May 2009

SWAMPED!!!
The South Texas Frank Padula Memorial Golf Outing Rescheduled!!

Our outing was postponed due to high water on the course. We have rescheduled for Monday, May 18th. All other details remain the same. We can accommodate 20 more golfers. Please contact Suzanne Biggs if you have registered and will be able to (or not be able to) make this date. We can also use many more hole sponsors.

Sign up now for the South Texas Frank Padula Memorial Golf Outing, Monday, May 18th at Timbercreek Golf Club in Friendswood. Details and registration can be found at www.spe-stx.org/GolfOuting.cfm. Join us for golf, lunch, dinner and door prizes - $130.

For those that are interested, there are sponsor opportunities from $150.

Meeting Announcement

Join us for our annual Awards Meeting and Past President's Night on Monday, May 11th. The meeting will be held at the Houston Engineering and Scientific Society (HESS) at 5430 Westheimer in Houston (ZIP Code 77056).

Dr. Ramanan Krishnamoorti of University of Houston will discuss his university’s plastics program.

The meeting starts at 5:00 PM and cost is $25.
Make reservations online at:
https://www.spe-stx.org/MeetingRegistration.cfm?MID=35.

President’s Message

On behalf of the South Texas Board of Directors, I hope that this letter finds you well. The 2008- 2009 program year has been quite an adventure. Hurricane Ike started our year off and it feels as if we have been scrambling ever since. We celebrated our industry and spent time together at another great Astro’s event with nearly 200 in attendance. Brandon Cleary and the program committee have provided outstanding programs including joint industry events where we collaborated with other industry associations such as the National Association for Corrosion Engineers, Plastics for Corrosion protection and prevention. We had a sell out for our Basics of Extrusion program hosted by Emery Jorgenson and Jorgenson Machinery. The economy has presented all of us with significant changes to our industry. These changes have impacted our careers and required greater focus on internal cost as well as revenue generating activities. Sharing in difficult times has given us the opportunity to pull together and support each other at the numerous informal breakfasts, lunches and various get-togethers.

In February, Don Witenhafer and the Polyolefins committee hosted an outstanding conference that proved the conference as a relevant priority with better than average attendance and exhibitor support than many conferences hosted this year. I want to again thank all of our sponsors and attendees for their support of the South Texas section education initiatives. Marti Husti, Shawn Smith and the education committee shepherded student section programs at 10 universities. We are all proud of the efforts of each of our University sponsors and students.

I am excited about the upcoming year. This year we developed a long term plan to help provide continuity in planning from year to year and developed collaborative programs bringing our regions most important industries and their challenges together with Plastic professionals. In addition to providing education to plastic professionals, we believe the SPE must create opportunities for participants to generate revenue for their sponsoring companies by creating venues to identify challenges and present solutions to the leading industries in our region. Dr. Kurt Hayden is our upcoming program chair and is already beginning the planning for a great year.

We have made a concentrated effort to recruit young professionals and add new board members to the section. I am eager to build relationships and grow our industry with the participation os these young professionals. I am excited about new ideas for participating in online social- and professional- networks as well as adding other venues. I want to encourage all the members in our section to find an opportunity to get involved. Come to a technical program, help promote an industry event, support your favorite college, or (and!) join the monthly breakfast or lunch bunch for an informal meal. You will find friendly faces willing to help you. You will find you get great return from a small investment in time.

It is not too late to catch our last two events of the program year. April showers postponed The Frank Padula memorial golf tournament until May 18. On May 11 we will host our last meeting of the program year where Dr. Hung-Jue Sue from Texas A&M Polymer Technology Center will be giving an update on the education program at Texas A&M along with an opportunity to recognize those that have served our section this year.

Thank you for the opportunity to serve as your president this year. It has been a pleasure to work with so many superb industry professionals and be a small part of helping students, individuals and companies reach their goals. I wish you all a fun, safe and prosperous summer and look forward to seeing everyone at the Astro’s game in September!

