Plastic parts are structural designs based on industrial modeling. First, check whether there are similar products for reference, then perform a detailed functional decomposition of products and parts to determine splitting, wall thickness, demoulding slope, and gap between parts. Main process issues such as transition treatment, connection treatment, and strength treatment of parts.

1. A complete collection of plastic parts design techniques, this is
very useful for plastic molds
 01. General steps of plastic parts design
Plastic parts are structural designs based on industrial modeling. First, check whether there are
similar products for reference, then perform a detailed functional decomposition of products and
parts to determine splitting, wall thickness, demoulding slope, and gap between parts. Main
process issues such as transition treatment, connection treatment, and strength treatment of
parts.
 Similar reference
Before designing, you should first look for similar products of company and peers, what problems
and shortcomings have occurred in original products, refer to existing mature structure, and
avoid problematic structural forms.
 Determine part breaks, transitions between parts, connections, gap treatments
Understand modeling style from modeling drawing and effect drawing, cooperate with functional
decomposition of product, determine number of parts breaks (different surface states are either
divided into different parts, or there must be over-processing between different surfaces), and
determine excessive treatment between surfaces of parts, determine connection mode between
parts, and fit gap between parts.
 Determination of part strength and connection strength
According to size of product, determine wall thickness of main body of part. Strength of part
itself is determined by wall thickness of plastic part, structural form (strength of plastic part in
shape of a flat plate is the worst), reinforcing rib and reinforcing bone. While determining
individual strength of parts, connection strength between parts must be determined. Methods of
changing connection strength include adding screw columns, adding stoppers, adding buckles,
and adding reinforcement bones for lower support.
 Determination of draft angle
Demoulding slope should be based on material (PP, PE silica gel, rubber can be forcibly
demoulded), surface state (slope of embossed surface should be larger than that of smooth
surface, and slope of etched surface is as much as possible 0.5 degrees larger than sample
requirements, so as to ensure that etched surface is not damaged and improve yield of products),
and whether part should be transparent or not (transparent slope should be large).
 Types of materials recommended by company's different product lines
N
o
Parts
classification
Recommended
material
Markup example Precautions
1 Wrench Flame
retardant ABS
Flame retardant
ABS
Color must be pumped, vacuum
plating, electroplating performance is
not good.
2 Small panel
class
Flame
retardant ABS
HF-606 Color must be pumped, vacuum
plating, electroplating performance is
not good.
3 Rail class PA66+glass PA66-RG25 Surface of parts looks rough, color is

2. fiber difficult to adjust, material is brittle,
toughness is not good, and small
structures such as buckles on structural
parts are easy to break, so rounded
corners should be added to structural
design to enhance strength.
4 Mirrors, light
guides
PMMA,PC PMMA 560F Transparent PC has good toughness, is
not easy to be brittle, expensive, and
poor in light transmission. PMMA is
brittle, has better light transmission
and lower price. Fluidity of PC and
PMMA is not good, design should be
fully considered.
5 Lens,
transparent
window
Transparent PC Dow 302-05
6 Dust net Silk woven
dust net
PPK15X13H/B
Zhuhai Jinhai
Sanxi Company
If ambient temperature is too high,
consider changing material.
Flame
retardant ABS
Flame retardant
ABS
7 Two-color
injection
molding sign
Anti-plating,
weather-resist
ant materials
PC Double injection molding process is
more complicated
Plating
material
ABS PA-757
 Surface treatment of plastic parts
No Type Process
essence
Surface effect Precautions
1 Plastic Raw
Glossy
Surface
Mold cavity
surface
polishing
Smooth and shiny
plastic surface
The higher dimensional accuracy of
mold surface, the higher surface
finish and the more uniform gloss of
part.
2 Plastic
original
grain
surface
On the basis
of polishing
surface of
mold, it is
etched again.
Plastic surface
presents tiny
textures, and
surface has
different gloss
states.
Etching pattern is divided into light
pattern, semi-gloss pattern and
matt pattern;
Coarse lines are generally not used
in conjunction with fuel injection;
The finer etched plate, the less
scratch-resistant it is;
Matte pattern is easy to scratch.
3 Plastic
surface
spray
Spraying
plastic surface
Different colors,
different gloss
states, different
handles, and
surfaces with
different degrees of
Spray paint has gloss effect and
matt effect, and spray paint can
significantly improve appearance
grade of surface of plastic parts, but
cost will also increase a lot.

