Plastics Training & Technology Blog

Converting Percentage to Pressure

By Andy Routsis on May 22, 2013 1:10 PM 0 Vote 0 Votes

It is relatively easy to convert a percentage setting on your controller to the actual pressure applied to the plastic. The most important factor is that you need to know the maximum pressure capacity.

Question: How do I convert a pressure measurement which is provided in percentage? When the panel says 50%, what is that in pressure?

My Response: Percentage is per-cent or per-hundred. This is a ratio with 100 as the maximum and the percentage as a fraction of that. 50% = 50/100 = half of the maximum. To calculate this, convert the percentage to a fraction 50% = 50/100 = 0.50 and then multiply this by the maximum machine capability.

Pressure = (percent / 100) x Maximum

-Andy

Documentation without 1st Stage Short

By Andy Routsis on May 20, 2013 2:15 PM 0 Vote 0 Votes

Essentially, you cannot generate reliable machine independent process documentation based on process outputs if the mold cavity fills completely during 1st stage injection.

Problem: I see many companies trying to utilize machine-independent process documentation while still filling the cavity completely during 1st stage injection. Since the mold cavity is both filled and packed during first stage injection it can only be repeated when the material has the identical viscosity at the time the process was documented. Each time the material viscosity changes, the amount of filling and packing during first stage injection will change, requiring adjustments to the overall process. For this reason, machine-independent process documentation is only a moderately helpful tool when the cavity is 100% full at the time of transfer.

Resolution: Fill the mold 90 to 95% full during first stage injection and then pack out the part using 2nd stage packing pressure. This will allow you the benefit of documenting important repeatable process outputs such as 1st Stage Time, Melt Temperature, and 1st Stage Short Shot Weight.

-Andy

Glass-Filled Nylon with Regrind

By Andy Routsis on May 15, 2013 9:20 AM 0 Vote 0 Votes

Since the regrind has variable sized pieces, additional drying time may be necessary to ensure the thicker chunks are fully dried. The regrind process results in irregular-sized pieces which have different cross sections.

Question: Most of our parts use an industrial grade resin ( recycled PA with GF) and often with 20% of regrind resin. This means that the viscosity can vary significantly as well as the size of the “pellet”. Any recommendation when molding with recycled PAGF resin?

My Response: Since the regrind has variable sized pieces, additional drying time may be necessary to ensure the thicker chunks are fully dried. You should always take measures to reduce the shear applied to this material. This will reduce the degree of polymer chain and additive degradation which takes place in the barrel. If the parts have thin walls, you will have to use higher 2nd stage packing pressures than you currently use, this can be more than the transfer pressure for many molds (assuming a 1st stage short shot is used).

-Andy

Hydraulic vs. Position Transfer

By Andy Routsis on May 13, 2013 11:50 AM 0 Vote 0 Votes

When filling the mold completely during 1^st stage fill, many processes respond well to pressure transfer when compared to position transfer. In either case, you will see even better results with a 1^st stage short shot (90-95%) using position transfer.

Question: I recently moved to another company and all the setups are using Injection Pressure as trigger to change from fill phase (Velocity controlled) to Holding ( Pressure controlled). Note that there since there is no pressure sensor in the mold but the machine injection pressure is used. A Engineer here tells me that this was done because while using a screw position switching point they had too much variation of the cushion leading to short shot and flash. What are the pros and cons of a pressure switching point versus screw position switch point?

Regarding Hydraulic Transfer: Hydraulic pressure transfer is used the fills mold completely during first stage. Once the mold fills, the pressure begins to increase rapidly and the pressure transfer method is used to identify this pressure spike and then transfer. When filling the mold completely during 1^st stage fill, many processes respond well to pressure transfer when compared to position transfer. In either case, you will see even better results with a 1^st stage short shot (90-95%) using position transfer.

My Recommendations: The process should run with position transfer to best compensate for the viscosity changes. The part should be 90 to 95% short shot based on part weight. If this is a multi-cavity mold, 100% full is the weight of all parts when the first cavity becomes full (this ensures all mold cavities are short shot). The packing pressure should then be used to compete mold filling and pack out the parts without flash. You will likely use a significantly higher packing pressure, but will gain more process consistency as a result.

-Andy

Improving Process Efficiency - Packing Time

By Andy Routsis on May 6, 2013 2:05 PM 0 Vote 0 Votes

A 2nd Stage Packing study is a critical aspect to optimizing any molding process. You should perform a gate seal test for each of your molds to ensure you are not using wasting energy with excessive packing time.

Gate Seal Time: To determine the time required for the gate to seal requires a gate seal study. To perform this test, you mold a series of parts using different 2nd stage times and weigh each part (without gates and runners). When graphed, you will see the part weigh increase with packing time until the gate seals. Once the gate is sealed, the part weight should stabilize for a cold runner mold. If a hot runner gates directly into the part, the part weight will stop curving and become linear at the time of gate seal.

Packing Time: When gate seal is desired, the optimal time used for 2nd stage packing is 10% greater than the gate seal time. When a non-seal condition is desired, the 2nd stage packing time should 10% lower than the gate seal or less.

-Andy

Molding Issues with Red PMMA

By Andy Routsis on May 1, 2013 2:10 PM 0 Vote 0 Votes

Design issues are very difficult to process around. Your defects indicate a potential venting problem which should be addressed first by reducing clamp tonnage and/or adding more vents.