Jeff Applegate
SPE South Texas President 2008-2009

http://www.facebook.com/group.php?gid=35280334773&ref=ts

Join the South Texas Section of the Society of Plastics Engineers on Facebook. Click on the above link and you will be directed to our group page. We hope that this will serve as another communication tool for our Professional Section and be a familiar platform to welcome and inform the Student Sections. It is easy and free, so join in.

Plastics Hall of Fame Taps New Member

By Bill Bregar | PLASTICS NEWS STAFF

CHICAGO (March 6, 4:45 p.m. ET) -- The Plastics Hall of Fame’s class of 2009 covers a global spectrum of leaders in materials, machinery, screws and packaging.

Donald Witenhafer

Witenhafer pioneered technical achievements at B.F. Goodrich Co. that helped save the PVC industry after it was discovered that vinyl chloride monomer causes cancer.

In 1969, Goodrich’s plant physician at its Louisville, Ky., plant called a news conference to announce that several workers had acquired a rare form of liver cancer. These were workers who entred the PVC suspension polymerization reactors after each batch and scraped polymer buildup off the walls. The doctor announced the VCM was a likely human carcinogen.

Goodrich’s board of directors created an emergency project to solve the problem. The team included Witenhafer, a scientist in the PVC polymerization research and development group.

The team thoroughly studied PVC production, looking at ways to eliminate emissions and reduce the residual, un-reacted VCM to very low concentrations before it left the plant.

Witenhafer made the key innovations to solve both of those problems.

He worked for Goodrich for 20 years and SC Johnson & Son Inc. as manager of polymer research for five years before starting a consulting business in 1992.

Bulletin Board

Plastics Information: Check it Out

The Houston Public Library on McKinney has resources on plastics and polymers. Check out their catalog at www.hpl.lib.tx.us. If you are not near the McKinney location, you can arrange to pick up your books at your local branch.

The Fondren Library at Rice University has the most complete collection of books on plastics and polymers. This is also a prime resource for patent and trademark information, as well as other US Government documents. You cannot check out books there unless you join Fondren Library [$50], but you can arrange for books to be sent to your library by inter-library loan. Use their catalog at http://library.rice.edu/.

The next best place to browse is at the MD Anderson Library at the University of Houston central campus. South Texas Section has donated many plastics books to this library. If you plan ahead, you can get a TexShare library card from a library where you are a member, which will allow you to check out books from any U of H library. Their catalog is at www.library.uh.edu/.

Book Bag

	Polymers, Patents, Profits: A Classic Case Study for Patent Infighting Heinz Martin, 2007, 294 pages Originally $72.00 NOW $49.00 Presents a 45-year long historic as well as scientific overview about the different national and international effects of the discovery of Ziegler´s catalysts and the hurdles to take on the way to its patent protection. It provides an impressive example of fights between academics and industry concerning patent rights and the economic utilization of academic research innovations.
	Influencer: The Power to Change Anything Kerry Patterson, Joseph Grenny, David Maxfield, Ron McMillan, Al Switzler Originally $25.00 NOW $15.00 Whether you're a CEO, a parent, or merely a person who wants to make a difference, you probably wish you had more influence with the people in your life. But most of us stop trying to make change happen because we believe it is too difficult, if not impossible. We develop complicated coping strategies when we should be learning the tools and techniques of the world's most influential people.
	Machinery's Handbook CDROM 26th Edition Originally $91.00 NOW $35.00 It provides Boolean and proximity searches, complete test and key word search; extensive support for bookmarking, journaling and annotation: inter-links leading to related topics and examples; zoom features for detailed viewing of figures and graphs; cut and paste capabilities for inserting text and figures into other documents and programs, and an expandable table of contents provides instant access to text, tables and illustrations. Besides the complete 25th Edition, this CDROM also contains material from earlier editions of the Machinery's Handbook.

