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This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

Intensification Ratios with Multiple Injection Cylinders…

I received this from a blog reader the other day…
JF
I need to be able to figure intensification ratios for my current presses. Right now I am stumped by the intensification ratios because some of the presses have to parralel cylinders exerting force on the screw…..do you multiply the diameter of both?
note: The intensification ratio is a factor which is used to convert the hydraulic pressure being applied to the hydraulic injection cylinders into the actual pressure being applied to the polymer melt at the front of the screw. For example, if a hydraulic pressure of 1000psi is applied on a machine with a intensification ratio of 12, then the plastic would receive (1000psi)*(12)=12,000psi.
My Response
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:
Iratio = Acylinder  Ascrew

For two cylinders:
Iratio = (Acylinder 1 Acylinder 2) / Ascrew
or:
Iratio = 2 * (Acylinder) / Ascrew

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

Relying on On-The-Job Training Alone

I heard this comment the other day from a restaurant owner. Although it was not my place to tell him how to run his restaurant, I did think it was a great discussion topic for this blog…

Owner
I can’t bother to train my staff… they learn what they need to know on-the-job.
My Thoughts
Expecting on-the-job training to train all your employees is a very questionable option. It requires time off the job for both the trainer and trainee, and in most cases, the information passed on is limited to a need-to-know basis – making it sparse and incomplete.
In the case of the restaurant, it is unnerving to consider, the employees may never have been taught the true importance of separating the meats from the vegetables, proper temperature settings on refrigeration equipment, or heating my chicken breast to 170ºF.
This got me thinking… how does a typical customer feel when their supplier says the same thing… do they have the same level of concern regarding the products they receive. Are they asking: Does Quality understand our critical factors…? Did the Material Handler understand why my nylon must be dried…? Did Shipping understand why our boxes are labelled in a special way…?
On-The-Job training is good, but it must be tempered with actual knowledge and skill development exercises so that the employees understand the concepts and can apply them to their daily job.
Anyways, the food was cooked (maybe a little too much), but I may never go back to that restaurant again.
-Andy

Calculating Material Usage Requirements

I just received a question which is a common frustration for any process whether it is injection molding, blow molding, extrusion, or  compounding…

Firdaus
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?
My Response
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.
Additional Thoughts
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

Understanding Torque…

I received this question the other day via telephone…

Steve
Can you explain the concept of torque…? and why using a torque wench is so important?
My Response
Basically… Torque is a measure of rotational force. In other words, the force being applied to rotate something is considered torque.
The way torque is calculated is by multiplying the force being applied times the distance it is being applied. This is typically represented as Newton-meters (N-m) in the Metric system or foot-pounds (ft-lb) for Imperial measurements.
For a more detailed definition of torque, please feel free to visit wikipedia:
Technicians and mechanics often use torque wrenches to measure the rotational force being applied to a screw when they are being tightened. This helps prevent the platen threads from becoming stripped or damaged.
For a more in-depth discussion on this topic, please visit my previous blog:

Proper Torque Value for Clamping Mold to Platen

Additional Thoughts
Engineers are often concerned with the amount of torque or rotational force being generated by a servo motor. Since the energy usage of the servo motor is measured in AMPS… it will directly relate to the rotational torque being produced.

-Andy

Producing Hollow Parts…

A molder in China just sent this unique question about producing hollow parts…

CJ
We are a molder in China and our customer asked us to prototype a decorative hollow part for a consumer application. What is the best method to do this?
My Response
In the prototype phase, the quickest, and least expensive way to do this is using either rotational molding or blow molding.
Rotational Molding: The material clamped within a mold, it is rotated about multiple axis while it is heated to melt the material and cover the circumference of the interior. The mold is then cooled and the part is eventually removed. This process is very slow, but is great for very low production prototypes.
Extrusion Blow Molding: This process uses an extruded, tube shaped, parison which is clamped within the mold and blown to size using pressurized air. The mold is more expensive than a rotational mold, but you are able to manufacture many more parts than with rotational molding.
You can learn more about Extrusion Blow Mlding with our blow molding series of training:
Additional Thoughts
You can also produce the part using injection molding along with an assembly process such as ultrasonic welding… but this seems far too expensive for a prototype of a basic decorative consumer part.
-Andy