3. wear resistance can
be obtained.
4 Screen
printing on
plastic
surface, pad
printing
Partial printing
on the surface
of plastic parts
Can print
characters,
patterns, can have
different color
states
Its effect mainly depends on ink and
color;
5 Plastic
surface
vacuum
plating
Vacuum
plating
It can have different
colors and different
gloss states, and
can make
semi-transparent
light mirrors or light
panels.
Gloss depends on original state of
plastic;
Color depends on vacuum plating
itself;
Vacuum-plated products are not
wear-resistant, and generally spray
UV after vacuum plating to improve
wear resistance.
6 Plastic
Surface
Plating
Plastic Plating It can have different
colors and different
luster, but it cannot
be made
translucent.
Gloss depends on original state of
plastic;
Color depends on plating itself;
Electroplating product itself has
good wear resistance;
Some plastic materials cannot be
electroplated.
7 Plastic
surface IMD
Process can be
understood as
covering a
layer of
printable film
on the surface
of plastic.
Film itself is
wear-resistant, and
basically effect that
can be achieved by
printing can be
achieved on the
plastic surface by
this method.
Film generally covers surface;
It can be covered with white film,
screen printed on film or behind
workpiece, pad printed and then
sprayed with UV paint.
This method has process design
requirements for acute angles and
bumps.
8 Plastic
surface IML
Film layer is
relatively
thick, which
can be
understood as
a separate
part, which
can be
superimposed
on the surface
or inside of
plastic.
Film itself is
wear-resistant, and
three-dimensional
effect can be
achieved by using
thickness of film to
stack multiple
layers.
Thin film generally covers surface.
This method has process design
requirements for acute angles and
convex and concave.
 02. Technical requirements for plastic parts

4.  Wall thickness selection of plastic parts
Plastic type Minimum wall
thickness
Wall thickness of
small parts
Wall thickness of
medium parts
Wall thickness of
large parts
ABS 0.75 1.25 1.6 3.2-5.4
Fireproof ABS 0.75 1.25 1.6 3.2-5.4
PA66+glass fiber 0.45 0.75 1.6 2.4-3.2
PMMA 0.8 1.5 2.2 4-6.5
Transparent PC 0.95 1.8 2.3 3-4.5
Plastic parts have requirements on uniformity of wall thickness, workpieces with uneven wall
thickness will have shrinkage marks. It is required that ratio of reinforcement rib to wall thickness
of main body should be less than 0.4, and maximum ratio should not exceed 0.6.
 Draft angle of plastic parts
In construction of three-dimensional drawing, it is necessary to draw slope where it affects
appearance and assembly, and reinforcement generally does not draw slope.
Draft angle of a plastic part is determined by material, state of surface decoration, and whether
part is transparent or not. Rigid plastics have a larger demoulding slope than soft plastics, the
higher part, the deeper hole and the smaller slope.
No Main aspects affecting draft angle
1 Influence of plastic material PE and PP can be forced to demould, and amount
of forced demoulding generally does not exceed
5% of maximum cross-sectional area of core.
2 Effect of etching In general, draft angle is 0.5 ° larger than
permissible for etched plate.
3 Effects of Artifact Transparency
Prevention
Transparent workpieces generally take 3°.
4 General value In general, take 0.5-1.5°.
 Recommended draft angles for different materials
Plastic type Cavity slope Core slope
ABS 40”-1.2° 35-1°
Fireproof ABS 40”-1.2° 35-1°
PA66+glass fiber 25”-45” 20”-40”
PMMA 35”-1°30” 30”-1°
Transparent PC 35”-1° 30”-50”
 3. Values with different precisions in different size ranges
 Dimensional accuracy of plastic parts
Generally, precision of plastic parts is not high. In actual use, we mainly check assembly size, and
mark the overall size, assembly size, and other dimensions that need to be controlled on plan.
In practice, we mainly consider consistency of dimensions, for example, sides of upper and lower
covers need to be aligned,
No Basic size
(mm)
Accuracy class
1 2 3 4 5 6 7 8
Tolerance value (mm)