Science/Engineering Fair of Houston

On March 13-14, 2009, we participated again in the Science/Engineering Fair of Houston as a Special Awarding Agency. Our judging team comprised Ana Estrada, Carlos De la Rosa, Jim Kersting, Ted Marker, Shawn Smith, and Bill Talbott. This year there were again a limited number of projects related to plastics and polymers but more than last year. We found worthy projects for awards in the Senior Division and the Junior Division. First place winners will receive a $400 award and trophy; second place, a $250 award and trophy and third place a $150 award and trophy. The judging team selected the following projects for awards:

Senior Division
First Place – Cameron Strong from the Academy of Science and Technology in The Woodlands for his project “Getting Carried Away III”, investigating the effect of degradation of polyacrylamide and polyvinylalcohol when used to prevent soil erosion in irrigation furrows.
Second Place – Bhavesh Jokhaker from the Harmony Science Academy Houston for his project “Building with Plastic”, investigating the use of reclaimed plastic in cement/sand simulated bricks.
Third Place -- Jeanette Ferrara from the Academy of Science and Technology in The Woodlands for her project “Finding the Fragility of Swimsuit Fibers II” including structural change in the fibers.

Junior Division
First Place – Hayden Hardegree from Westbrook Intermediate for his project “Abrasion Resistance of Kayak Materials”.
Second Place – Jacob Neff from Clifton Middle School for his project “Cleaning Oil Spills at Sea” investigating the use of a polymer to absorb the oil.

Mr. Strong was one of our awardees last year in the Senior Grade Division for the second part of his project on soil erosion. Ms. Ferrara was one of our awardees last year in the 9th-Grade Division for the first part of her project.

We will mail the awards and trophies to the students.

(photo courtesy of Marc Nathan Photographers)
Bill Talbott, friend substituting for Mr. Jokhaker, Jeanette Ferrara, and Cameron Strong.
Mr. Hardegree and Mr. Neff were not present for the photo.

SPE-STX Board Meeting

Location: Spaghetti Warehouse, Houston, TX
Date: March 9, 2009

Designing Plastic Products for Injection Molding

Jan Spoormaker, Spoormaker-Consultancy, Leiderdorp, The Netherlands
Anton Heidweiller, Delft University of Technology, Delft, The Netherlands

Abstract

Designing plastic products for injection molding requires knowledge about strength of materials, heat transfer,
mold making, injection molding and costs.

The product design can then be optimized for minimal mold and processing costs. Students of Industrial Design Engineering at the Delft University of Technology are educated in designing consumer products for small series till mass production. Cost awareness is important and in the lectures we present practical engineering design rules.

Introduction

The influence of the engineering design on the mold design, cooling time, shrinkage and warpage of products is large. Unfortunately little about these aspects is taught in engineering education in The Netherlands. The majority of students in mechanical engineering faculties do not receive any education at all in designing in polymers. Most students do not realize this because of lack of experience.

In most engineering educations students learn little about costs and often they will experience this aspect by trial and error. The Engineering Department of the Faculty of Industrial Design Engineering educates students in designing plastic products for injection molding. Students must be aware of the costs of molds and the cost of processing time.

In this paper 10 rules for designing plastic products with respect to injection molding are given.

1. Carefully select the location of the gate and ejector pins.
2. Design equal wall thickness.
3. Make wall thicknesses as small as possible.
4. Use large radii.
5. Stiffening by curvature instead of ribs
6. Mold partitioning as simple as possible.
7. Allow sufficient tapering to easily eject a product and avoid scratching of etched surfaces.
8. Account for achievable dimensional tolerances and take care that dimensions with low tolerances are determined by 1 part of the mold.
9. Avoid undercuts.
10. Avoid weak spots in molds and products.

General Considerations

Plastics are not very cheap, but because functions can be integrated plastic products can become cheaper than metal products.

