A complete collection of plastic parts design techniques, this is very useful for plastic molds

Time：2023-07-19 10:08:26 / Popularity： / Source：

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

No	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 fiber	PA66-RG25	Surface of parts looks rough, color is 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.
6	Dust net	Flame retardant ABS	Flame retardant ABS
7	Two-color injection molding sign	Anti-plating, weather-resistant materials	PC	Double injection molding process is more complicated
7	Two-color injection molding sign	Plating material	ABS PA-757	Double injection molding process is more complicated

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 wear resistance can be obtained.	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.
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

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)
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
Plastic type	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 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.

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

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

Gud Mould Industry Co., Ltd