Question: I am processing both clear and red transparent PMMA of the same grade. For the red part, the thickest area is 4mm compared the average wall thickness 2.5 mm. It's easy to get shrinkage in the 4 mm position. Each component gets one hot runner gate. Thick area is far from gate compare the conjunction area with clear component. Here is the problem. When you increase the holding pressure or holding pressure time, it easily leads to shining line even break in the conjunction area. I also try to set the holding pressure very high at the first 0.3 second till reduced pressure to 13 seconds. But it doesn't work really. The mold gets some problem, but I want to compensate it with the processing.

My Response: You are correct, this is a design problem which is difficult to process around. The shiny line and breakage defects may indicate a potential venting problem which should be addressed by reducing tonnage and/or adding more vents. The best way to attempt processing this part is to fill the mold with a short shot during 1st stage fill, and then use one pressure for the entire second stage packing until the gate is sealed. To determine the best pressure, perform a study to determine the highest and lowest pressure which will provide an acceptable part.

Improving Process Efficiency - Packing Pressure

1st Stage with Multi-Cavity

-Andy

Recovery Issues with Red PMMA

By Andy Routsis on April 29, 2013 11:15 AM 0 Vote 0 Votes

The red colorant will affect the material conveyance within the feed zone of the screw. Different colorants can cause many intentional and unintentional purposes including a plasticizer, lubricant, slip agent, insulator, etc.

Question: I am processing both clear and red transparent PMMA of the same grade. My question is why is the plasticizing of the red material inconsistent. Both the temperature setting is from nozzle the the feedthroat, 240, 240, 230, 190, 60 degree. The plasticizing speed is 0.1 mm/s, back pressure is 15 bar. The decompression after plasticising is ok. However, the red component isn't stable compared to the clear one. Also I hear the screaming when in plasticising. But others for PMMA, it's normal.

My Response: The red colorant may be affecting the material conveyance in the feed zone. In such a case, you will need to perform a rear zone temperature study (currently 190) Essentially, the red color may need a different rear zone temperature to get the desired material conveyance in the feed zone.

When to Optimize the Rear Zone

-Andy

Improving Process Efficiency - Back Pressure

By Andy Routsis on April 22, 2013 10:00 AM 0 Vote 0 Votes

Unless there is a mixing issue, you should use a low back pressure when possible. This approach will minimize the energy consumed in recovering the shot.

Back Pressure: Any additional back pressure will cause the screw to rotate additional turns due to the polymer backflow over the flights of the screw. Using more back pressure than necessary to create a consistent shot can causes a 5-10% increase in the energy consumed during recovery.

Recovery: In most processes, there is more energy consumed in screw rotation than any other aspect of the process. As a result, any reduction in energy consumption during screw recovery will benefit the overall process.

-Andy

Dry vs. Wet Purging

By Andy Routsis on April 15, 2013 9:45 AM 0 Vote 0 Votes

Materials with poor conveyance tend to be more successfully purged using wet purging option. Materials with good conveyance tend to be purged successfully using either method.

Question: Everybody purges differently, some claim dry purging is best while others tell me it is a bad idea... what is the best method?

note: 'Dry' purging refers to purging all the material possible out of the barrel before adding the next material. 'Wet' purging involves retaining the previous material in the feed zone of the screw when adding the next material.

My Response: There are many theories regarding purging, and most have validity in some cases. For example, some people state that dry purging will increase screw and barrel wear, while many purging companies claim this is not likely as there is always material left in the screw and barrel. In practice we have found the following to be true, dry purging tends to be more successful than wet purging with most materials and purging compounds. The biggest caveat relates to plastics which are difficult to feed. Such materials are tend to purge better with wet purging as the subsequent material will help convey and remove the previous material through the screw. This works especially well with purging compounds which have expanding components which help material conveyance.

-Andy

Improving Process Efficiency - Cooling Time

By Andy Routsis on April 9, 2013 7:20 AM 0 Vote 0 Votes

In my experience, most molders do not take the time to determine the optimal cooling time. This should be done after the minimal acceptable mold temperature is determined.

Cooling Time Study: Essentially, you should incrementally step down the cooling time and allow the process to stabilize between each step. The parts produced at each step should be tested for quality to determine the lowest time which produces an acceptable part.

Optimal Cooling Time: The optimal cooling time setting is 10% above the minimal acceptable time. This provides a buffer to compensate for normal fluctuations in mold and melt temperature. You should allow the process to run for a while at this time while checking quality to ensure you have allowed for process stabilization.

-Andy

Global Skills Gap

By Andy Routsis on April 4, 2013 10:40 AM 0 Vote 0 Votes

In the global picture, employers in the USA actually fair relatively well in locating skilled production employees and engineers. Believe it or not, finding skilled employees is a world-wide phenomena.

Problem: Managers often say..."In our town, it is tough to find skilled production workers" or " There is a significant lack of engineers in our state". Basically, the lack of industry-specific trained personnel is a global issue in most developed countries. In difficulties in finding Engineering talent US ranks 6th behind Brazil, Germany, India, Japan,and the UK (based on research by Debra Auerbach from Careerbuilder.com). Much of this stems from the fact that there is an ever-increasing amount of specialization in each branch of every industry. For example, an employee working as a technician in one packaging company is unlikely to have all the skills necessary to work at another facility without some training.

Solution: It is the responsibility of each company to develop in-house training systems so you can develop talent in-house. This includes both the employees who are currently in your facility who want to advance as well as those who you hire and need to get up-to-speed.The most efficient in-house programs provide metered training and hands-on exercises over an extended period of time. This will instill the desired practices, habits, and behaviors in your employees.

