one or two routine tasks which take place monthly or quarterly such as a tool design review or a molding trial… having a consultant on retainer may save you money in the long term.
4) If you have an established routine or series of complex tasks which need to be performed, hiring an engineer or technician is most likely the best course of action.
Additional Thoughts
Always try to hire employees and consultants with the intention of exploiting their talents… for example, hiring a consultant to measure and weight a large number of parts may not be cost effective… but hiring a consultant arrange outside testing, or to evaluate the results and help draw conclusions may be very helpful. Adversely, when our consultants help a company develop a strategy for training their employees, their engineers, managers, and supervisors are often the best people to implement and carry out this training with their employees.
-Andy

Profiling Injection Due to Moldflow Results…

I saw this unique question in my e-mail yesterday…

MJ
What is your opinion about profiling the injection based on the results of a Moldflow analysis?
My Response
Mold filling analysis programs such as moldflow can provide a variety of options for filling. For example, you can use the software to profile in order to minimize variations in fiber orientation, shear rate, shrinkage, etc. Many seasoned injection molders will agree that the simulation information is very helpful, but the final process will depend more factors than can be programmed into the computer… The biggest factor being Quality.
Overall, I find these simulations extremely helpful in establishing the mold design, and to determine the general approach to processing the final tool. If such a program suggests an injection profile, then I would use that information to better understand potential complications with the process. For example, if the software recommended a lower speed through a thin section and a faster speed through a thicker section… I would personally evaluate whether to first try one speed through both, or profile my speeds during the initial setup.
One thing to keep in mind when processing… each injection speed is a variable… the more variables you introduce into the process, the more variation you potentially introduce.
My Response
All data such as blueprints, analysis, and customer requirements are helpful to the person establishing the process. Additionally, the more complex the part and tool, the more useful this data can become.
-Andy

Adjusting for Differences from Machine to Machine

This is a topic which often occurs when you are processing with various machines.
M.

How do I compensate for the differences in how my machines handle the transfer from fill to pack?

My Response
Although we often talk in generalities… each brand and generation of machine has it’s quirks.
For example, when using an older ____ controller… I often have use profiling to incrementally drop the injection speed at the end of fill to slow down the large ram so it does not overshoot during pack.
Also, I recently used a ____ all-electric molding machine which would stop the ram when I turned off second stage pack, but would allow the ram to overshoot once pack was turned on. In that case, I had to turn on pack… but reduce the pressure to the minimum setting to get a realistic performance.
Additional Thoughts
Think of it like a car, an older truck may need 3rd gear to climb a hill at 40 MPH… while a newer sports car could easily climb it in 2nd, 3rd, or 4th gear. Some choices are preference… while some are from necessity.
The key is to establish a way to document the process in a manner which can be duplicated and transferred to your different molding machines.
-Andy

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