February 2010 Archives

I just heard another common excuse for not training...

HR Manager
We are waiting until things pick up before we start employee training.

My Thoughts
Believe it or not, slow times are the optimal time to start your employee training initiative. These are great opportunities to get your current employees up to speed, and prepare them for the next increase in production.

This is especially true if you hire temporary employees since you will rely heavily on your current employees to being new and temporary hires up to speed when things get busy.

-Andy

The Purpose of Cushion

Andy Routsis
Vote 0 Votes
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



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

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
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



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
I was at a customers facility the other day and they had a problem which is becoming more common with today's electric molding machines...

Joe
We bought these fancy, new, direct-drive electric molding machines. Overall, we like them, but we have already snapped three screws and the manufacturer has no idea why this is happening. Do you have any ideas?

Note: After further discussion and a review of their processes, I determined that they were not using sufficient screw delay prior to recovery... In some cases they were not using any screw delay at all.

My Response
Immediately after 2nd stage pack, there is typically a large amount of pressure present in front of the screw. If this pressure is not relieved, it will increase the torque applied during screw rotation. This additional force can quickly weaken, twist, or even break the screw within the barrel when screw recovery starts.

This concern is even greater with electric injection molding machines, since the servo-motor can apply a great amount of torque instantaneously.... especially on direct-drive machines where the minute flex of the belt is non-existent.

To relieve the pressure that’s at the front of screw and to prevent unnecessary screw damage, the ‘screw delay’ or ‘screw decompression’ option should be used. The screw delay option allows you to add a delay after 2nd stage packing to relieve the plastic pressure on the screw before recovery. The decompression will actually back up the screw to relieve the pressure... similar to decompression after recovery.

Additional Thoughts
Hydraulic molding machines often avoid this issue since they have an inherent buffer due to the slight buildup of pressure as the valve opens, and a period of pressure stabilization that occurs before the screw reaches full speed. Adversely, the electric servo-motors apply a high amount of torque to the screw virtually instantaneously. People who are are familiar with hydraulic molding machines often have difficulty adjusting to the instantaneous response of an all-electric molding machine.

-Andy

Tolerance For Screw Wear

Andy Routsis
Vote 0 Votes
An frequent e-mailer recently asked this question...

JM
What is the downside to a worn screw?  Why should I measure it, and why should I replace it if it is worn beyond tolerance?

My Response
Each process and company has a different tolerance for wear. A company molding medical devices needs much more control over melting and residence time than a company molding commodities.

The benefits to regularly measuring your screw and barrel is that you can determine what level of wear tends to indicate inconsistent processes. Ultimately, an excessively worn screw will increase residence time, decrease melt consistency, and reduce melting efficiency.

Additional Thoguhts
Many companies will have a couple 'junk' machines which are not worth replacing the screw and barrel… but can run a handful of jobs well enough. Other companies keep every machine in perfect shape. Remember, you don't need a race car to get your groceries… but you also don't want to enter a race with a Yugo.

-Andy
I was recently asked this training-related question...

Joan
I am having a hard time explain the importance of training to new hires, how might I approach this?

My Suggested Verbiage
Training plays an important role in the success of any molding facility, and provides benefits to both the employee and employer. It is important that personnel at all levels participate in a structured in-house training program. Such employees increase their knowledge base and learn applicable skills to enhance their performance in a production environment.

Training also helps the employer satisfy their customers - and ensures future customers that they’ll be able to handle new challenges. Such training is essential to performing your job correctly. In addition, training can help achieve your professional and financial goals by providing job advancement opportunities.

Additional Thoughts
Don't be afraid to be frank, especially with new hires... Any employee who is unwilling to learn and improve should be concerned about their longevity at your company.

-Andy
I received a question which relates to an issue that many injection molders face...

MJ
Is there a technique for optimizing recovery speed?

My Response
Basically, there is a way to optimize recovery speed... but it is a two step process:

1) First, determine the optimal feed zone temperature for your process by performing a feed zone temperature study. The purpose of this study is to determine the optimum feed temperature by graphing feed zone temperature versus screw recovery time. Starting with a low feed zone temperature, incrementally increase the temperature and document the screw recovery time at different increments. When graphed, the screw recovery time will drop and then rise as the temperature is increased. The optimal feed zone temperature is the temperature at which the screw recovery time is the lowest. This is the temperature where the polymer sticks best to the barrel, causing it to convey most efficiently.

2) Once the optimal feed zone temperature is determined, you should adjust the rotational speed of the screw so that screw recovery consumes 80 percent of the overall cooling time. Note that the back pressure used during screw recovery should be high enough to provide a consistent recovery time and consistent mixing. Your recovery times should not vary more than 5% from shot to shot, and 10% from material lot to material lot.

Additional Thoughts
Many older molding machines cannot maintain consistent screw speeds at low RPM. In such a case, you may want to consider a longer delay before recovery to ensure the machine can maintain the desired consistency.

-Andy
I got a multi-part question the other day regarding a couple aspects of mold clamping...

Rob
Do you have a suggested torque specification for 5/8-11 and 3/4-10 bolts? I know you are suppose to refer to the manufacturers torque specification, but does that apply to a clamp bolt and clamp? The clamp is a lever and flexes a little like a spring, so would that require the same torque as a bolt holding two flanges together? Also, would you recommend or discourage using a lubricant on the clamp bolts?

My Response
I will address this question in a few parts...