Please feel free to read the article mentioned in this post:

http://msn.careerbuilder.com/Article/MSN-3313-Job-Info-and-Trends-Infographic-Skills-gap-impacting-employers-across-the-globe/?SiteId=cbmsn43313&sc_extcmp=JS_3313_advice

-Andy

Good Part vs. Good Process

By Andy Routsis on April 1, 2013 9:00 AM 0 Vote 0 Votes

There is a big difference between making good parts and developing a good process. The problem with traditional troubleshooting is the focus on making good parts.

Good Parts vs. Good Process: Just because you are making a good part does not necessarily mean you have a good process. If your technicians only focus on making good parts, then they are just concerned with making an acceptable part at the moment. Unfortunately, if there is not a good process making a good part, then the process may not be robust enough to compensate for variability. A good molding process is designed to compensate for the natural variability of plastics.

-Andy

Relevant Training Will Help

By Andy Routsis on March 25, 2013 9:50 AM 0 Vote 0 Votes

It is never a bad time to train your employees. If you are having a problem keeping up with production, then effective, relevant workforce training will help you get more out of your production facility.

Training: Improving the knowledge of your employees will provide them with a better ability to handle adversity and make educated decisions. There is a difference between good parts and a good process. If your employees do not know how to develop a good process, then they are going to constantly adjusting the process to make good parts.

-Andy

Improving Process Efficiency - Mold Temperature

By Andy Routsis on March 18, 2013 9:50 AM 0 Vote 0 Votes

Since cooling time typically consumes 40-60% of the overall cycle time, it is very important to optimize the mold temperature. Although there are many aspects to mold temperature, this will give you some steps you can take to begin the optimization process

Find the Specifications: The first step in the process is to determine the recommended mold temperature range for your material. This should be available from the manufacturer, design guidelines, processing guidelines, online, or in a guidebook.

Optimize Process: You should initially try to produce acceptable parts using the lowest recommended mold temperature. If this is not possible, then increase the temperature and try again. Essentially, the lowest mold temperature which provides acceptable parts has the highest likelihood to provide the lowest cycle time.

-Andy

US Manufacturing Tax Incentive

By Andy Routsis on March 11, 2013 10:40 AM 0 Vote 0 Votes

The popular Section 179 deductions on equipment purchases have increased from 125,000 to $500,000 in 2013. This rapid deduction allows companies to deduct up to one half a million dollars in just one year without having to depreciate the equipment. The idea behind such a tax credit is to encourage companies to purchase new equipment because as they purchase new equipment and thus hire more people.

Concern: Our industry has a habit of buying new equipment such as electric molding machines and low RPM grinders, but are using them properly. It is critical to know how to get the most out of your investment. To get the most out of your molding machine, you should optimize the fill, packing, recovery, temperatures, part removal, and documentation. These high performance machines are a great investment, but improper processing is like driving a Ferrari around in first gear.

For more information on the tax incentive visit:

http://www.section179.org/

-Andy

Shorts, Sinks, & Flash

By Andy Routsis on March 6, 2013 12:20 PM 0 Vote 0 Votes

During a recent discussion, a manager related to me that their most common defects are shorts, sinks, and flash. She then stated that this is inherent to the molding process could not really be changed.

Shorts, Sinks, & Flash: Whenever these three are the top defects at a facility, it is because the molds are being filled too much during 1st Stage Fill. When the mold fills too much during first stage fill, the packing pressure is primarily used to ensure gate seal since the mold cavity is full. This only works when the material viscosity is identical to the time the process was established. When the viscosity drops, the mold will flash, when the viscosity rises, the mold will short or sink.

For more about this, please feel free to read:

1st Stage with Multi-Cavity

More Discussion about Open vs. Closed-Loop 1st Stage Injection...

-Andy

Improving Process Efficiency - Packing Pressure

By Andy Routsis on March 4, 2013 12:25 PM 0 Vote 0 Votes

Process efficiency is one of the largest concerns in the industry today. Improving the efficiency and reliability of a process is a great way to reduce overhead, this post specifically addresses optimization of packing pressure.

Packing Pressure Window: Assuming the transfer is developed with a 1st stage short shot, you should determine the minimum and maximum pressure which provides an acceptable part to determine the process window.

Packing Efficiency: Once you determine the range or acceptable packing pressures you can select the best packing pressure based on your application. For example if it is an engineering part, you might select a pressure in the middle. If it is a commodity part, you may use a pressure near the lower end of the window to reduce part weight and material usage by as much as 4% for thick walled parts.

-Andy

Improving Process Efficiency - Transfer

By Andy Routsis on February 26, 2013 3:50 PM 0 Vote 0 Votes

This is another installment in a series of posts on improving process efficiency. Using the appropriate transfer from 1st Stage Injection to 2nd Stage Packing will help reduce equipment wear and overall energy consumption.

Regarding Energy: When you transfer when the mold is completely full, the pressure in the barrel increases sharply. This increase is caused by the machine's attempt to maintain the programmed screw velocity. This increase in pressure causes a significant rise in cavity pressure during injection. This rise in pressure causes the need for additional clamping force to counteract injection forces.

Regarding Process Consistency: The process should be designed to use a 5-10% short shot at the time of transfer with enough packing pressure to pack out the part without flash. When normal viscosity fluctuations occur, the process should remain short during injection while enough pressure is available during packing to fill and pack the mold. This will provide a more robust and reliable process that compensates for normal variation.

