LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE
KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
RHEINISCH-WESTFÄLISCHE TECHNISCHE HOCHSCHULE AACHEN
UNIV.-PROF. DR.-ING. J. FELDHUSEN
The Grafton Acrylic Alto - Saxophone (1950)
“It was the largest piece of injection moulding attempted up to that time”
(Wally Horwood: The Grafton Story)
Handout “Design of Plastic Parts“
Within the course
Systematic Engineering Design I
Assignment
Script und presentation: Dr.-Ing. A. Brezing
LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
RHEINISCH-WESTFÄLISCHE TECHNISCHE HOCHSCHULE AACHEN
1
Theory and literature
This exercise deals with some examples on designing injection moulded plastic parts. It’s only
a part of the field of designing plastic parts - albeit an important one. The theory falls into two
parts; first, design guidelines are discussed that result from shrinkage of the material in the
cavity. Secondly, typical features are treated that are feasible because of the material’s
physical properties. “Design for Plastics” does not only mean to take into account specific
restrictions, but also to utilize the material’s full potential.
Most of the figures have been taken from these sources:
[HOECHST85]
HOECHST AG: Technische Kunststoffe: Berechnen - Gestalten - Anwenden;
C 3.4: Richtlinien für das Gestalten von Formteilen aus technischen
Kunststoffen. Eigenverlag, Frankfurt a. M., 1985.
[TRES06]
Paul A. Tres: Designing Plastic Parts for Assembly, 6th ed. Carl Hanser,
Munich, 2006.
1.1
Embodiment concepts
As an entry into the complex field of shaping plastic parts, the author suggests a categorization
of all possible shapes into two basic shapes that result from the fundamental restriction that
cast parts must be thin-walled:
•
Shell Shape or
•
Plate Shape, Figure 1.
Deciding on either of these concepts at the beginning of the design phase supports an efficient
work progress, especially when using a 3D-CAD modeller. Thin walls are necessary not only
for reasons of reducing the amount of used material, but also because of quick cooling phases
(cycle time is the predominating cost factor with injection moulded plastic parts) and lower
residual stresses that result from uneven shrinkage over wall thickness.
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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Figure 1:
1.2
A cuboid body, transformed to a shell shape (left) and a plate shape. The plate shape lacks the rounds on the
inner geometry. However, rounds aren’t as critical on plastic parts as they are with cast steel parts. Drafts and
rounds are exaggerated on these examples.
Shrinkage
It is an unsurprising fact that the material shrinks while cooling down inside and outside of the
cavity, which on principle is not a problem. What is critical is uneven shrinkage in the part, as it
results in deformation or even fracture. As thermoplastics have an amorphous crystalline
structure, their density at the end of the cooling process depends on how much time the
molecules have to restructure before the material solidifies. As a conclusion from this, the
faster the molecules are “frozen”, the lower the amount of shrinkage. A low wall temperature
will consequently reduce shrinkage. A high pressure within the cavity will also reduce
shrinkage as it impedes the ordering processes of the molecules, Figure 2.
Figure 2:
Shrinkage as a function of pressure and wall temperature within the cavity [HOECHST85]
Pressure and temperature can be controlled with the injection moulding machine’s control unit,
but only as global parameters. Local differences within the cavity directly or indirectly result
from its shape: the local wall temperature depends on the path of the coolant bores; uneven
pressure results from the flow resistance as a direct result from the part’s shape and the sprue
position, Figure 3. Therefore, the main determining factor for the part’s achievable quality is not
the design of the tooling but the embodiment design of the part itself.
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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Figure 3:
1.3
Shrinkage and pressure drop within the cavity over the length of a plate [HOECHST85]
Typical deformations and derived design guidelines
All of the typical deformations of plastic parts and their remedies listed below directly result
from the interrelations described in 1.2. For example, the design guideline resulting from
Figure 4 is to strive for constant wall thickness.
Figure 4:
As shrinkage in the thinner section of the plate is lower than in the thicker section, the part gets warped if there is a
step in wall thickness [HOECHST85]
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
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Figure 5:
Intended shape and typical deformation on a corner [HOECHST85]
Figure 6:
Design guideline to prevent deformation and blowholes on corners [HOECHST85]
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
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Figure 7:
Intended shape and typical deformation on a T-junction [HOECHST85]
Figure 8:
Design guideline to prevent deformation, shrink marks and blowholes on T-junctions [HOECHST85]
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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Figure 9:
Intended shape and typical deformation on a box with floor plate [HOECHST85]
Figure 10:
Design guideline to prevent deformation on a box with floor plate [HOECHST85]
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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Figure 11:
Intended shape and typical deformation on a box with dividing wall [HOECHST85]
Figure 12:
Design guideline to prevent deformation on a box with dividing wall [HOECHST85]
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
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1.4
Typical features
Listed below are some typical features that are feasible because of thermoplastics’ material
properties. The low module of elasticity allows for snap fits (positive fits) without any additional
parts while the low melting point makes different kinds of material fits such as welding
possible. Especially the living hinge is a good example for a cost-efficient design that allows
reducing the number of components by utilizing thermoplastics’ high ductility.
There are design guidelines for each of these features available in literature. Some examples
are shown here.
1.4.1 Snap fits
Figure 13:
Snap hooks for one-way (left) and two-way snap fits [TRES06]
Figure 14:
Mounting and un-mounting of snap fits [various websites]
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
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Figure 15:
Unmolding concepts for snap hooks [various websites]
Figure 16:
Snap fits: variation of working motion/surface [TRES06]
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
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1.4.2 Welding and other material fits
Figure 17:
Ultrasonic welding
Figure 18:
Design guideline for ultrasonic welding joints (example taken from [TRES06])
Figure 19:
Combination of principles for a joining task: Hewlett Packard laptop housing (example taken from [TRES06])
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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Figure 20:
Staking [www.sonitek.com]
Figure 21:
Shape variations and design guidelines for staking (example from [TRES06])
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
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1.4.3 Living Hinges
Figure 22:
Living hinges: application example and tooling configuration [www.immnet.com]
Figure 23:
Design guideline for living hinges [TRES06]
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
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LEHRSTUHL UND INSTITUT FÜR ALLGEMEINE KONSTRUKTIONSTECHNIK DES MASCHINENBAUS
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2
Assignment 1
Design in detail a plastic stabilizer link as outlined in the solution sheet, scale 1/1. The medium
wall thickness should be about 4mm. The seating of the plastic bearing race (given) will be
milled for reasons of dimensional accuracy. The basic layout of sealing boot and clamping
rings shall be used in the design; the bearing cover and ball pin must not be changed.
Figure 24:
3
Application example and detailed view of a stabilizer link
Assignment 2
Design in detail a rectangular, moulded plastic box/housing, l*w*h=100*60*60. It must consist
of two identical parts, divided in a horizontal plane, that are snapped together using one-way
snap-fits. No lifters/undercuts are allowed; however, the housing may have openings.
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
14
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Solution sheet to assignment 1
Handout “Design of Plastic Parts“
© IKT, RWTH Aachen, 2008
Unauthorized reproduction prohibited
15