Regarding Torque Ratings: Many bolt manufacturers will provide recommended torque values. You must remember that these bolts are not manufactured or designed specifically for injection molders. The same bolt you are using to hold your mold in place may also be used to secure the rafters in a stadium. The torque rating is based on what the bolt can safely sustain... not necessarily how it should be used. Since the machine platen is typically cast, the threads are significantly weaker than the threads on the bolt.

Regarding Torque Recommendations: Most injection molders using similar bolts use a torque value around 50-60 ft-lbs. For more on this, please read our past blog: Proper Torque Value for Clamping Mold to Platen

Regarding Lubrication: You should not need lubrication to remove the bolts from the platen unless you are using a small amount of anti-seize. If you are having problems removing the bolts from the platen, it is likely that your die setters are using too much torque on the bolts or your platen threads may already be damaged, burred, rusted, or dirty. If this is the case, you will need to repair or re-tap the platen holes to ensure proper mold clamping. Lastly, always ensure the platen is smooth, flat, and clean each time you change molds.

Additional Thoughts
Your technicians will get much more support through the use of more clamps rather than using more torque. For more on this topic, I recommend reviewing the following post: How Many Clamps To Use?

-Andy

Yesterday, I received this question from a frequent blog contributor...

Milan
What is the difference between molded-in compressive stresses and molded-in orientation stresses?

My Response
I will first define the two concepts in practical terms, and then address the differences between the two...

Molded-In Compressive Stress - As the polymer fills the mold and cools, the polymer begins to shrink. As the polymer shrinks, additional polymer is then forced, or packed, into the mold to compensate for this shrinkage. As additional polymer is packed into the mold, internal pressure can build up causing general compression. Some molders will refer to a part with too much molded-in compressive stress as over-packed. These stresses most-often occur when the mold temperature, melt temperature, or packing pressure is too high allowing additional packing to occur. 

Molded-In Orientation Stress - As the polymer fills the mold, the long polymer chains tend to orient in the direction of flow. Basically... more of the polymer chains are aligned in the direction of flow than are aligned perpendicular to the direction of flow. Immediately after the polymer fills the mold, the polymer chains try to orient themselves randomly (their preferred state). Since the polymer chains themselves tend to shrink more along the length of the chain, than perpendicular, the differential shrinkages tend to cause stress within the part. Additionally, the polymer chains are not in the preferred random orientation, so there will always be some internal stress due to orientation. Even though these stresses will always exist, they are made worse by factors such as low mold temperatures and high injection speeds.

Basically, compression stress is created during packing while orientation stress is created during mold filing. As a result, you can have a part with both molded-in compressive stress as well as molded-in orientation stress. Although both can cause a bad part, a good molding process is typically a balance of minimizing these affects and maximizing productivity.

Additional Thoughts
There are many applications where a molded-in compressive or orientation stress is beneficial to the products performance. For example, hinges and cable ties get their strength and longevity from a high degree of molded-in orientation. Likewise, many molders improve the useful life and performance of shock absorbing components when they have internal compressive stresses.

In some engineering applications, the molder will heat treat, or anneal, the molded parts to allow the polymer chains to align themselves in a more random state to reduce the internal stresses.

-Andy
I just got this quick question from one of our blog readers...

Larry
We have a small nozzle mold (0.2oz of Lexan PC) that traps air, but we can't seem to find a way to vent it. The only success we have had is to reduce the clamp tonnage. Can you briefly state a procedure for identifying where to vent?

My Response
Since a clamp tonnage reduction helps your part quality, you have correctly determined that it is a venting issue. Additionally, it is most likely that you only need venting on the parting line to correct the issue.

The easiest way to determine the vent location is to mold a short shot which will allow you to see where the end of the flow front is located. In most cases, this is where the venting should take place.

Additional Thoughts
Since you are molding PC, your application may be optimized with a large vent encompassing most of the parting line.

In any case, your mold should have a deep channel behind the vents to ensure the venting can leave the mold easily and quickly.

-Andy


This e-mailer had a long question about texture and shininess. I have shortened it to include the critical portions...

Nathan
The largest part quality issue is shininess in the textured portion of the part. A particular part is molded out of PC/ABS C6600, we have performed some studies and came up with the following conclusion: 

The draft angle on the texture should be modified to allow the gas freely from the texture grains. This would allow the melt to attain the shape on the texture depth. Even though proper venting is done, the gas must escape from the texture grains is so important than others.

Does this sound correct?

My Response
You are correct in believing that the melt is not properly attaining the shape of the texture. Basically, proper surface texture reproduction is the result of good interaction between the polymer melt and the mold surface during mold filling. If the melt cannot properly reproduce the mold texture, it obtains a shiny appearance. As a result, the primary factors which improve this are an increase in melt temperature or mold temperature at the point of contact.

Regarding mold temperature: There is really only one way to increase mold temperature... by increasing the coolant temperature, or reducing the coolant flow.

Regarding the melt temperature: Since we are only concerned with the temperature of the melt as it contacts the mold surface, there are five ways to improve this: 1) increase barrel temperature 2) increase screw recovery speed 3) increase back pressure 4) increase injection speed and 5) decrease the gate size.

Options 1-3 are most helpful if your melt temperature is too low when measured by a melt pyrometer.

Options 4 & 5 are often the most helpful because they avoid excessive heating and degradation inside the barrel, and only provide heating through shear at the time of injection.

Additional Thoughts
If you strongly believe venting is the cause, you can always seal the parting line and add a vacuum pump to investigate that prospect further.

-Andy

About this Archive

This page is an archive of entries from February 2010 listed from newest to oldest.

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