5. 1 ＜3 0.04 0.06 0.09 0.14 0.22 0.36 0.46 0.58
2 ＞3-6 0.04 0.07 0.10 0.16 0.24 0.40 0.50 0.64
3 ＞6-10 0.05 0.08 0.11 0.18 0.26 0.44 0.54 0.70
4 ＞10-14 0.05 0.09 0.12 0.20 0.30 0.48 0.60 0.76
5 ＞14-18 0.06 0.10 0.13 0.22 0.34 0.54 0.66 0.84
6 ＞18-24 0.06 0.11 0.15 0.24 0.38 0.60 0.74 0.94
7 ＞24-30 0.07 0.12 0.16 0.26 0.42 0.66 0.82 1.04
8 ＞30-40 0.08 0.14 0.18 0.30 0.46 0.74 0.92 1.18
9 ＞40-50 0.09 0.16 0.22 0.34 0.54 0.86 1.06 1.36
10 ＞50-65 0.11 018 0.26 0.40 0.62 0.98 1.22 1.58
11 ＞65-80 0.13 0.20 0.30 0.46 0.72 1.14 1.44 1.84
12 ＞80-100 0.15 0.22 0.34 0.54 0.84 1.34 1.66 2.10
13 ＞100-120 0.17 0.26 0.38 0.62 0.96 1.54 1.94 2.40
14 ＞120-140 0.19 0.30 0.44 0.70 1.08 1.76 2.20 2.80
15 ＞140-160 0.22 0.34 0.50 0.78 1.22 1.98 2.40 3.10
16 ＞160-180 0.38 0.55 0.86 1.36 2.20 2.70 3.50
17 ＞180-200 0.42 0.60 0.96 1.50 2.40 3.00 3.80
18 ＞200-225 0.46 0.66 1.06 1.66 2.60 3.30 4.20
19 ＞225-250 0.50 0.72 1.16 1.82 2.90 3.60 4.60
20 ＞250-280 0.56 0.80 1.28 2.00 3.20 4.00 5.10
21 ＞280-315 0.62 0.88 1.40 2.20 3.50 4.40 5.60
22 ＞315-355 0.68 0.98 1.56 2.40 3.90 4.90 6.30
23 ＞355-400 0.76 1.10 1.74 2.70 4.40 5.50 7.00
24 ＞400-450 0.85 1.22 1.94 3.00 4.90 6.10 7.80
25 ＞450-500 0.94 1.34 2.20 3.40 5.40 6.70 8.60
 Economical precision for different materials used
Plastic type Recommended Accuracy Classes
High precision General accuracy Low accuracy No tolerance
ABS 2 3 4 5
Fireproof ABS 2 3 4 5
PA66+glass fiber 3 4 5 6
PMMA 2 3 4 5
Transparent PC 2 3 4 5
PC+ABS 2 3 4 5
 04. Values with different precisions in different size ranges
 Surface roughness of plastic
1) Roughness that cannot be marked on etched surface. In places where plastic surface finish is
particularly high, circle this range and mark surface state as a mirror surface.
2) Surface of plastic parts is generally smooth and bright, and surface roughness is generally
Ra2.5～0.2um..
3) Surface roughness of plastic mainly depends on roughness of surface of mold cavity.
Roughness of mold surface is required to be one to two grades higher than surface roughness of

