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Lesson 1

Designing the Nominal WallLesson 1

I. Nominal Wall

All injection molded plastic parts have a nominal wall. The nominal wallforms the frame or backbone of the part. All other design details areattached to the backbone of the part. It is very important to design thenominal wall of a part correctly. If the backbone of the part isn’t right, thewhole part will suffer.

II. Nominal Wall Requirements

A. The nominal wall must meet the functional requirement for which thepart was created.

B. The nominal wall must be designed so that it can be manufacturedefficiently at a high quality level.

NOTE: It is often difficult to satisfy these two requirementssimultaneously; however, every effort must be made to strikethe best compromise between these two requirements.

Plastic Part Design for Injection Molding

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A. Functional RequirementsThe functional requirements of a part include all those things that a parthas to do in the marketplace and in the hands of the consumer.

1. Over designed part - useless if the thick wall is so costly that noone will purchase the part for its intended purpose.

2. Under designed part - useless if it doesn’t perform satisfactorilyand the consumer doesn’t repurchase.

3. Nominal wall thicknessa. Insulating properties of a product are directly related to the

part’s nominal wall thickness.1. For example, electrical, heat, sound, light and

vibration insulation are all related to the thickness ofthe part.

b. The precision of a specific injection molded part relatesprimarily to the specific material being used. However, thenominal wall thickness has an important effect on a part’sdimensional stability.

B. Manufacturing Requirements - Flow, Cooling, Ejection, Handling

1. Flow - The nominal wall must be thick enough to allow for theflow of the melt through the cavity.

a. A very thin part in a high viscosity material such aspolysulfone may satisfy the product requirements.However, the nominal wall will have to be thickened inorder to allow the melt to flow to the extremities of thecavity.

2. Cooling - The cycle time required to cool a plastic part is directlyrelated to the nominal wall thickness and the specific plasticmaterial.

a. The thicker the wall, the longer the cooling portion of thecycle will be.

3. Ejection - The nominal wall thickness also has an effect on theremoval of the part from the mold.

a. Plastic parts similar to extremely thin-walled disposableproducts, often cause ejection problems. The walls are oftenso thin that there is no area large enough for an adequateejection mechanism. In these cases, the nominal wall maybe thickened, ribbed or otherwise reinforced in order toimprove ejection without waiting for the part to cool enoughto accept the force of ejection.

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Lesson 1

4. Handling - The nominal wall thickness of a part will also have aneffect on the handling, shipping and assembly cost of a plasticproduct.

a. The designer’s natural tendency to minimize a part’snominal wall thickness can have a negative effect. Thinwalled, fragile parts require careful handling and protectivepackaging. A slightly thicker wall might eliminate layerpackaging or hand unloading of the mold.

NOTE: Most piece-part designs are a compromise between themanufacturing and the functional requirements. Bothrequirements are of primary importance. However, in the finalanalysis, the functional requirement must take precedence. Thebest design is always the one that strikes the best compromisebetween the end-use and manufacturing requirements.

III. Nominal Wall - Flat, Curved, Angled

The nominal wall of a part can be flat, curved, or angled. It can have athickness which is uniform, gradually increasing or sharply stepped. Nomatter what shape the nominal wall assumes, its design requirements remainbasically the same.

A. Nominal wall with uniform thickness:

1. Fills evenly2. Minimizes localized over-packing and molded-in stress3. Results in a more uniform cooling rate and uniform mold

shrinkagea. Produces a part which has a lower level of molded-in stress

and an economical cooling cycle.

Plastic Part Design for Injection Molding

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NOTE: All other things being equal between two plastic parts, the onewith the more uniform wall will have improved strength,dimensional stability, appearance and cost.

B. Short Shots

As you can see in this series of short shots, the thicker section completely fillsout before there is any flow into the thinner section of the block. That portionof the melt that starts into the thinner section, stopped flowing and sat idlewhile the thicker section of material filled out completely. The leading edgeof the melt that sat idle for a period of time has already started to cool. Thatcool segment of the melt becomes a part of the thinner half of the piece. Ifthat part of the melt has cooled so much that it does not become a part of thehomogenous mass of the melt, it will become an inherent weak spot in thefinal part.

1. Melt that has sat idle and cooled can contribute to:a. weak weldlinesb. unexpected flow marks in the part (especially in parts with

non-uniform thickness)

NOTE: This condition will become progressively worse as the size ofthe part increases

2. The flow of melt through progressively thinner sections results in:a. higher levels of molecular orientation

1. The thick and thin sections of a part can thereforeexhibit different physical properties.