January 2012 Archives

Setting Final Cushion...

Andy Routsis
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During some classroom training the other week, a student asked me the following question...

John
What value should you set the cushion at?

My Response
A good rule of thumb is 10% of the initial shot size. For example, if the overall shot size is 100mm, then the cushion should be about 10mm. This avoids pressure losses from too much material in front of the screw, yet provides enough room in front of the screw for normal variability.

Related Posts:

-Andy

Teaching Tolerances...

Andy Routsis
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I was asked this question during a recent on-site visit...

QA Manager
What is an easy was to explain tolerances to new employees?

My Response
Focus on the fact that tolerances are commonly expressed in a way which can be easily broken up into simple math equations.

Dimension: 1.00 ± 0.01

Upper Limit: 1.00 + 0.01 = 1.01
Lower Limit: 1.00 - 0.01 = 0.99

Additional Thoughts
When possible break ideas into their simplest form such as an equation or concept. Keep in mind the issue is very often math not tolerances. In this case, focus the attention on basic math and calculator usage first before moving on to part tolerances.

-Andy 
As a trainer, this is one of the more common questions I receive...

Manager
How much training a week should I give my employees?

My Response
We typically recommend between 1 and 2 hours a week. It is always much better to provide your employees a slow, steady, & consistent stream of information over an extended period of time. One hour a week for 40 weeks can be much more effective than one 40-hour week in altering the behavior and improving the skills of your employees.

Additional Thoughts
There is always a place for multi-day training sessions, but they are most effective in teaching a specific skill rather than relaying general knowledge.

-Andy
I was asked this question by a molder who was having trouble balancing a hot runner system...

Bob
We have this new mold which is running 16 cavities in PBT. Usually we can balance the mold using the drop temperatures, but that does not seem to work on this mold. Any thoughts?

My Response
Essentially, semi-crystalline polymers with a high degree of crystallinity such as PBT and nylon are not highly affected by temperature. In such a case, you will have to adjust the gates or the hot runner system itself to balance the shear of the mold.

Additional Thoughts
Cold runner systems are the best for these materials. I know many molders who use a press-side low-RPM grinder to effectively grind up the material and re-introduce PBT back into the process.

-Andy
I was in a discussion recently with a GM and he had this problem...

Manager
My technicians are not willing to take the time to document their changes. They claim it takes too long?

My Response
The issue here is not time, but education. If your employees are not taking time to document, it is likely that they do not fully understand the importance and benefits of good documentation. If they understand the ways it can simplify their job and improve their effectiveness, they would be more interested in compliance. Unfortunately, this is not a simple fix... it require the employees to not only understand the benefits, but also know what to do with the information they are given.

-Andy
I was recently at a company who purchased some training a while back from a different provider under the guise of having a 'resource' for all their employees to use...

My Comments
Training is good, but it is critical that you tie the training to each job function through the use of a training plan. Without a structured training plan focused on job title you will not be able to make a change to your workforce. In general, your employees will not go and train unless there is a structure to show them what they are supposed to do when they train.

-Andy
I was recently at a molding facility during a training implementation was in need of some training due to a lack of fundamentals among the employees...

Company President
What is the best way to process, and why don't we just have everybody process that way starting tomorrow?

My Response
I told her that there were ways that there are many ways to process, and there are a few methods which are far superior to others. Unfortunately without a strong background in processing knowledge your employees will have no reason to understand why this method is better.

Essentially, people who have been making a living molding a particular way do not just change their entire approach to processing because someone tells them it is better. You need to educate them on plastics, machinery, materials, processing, and tooling to ensure that they know why it is a better way.

Additional Thoughts
There are always steps which your employees can take to improve the process. As they see the success they will continue to employ more cost saving, and process improving techniques. This is one of the reasons why it can be easier to teach someone who has never processed that to teach someone who has processed for years.

-Andy
I received this excellent question last week...

Saeed
In one of your videos, you state that higher melt temperatures will increase part weight. To my understanding, a less viscous material is pumped in the cavity with a relative ease thus increasing the part weight. Could you please guide me as to whether or not a decrease in density negates the above?

My Response
You are correct that you will be injecting a less dense material in this scenario… but the higher melt temperature causes many things to occur, but the 2 major factors increasing the part weight and density are listed below:

1) There will be a higher amount of material flow and in-mold pressure during packing resulting in more material entering the mold during both 1st and 2nd stage.

2) A higher melt temperature promotes an increase in part shrinkage resulting in a higher overall part density.

Additional Thoughts
This question points out how complex the interactions between the mold, machine, material, and  injection molding process can become.

-Andy
I received an email from a blog reader about another benefit to increasing injection velocity...

Harry
Another reason we use a higher injection velocity in our facility is to increase the amount of packing that is possible.

My Comments
This is correct because of two reasons. First, the gate begins to freeze the moment the material enters the gate. The shorter time required to fill the mold allows more time for packing. The second reason is because the pressure loss within the mold cavity is decreased, allowing more packing pressure to reach the extremities of the mold during packing.

-Andy

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This page is an archive of entries from January 2012 listed from newest to oldest.

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