Design of general plastic parts
1. Terms and Qualifiers
1.1 Shrinkage and shrinkage marks
Phenomenon of depression on the surface of product is caused by shrinkage of plastic volume, which is common in local thick area, such as intersection area between reinforcing rib or column position and surface.
1.2 Shrinkage holes
Vacuum bubbles caused by volume shrinkage of local thick part of product during cooling process are called shrinkage holes.
1.3 Bubbles
When plastic melt contains air, moisture and volatile gases, air, moisture and volatile gases enter interior of product during injection molding process and remaining cavities are called bubbles.
1.4 Lack of glue and incomplete mold
Plastic melt does not completely fill cavity.
1.5 Burrs and burrs
When plastic melt flows into parting surface or insert mating surface, clamping force will be sufficient, but a thin film of excess rubber will be produced at junction of main runner and branch runner.
1.6 Burning
Generally speaking, so-called burning includes discoloration of product surface due to plastic degradation and charring of filling end of product; burning refers to air trapped in cavity that cannot be quickly discharged (trapped air) when plastic melt is filled, is compressed and significantly heated, burning material.
1.7 Welding marks and water inclusion marks
When mold adopts a multi-gate pouring scheme, flow fronts of rubber material merge with each other; in hole position and obstacle area, flow front of rubber material will also be divided into two: uneven wall thickness will also cause welding marks.
1.8 Spray marks and snake marks
Plastic melt passing through gate at high speed directly enters cavity, then contacts cavity surface and solidifies, and is then pushed by subsequent plastic melt, leaving snake marks. For side gates, when there is no material retention area or insufficient material retention area after plastic passes through gate, spray marks are likely to occur.
1.9 Silver wires and silver stripes
Silver-white stripes appear on the surface or near surface of product along flow direction of plastic. Generation of silver wires is generally caused by vaporization of water or volatiles in plastic or water attached to mold surface. Sometimes silver stripes are produced when injection molding machine screw is entrained in air.
1.10 Cracks and crazing
Severe and obvious cracks on the surface of product are called cracks, hair-like cracks on the surface of product, and this phenomenon is often seen at sharp corners of product, which is called crazing, also often called stress cracking.
1.11 Poor surface gloss
Surface of product loses its original gloss, forming a milky white film, a blurred state, etc., which can be called poor surface gloss.
1.12 Warping deformation
Due to wall thickness or uneven cooling during forming, shrinkage ratio of product is different, resulting in deformation or distortion of product.
1.13 Flow mark
Trace of plastic melt flow, which is a striped wave shape centered on gate.
2. Material selection
There are many types of plastic materials, and different requirements are required for different types of products. For design of our company's plastic products, we must fully consider occasions and working environment of product, such as meeting temperature requirements, fire protection or biocompatibility requirements. For those with fire protection requirements, there are different fire protection levels of materials (including GE, Chi Mei, TORAY, BAYER, Formosa Chemical). When fire-resistant materials are required, we only need to ask supplier for material certification number.
3. Choice of wall thickness
Wall thickness of plastic parts has a great influence on quality of parts. If wall thickness is too small, flow resistance of molding is large, large and complex parts are difficult to fill cavity. Minimum size of plastic wall thickness should meet following requirements:
Sufficient strength and rigidity.
Can withstand impact and vibration of demolding mechanism during demolding
Can withstand sufficient tightening force during assembly
Plastic parts have a minimum wall thickness value, which varies with type and size of plastic. For shell parts, following wall thicknesses are recommended: ABS, PC+ABS, PC, transparent PC, transparent ABS, wall thickness: 2.0-3.5mm Some small appearance parts (such as button caps, lamp holders, knobs) can be 1.2-2.0mm. Wall thickness of same plastic part should be as consistent as possible, otherwise wall thickness may shrink due to uneven wall thickness.
4. Design of draft angle
Sufficient draft angle should be designed along demoulding direction on inner and outer surfaces of plastic parts, otherwise it will be difficult to demould, or plastic parts will be scratched or damaged during ejection. Another point is that if draft angle is small, etching will be shallow, which will make appearance parts easy to get dirty, so following draft angle is recommended: (design angle is 0.5 degrees larger than required slope of the pattern, and each supplier has its own pattern plate)
5. Column design
5.1 Function of column: Usually, plastic column is used to support PCBA, fix PCBA or plastic parts, fix electronic components, or connect front and back shells of products. One of its biggest advantages is that height is easy to adjust. Therefore, under normal circumstances, we should try to use plane formed by the end face of column as support surface. Plane is formed by several ribs.
5.2 Draft and height of column: When column height is greater than 10mm, it is usually ejected by a sleeve, so its draft angle can be very small or 0 degrees. Inner hole can be taken as 0 degrees and outer surface can be taken as 0.25 degrees. When column height is less than 10mm, inserts may be used on mold, draft angle can be taken as 0.5 degrees for inner hole and 1 degree for outer surface. If you want to adjust height, please indicate it on drawing, and consider adjusting height when requiring mold processing. Under normal circumstances, height of M3 self-tapping screw column should not be greater than 30mm. If it is too high, column's sleeve pin will be easily bent and deformed by glue flow.
5.3 Column size: Since columns with a height greater than 10mm are usually ejected with a sleeve, the outer and inner diameters of the column are limited.
Outer diameter series of commonly used sleeves are: 4.0, 4.5, 5, 6, 6.5, 7, 8, 10, 12
Commonly used sleeve pin series are: 1.5, 2.0, 3.0, 3.5, 4.0, 4.5, 5.0

