BDD 40903 Injection Mold Design Chapter 2

BDD 40903INJECTION MOLD

DESIGN

BDD 40903 Prepared by : Mohd Hilmi Othman 1

CHAPTER 2

2. Plastic Part Design(2 hours)a) Product design

b) Design requirement

c) Rib design

d) Boss design

e) Corner design

f) Draft

g) Undercuts

h) Surface finish and textures


Product design


To insure a quality final product, it is necessary to start out

with quality components. Injection molded parts can be

molded to a high quality standard by focusing on these areas

of plastic technology:

1) Correct Part Design

2) Accurate Selection of Material

3) Processing Plastic Processing

Only by drawing on expertise from these three areas of plastic

technology can a product designer create quality molded parts

that maximize performance and are cost effective. The

purpose of this design guide deals with the first of these three

issues - part design.

http://injectionmoldingva.com/Plastic-Part-Design.php


Design requirement

Correct Part Design Guidelines

If there was only one rule for the injection molding process it would have to be to maintain uniform wall

thickness. Here are some examples of problems associated with part designs that feature a non-uniform wall

thickness.


Design requirement (2)

Rib design


Many times the stiffness of a part must increase because of the load applied to the part design. One of the

easiest ways to cure this problem is change the part geometry by adding ribs. The use of ribs is a practical

way and economical means of increasing the structural strength of a part. But there are guidelines that govern

adding ribs without causing sink marks or surface blemishes to the molded parts.

1) Rib thickness should be less than wall thickness. A rib

thickness of 60% to 80% of nominal wall thickness is

recommended.

2) To increase stiffness increase the number of ribs or

'gusset plates', another feature designed to strengthen

the plastic part.

Rib design (2)


3) For a given stiffness, it is better to increase the

number of ribs, not the height

4) For thick ribs 'core out' the rib from the back. This

creates a hollow space underneath the part and

maintains a uniform wall thickness.

Height: Maximum height of three time nominal wall

thickness of part.

Spacing: Minimum of two times nominal wall

thickness of part between ribs

Rib design (3)


Boss design


Bosses are used for locating, mounting, and assembly purposes. There are boss design guidelines that must

be followed to insure the highest quality in molded parts. Again, one of the main points to consider is

nominal wall thickness. Too many times bosses are designed with thick wall sections that can affect the

appearance of the plastic part and the final product.

Boss design (2)


Rule of thumb: the wall thickness around a boss design feature (t)

should be 60% of the nominal part thickness (T) if that thickness

is less than 1/8". If the nominal part thickness is greater than 1/8"

the boss wall thickness should be 40% of the nominal wall.

Boss diameter, wall thickness, and height design parameters.

While boss heights vary by design, the following guidelines will

help avoid surface imperfections like sink marks and voids: the

height of the boss should be no more than 2 1/2 times the

diameter of the hole in the boss.

Please observe the "60/40" rule (see above) for the wall thickness

at the bottom of the boss.

Corner design


Radius All Corners

For any plastic part featuring corners, strong part design is heavily dependent upon radius. Sharp corners create

high stress areas; when a plastic part breaks, the break often occurs at or near a sharp corner. The radius of a

corner generally should be designed as large as possible without inadvertently creating a thick wall section.

Draft


Plastic parts are always designed with a taper or draft in

the direction of mold movement to allow part ejection or

removal from the mold. Since plastics shrink when

cooled, it is common for parts to shrink (or grip tightly)

to cores.

A good definition for draft would be the degree of taper

of a side wall or rib needed to allow the molded plastic

part to be removed from the metal mold.

Without proper draft, plastic parts may be difficult to

remove from the mold. A draft angle of 1/2 degrees is

regarded as minimum for most applications. Draft angles

of 1 1/2 to 2 degrees per side are considered normal for

plastic injection molding.

Undercuts


1. Any indentation or protrusion that prohibits an ejection of a part from a one-piece mold.

2. Reducing the number of undercuts often helps to reduce the initial cost of tooling.

3. If possible utilize other mold design features that can offer similar operation without the need of moving

slides.

4. Most commonly categorized by either an internal undercut or external undercut and requires an extra

component of the mold to capture the detail.

5. Undercuts are features that prevent a straight ejection at the parting line of mold.

6. Undercuts typically lead to increased mold complexity and can lead to higher mold construction costs.

7. A simple re-design or modification can often eliminate or reduce the number of undercuts a part has and

can potentially save in tooling costs, leading to better designed plastic parts and a more efficient molding

process.

8. When an undercut feature simply cannot be removed from the part design, the tooling will most likely

require internal mold mechanisms to help facilitate the ejection.

http://www.xcentricmold.com/resource/injection-mold-undercuts.php

Undercuts (2)


Surface finish and textures


1. During the part design consider the relationship

between cost, lead time and surface finish.

2. Typically higher finishes will increase the tooling

cost.

3. Surface finishes play a critical role in both

functionality as well as the cosmetic look and feel.

4. Parts that are hidden may not require a surface finish

at all, where exposed components might need to be

visually appealing.

5. If the parts will not be visible in the end product, a

lower finish option is typically selected.

Surface finish and textures (2)


6. Manufacturer may offers a wide range of

materials with multiple surface finish options.

7. Most resins are available in many colors and

manufacturers also capable of creating custom

colors to match customer requirements.

8. If the customer are working on a project that

may require painting as a final process,

considering molded-in color can often be

achieved for a much lower price than

traditional painting labor and material costs.

9. If the customer must paint their plastic parts ,

select a resin that paints easily and preferably

one that does not require surface etching and/or

primer.

Surface finish and textures(3)


http://www.protolabs.com/injection-molding/fundamentals-of-molding/molding-surface-finishes



http://www.quickparts.com/LearningCenter/BasicsofInjectionMoldingDesign.aspx

1. Texturing is a process used to apply patterns to a mold

surface.

2. This process allows flexibility in creating the final

appearance of your parts.

3. Texturing is an integral piece in overall product

development and should be considered during the design

process to achieve the desired results.

4. Texture can be a functional component of design as well.

5. Imperfect parts can be camouflaged by the right texture.

6. Is the part designed for frequent handling?

7. Texture can be used to hide finger prints and improve the

grip for the end user. Texture can also be used to reduce

part wear from friction.




A wide variety of textures are available for injection

molded parts such as:

1. Natural/Exotic

2. Matte Finishes

3. Multi-Gloss Patterns

4. Fusions

5. Graphics

6. Leather Grains/Hides

7. Wood grain, Slate & Cobblestone

8. Geometric & Linens

9. Layered Textures to Create New Looks

10.Images or Logos Incorporated into the Pattern




1. When applying a texture to a part, the CAD drawing must

be adjusted to accommodate for this surface variance.

2. If the texture is on a surface that is perpendicular or

angled away from the mold opening then no draft changes

are necessary.

3. If the texture is on a parallel surface with the mold

opening, however, increased draft is necessary to prevent

scraping and drag marks that could occur during part

ejection.

4. Different textures have different impacts on the molded

part.

5. The rule-of-thumb when designing for texture is to have

1.5 degrees of draft for each 0.001” of texture finish

depth.

Documents

BDD 40903 Injection Mold Design Chapter 2