This document helps to explain the basic and important guideline while designing a plastic part. If this ideal practice refers while designing part then the designer will deliver a product which is suitable as per the manufacturing and production point of view.

1. “IDEAL PRACTICE WHILE DESIGNING INJECTION MOULDED
PLASTIC PART”
Presented by
Mr. Narendra Gupta
E-mail:-narengupta124@gmail.com Contact:-8080007801

2. Introduction:
-
• It is necessary to understand the limitation and criticality of the
process before designing a product which is going to develop by
that process.
• If there is product which is going to develop by injection
moulding then the product should be design as per that.
• If not then product tool manufacturing and production may time
consuming, costly, complex or equal to not possible.
• Product should be design as per the DFM of injection moulding.
• Designer should be aware of the tool design ideal practice.
• This presentation explain the basic and important guideline while
designing plastic part.
• If this ideal practice refer while designing part then designer will
delivered product which is suitable as per the manufacturing and
production point of view.

3. Injection moulding
Advantages of Injection Moulding
• Low Waste
• Labour Costs Low
• Material and
Colour
• Flexibility
• Fast Production
Disadvantages of Injection Moulding
• High initial tooling cost
• Part design limitation

4. Problem with the injection moulding part design
The process of injection moulding means there are some restrictions with
regards to part design. After designing a part there may be need to make few
changes to parts so that can make or decide on a different manufacturing
technique. The most important thing to realise is that a mould tool is made
from two halves that need to pull apart and the part needs to be able to be
released from the tool. This is simple, but massive. It has all sorts of
ramifications down the line in terms of tool design like:
• If there is no draft to part then it is difficult to release the part from the tool.
• Under-cut requires side-core with is cost effective.
• Uniform wall thickness required for easy flow of plastic material.
• Sometime Sharp-edges are very difficult to fill and manufacturing in tool.
• Limitation of Straight Pull.
• Part size restriction.

5. The below image helps to understand the terminology of typical injection moulded plastic part
Injection moulding-
Electronic control box

6. • Warping:- Thick sections cool slower
than thin sections. The thin section
first solidifies, and the thick section
is still not fully solidified. As the
thick section cools, it shrinks and
the material for the shrinkage
comes only from the unsolidified
areas, which are connected, to he
already solidified thin section. This
builds stresses near the boundary of
the thin section to thick section.
• Sink marks:- Due to uneven
thickness when the thicker material
get cool, it start shrinking result of
that pull the surface material down
which results in the surface area
mark know as “sink mark”
Warping
Sink marks
• The optimum uniform wall thickness is often needed in plastic part
• The wall thickness should neither be too low nor too high as it could lead to some
flowing issues, or some structural and aesthetic issues
Issue with non-uniform wall thickness
1 Wall-thickness

7. Voids
Air-trapping
• Voids:- Voids are holes enclosed
inside a part. These can be a single
hole or a group of smaller holes.
Voids are caused when the outer skin
of the part is stiff enough to resist
the shrinkage forces thus preventing
a surface depression. Localized
shrinkage of the material at thick
sections without sufficient
compensation result in Voids.
• Air-trapping:- Trapped air is a root
cause of bubbles as well as
blisters. The air may be trapped in
ribs, threads or non-vented
projections off the nominal wall.
Melt flow pattern is a major cause of
bubbles. Processors should examine
each parts flow pattern via short
shots to see if the plastic flow front is
coming around on itself.
1 Wall-thickness
Issue with non-uniform wall thickness

8. 1 Wall-thickness
What if you cannot have uniform walls, (due to design limitations)?
• Coring:-Coring is a method where plastic is removed from the thick area, which helps to keep
Wall sections uniform, eliminating the problem altogether.
• Gradually smooth changing:-If design limitations make it impossible to have uniform wall
thicknesses, the change in thickness should be as gradual as possible.

9. 1 Wall-thickness
What if you cannot have uniform walls, (due to design limitations)?
• Gussets:- Gussets are support structures that can be designed into the part to reduce the
possibility of warping. What if you cannot have uniform walls (due to design limitations)?

10. • Mould drafts facilitate part removal
from the mould.
• The draft must be in an offset angle
that is parallel to the mould opening
and closing.
• The ideal draft angle for a given part
depends on the depth of the part in
the mould and its required end-use
function.
• For a rib a least 0.5° draft angle
required per side for easy
removal from the tool.
• If there is not sufficient draft
give to part then part may not
eject from the tool or we found
rubbing mark of tool on the.
• Positive draft helps in machine
operation of the tool.
2 Draft-angle

11. • Sharp corners greatly increase stress
concentration, which, when high enough,
can lead to part failure.
• Corner radius reduced the stress
concentration
• In addition to reducing stresses, the fillet
radius provides a streamlined flow path for
the molten plastic, resulting in an easier fill
of the mould.
• A bigger radius should be used if part
design allows.
• Radius must be design properly to make
uniform flow of molten plastic
3 Fillet/Radii

12. 4 Bosses
• Bosses are used for the purpose of
registration of mating parts or for attaching
fasteners such as screws or accepting
threaded inserts (moulded-in, press-fitted,
ultrasonically or thermally inserted).
• Wall thicknesses for bosses should be less
than 60 percent of the nominal wall to
minimize sinking.
• The base radius should be a minimum of
0.25 X thickness.
• Bosses can be strengthened by
incorporating gussets at the base or by
using connecting ribs attaching to nearby
walls.
• Bosses should not be located along any
edge. If rigidity is required the bosses can
be linked to the edge by ribs: see below
pictures for some examples.

