Preface

Norman C. Lee

Blow Molding Design Guide

ISBN: 978-3-446-41264-4

For further information and order see

http://www.hanser.de/978-3-446-41264-4or contact your bookseller.

© Carl Hanser Verlag, München

Preface

Designing plastic blow molded parts can be an extremely diffi cult task because of the complexities of part geometry and the molding processes. It is challenging to even experienced designers. To produce an effective design it should be:

Functional and achieve the objective for which it is intended, •

aesthetically pleasing, within the limits of the process, •

practical, utilizing the right material, maximizing the benefi ts of the process, •

cost effi cient, with consideration of tooling cost and methods and run quantities. •

Therefore, the objectives of this book are to give the reader an understanding of plastic blow molding, materials and processes, thus enabling him to design a blow molded part that optimizes the effectiveness of the plastic materials used, process employed, as well as the function of the part. It includes the application of bottles, industrial and structural parts. It is intended to be a no-nonsense, practical hands on book, that forgoes a scientifi c language that most ordinary people do not understand and concentrates on real life, day to day problems faced by those working to create cost-effective blow molded parts. It is a good introduction to the overall picture, for those who then wish to delve into more detailed and academic aspects of anyone of the many processes discussed.

Because the work includes so many diverse subjects it is not possible for one individual to be an expert in them all, thus, I have relied heavily on experts in their relative fi elds for information and advice. These of course are acknowledged at the end of each chapter. In many cases I have drawn from the published work. I make no apologies for this, since I am not able to improve on the original work. Also, much of the matter presented is leading edge technology and development by the originators.

I also acknowledge several who have helped me put this manuscript together. Loretta Lee, graduate student at A & T State University, a UNC System in Greensboro, NC, who took it on as a Masters project. Dr. Brent Strong, Brigham Young University, reviewed the manuscript, corrected errors, and made suggestions for changes. The review committee of the S.P.E. blow molding division headed by James Parr, Exxon Company, Robert Gilbert, Equistar; and Robert Read, Dow Brands, made valuable suggestions for improvement. Several diagrams were drawn by Sam Huffi ne, Huffi ne Associates, Greensboro, NC and Auto-CAD drawings by James lee, Ashboro, NC.

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Sample Pages

Norman C. Lee

Blow Molding Design Guide

ISBN: 978-3-446-41264-4

For further information and order see

http://www.hanser.de/978-3-446-41264-4or contact your bookseller.

© Carl Hanser Verlag, München

13 Finishing

Finishing of a blow molded part should be considered in product design, mold engineering, and process planning stages.

13.1 Layout the Entire Process, Step by Step

Finishing the part to the maximum degree possible, even the packing of it, in the molding room, adjacent to the molding machine, promises the best economics, because it eliminates the labor involved in warehousing, transport, and storage.

Consider:

Removing a dome or other sections from part body. •

Removing the fl ash, drilling and machining operations. •

Decorations – hot stamping, heat transfer and serial numbering by hot stamping method (see •Chapter 12).

Part handling. •

13.2 Product Design

The following considerations should be made in the planning for the fi nishing of a blow molded parts:

Radii: no square corners in inside or outside. •

Whenever possible, include orientation and register features, for positioning and holding in •downstream tasks.

Lay out fl ash pockets in the product drawing stage. Make sure that they are wide enough to •accept normal fl ash variations. Circumferential fl ash is to be avoided, if at all possible. When circumferential fl ash is unavoidable, compensate in the mold design.

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190 13 Finishing

13.3 Mold Engineering

The following are essential before cutting steel for a mold:

Can the parts be molded together (‘Siamese’) •

Is the application suited for ‘family’ molds? •

Are fl ash pockets provided? •

1. Depth: calculate and plan ‘metal safe’.

2. Make sure to keep fl ash pockets depth tight enough to get cooling contact, so that the fl ash is fl at and rigid.

3. Consider ‘corrugating’ the fl ash, especially head and tail tabs. It is more expensive, but provides superior results, and can decrease cycle time.

4. Evaluate ‘blowing fl ash’, which provides a dimensionally stable, cool fl ash. Again, a fringe benefi t can be decreased cycle times.

Accommodate circumferential fl ash. Consider employing ‘fl ash breaker’ pinch-offs, so that •fl ash can be removed in sections. This is especially important when de-fl ashing is performed at the machine.

Pinch offs – extremely important: •

Width: keep to minimum, 0.39 mm recommended.

Hardness: Beryllium copper, preferably, or steel, preferred over aluminum.

Incorporate in machine replaceable inserts, whenever wherever possible.

Part ejection: confi gure mold to eject part so it clears the mold without human intervention: •

Utilize pre-pinch mechanism as strippers.

Pre-plan to include mechanisms to retain the part in the preferred mold half, and then include ejection features.

13.4 Trimming and De-Flashing

In some plants and for some products, CNC routers are used to trim fl ash and remove unwanted material automatically rather than manually, in particular when part quantities are high enough to justify the cost (see Fig. 13.1).

Even with the introduction of robotic devices for fl ash removal and trimming, manual removal is the most common method of fl ash removal in large industrial parts, using a knife to trim and hammer to remove the parison tail.

Increasingly in the high-speed production of containers, machine de-fl ashing is used. A common de-fl ash press uses a female bottom nest which includes a die or cutout.

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19113.5 Removing Domes and Other Sections

The top tooling includes a punch. De-fl ashing presses may be either top or bottom acting. The part, cradled in the nest, contacts the de-fl ashing die, which over-strokes slightly. A top acting stabilizer/ejector member is generally required.

Irregular parting lines are better handled by ‘opposed chisel’ tooling, which works by shearing the plastic between two sharp edges.

A feature common in de-fl ashing machinery is the guillotine, which is used to chop off channels and neck-like areas. Because of their size and diffi culty of adequately guarding the blade, they are carefully designed to include several safety devices.

There is a category of secondary equipment, referred to as fi xtures. Generally, they are job specifi c for a particular part and designed for secondary operations. Some fi xtures include de-fl ashing, both partial and complete exterior de-fl ashing.

13.5 Removing Domes and Other Sections

Rotating saw blades are often used to remove domes or other sections from the part body. Disad-vantages are:

They are very noisy. •

Chips and fl uff made by the saw blades when cutting the part create a mess in the work area •and may require the wearing of respirators.

There is a danger from exposed saw blades and chip being fl ung so that they may get into the •operators eyes.

Finished cuts are not very smooth and require extra trimming. •

Figure 13.1 Fanuc Robotics’ new M-6iB robot is performing robotic defl ashing and trimming of a blow molded plastic child seat with AccuDefl ash (Fanuc Robotics North America Inc.)

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192 13 Finishing

A better way is to use a knife cut when possible (see Fig. 13.2), which shows a groove for a knife cut. Groove size will depend on part size and wall thickness. Generally, the height and depth of the groove should be twice the wall thickness.

As with all cutting devices, the knife must be guarded during operation. Always turn off the machine and lock out the knife movement mechanism before working on it, and wear cut-resistant gloves when working on or around trimming knifes.

13.5.1 The Cutting Machine – Round Parts versus Parts with Corners

Round parts can typically be cut on a machine, which rotates the part. The blade may be brought in manually or automatically.

Squared parts that are rotated need some type of control to maintain the depth of the blade into the part, something to automatically bring the knife in and out, and to stop the machine in the same orientation each time. Other operations can also be added to the cutting machine if needed.

2t

2t

t - THICKNESS

SHARP CORNERS

KNIFE BLADE

Figure 13.2 Groove for knife cut

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