The larger the series of a plastic product the more important becomes the price of the plastic and the cycle time.

Plastics shrink in molds during the cooling process. Shrinkage for amorphous thermoplastics is less than for semi-crystalline because they have a denser packaging. The shrinkage in the thickness direction is larger than in the other directions. The plastic is oriented in the flow direction and in these directions the shrinkage is restricted by the mold. The shrinkage through the wall is most of the time not restricted.

Fillet radii should be not small to reduce stress concentrations, but also that the fillet radius does not disappear due to shrinkage [a]. The wall thickness should be constant to reduce cycle times and also to reduce stress concentrations.

The cooling time is proportional with the square root of the thickness. This can easily be learned from the Fourier number:

• a is the thermal diffusivity [m2/s]
• t is the characteristic time [s]
• h wall thickness [m]

which students know from Fluid Mechanics.

Many problems with molds can be prevented by carrying out design reviews in early stages of the design process. However, when mold manufacturing is carried out in low cost countries this can be a big problem.

Location of Gates and Ejector Pins

The location of gates determines easiness of filling the mold, the location of weld lines and the roundness of a product. This is very schematically depicted in Figure 1 and from this figure it can be learned that the location of the gate is very much of influence on weld lines. With mold filling simulations software it is rather easy to optimize the locations of gates.

To eject complex mouldings with bosses, ribs and other features, ejector pins are generally used because they are economical and easy to install. However, the ejector pins can cause high local stresses and strains in the moulding at the stage of ejection, leading to part deformation and damage. Thus, the proper layout of ejector pins is very important in mould design. A method is described in and article by S. Kwak et al. [1] for determining the layout and size of the ejector pins required ejecting thermoplastic mouldings, minimizing part deformation and damage.

Equal Wall Thickness and Differences in Thickness

This has already been mentioned in the general considerations. This causes another problem because a thickening occurs at the corner as is depicted in Figure 2A. The cooling time is proportional with the square root of the thickness and hence a design as depicted in Figure 2 C is the best solution.

In Figure 2B a design of a frame with different wall thicknesses is depicted. The thick part (A) in the middle this bar solidifies last of all and pulls the vertical parts inside. A thin part (B) in the middle this part solidifies first of all and results in a convex product. Only equal wall thicknesses will result in a straight product.

Wall Thicknesses as Small as Possible

The maximum thickness of plastic products should be less than 4 to 5 mm because of void formation and cooling times. It is preferred to make the wall thickness as small as possible and this is possible with ribs and corrugated structures.

In Figure 2C a product with an equal wall thickness is given. For the same stiffness ribs could be applied or a large thickness as is shown in Figure 2 C (detail A).

In Table I the mass and cooling time for the same stiffness are presented. From table I it can be learned that thick sections require high cooling times and far more material.

Transitions in Wall Thickness

Abrupt transitions in wall thicknesses cause high stress concentrations, but also disturbance in flow of the melt.

Design A in Figure 3 is worst and design C is the best because radii have been applied.

Students should be aware of the fact, that sometimes mold makers do not provide radii, because this is cheaper.

This must often explicitly be specified especially when molds are manufactured in low cost countries and communication is a problem.

Simple Mold Partitioning

Molded parts are used extensively because they produce products that require minimal secondary operations. On the basis of the number of parting surfaces in a mold, molds can be divided into two-piece molds and multipiece molds. Multi-piece molds refer to molds having more than one parting surface and hence more than two components. These molds can produce complex parts that cannot be made using two-piece molds. They enable the use of molding for making parts that were previously manufactured using other processes. Since they have more than one parting surface, they can be decomposed along different directions and thus can be used to make geometrically complex parts.

In Figure 4 some portioning possibilities are depicted.

The complex mold partitioning A is not recommended and solution C is efficient. This because it has a plane partitioning and hence:

• the mold will be cheaper
• the lifetime of the mold will increase
• it is simpler to have more cavities

Use Curved Surfaces

From Figure 2C it was learned that for the same stiffness ribs and corrugated structures require less material than thick cross sections.