 

Corresponding relationship is shown in the table above
Inner diameter of column position is obtained by grinding sleeve needle. As can be seen from table above, we should not design columns with a wall thickness less than 1.0mm, and it is best not to design columns with an outer diameter that is not a standard value.
5.4 Treatment of root of column: Usually two situations are prone to occur: shrinkage or shadow (or water mark). In order to prevent shrinkage, it is necessary to reduce glue at root of column and lengthen sleeve needle. If a shadow (or water mark) has appeared, it is necessary to add glue to root of column or shorten sleeve needle. Therefore, for a wall thickness of 3mm and a column diameter greater than or equal to 6mm (screw M3 self-tapping screws), in order to prevent shrinkage, first reduce glue at the root of column during mold processing. If a shadow appears during trial mold, add glue. (Pay attention to requirements at the root of column, ejector surface and crater surface are not at same height to reduce stress)
5.5 Selection and use of embedded studs: If frequent disassembly and assembly are required, or if maintenance is required, embedded (hot-pressed) studs should be used, and ordinary screws should be used instead of self-tapping screws. After hot pressing or embedded nuts, glue column should not crack or overflow.
5.5.1 Cracking problem of embedded studs and plastic parts:
Due to large difference between size change of embedded studs during cold setting and shrinkage value of plastic parts, a large internal stress will be generated around embedded studs, causing cracking of plastic parts, which is more serious for high-rigidity engineering plastics. Due to relatively large internal stress, PC is normally prohibited from embedding, and PC+ABS is not recommended, especially when number of embedded screws is more than 4, and studs are not of same height, resulting in too many types of studs and easy errors; at the same time, because of long placement time, material in injection molding machine barrel will deteriorate due to high temperature for too long. Following lists M2, M2.5, M3, and M4 self-tapping screw columns for reference only. For specific design parameters, please refer to general requirements for design dimensions of plastic screw holes.

 

6. Design of rib position
Function of rib is to increase strength of plastic parts and avoid deformation of parts. If strength of plastic parts is improved by increasing wall thickness alone, it is often unreasonable. First, shrinkage is easy to occur, and second, injection molding cost is increased. Rib should not be designed too thick, otherwise shrinkage is easy to occur at its root. It should not be too thin, otherwise uneven glue flow is easy to occur. Recommended thickness is: when mold is processed as a large water outlet, root thickness of part rib is less than 1/2 of wall thickness. Top thickness of rib should not be less than 1mm. When mold is processed as a small water outlet, root thickness of part rib is less than 2/5 of wall thickness, and top thickness of rib should not be less than 0.8mm. About draft angle of rib: For ordinary ribs, draft angle on both sides can be taken according to above thickness requirements. For ribs with special height requirements, 0.1-0.25 degrees on both sides can be taken. At this time, ribs will be made into inserts. If possible, adding a column position to rib can help demolding. (As shown below)

 

Reinforcement ribs on column: Plastic column must be designed with reinforcement ribs if structure allows. Reinforcement ribs on column can be much higher than ordinary reinforcement ribs because they are ejected together with column. It is only necessary to be 1-3mm lower than end face of column in height direction. At the same time, reinforcement ribs on column should be processed as symmetrically as possible to ensure that column deformation is as small as possible. Its shape is shown in Figure 1, and indicated slope is D=3-5 degrees.
7. Design of decorative seams (art lines), stoppers, forks, buckles, and stoppers
For industrial products, especially fixed lighting products, if environmental conditions allow (for some products, especially products in various dust or dust environments, there should be no decorative seams), it is best to design decorative seams. Design of decorative seams is to make up for appearance defects caused by deformation of plastic parts. In order to ensure that shapes of plastic parts match each other well and are easy to assemble and disassemble, it is necessary to design stoppers and forks in the places where they match each other. Designs of stoppers and forks are varied. Shapes of stoppers and forks are recommended as shown in Figures 2 and 3. Pay special attention to reducing glue evenly and gradually, without sudden changes, otherwise shadows will easily form on the surface.