-Andy

Improving Process Efficiency - 1st Stage Injection

By Andy Routsis on February 19, 2013 12:00 PM 0 Vote 0 Votes

This is the first in a series of posts I will make on easy ways to improve efficiency and reduce costs. Reducing the 1st Stage Injection time is one of the easiest ways to reduce both the cycle time and energy required to inject the polymer.

Regarding Energy: Increasing the rate of injection will increase the shear applied to the polymer. As with all polymers, as the shear rate increases the polymer viscosity is reduced due to shear thinning. This reduction in viscosity causes the overall amount of energy required to fill the mold cavity.

Regarding Cycle Time: Decreasing the time required to fill the mold will cause the polymer within the mold to begin cooling faster. This will reduce the overall amount of cooling time necessary to prepare the part for ejection.

-Andy

A Note to Educational Institutions

By Andy Routsis on February 15, 2013 3:25 PM 0 Vote 0 Votes

Whenever you are soliciting donations from industry, always make the donor feel that you are extremely grateful. It is the job of every educational institution to ensure that their contributors feel appreciated and that they are not sending their money down a black hole.

Make it Visible: Place placards or signs on donated equipment and material stating who donated it and when. Always keep the area around the equipment clean and professional looking to demonstrate respect. If a piece of equipment is no longer needed, offer it back to the company before taking it off display (they might offer you something useful in exchange).

Show Your Appreciation: Give the donor a placard, certificate, etc. that they can display in their lobby or conference room. This keeps your institution in their minds and gives the added benefit of others becoming aware of your institution. If this is dated, it will prompt the donor to keep their placards up-to-date.

Spread the Word: When possible, get the contribution written up in either in a paper or on a website. You should also have a special section on your website where all donors are featured. This will give exposure as well as create some potential competition between donors.

-Andy

Slowing Down the Screw

By Andy Routsis on February 6, 2013 7:50 AM 0 Vote 0 Votes

If you have to slow the injection speed down to prevent overshooting or gas entrapment during injection; then you are probably filling the mold too much during first stage fill.

Comment: We can use one speed to fill most of the mold, but we always have to slow the screw down as it gets to the the end of fill to prevent flash and gas entrapment.

My Response: The best approach to complete the filling of the mold is to transfer to pressure-controlled packing and let the screw slow down as the mold fills and the pressure builds in the cavity. Actually, the point where you start your slowdown is likely to be a good transfer position.

Additional Thoughts: Keep in mind, you should transfer from fill to pack at the position where the mold cavity is 90-95% fill. This is based on the mold cavity being 100% full with 1st stage only (when the first cavity fills if multi-cavity).

-Andy

High Injection Pressures

By Andy Routsis on January 31, 2013 11:40 AM 1 Vote 0 Votes

It is always to avoid running a pressure-limited process. Any process which is pressure limited will have and inconsistent 1st stage injection weights and 1st stage injection times.

Question: The company I work for now, wants all injection/boost pressures set at max 2,000 to 2,300 psi. I was told that it insured a more consistent shot and was also told that this would blow through any nozzle tip or gate freezing. Wouldn't this be harder on the check ring?

My Response: The company uses max pressure to ensure the machine always operates under velocity control which is very important. Not all companies need to use full pressure, typically 20% more pressure than what is necessary will work on most machines. This would not be the cause of the early check-ring wear, it is very possible that other causes such as abrasive additives or inferior check rings could be causing this.

-Andy

When to Optimize the Rear Zone

By Andy Routsis on January 15, 2013 8:45 PM 0 Vote 0 Votes

The optimal rear zone temperature is specific to each machine and material combination. For this reason, a rear zone temperature study should be performed each time a new material is molded on a particular machine.

Question: In my case we have machines from different brand, so a study in the rear zone temperature must be performed. So, in a situation like that, how do we perform that study? Under which conditions could we perform the procedure?

My Response: Since the rear zone temperature has no significant affect on the polymer melt, the following test can be performed during production. This can allow you to adequate time to allow the rear temperature zone to stabilize and obtain excellent results.

Rear Zone Temperature Study Procedure:

Set the molding machine to manufacture stable product
Set the rear zone temperature to the manufacturers recommended minimum
Allow the rear temperature to stabilize
Record the 'Rear Zone Temperature' and the 'Recovery Time'
Increase the rear zone temperature incrementally
Repeat steps 3-5, but do not exceed the manufacturer's recommended maximum
Graph the data: Time (Y axis) vs. Temperature (X axis)
Evaluate the graph and select a temperature with a low recovery time

-Andy

Always Look for Correlation...

By Andy Routsis on January 6, 2013 11:00 PM 0 Vote 0 Votes

When one process output is inconsistent, check the process data in case there is not correlating data which can help pinpoint the actual cause of the problem.

Problem: Screw recovery time variation in PBT material.

notes: The questioner sent a screenshot of the SPC data for the past 200+ shots. In this data, it was shown that the shots with a low recovery time had a high cushion, very often above the transfer or near position. Along with the inconsistent cushion and recovery time, the transfer pressure and revolutions per recovery were also unstable. Additionally the average cushion is almost 40% of the shot size.

My Response: With only recovery data, one would initially infer a feeding problem, but when you combine all the available data, it is clear that the whole process is inconsistent. The cause could be due to many issues, but the core concern here is that the inconsistent recovery was a symptom of the problem, and not the problem itself.

-Andy

Checking Your Check Ring Performance

By Andy Routsis on January 1, 2013 1:40 PM 6 Vote 0 Votes

All check rings leak, but over time the check ring will wear and leak more. The most important issue is how consistently it leaks during the velocity-controlled injection phase of the process.