Students often are not trained in distinguishing between in plane or axial stiffness and bending stiffness. For a bar with a rectangular cross-section the ratio between the axial stiffness kA and the bending stiffness kB is:

For a bar with a length of 100 mm and a thickness of 4 mm the ratio is more than 1 000.

For curved surfaces like a partial spherical shell [3] as depicted in Figure 5 the stiffness is:

where:

and C depends on the value of a:

For a curved circular plate as depicted in Figure 5 the stiffness increases very rapidly with the height h.

As an example a curved circular plate is considered with outer radius r = 100 mm and wall thickness t = 3 mm. It is easy to calculate that for a height h of 20 mm the stiffness is 8 times higher than for a flat circular plate. The additional material costs are very low, the mold is not as complicated as with ribbing and sink marks are avoided.

Sufficient Tapering

Plastic products shrink and hence they clamp on parts of the mold they cover. Ejection with damaging the product and the mold is achieved by tapering the part covered.

There are practical rules for the draft angel for smooth and etched surfaces. For shallow products (H < 10 mm) the draft angle is expressed in:

and for taller products:

For etched surfaces the draft angle should be 1º for
0.02 mm of etch depth. It is important to polish.

Achievable Dimensional Tolerances

Small tolerances are very expensive. Keep in mind that products of amorphous polymers have less shrinkage than products of semi-crystalline polymers. Moreover fillers also reduce the shrinkage.

It is also important, that dimensions with small tolerances are determined by one half of the mold. This is illustrated in Figures 9A and 9B.

Avoid Undercuts

Undercuts as depicted in case A in Figure 7 result in an expensive mold because side cores must be used. Design solutions without undercuts are shown in cases B and C in Figure 7.

Avoid Weak Spots in Molds and Products

In Figure 8A-A a design is depicted with sharp corners and a fastening hole far away of the wall and without a rib. This is weak design because the bending moment is high and the stress concentrations are high.

In Figure 8A-B an improved design is presented with sufficient fillet radii, a fastening hole closer to the wall and a rib to transfer the load more gradually to the wall.

Thin walls in molds must be avoided because a crack might occur during the hardening of the mold. In Figure 8B-A the arrow points to the weak spot in the mold between the pin and the wall of the mold. In Figure 8B-B the wall thickness of the mold has been increased.

Conclusions

It is important that students become aware of the relationship between product design and injection molding to obtain optimal products.

Very much information is available about mold design, but far less about the relation between product design and injection molding.

References

1. S. Kwak, Proceedings of the Institution of Mechanical Engineers. Part B. Journal of engineering manufacture Proc. Inst. Mech. Eng., B J. eng. manufacturing, ,.217,n4,463 (2003)
2. W. Michaeli, Training in Plastics Technology, SPE books from Hanser Publishing.
3. R.J. Roark, W.C. Young, Formulas for Stress and Strain, Fifth edition, (1983).

Lecture notes

1. A. Anemaat, and J.L. Spoormaker, Designing in plastics (in Dutch), Delft University of Technology.
2. J.L. Spoormaker, Compendium “Strength of materials” (in Dutch), Delft University of Technology.

Useful websites

Efunda Design Guidelines
www.efunda.com/DesignStandards/plastic_design/plastic_intro.cfm

Dupont Design Information:
http://plastics.dupont.com/myplastics/

BASF Design Solution Guide
www.basf.com/PLASTICSWEB/displayanyfile?id=0901a5e18000486d

Ticona Engineering Polymers
http://www.ticona.com/index/tech/design.htm

DSM Design Information
www.dsm.com/en_US/html/dep/design_homepage.htm

Solvay Advanced Polymers
http://solvayadvancedpolymers.com/

Key Words: shrinkage, product design, mold design

Nomenclature