6. plastic parts. Surface of mold can reach Ra0.05 by ultrasonic and electrolytic polishing.
 Fillet
Fillet value of injection molding is determined by adjacent wall thickness, generally 0.5 to 1.5
times wall thickness, but not less than 0.5mm.
Position of parting surface should be carefully selected with rounded corners. There are rounded
corners on parting surface, and rounded part needs to be on the other side of mold. It is difficult
to make, and there are subtle traces at rounded corners. But rounded corners are required when
cut-resistant hands are required.
 Rib problem
Injection molding process is similar to casting process, and unevenness of wall thickness will
cause shrinkage defects. Generally, wall thickness of rib is 0.4 times thickness of main body, and
maximum is not more than 0.6 times. Spacing between ribs is greater than 4t, and height of ribs
is lower than 3t, as shown in Figure 6-1. In method of improving strength of part, reinforcement
is generally added without increasing wall thickness, as shown in Figure 6-1.
Rib of screw column should be at least 1.0mm lower than end surface of column, and rib should
be at least 1.0mm lower than surface of part, or parting surface.
When multiple ribs intersect, attention should be paid to uneven wall thickness caused by
intersection.
 05. Rib Design for Plastic Parts
 Bearing surface
Plastic is easy to deform, and should be classified as positioning of blanks in terms of positioning.
Positioning area should be small, such as plane supports, which should be changed to small
bumps and convex rings.
 Sloping roof and slider problem
Lifter and slider both move in parting direction and perpendicular to parting direction. There
should be no rubber position to block movement of lifter and slider in direction perpendicular to
parting mold, and there must be enough space for movement, as shown in figure below.
 06. Lifter and slider problem
 Processing method of limit process problem of plastic
 1) Special treatment method for wall thickness
For particularly large workpieces, such as toy car shells, method of multi-point glue injection can
make wall thickness relatively thinner. Local glue position of column is thick, and method as
shown in figure below is used to deal with it.

7.  2) Treatment of small slope and vertical surface
Mold surface has high dimensional accuracy, high surface finish, low demoulding resistance and
small demoulding slope. In order to achieve this purpose, small slope of workpiece is individually
inlaid, insert is cut by wire and processed by grinding, as shown in figure
In order to ensure that side wall is vertical, a running position or a slanted roof is required. There
are interface lines in running position. In order to avoid obvious interfaces, wiring is generally
placed at junction of rounded corner and large surface.
To ensure that side wall is vertical, a running position or a slanted roof is required. There are
interface lines in running position. In order to avoid obvious interfaces, wiring is generally placed
at junction of rounded corner and large surface.
 07. Problems that often need to be solved in plastic parts
 1) Transition processing problem
Precision of plastic parts is generally not high, there must be transitional treatment between
adjacent parts and between different surfaces of same part.
Small grooves are generally used to transition between different surfaces of same part, small
grooves can be used between different parts to deal with high and low staggered surfaces, as
shown in figure
 2) Clearance value of plastic parts
Direct assembly between parts, generally take 0.1mm if there is no movement between them;
Seam is generally taken as 0.15mm;
Minimum gap between parts without contact is 0.3mm, generally 0.5mm.
 3) Common forms and gaps of plastic parts are as shown in figure

8. Hole diameter of plastic parts corresponding to self-tapping screws
Self-tapping screw specifications ST2.9 ST3.5 ST4.2 ST4.8 ST5.5 ST6.3
Aperture of plastic parts 2.3 2.8 3.6 4.1 4.8 5.8
 08. Plastic parts are entering field
1. Replace sheet metal case cover with a plastic case cover;
2. Replace electroformed sign with a double-color injection sign;
3. Company is replacing metal anti-static small panel with an anti-static plastic small panel;
4. Company is replacing small metal shielding panels with plastic shielding panels