January 2010 Archives

Although there are a variety of nozzles available to the industry, most have a large diameter opening where they attach to the barrel with a significant reduction to the final orifice where it meets the sprue bushing.There are three common ways the internal dimensions are constructed at the sprue bushing end of the nozzle:

1) GP (General Purpose) Nozzle - This nozzle typically uses a straight land area where the polymer enters the sprue bushing. For example, if the nozzle orifice diameter was .100" or 2.54mm the orifice would maintain that diameter for the length of the land. The benefit to this is the polymer tends to be pulled from the nozzle during mold opening, providing a small area for material to drool between cycles. One disadvantage to this design is that the amount of material that is removed from the nozzle can often be inconsistent. The other disadvantage to the long land area is the increased shear rate that occurs in this region. The general purpose nozzles are often helpful for molding materials which tend to exhibit small amounts of drool.

2) Full-Taper Nozzle - These nozzles have a nozzle orifice diameter which is smallest where the nozzle meets the sprue bushing. Unlike the GP nozzle, the polymer gets the least resistance to flow. The advantage to this design is it provides the most flow with the least shear. The disadvantage to this design is that any drool has the potential of causing cycling issues. Most molders use such nozzles for amorphous materials such as PC and ABS. 

3) Reverse-Taper Nozzle - This nozzle uses an orifice which has an opening larger than the inner dimensions of the nozzle. The purpose of this reverse-taper design is to promote the removal of material from the nozzle as the mold opens. This can be very advantageous for low-viscosity semi-crystalline materials such as nylon and PP which are prone to drool.

To answer your specific question, GP nozzles are bad for some materials, great for certain applications and OK for others. It may be a good idea to educate your employees on the differences, and start using more appropriate designs for applications which could benefit from reduced shear and increased flow.

Whenever possble, I opt for a nozzle which is best suited to the application. This decision would incorporate some of the following factors:

1) What is the maximum nozzle orifice diameter I should use?

2) What is the shortest nozzle I can use?

3) Would a full, straight, or reverse taper be best?

4) How long of an orifice land do I need?

-Andy

Although there are a lot of questions, I think it can be handled from a couple different angles.

1) Pressure is created by resistance to flow - During injection, a significant amount of pressure is being created in front of the screw. Since polymers are compressible, there is a counteracting pressure pushing the screw backward. If your hold pressure is less than the polymer pressure in front of the screw, then the screw will move backward.

2) Excessive mold filling during first stage - Ideally, your mold should not be completely full during first stage injection. If you completely fill the mold during first stage, then there is a spike in injection pressure as the polymer begins to pack out the mold cavity. When this spike in pressure is followed by a lower packing pressure, there is often a backflow of material out of the mold cavity... resulting in screw bounceback. Overall, this is not a good approach to processing because changes such as material viscosity or melt temperature will cause significant variation in your process. 

Your best approach is to:

1) Increase your transfer position to obtain a short shot during first stage.

2) Increase your hold pressure until you obtain appropriate part quality.

3) Perform a gate seal study to ensure the 8 sec. hold time is appropriate for the adjusted hold pressure.

-Andy

I was wondering if you help me locate some references on Processing HDPE and PP with 1% chemical foaming agent? All the information I find focuses on Structural Foam which is a low pressure process. We primarily use high pressure injection molding, using the the chemical foaming agent for weight reduction and cosmetics.

Since the banning of foaming agents containing CFC's, the use of chemical blowing agents has obtained an inappropriate stigma in the marketplace. Believe it or not, their use is not as rare as it may seem. Many injection molders will use a small amount of blowing agents to eliminate sinks, lower material costs, and even enlarge the part to help meet dimensional requirements.

In your case, there are many places to find information... when searching online, use terms such as:

or get more specific:

There are also some good articles available online:

or

Since structural foam molders use the largest volume of additives, much of the literature is focused on their needs... but you will find that many of these resources will also provide great high-pressure molding information... click for an example of this from Bergen.

You may have to register to see some of the materials from the Suppliers, but this is typically free.

Keep in mind, your material providers can be great resources if you ask the correct questions. Some additive providers provide agents specifically formulated for your type of application.

Basically, when you are conducting a high pressure molding process with blowing or foaming agents, you would melt and inject the material in a manner similar to a traditional process, but apply many of the packing and cooling strategies of structural foam. Ultimately, the more blowing agent you add... the more the polymer will behave like a structural foam after it is injected. 

-Andy

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.

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

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.

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

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.

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

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.