13. 4 Bosses
• The standard rule for the cylindrical bosses
with holes to receive screws, threaded
inserts, etc. is to keep the outside diameter
between 2 and 2.4 times the diameter of
the screw or insert as shown in following
picture (left).
• Regarding the depth of the hole, the
bottom of the hole should extend
approximately to the base wall level (below
picture, right).

14. 5 Ribs
• Ribs in plastic part improve bending
stiffness (relationship between load and
part deflection) of the part and increases
rigidity.
• It also enhances mouldability as they
melt flow in the direction of the rib.
• Ribs are placed along the direction of
maximum stress and deflection on
nonappearance surfaces of the part.
• Proper rib design involves five main
issues: Thickness, Height, Location,
Quantity, and Mouldability.
• Consider these issues carefully when
designing ribs.

15. 5 Ribs
Height-The height of a rib should be limited
to less than three times its thickness. It is
better to use multiple ribs to increase
bending stiffness than to use one very tall rib.
Thickness-Rib thickness should be less than
wall thickness to minimize sinking effects. The
recommended rib thickness should not
exceed 60 percent of the nominal thickness.
Plus, the rib should be attached with corner
radii as generous as possible.
Mouldability- Draft angles for ribs should be
minimum of 0.25 to 0.5 degree of draft per
side.
Location-The rib orientation is based on
providing maximum bending stiffness.
Depending on orientation of the bending
load, with respect to the part geometry, ribs
oriented one way increase stiffness.

16. 6 Sharp corner
• Avoid sharp corners as it could lead to
high stress concentration, or in some case
create air traps.
• Below picture (left) shows the stress
concentration as a function of the radius
to thickness ratio: a ratio of approximately
0.15 gives a good compromise between
performance and appearance.
• Corner are always complex to
manufacture
• Removing sharp corner can reduce the
chances of damage and accident

17. 7 Thermal Heat of expansion
• Most of the plastics have higher
coefficient of thermal expansion
than metals. Hence a plastic part
that is assembled on a metallic part
can show some excessive stress at
its fixing points due to the
difference in thermal expansion.
• On the long term this can lead to
some damage.
• This problem can be addressed by
using some slots rather than round
holes as shown in below picture.

18. 8 Parting line
• Avoid locating the parting lines on
curved surfaces.
• Do the draft analysis
simultaneously while selecting the
parting line.
• In the case of a shaft for example, it
is recommended to create a flat as
in below picture.
• Parting line selection very
important for tooling point of view
• Parting surface should be free from
radius to avoid flash in production

19. 9 convert metal to plastic
• How to convert metal part in
plastic? There is no direct method
or rule. Here are some example
which give basic idea for
conversion.
• These 2 images gives the rough
ideas about the conversion of
metal part with plastic.
• The metal part is design according
the operation by which it is made,
while converting we have to be
careful from where we can remove
the material and where to add the
material but without disturbing the
function and the strength of the
part.
• As if you notice the second picture
you will notice in plastic part
material is removed by the coring
but keeping the function and
strength same.

20. 10 Straight-pull
• Design Parts that qualify for Injection
Moulding must be designed as
straight-pull parts.
• A part made with a straight-pull
mould is designed such that when
the two halves of the mould pull
straight away from each other, there
is no mould metal that wants to pass
through the part plastic (an
impossible, ‘die locked’ situation).
• Undercuts on the part require mould
pieces to pull out sideways,
perpendicular to the direction of pull.
These are called side actions.
• Parts with undercuts are not
available within the Injection
Moulding process. However,
undercuts are easily produced using
either slider or lifter.

21. 11 Under-cut
• In plastic injection moulding
industrial, it refers to part features
that prevent straight ejection at the
parting line, which cause much
mould complexity and lead to higher
mould construction and
maintenance costs.
• Whenever it's possible, redesign the
part to avoid undercuts.
• Minor part design changes can often
eliminate undercuts in the mould.
• For example, adding through holes
can give access to the underside of
features that would otherwise be
undercuts, simple modifications
enable the mould to form a hole in
the sidewall rather than with a side-
action mechanism. See below figure.

22. 11 Under-cut

23. Reference
• James M. Margolis’s “Engineering Plastics Handbook 2006” McGraw-Hill Education
• Sanjay K Nayak, Pratap Chandra Padhi, Y. Hidayathullah “Fundamentals of plastic
Mould design” Tata McGraw-Hill Education
• Robert A. Malloy “Plastic part design for injection moulding” by SPE Books from
Hanser Publishers
• “Designing Plastic Parts for Assembly” 7th Edition by Paul A. Tres SPE Books from
Hanser Publishers
• “The Complete Part Design Handbook: 'For Injection Molding of
Thermoplastics” by E. Alfredo Campo
• http://www.efunda.com/designstandards/plastic_design