Question: I have an issue with a 360 ton machine. Over the last three months, I have had most of my process' have had their shot sizes grow. Same set process' as before but as much as 10% more shot is required to fill part. The same tools in different machines are running spot on. Could this be the check ring or the barrel causing this issue?

My Response:I strongly recommend performing a Check Ring Repeatability test on all your machines as a baseline for comparison.

To perform this test:

1. Mold 10 short shots

2. Weigh each shot

3. Perform the following calculation:

100% x (Heaviest-Lightest) ÷ (Average) = %Variation

Some high precision molders will change the check ring when it exceeds 1 or 2%, while some lower precision molders might wait for it to reach 3 or 4%.

-Andy

Silverstreaking in PVC

By Andy Routsis on December 26, 2012 12:40 PM 0 Vote 0 Votes

PVC polymer chains break easily when they encounter shear. Any processing or equipment changes you can make to reduce the shear on the material will significantly improve your process.

Question: We have tried increasing and decreasing the mold temperature. We have also tried to reduce the barrel temperature with the use of a blower fan but we cannot eliminate the silverstreaking by the gate.

Additional information provided:

Defect: Silver Streak on the surface

Material: Rigid PVC (black)

Screw: GP screw (not for PVC)

My Response: The defect appears to be the result excessive shear on the material throughout the process. This assumption is reinforced by the need for the additional cooling fan on the machine barrel.

PVC polymer chains break easily when they encounter shear. The best approach to this would be to minimize shear and shear heating as much as possible. We will address this is two parts 1) processing and 2) equipment. Although the process may improve the situation, your assumption about the equipment is likely the root of the problem and may or may not have to be addressed.

1) Using a lower screw speed, lower back pressure, or lower injection speed may reduce the shear. In some cases, a higher rear zone temperature may provide more conductive heating, but this must be done very carefully with PVC.

2) A PVC screw has some of these characteristics... low compression ratio, long transition zone, and a smear tip.

If you already have another screw or tip available with any of these characteristics, you should consider trying it in that machine.

-Andy

Practice what you Learned

By Andy Routsis on December 21, 2012 1:30 PM 0 Vote 1 Vote

Just because you know the parts is supposed to 90-95% full does not mean you are actually practicing it. In a recent on-site training session the participants all understood the importance of molding a part short during 1st Stage Injection, but learned an important lesson in the lab.

Situation: In lab, we were establishing the first stage fill. We made a part which was just full and weighed the shot. In increasing the transfer position, they made a part which they thought was 90-95% full and weighed it. It turned out to be 98.5% full. They were amazed and learned a valuable lesson about why it is important to measure the parts and not just assume. The lesson is, you must practice what you learned so that you understand the application.

-Andy

Check the Melt Temperature

By Andy Routsis on December 18, 2012 8:50 AM 0 Vote 0 Votes

Unless you actually measure the melt temperature, you really do not know what is happening with the material. Recently at a molding facility, we encountered a 65 F (36 C) difference between the melt temperature and barrel temperature.

Comment from Technician: We always have to to run this mold with a hot material to get it to fill. Typically , we have to run the Polypropylene around 500°F (260°C).

Response: When measured, the melt temperature was actually only 434°F (223°C). In reality, this machine as designed with improperly located heater bands giving the impression that the material is actually much hotter than it really is. In fact, the material is being processed at a much lower temperature than most of their other machines.

-Andy

Is MFI Better than Nothing?

By Andy Routsis on December 12, 2012 11:25 AM 0 Vote 0 Votes

Having the Melt Flow Index of a material does not provide a real picture of how the polymer will actually process, but it can be a helpful indicator in some cases. This post will outline an example of how the data can be of use to a technician.

Question: MFI is not supposed to be an accurate measurement of viscosity, so should we stop testing it?

My Response: The melt flow index is a low pressure & low shear rate test with a single data-point as a result ( grams extrudate per 10 minutes). Injection molding is a very dynamic process which typically involves very high shear rates and shear stresses. The only real value MFI data provides is for relative use in material comparison for material selection.

Example: Let's suppose you are making good parts with one lot of material which has a MFI of 40 g/10min. If you are running out of material and have to change lots and you can choose between one grade with a 45 MFI and another which has a 25 MFI. You might choose the 45 MFI material because it has the highest likelihood of molding similar to the 40 MFI lot.

-Andy

Rear Zone Temperature Study

By Andy Routsis on December 10, 2012 12:50 PM 2 Vote 1 Vote

It is critical that your perform a rear zone temperature study on each of your molding machines. Just because two machines have the same brand and are running the same material, the optimal rear zone temperature is most often different.

Question: We run the same grade of PP on all our jobs and all the machines are the same brand. Do I really need to perform this test on each machine?

My Response: Yes, each machine may or may not be the same optimal rear zone temperature. I have seen a difference of 50°F using two similar machines with the same grade of material. Keep in mind, you also need to check the melt temperature since the temperatures are likely to be different.

-Andy

Monitoring Power Consumption

By Andy Routsis on December 5, 2012 5:25 PM 0 Vote 0 Votes

One great way to get your employees excited about energy reduction is to provide a method of monitoring energy consumption. If your employees can actually see that a change, such as increasing 1st stage injection speed or reducing 2nd stage packing time, actually reduces the power consumption it can become a very empowering experience. There are many options available for this, but one which allows you to track consumption for individual machines over time would be most helpful.

-Andy

Cooling Time or Temperature?