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

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

I recently received this follow-up question regarding an earlier blog entry...

To establish a molding parameter, what is the normal percentage tolerence to be used for the injection pressure and other parameters? Currently my process does not have any tolerence and sometimes this may cause difficulty in troubleshooting which will result short mold and etc.

In general, a well-established process encounters approximately 10% variation. For this reason, it is critical to ensure you have enough room to adjust your process inputs for this. 

For example, if first stage injection becomes pressure-limited, the machine can no longer maintain the desired injection rate, resulting in an inconsistent fill rate and injection time. This generally leads to unwanted short shots, sinks, and flash on the final part.

To avoid a pressure-limited process, you should always have more pressure available to fill the mold than is actually necessary. This will allow the machine to maintain the ‘injection speed set point’, ensuring the highest possible repeatability.

The problem you may encounter is the fact that many machines actually need an additional buffer to perform properly. For instance, a process may reach a peak pressure of 10,000 psi during first stage fill... yet, if the machine has a maximum setting below 10,500 psi, the process could become pressure limited.

The best way to approach this is to do the following...

1) Establish a good process with significantly more pressure than is necessary.

2) Reduce the maximum injection pressure until it affects the injection time by increasing the 1st stage fill time.

3) Increase this maximum by 10-15% to accomodate for material variablity.

I also recommend you review a few of our related posts... including

If you are running a lot of regrind, or off-spec material, you may want to increase this buffer to as much as 20%. In such a case, it is imperative that you use a short-shot during 1st stage fill.

-Andy

When asked 'How many clamps should I use?' one plant manager I know likes to respond with 'As many as you can'. In reality, each clamping system, mold base, and platen is different. When discussing standard adjustable height mold clamps, I prefer to use six clamps per half on smaller molds, and as many as a dozen for larger molds. In general, the spacing of the clamps are usually dictated by the platen hole location and rail size.

When tightening the bolts, I recommend using an alternating pattern which alternates back and forth as well as up and down. For example, when using a four clamp setup, starting with the top right hand corner, you would tighten in this order:

The intent is to balance the force being applied to the mold base to prevent any uneven stress on the mold base or platen.

Ultimately, the more clamps that you use, the less stress that is applied to any specific clamp point. This will increase the life of the clamps, and help prevent damage to the platen holes. 

-Andy 

I received this from a blog reader the other day...

You are close... when you calculate the intensification ratio, you need to add the surface areas of the two cylinders and use that value in your calculations.

For one cylinder:

Determining the intensification ratios for each machine is a critical step in obtaining real process output data from your molding machines.

-Andy

I can't bother to train my staff... they learn what they need to know on-the-job.

I have a problem calculating the material usage on the production floor. When I weigh 10 shots the average weight is 5.2g per shot for an 8 cavity mold. We originally calculated a material requirements of 70.2kg of polypropylene to mold 108,000 parts. When the run was complete, we used more than 70.2kg of material to mold the parts, and all 108,000 parts weighed more than 70.2kg. Can you please advise us what is the real calculation for the material usage?

Plastics materials have a tendency to exhibit variability. Using this scenario, I will suggest a few factors to incorporate, and a few strategies which may also help increase your accuracy.

Compensate for Startup and Shutdown - All processes have some amount of loss when starting up and shutting down the molding machine. Most companies know the average startup time required to initiate a production run as well as the time to shut down the machine. During this time, material is purged, and scrap parts are being generated. A good starting point for losses is to assume the machine is molding scrap parts throughout this time. Remember, if the machine is scheduled to be shut down and re-started during the run, these processes also need to the considered.

Compensate for Scrap - Since virtually every process creates scrap, you should compensate for the expected scrap rate by adding that loss to your expected amount of material usage.

Compensate for Troubleshooting - When troubleshooting, technicians tend to put more material into the mold as time progresses. For instance, when sink marks occur, the most common action is to increase packing pressure... likewise, when flash occurs, they tend to increase clamp force rather than adjust the transfer position. As a result, the part weight of the last part tends to be higher than the first parts that are produced. Therefor, if part weight is not measured and monitored regularly, I suggest you add a 10% variability factor.

Additionally, polypropylene is a highly semi-crystalline polymer. When the initial shots were measured to calculate the material usage, it is likely that the mold temperature had not yet stabilized. As a result, the mold temperature increased a little, causing more material to be packed into the mold cavity.

Ultimately, the best way to control material usage, and limit costs, is to routinely monitor the shot weight as well as minimize scrap and downtime.

-Andy