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The Purpose of Cushion

I just received this question regarding a recent posting on my blog…

Blog Reader
You mentioned pressure loss with respect to the importance of maintaining a consistent cushion, could you quickly expand that thought?

My Response
All plastics are compressible… therefore there is a compression that takes place with the polymer in front of the screw. There needs to be a cushion to compensate for variability, but too much cushion results in too much pressure loss before the nozzle.
Additional Thoughts
Although a consistent cushion will help ensure a more reliable process… cushion control is not a reliable method of achieving this. You are best to monitor the cushion and adjust the shot size and transfer yourself when necessary.
-Andy

Understanding Back-Flow Within the Mold…

I received this question yesterday…

MJ
Could you explain the phenomena of back-flow inside the mold? I have in-mold sensors, but I don’t think I am seeing this.
My Response
Basically, there are two areas within the molding process where back-flow typically occurs:
1) Transfer From Fill To Pack: If the mold is completely full during first stage, the mold will begin to pressurize as the machine tries to continue pack the mold with first stage fill. When the machine transfers to pack using a lower pressure, a little amount of the pressurized material within the mold will tend to back-flow. In some cases, this back flow will cause sinks or voids on the molded part… in other cases, you may see delamination occur. If you are using a pressure monitoring system, you may see a sharp drop in cavity pressure at the point where the machine transfers from fill to pack, especially near the gate.
2) At The End Of Pack: Insufficient packing time results in a gate which is not completely sealed. As a result, some of the pressure in the mold cavity will become relieved by a small back-flow of material across the gate into the runner system. This most often results in sinks or voids near the gate area. If you are using a pressure monitoring system, you should see a significant drop in pressure at the cavity pressure sensor near the gate when the packing pressure times out.
Additional Thoughts
Keep in mind… if your process is properly established, you should notice a nice gradual rise and fall of pressure within the mold cavity and little to no back-flow within the mold cavity.
-Andy

Targeting Cushion Size vs. Transfer Position

I was recently asked this question…

e-Mailer
We currently use a target transfer position of 0.25″ for all our processes. to avoid uneven barrel wear should we adjust the our standard transfer to something larger?
My Response
A standard cutoff is helpful when initially establishing your fill… but you ultimately want to target a percentage of shot size. When you target a specific cushion value, then you tend to have the same amount of material, and pressure loss, in front of the screw during pack.
Many companies who mold similar products in similar sized machines sucessfully target a specific cushion value. Most molders have the greatest success in targeting a percentage, such as 5% or 10%, of the full shot size.
Additional Thoughts
The transfer position should not have a significant influence on screw and barrel wear… but focusing on a cushion rather than a specific transfer will improve your ability to maintain a solid and reliable injection molding process.
-Andy

Torque Values For Larger Molds…

I received a few follow-up questions from a previous post, so I will address the most common one…

JMG

I was wondering if you have a chart showing proper torque spec for mold c

lamps and bolts for 250 up to 1100 Ton machines? 


My Response

Sorry, but I do not have such a chart, but I suggest that you look at the torque value based on bolt diameter. Platens accommodating bolts around 5/8″ diameter typically handle torque values around 50-60 ft-lbs… while larger platen threads above 1″ diameter can usually handle torque values around 80-120 ft-lbs.


Unfortunately, all platens are not manufactured equally. Machine age, manufacturer, steel type, and thread quality will factor into how much torque the threads can actually bear. In many cases, newer manufacturers will actually provide maximum allowances verbally, but will not publish these values since they don’t want to be liable for any platen damage.


As a general rule of thumb, it is best to torque the bolts based on their diameter (and machine in some cases). Always use more clamps when a larger mold is present. Personally, I like to use as many clamps as possible to ensure the mold base stays in place, and to ensure no bolts loosen during production. Clamps are inexpensive compared to one damaged mold, and inconsequential when compared to an injured employee.


Many molders will not go above 100 ft-lbs as it is easy to injure yourself when removing an over-tightened bolt. 


Additional Thoughts
Unfortunately, most molders do not torque the bolts on their molding machines. They will usually tighten the bolt manually, applying pressure based on the size of the tool. For instance, they will crank down on the bolts if it is a large 1 ton mold, but apply less pressure if the mold is smaller. 

Arm, back, and rotator-cuff injuries are not uncommon for these molders.


-Andy


The ‘Too Busy To Train’ Excuse

I was asked this question during a recent webinar…

Neil
How do I overcome the ‘Too Busy To Train’ problem?
My Response
I often encounter this question when upper management does not put a priority on employee training. Many managers will state that employee training is important to them… but often neglect to set aside resources such as time, materials, or manpower to make it happen.
For example, many companies try to use their conference room for training. Unfortunately, such rooms are often so busy, that there is never time to schedule employee training. Often training will get cancelled whenever someone in management wants to hold a meeting. As a result, the employees get the impression that training is not important, and therefore will not make the time.
Basically, employee training needs to become a permanent fixture in your company. You may train a couple hours a week, or a few hours a month… but it needs to be maintained and supported by management. In the example mentioned above, if a manager wanted to interrupt training to use a conference room, he or she should offer the use of their own office to help ensure that the training takes place… this would demonstrate that employee development is important and must be maintained.

Additional Thoughts
As with all initiatives, training needs to be both supported and encouraged by the upper management.
-Andy

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