By Andy Routsis on December 3, 2012 7:00 PM 0 Vote 0 Votes

You may still have to make a few more adjustments to the cooling time and coolant temperature to get the optimal settings, but this will get you close in a very short period of time.

Question: Which do I work on setting first, cooling time or mold temperature when performing initial tool trials?

My Response: The most efficient approach is to first start with a long cooling time and determine the lowest mold temperature which allows you to mold an acceptable part. Once this is complete, you can determine the lowest acceptable cooling time based on part quality and part ejection.

-Andy

Calculating Screw Speed

By Andy Routsis on November 29, 2012 9:55 AM 0 Vote 1 Vote

The following is a formula which can be used to quickly determine the screw speed during setup. This is based on consuming 80% of the cooling time for shot recovery.

Question: Could you provide me with a quick formula my setup techs can use to estimate the final screw speed?

My Response: This is a quick set of calculations for determining the screw speed to consume 80% of the cooling time:

(RTdesired)= (tcool) x (0.8)

(RPMfinal) = (RPMcurrent) x (RTcurrent) ÷ (RTdesired)

Symbol Key

RTdesired = Desired Recovery Time

RTcurrent = Current Recovery Time

RPMfinal = Final RPM Setting

RPMcurrent = Current RPM Setting

Additional Thoughts

If you put this into a spreadsheet, then the technician would only have to enter the Current Screw Speed, Current Recovery Time, and Cooling Time.

(RPMfinal) = (RPMcurrent) x (RTcurrent) / (tcool x 0.8)

-Andy

1st Stage with Multi-Cavity

By Andy Routsis on November 26, 2012 10:10 AM 0 Vote 0 Votes

The intent of the short-shot approach is to help ensure that all cavities are short during 1st stage fill. This ensures none of the cavities are packed during fill and therefore receive the same amount of packing in all cavities during 2nd stage.

Question: How do I determine 90-95% fill for a multi-cavity tools? I'm not looking for theory, just a practical procedure.

My Response: There are many opinions on this matter, but I have had great results following the guidelines below:

Turn off 2nd stage packing and mold a short shot with all cavities short
Incrementally decrease the transfer position and mold parts until the first cavity fills completely
Weigh and record the shot weight of all cavities without the sprue or runners.
Multiply that weight by 0.90 and 0.95 to determine the lower and upper limits of your target weight
Adjust the transfer position until you achieve a 1st stage part weight between those limits

-Andy

Part Sticking and Mold Temperature

By Andy Routsis on November 21, 2012 10:30 AM 0 Vote 0 Votes

The documentation of coolant temperature both entering and leaving the mold is critical to good documentation. This reduces the time required to get to first piece approval during mold start-ups. Troubleshooting will also become much easier, faster, and efficient.

Question: We encountered part sticking after the mold ran a while. In this part, this situation is typically caused by a warm mold. We checked the water lines and ensured there was flow. We then re-configured cooling lines and even tried to put a portion of the mold on chiller water, yet the parts were sticking after about 30 minutes each time. In the end, it turned out the problem was air in the thermolator affecting the coolant flow.

My Response: Documenting coolant temperature entering and leaving the mold would significantly reduce the time it took to troubleshoot the problem. If, at the time of part sticking, the technician would first check the coolant temperature since it is already know part sticking was related to the mold temperature. The tech would immediately notice a difference between the current coolant temperatures and those of the documented standards. This difference should trigger the technician to start troubleshooting the thermolator and cooling line configuration to identify the source of the difference.

-Andy

Blocking Mold Cavities

By Andy Routsis on November 19, 2012 12:45 PM 4 Vote 0 Votes

Blocking off mold cavities will quickly cause you to quickly lose the profit margin on your existing production. Just 1 out of 8 cavities will require the mold to operate 12 percent longer to meet the same production goals.

Question: We run many multi-cavity molds and when there is a complication with a cavity, we just block it rather than fix it. We typically block 20-30% of the mold cavities during production. Is this a bad practice?

My Response: It may be necessary to block a particular cavity if the production demands delay the repair for a short while; but it is difficult to justify the blocking of mold cavities as a standard operating procedure.

When you block 25% of your mold cavities the you must run the mold 33% more cycles to achieve the same amount of production. The main reason a molder uses a high cavitation is to increase productivity due to either customer demands or profit margins. Blocking off cavities generally counteracts the benefits of the high cavitation. This most often results in either a break-even or profit-loss for the production run. In today's tight market, it is difficult to envision such a company remaining competitive over the long term with such a high margin of loss. In any case, it is critical to institute a repair policy to begin returning the tools to proper operating condition. Without such a plan, your facility will never be prepared for growth as it's production capacity is hindered by the artificially high production runs.

In the case that a molder must block off a cavity, it is critical that they maintain the same process outputs such as fill time and packing pressure. The only process output adjustments will be a reduction in 1st stage fill weight and final part weight due to the reduce number of cavities. In this case, the molder should schedule the mold to be repaired at the earliest convenience.

-Andy

Learn and then Teach Others...

By Andy Routsis on November 12, 2012 11:40 AM 0 Vote 0 Votes

Just a quick comment today...

I am often asked how to best ensure employees are learning the information provided in hands-on training. There are methods of testing which can be used effectively, but the best method is to have your employees teach the hands-on portion to others. If someone learns how the training relates to their job, and then demonstrates that information to someone else, then the information becomes ingrained deep in their mind.

-Andy

Just Start Documenting...

By Andy Routsis on November 7, 2012 10:30 AM 0 Vote 0 Votes

I was recently asked this question...

