March 2010 Archives

There are always exceptions to the rule, but it is more important to teach your employees the fundamentals first... and then explain the exceptions that might apply to your application.

When you handle the teaching of an exception, it is most important that you explain why this exception occurs. 

In most cases, an exception occurs as a result of factors such as a shortcoming in the mold or part design or possibly a need to overcome a deficiency in the processing equipment.

-Andy

Yes, back flow does occur within the barrel in three ways:

1) Material will flow back over the check ring - During injection, all check rings will leak a little, causing some back-flow along the screw.

2) Material will flow back down the screw - During injection the material that leaks around the check ring will push polymer back down the screw channel.

3) Material will flow over the screw flights - During screw recovery, polymer will pass over the flights of the screw. This mixing will increase with an increase of back pressure.

All this back-flow exists in virtually all reciprocating-screw molding machines. If you are processing a heat sensitive material... you need to do your best to minimize these affects by maximizing the efficiency of your screw recovery, and monitor the performance of your check ring.

-Andy

Note: Nathan also provided many photos and specifications for both the tool and parts which we will avoid displaying these to protect his customers.

The three biggest factors that hinder part ejection and hold the core to the part are: the force holding the part to the core, the vaccuum forces holding the part to the core, and the coefficient of friction between the core and the part.

First, the hoop stresses due to shrinkage causes the part to contact the core even after it begins moving off the core. The warmer the part is during the time of ejection, the less this stress will hinder part ejection.

Second, highly polished surface of the core can create such a good seal with the core, it can help to maintain the vacuum seal during part ejection. Applying a very slight texture or surface treatment to the core surface can often reduce this seal and help gas get under the part, thus reducing these vacuum forces. Many molders will also use porous steel and air poppet valves to help facilitate this removal process.

Third, the coefficient of friction for most tool steels can be improved significantly through the use of dry lubricating surface treatments such as dicronite. This is one of quickest ways to overcome friction and improve part ejection.

One great benefit to surface treatments such as dicronite, you can see the coating on the core, so any wear or damage to that coating can be easily tracked and monitored.

-Andy

Personally, I view a 100% full part as the weight of one where the polymer just reaches the end of fill… such a part would have little to no packing. Some people view it as a percentage of a complete filled and packed part, which works well for thin wall molding. Unfortunately, this not work as well in thick parts where the packed material can be a significant percentage of the overall part weight.

The most important aspect is that you achieve a visible short at the end of first stage.

-Andy

note: Cashew gates, or banana gates, are semi-circular gates which tunnel under the parting line to gate on the underside of the part. During ejection, they detach from the part and curl out of the mold. Since these gates cannot be machined directly into the core, they are difficult to machine. As a result, the gate has to be machined into a pair core inserts, each insert contains half of the cashew gate detail. This is done using a CNC milling machine or a die sinker EDM machine.

There are two complications with this gating system which must be overcome before it becomes a viable option.

1) The material must be flexible enough to curl out of the mold during ejection, yet it must be strong enough to not break. Very soft elastomers, for example, will generally break off, leaving pieces of debris inside the gate during the gate removal process. This can either block the gate... or create solids which may act as contaminants for the next shot.

2) The underside of the part must not be cosmetic. Since the gate enters the mold cavity perpendicular, the gate removal process often leaves a large and inconsistent amount of gate vestige behind.

When implemented properly, cashew gates can be a good way to disguise the gate location.

-Andy

One machine has a 30mm screw (3100 bar max pressure), and one has a 40mm screw (1800 bar max pressure). Both have the same size injection cylinders. When I transfer from the 30mm to the 40mm machine, do I need a higher pressure pump to move the screw the same distance.

In reality, the larger diameter screw changes all the aspects of pressure, velocity, and distance.

Since the surface area of the 30mm screw is 56% of the 40mm screw, you need to approach the process with the following assumptions:

1. To maintain the same injection volume, the 40mm screw will need to travel approximately 56% of the distance.
2. To maintain the same injection rate, the 40mm screw will need to travel at a speed approximately 56% of the 30mm screw.

In most cases, you will need more pressure only if the machine is maxed out... and you must run a pressure limited process to make a good part.

