Andy Routsis: July 2010 Archives

There are many helpful charts and factors, but these use too many assumptions and are just ways to avoid proper circulations.

Proper cooling time estimations contain the following considerations:

One of the most common calculations are as follows:

tc = [(h^2)/()(π^2)] ln |(4/π)[(Tm-Tw)/(Te-Tw)]|

h = wall thickness

If you perform a search on the internet, you can find automatic cooling time calculators which will quickly take all these factors in consideration once entered.

Keep these factors in mind...

• Always use your thickest portion of the part for these calculations
• Assume your actual mold surface temperature will be higher than the temperature setting
• Give yourself a 10-25% buffer based on the level of geometric complexity and amount of plastic shrinkage

-Andy

When discussing employee training, it is critical to focus on how it will affect the employees. For example, if operators are not involved in the training initiative during the first couple months, let them know when they will be trained, and who will be taking the training first.

Explain a few of he specifics such as, how many hours a week or month they will train, the types of information they will learn, how the training will benefit the company as well as how it will benefit the employee.

With any initiative, employees will look to management for honesty and their commitment to maintaining the program. Keep the program transparent and track the progress visibly so everyone can see the program actually being used.

Many companies start a new program and then let it fall by the side when it looses visibility. Always update the employees on the progress of any company initiative.

Don't be afraid to make this process fun. Banners, luncheons, gifts, and announcements are great ways to inform your employees.

-Andy

I received this question over the weekend...

A spotting press is great for checking the parting line. This is a large hydraulic press specifically designed to open and close the mold. 

Spotting fluid is placed on the parting line and the mold is closed under pressure using the press. Areas where the spotting fluid transferred from one half to the other are areas where the parting line is shutting off successfully. A parting line mismatch exists in areas where the spotting fluid does not transfer from one side to the other.

I recommend that any molder either have a spotting press, or access to a spotting press. If you have a toolmaking department, it is a necessary investment. 

If you are a small molder, or have a small tooling department just for repairs, then it might be a price advantage to bring the tool to a local tool shop and borrow their press (this will also expose you to their expertise in evaluating the results).

-Andy

As long as the flow rate and gate dimensions are in the same units (mm, cm, or in) they will cancel out and you will receive a a resultant in the units of reciprocal seconds which will represent the rate of shear.

-Andy

Basically, you need to increase the contact force between the nozzle and the sprue bushing. This will often relate to the carriage forward force or position.

-Andy

Electric molding machines do not have intensification ratios because they lack hydraulic pressure. Your machine should be configured to provide the actual injection pressure.

Basically, an electric molding machine measures the injection force using a conversion from the torque applied during injection. This conversion provides the actual injection pressure on the control panel. 

he only time you may need a conversion is if the screw and barrel were replaced, and the machine was not re-programmed to accomodate.

-Andy

The general recommendation from most manufacturers is between 0.25-0.5mm or 0.010-0.020in.

In reality, this specification refers to the clearance between the top of the ejector pin head and the counter-bore in the ejector plate. Most mold makers use a similar ejector pin clearance even when a sensor is not being used.

The purpose of this clearance is to ensure there is no preload on the sensor, especially if any dirt gets beneath the sensor. Such a preload will destroy the sensor and provide erroneous pressure readings.

-Andy

In general, once a material is contaminated with a hard material such as sand, it will become highly abrasive to the grinder, screw, barrel, hot runner, and mold. To regrind that much material in such a state will most likely cause significant damage, requiring repairs and possible replacement of many components. Likewise, such an abrasive material would damage the screw and most likely require a new check ring after re-processing.

If the reground material is sold or any products are sold molded with this material without the customer's approval such an act would be unethical and most likely illegal.

Since many of the costs associated with regrinding this material depend on the material, grinder, region, as well as repair costs, it is impossible to estimate with any degree of accuracy.

This is one of the reasons it is critical to teach your employees the importance of proper handling of materials, parts, and regrind to ensure that contamination does not occur.

-Andy

Pressure is a measurement of force spread out over an area.

Pressure = Force / Area

This means, if you apply the same force on a smaller area, you get more pressure.

An easy way to demonstrate this by holding up a piece of paper.

First, ask the employee to push on the paper with the palm of their hand. Explain that they can apply a lot of force without the paper breaking. This is because the force they apply is distributed over the area of their palm.

Second, ask the employee to push on the same piece of paper with a pen or pencil with the same amount of force. Explain that the paper breaks because the force is applied over a much smaller area resulting in a much higher pressure.

-Andy

This question brings up the importance of machine independent documentation that includes part specific data such as part weight, fill time, coolant and melt tempertaures as well as the actual pressures applied to the polymer.

When you do this, you can ensure the actual process output from two different machines match. The most important factor is not the parameters that go into the machine, but the parts and associated outputs that result from the process.

This topic is also addressed in the following blog: The Symptoms Of Machine Dependent Documentation

 

-Andy