Manager

My technicians are now excited about doing some more advanced process documentation. When should we start?

My Response

Today. You don't have to make a big production about it, just get started with an approach which fits your companies culture.

Additional Thoughts

I have seen three approaches work with great success.

All-In Approach - In this approach, the company starts using an entirely new machine independent process documentation system. All older process documentation sheets are systematically replaced and removed. This is the fastest way to convert, but is often the most contentious, especially with the more seasoned molders.

Dual Approach - In this approach, the company creates machine independent process documentation which is associated with the mold. Typically, the company uses this approach to allow the technicians to reference both machine dependent and machine independent documentation. This approach is typically the easiest to implement, but the most confusing with respect to which documented process is the actual standard.

Gradual Approach - In this approach, machine independent parameters are gradually added to the process documentation while machine dependent parameters are gradually removed until fully machine-independent documentation becomes commonplace. This is the most common approach I have seen in the industry.

-Andy

Again, Don't Discount Your Employees...

By Andy Routsis on November 5, 2012 10:35 AM 0 Vote 0 Votes

Often I am approached by employees who want to improve their skills, yet their employer does not want to invest in training. Remember, each employee wants to do a good job... if they did not want to do a good job, you would not have them on your staff.

Always Nurture a Desire to Learn

When your employee is actively trying to improve their skills knowledge, pay attention because that is the type of employee motivation employers want. Your best course of action is to investigate what they want to learn and evaluate the benefit to the company. If their desired learning tract is not exactly in-line with the company goals, then try to find a similar form of training which will also benefit the company.

-Andy

Mold Trials Under Realistic Conditions...

By Andy Routsis on October 31, 2012 12:00 PM 0 Vote 0 Votes

I was recently involved in a consulting project with a common issue...

Situation

The company has purchased a new mold and trials were run. Unfortunately, the PP parts were failing performance tests and exhibiting cracking. The company was investigating the tool in hopes of determining what is different with the new tool.

Problem

Upon investigation, the tool was not much different, but they were not using a process similar to that of the production tool. They were using less than 5% of the overall cooling time to recover the next shot causing the material to become over-stressed during melting. Additionally, they were using time for 1st stage transfer causing over-packing during 1st stage fill. This creates additional molded-in stresses in the part contributing to the part failure.

Additional Thoughts

Just because you are running trials does not mean you can rush and cut corners. Always take the time to develop a good process so you can derive applicable data form your efforts.

-Andy

Why Optimize Packing Pressure...

By Andy Routsis on October 29, 2012 10:05 AM 0 Vote 0 Votes

I was asked this question during a training class...

Student

Does it really matter if we optimize the packing pressure?

My Response

Although it depends on the part geometries, an excessive packing pressure can easily add 1-3% to the overall part weight. In an commodity market, such an increase in material consumption may have a large impact on the overall profitability of the process. In many packaging markets, a 2% increase in material consumption can result in an overall profit loss.

Additional Thoughts

In today's globally competitive market, molders should take every opportunity to improve efficiency... especially when there is no capital investment necessary. Every step in optimizing your filling, packing, cooling, recovery, and part removal results in direct increases in your overall efficiency.

-Andy

Inconsistent Dryer Dewpoint Readings...

By Andy Routsis on October 26, 2012 9:50 AM 0 Vote 0 Votes

I was asked this question recently...

Kevin

We were encountering complications with the drying of nylon. We have attached a dewpoint meter to our desiccant dryer and the measurements range erratically between -37 and -10°C while operating. The desiccant regeneration is set to 3200 second. There seems to be plenty of suction on the return line from the hopper to the unit. Could you recommend What is your interpretation of the data and how do you think it will affect the performance of our equipment?

note: Kevin attached photos of the dryer and the dewpoint measurement graph showing an erratic fluctuations with a slight incline at the regeneration period.

My Response

You should consider shorter bed times and/or replacing the desiccant bed if it is older.

It would be worthwhile to check all the seals, connections, and filters to see if there is a leak causing moisture from the atmosphere to leak into the hopper.

Additional Thoughts

If you are not getting adequate support and guidance from the dryer manufacturer on this matter, I strongly suggest considering another brand.

-Andy

More on Venting...

By Andy Routsis on October 22, 2012 10:35 AM 0 Vote 0 Votes

I have received a few follow-up emails on venting, and would like to add the following comments and links for information...

Alternate Venting Techniques

Ejection - You should always consider venting your moving components such as ejector pins, ejector blades, lifters, stripper plates, and slides are all great components to vent. Since these move each cycle, they usually act as self-cleaning vents. Components such as vented ejector pins are already available commercially through many suppliers.

Porous Mold Steel - As long as you can vent to atmosphere, porous steels can be a great option for improving venting. These materials allow air to actually vent through the steel. For some design guidelines for such materials, please feel free to visit this helpful site:

http://www.imsteel.com/plastic_mold_steels/porcerax/porcerax_design_guidelines.htm

Vacuum venting - To improve the efficiency of venting, many molders are turning to 'negative cavity pressure' techniques. These systems help draw the air out of the mold cavity faster allowing you to inject faster and maintain a more repeatable process. For a design guideline for vacuum venting, please feel free to visit this helful site:

http://www.moldvac.com/tooling-modifications.php

-Andy

You Can't Over-Vent...

By Andy Routsis on October 17, 2012 10:10 AM 0 Vote 0 Votes

I received this commentary form one of our frequent blog contributors...