Basically, this is why it is critical to document your process based on process outputs such as fill time, part weight at the end of fill, and plastic pressure at transfer. Such an approach will eliminate the need to make such conversions.

-Andy

note: after a few emails, it was determined...

As a general rule, a 3-4% variation in fill time is not unusual with a hydraulic molding machine.

Although common, one way to improve this variation is to tailor the process to the machine's capabilities...

1) Make sure the machine has adequate maximum injection pressure. Also read: Providing a Buffer To Accommodate for Variation

2) Make sure the hydraulic valves and transducers are in good functioning order. Also the hydraulic fluid should be sent to a lab every six months to check for contaminates and breakdown. Also, the filters should be checked and replaced routinely.

3) Your machine may have a hard time during velocity transitions, causing a pattern or cycle of overshoot and undershoot. This may also be seen at the beginning and end of fill. In these cases, a stepped velocity profile will adjust the velocity to one which is easier for the machine to maintain.

Although closed loop controls are very helpful, it is often necessary to adjust the process to compensate for a machine's shortcomings. Keep in mind, once you identify an issue with your molding machine, you should work with the manufacturer to better understand the cause. They might have encountered the same issue with another customer... and might already have a fix which you might not have considered.

-Andy

One of the most common vent drop is a deep channel machined around the perimeter of the mold cavity. This method is often preferred because it is easy to add more venting since it will automatically transfers the gas to the vent drop.

Another common vent drop method is to machine a channel 5-10 times deeper and at least 2 times wider than the actual vent.

If runners or actions prevent the vent drop from reaching the to the perimeter of the mold through the parting line, holes are often be drilled through the core to vent the air to the ejection housing.

Many mold designers make the mistake of assuming that the air will vent away from a core block which is 'proud' (sticks out from the base a little). When clamped, most of this metal is designed to focus the clamp tonnage, and does not provide enough clearance to properly vent the gas from the mold during injection.

-Andy

Actually, near and far field refers to the distance from the ultrasonic horn to the weld joint. Typically, when this distance is less than 0.25in or 6mm it is considered 'Near Field' or 'Close Range' ultrasonic welding. Distances greater than that are generally considered 'Far Field' or 'Long Range' ultrasonic welding.

As a general rule, the greater the distance, the more difficult it is to get the vibrational energy of the horn transferred to the point of the weld. Materials which are softer often get less polymer chain entanglement at the interface since much of the vibrational energy is absorbed by the part before it reaches the joint.

Parts being designed for ultrasonic welding should take factors such as horn location and joint design early in the design process to ensure success.

-Andy

Basically, the purpose of a higher feed temperature in a reverse profile is to ensure the pellets soften enough to stick to the barrel so they convey to the transition zone for melting.

You should conduct a test to ensure what feed zone temperature is best for your material as different heater bands, screws, and barrels behave differently.

There are many times I have found that the same temperature can be used for both the feed and middle temperature zones.

Processing guides can be a great starting point when setting your temperature zones, but do not always reflect the best settings for your machine.

-Andy

In a D1 (DECOUPLED MOLDING 1) process the first stage is filling and packing, yet a D2 process isolates filling in the first stage. Could you please explain the difference?

A reverse profile is typically used when processing materials such as nylon and LCP. In fact it is quite common for many semi-crystalline polymers. Basically, such polymers have a melting point. The high rear temperature often helps improve conveyance as well as melt the polymer quickly, Once melted, the polymer can often be cooled a little in the front to improve the overall cycle time and meet the desired melt temperature.

Usually the need for a forward or reverse temperature profile should be approached scientifically. The middle barrel temperatures should be set to the desired melt temperature. The front zones are typically adjusted to obtain the desired melt temperature. The rear temperature should be set  to optimize feed and melt consistency.

Historically, low viscosity polymers with high crystallinity tend to process best using a reverse profile... but you should always look to optimize this, since everyone has different screws, barrel, heater bands, thermocouples, and temperature controllers.

Try not to confuse the front temperature with the actual nozzle temperature. The nozzle temperature will be adjusted independently to meet the needs of the specific process, such as avoiding drool or freeze off.

-Andy