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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.

The Pitfalls of 98% Full…

When your mold is 98% full, packing is already beginning to take place. This makes it nearly impossible to completely separate filling from packing.

Question: I configured my mold with a 98% fill and finish with packing. Why do I still get sinks and short shots?
My Response: We always recommend 90-95% because 98% fill causes the following problems:
1) Since only a small portion of the mold is filling at 98%, packing has already begun to take place in most of the mold cavity. This results in a blending of packing and filling. Whenever the fill varies, the degree of packing will also vary again.
2) Plastics tend to have 5-10% normal variation in viscosity. A 98% fill can only compensate for small variations in material viscosity. In such a process, a moderate drop in viscosity is likely to cause flash, overpacking, overweight parts, warpage, large dimensions, cycling problems, or part sticking. Likewise, a moderate rise in viscosity can cause shorts, sinks, voids, warpage small dimensions, and cycling problems.
3) Common variation in a ‘healthy’ check ring is typically between 1-3%. This means a process established with a 98% fill can vary between 96.5% and 99.5% full with a ‘healthy’ check ring. The parts produced from a 99.5% fill are going to be much different than those produced from a 96.5% fill.
4) When the fill reaches 98% full, the mold cavity is beginning to pack. This results in a higher requirement for clamp tonnage since it is required to overcome injection pressure rather than packing pressure.
For more about why 90-95% fill, please feel free to read the following:
-Andy

New Injection Molding App…

We have a new Mobile App that is available for your entire staff at no cost. 
This handy Scientific Molding “Pocket Guide” contains essential processing information for all injection molders.
The guide now available as an app for both Apple™ & Android™ devices, or pdf.
The Scientific Molding Reference Guide includes:
Understanding Plastics
Plastic Materials 
Material Properties, Additives & Preparation
Establishing a Scientific Molding Process
Seven Steps to Scientific Troubleshooting
Molded Part Defects
Basic Mold & Part Design Guidelines
Frequently Used Calculations
The Importance of Training
To download, simply click on the following link:
Spread the word and share this link with everyone.
I’d love to hear your feedback in the comments section.
-Andy

Increasing Part Density…

Polymers are compressible, so there are many ways to increase density of your molded part. This is especially true for semi-crystalline polymers.

Question: Our customer is complaining our product density is lower than other suppliers based on a 3D CT SCAN inspection of our product. How do we increase the density of the PA66+30%GF product.
My Response: There are a few aspects to this, but you should always start with the basics:
 
1) A higher melt or mold temperature will slow cooling, resulting in more semi-crystallinity.
2) A short shot during fill will allow you to use more packing pressure without flashing the mold.
 
3) A higher injection speed will reduce the pressure drop during fill and allow more packing to take place.
4) A shorter cooling time will allow the part to have more cooling after molding.
-Andy

Fixing Mold Sweat…

In a non-climate controlled environment, cold molds will often ‘sweat’ on humid days due to the mold surface temperature being lower than the dewpoint of the surrounding air.

Question: We have to warm up our molds during the summer to keep the molds from sweating. This results in much longer cooling times. Management will not add climate control to the facility, but can the molds run faster?
My Response: If you cannot control the plant environment, you can still control the mold environment. Many molders will put dehumidifiers next to the machine to provide a dry air environment within the clamp area.
More Comments: If the sweat is mild, you can also gently blow compressed air at the core and cavity surface since this air typically has a dewpoint of -10 to -20 degrees.
-Andy

Skill Development Requires Practice…

Knowledge of a topic or concept is critical to understanding, but practice is the key to skills development.

Question: I want my engineers to learn scientific molding skills, but can’t they just take the online courses? Do they really have to bother with the hands-on worksheets?
My Response: If someone reads about Scientific Molding in a book, they may become knowledgeable in the topic. to become skilled in Scientific Molding, he/she must actually go out on the production floor and do it. This gives them a concrete understanding of the application of the knowledge to the production environment. This is why we only perform classroom training in conjunction with hands-on processing equipment.
Additional Thoughts: More According to the Oxford Dictionary, a skill is the ability to do something well.
Essentially, a person has to have a certain level of competency and proficiency to be ‘skilled’.
-Andy