Nathan

I expect you to give a very strong message on tool venting in your Plastic Training and Technology Blog. Basically, many of the Moulders are not aware of venting. What should be the best way to make them understand?? Even though they agree, they still have a strong concern on venting which may cause flash on the product.

My Response

The best rule of thumb is 'You can never over-vent your tool'.

Since the melted polymer will not flash if the vents are of appropriate depth, the number of vents has no affect on the likeliness of flash.

When I am involved in a tool design or revision, my motto is to vent everything... runners, cavities, parting lines, ejector pins, slides, and lifters. Drawing a vacuum on the mold or incorporating porous materials such as Porcerex are very helpful options.

Additional Thoughts

Poor venting often contributes to defects such as burning, poor surface finish, short shot, and part hang-up. This also restricts your ability to increase injection velocity to help correct many defects including poor surface finish, short shot, sinks, runner flash, voids, large dimensions, and warpage.

Essentially, inadequate venting restricts your ability to develop and maintain a good Scientific, or Decoupled, molding process.

-Andy.

Material on the Screw...

By Andy Routsis on October 9, 2012 9:40 AM 0 Vote 0 Votes

Someone asked me this during a classroom session last week...

Participant

How long can material remain stuck to the screw?

My Response

Indefinitely. Essentially, if the material adheres to the screw, it can degrade and become baked onto the surface of the screw. I have seen this with materials and additives such as colorants. This is one of the reasons you should take great care of the polymer during screw recovery to promote smooth material conveyance and reduce excessive back-flow and stagnation.

Additional Thoughts

Although there are many advances in purging compounds over the years, the most effective method of cleaning the screw is still to actually remove the screw and manually clean the screw surface.

-Andy

Process Outputs for Blow Molding...

By Andy Routsis on October 1, 2012 6:45 AM 0 Vote 0 Votes

I was recently asked this interesting question via email...

Technician

I hear all this discussion about process outputs for injection molding. Is this also important for blow molding?

My Response

Yes, process outputs are important parameters to document. These could include parameters such as melt temperature, coolant temperature, part weight, screw RPM, and flash weight can all be useful.

In the extrusion-blow process, the outputs from both the injection and blow stages can be very helpful in both setup and troubleshooting.

-Andy

Caring for Your Material...

By Andy Routsis on September 25, 2012 6:25 PM 0 Vote 1 Vote

Lately I have noticed a great deal of companies ignoring the integrity of the material during screw recovery. During a recent on-site training visit, one student was encountering significant quality issues with a particular product molded from TPE. In reviewing the situation, it appears the material may be damaged or degraded during recovery which will contribute to the quality issues they are encountering.

When recovering the next shot, it is important that you consider the following factors.

1) Melt Temperature - Don't rely on your barrel temperatures alone, actually measure the temperature of the polymer melt once the process has stabilized.

2) Rear Zone Temperature - The rear zone temperature has the highest influence on the material conveyance through the feed zone to the transition zone.

3) Screw Recovery Time - The recovery time should consume about 80% of the overall cooling time to maximize melting efficiency.

4) Back Pressure - The back pressure should remain low unless there are specific mixing issues or inconsistent recovery times.

-Andy

Only 2% Improvement...?

By Andy Routsis on September 19, 2012 9:15 AM 0 Vote 1 Vote

In response to a recent class, a participant encountered this result...

Participant

We have very fast screw recovery times, typically less than 2 seconds. When performing the rear zone temperature study, there was a steady increase in recovery time from 1.78 sec. @ 225°C to a result of 1.82 sec. at 260°C. Does this seem like a valid result?

My Response

This is a perfectly normal result, especially when the screw is properly designed for the application. In actuality, the lower rear zone temperature demonstrates a 2% drop in screw recovery time which usually translates to a 2% drop in the energy required to melt the material.

Additional Thoughts

Screw recovery typically consumes the greatest amount of energy during the molding cycle. A 2% drop in energy consumed during recovery might result in a 1% drop in overall energy consumed... this can become very significant to the bottom line of a high-speed consumer molder.

-Andy

Understanding Delamination...

By Andy Routsis on September 17, 2012 9:50 AM 0 Vote 1 Vote

I occasionally see employees confuse blisters and delamination and wanted to clarify this here.

Delamination

Delamination is the layering of the polymer as it fills and packs the mold. With this defect, different unconnected layers of plastic develop and form within the part.

Blisters

Blisters are gas bubbles which appear as small bumps on the surface of the part. This is caused by either moisture, air, gases or volatiles being present in the resin or on the mold surface.

Causes

Both defects can be caused by moisture in the polymer or material degradation. Blisters are more commonly causes by poor material handling while delamination is more commonly the result of degraded material or high injection pressures.

-Andy

Learn and Apply...

By Andy Routsis on September 7, 2012 9:40 AM 0 Vote 0 Votes

I wanted to bring up a very important aspect of employee training...

Learn and Apply

Training is most effective when it has at least two components. The initiative should have aspects intended to teach the important underlying concepts to the employee. These are intended to provide the 'why' aspect of the learning. In general, people are less likely to implement a concept if they do not understand the underlying concept behind the desired action. For example, if your employees do not understand the basic concepts of material feed and conveyance, it is difficult for them to understand why a rear temperature study should be performed.

Likewise, the training must have a practical aspect to teach the participant the desired skills. This backs up the preceding 'why' instruction with the all-important 'how' instruction to ensure the employee knows how the information they learn applies to their actual job.

-Andy

This informative blog allows plastics professionals to discuss Plastics Training & Plastics Technology. Please email blog@traininteractive.com or call (978) 957-0700 to open up a new discussion.

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