 

7.1 Design experience of stopper
7.1.1 Stopper form 1
From perspective of appearance, it is recommended that part 1 is front shell and part 2 is back shell. At the same time, if front and back shells are not flush after mold is processed and mold needs to be repaired, it is recommended to add more to front shell, so that front shell is larger than back shell (D1>D2), which is better than back shell being larger than front shell (D2>D1) in appearance. It is recommended to design front shell D1 with positive tolerance and back shell D2 with negative tolerance. Features of this stopper: beautiful appearance, large decorative seam, suitable for larger parts, appearance of thin shell parts has shadows, and sealing is slightly poor.
7.1.2 Stopper form 2

 

Good sealing, small decorative seam, beautiful, suitable for small structures, and easy to show seams for large parts.

 

7.1.3 Stopper form 3
Hand feel is slightly worse, suitable for larger parts, is conducive to covering size error of front and rear shells.

 

7.1.4 Stopper form 4
Beautiful appearance, decorative seams can be large or small, good sealing (sealing strips can be added), requiring thicker wall thickness and complex structure.

 

7.1.5 Buckle design between front and rear shells
In design of plastic parts, in order to simplify assembly and reduce number of screws, elasticity of plastic can be used to design buckles. There are many buckle forms. Here we should pay special attention to connection buckle between front and rear shells. Recommended buckle forms are as follows:

 

Structure shown in Figure 4 is generally made on the back shell, and structure shown in Figure 5 is generally made on the front shell. Pay special attention to uniform and gradual reduction of glue, not sudden changes. Otherwise, shadows or shrinkage will appear at the place where glue is reduced.
8. Design of rounded corners
Except for sharp corners required for use, all other corners of plastic corner parts should be rounded as much as possible, because sharp corners of plastic parts are prone to stress concentration, and cracking will occur when subjected to force or impact vibration. Even cracking occurs during demolding process. It is generally recommended to use following principles for processing rounded corners:
Top ends of ribs, R>1mm.
Square hole in the middle of plastic part is rounded around, and rounded corners are greater than 1/4 of wall thickness of part
Four corners of square part that matches hole above plastic part are rounded
Root of column that is particularly stressed is rounded, about R0.5
9. Design of commonly used transparent parts
Transparent materials: There are many types of commonly used transparent materials. Transparent ABS, PC, and PMMA are commonly used in our design.
Due to poor fluidity of transparent materials, it is easy to produce appearance problems during injection molding. For example, top of products such as transparent button caps is prone to water marks. Products such as lenses are prone to nail marks and snake marks near gate. In view of above problems, please pay attention when designing: For parts such as transparent button caps: top thickness should be designed to be thicker than surrounding area. If cavity is too deep, glue should be reduced locally near gate to allow glue to flow to top of part first and then to surrounding area.
Decorative sheets, lens parts: Please reserve a wider (greater than 6-8mm) and thicker gate position (greater than 1-1.5mm) when designing. When processing mold, use a wide fan-shaped gate as much as possible, runner should be short and thick.
Key points in plastic parts structural design
Appropriate and uniform wall thickness

 

Appropriate and uniform wall thickness

 

There should be transition parts between walls of different thicknesses

 

 

Avoid collapse

 

 

Elimination or concealment of surface dents

 

There should be enough draft angle

 

Plastic parts should have symmetrical structure

 

Adopting combined structure

 

Reduce deformation of corner parts

 

Avoid compression of slender rods

 

Avoid inscribed structures

 

 

Avoid side hole structures

 

Avoid lateral depressions or bosses

 

Using reinforcing ribs to increase strength and rigidity of plastic parts

 

Reasonable arrangement of reinforcing ribs

 

 

Avoid rib convergence

 

Avoid deformation of top and bottom of large shells

 

Avoid sharp edges

 

Avoid seams on the surface

 

Avoid leaving weld marks on plane

 

Reasonable design of structure of supporting surface

 

Reasonable design of corner braces

 

Reasonable design of boss

 

 

Reasonable design of through holes

 

 

Reasonable design of blind holes

 

1-core 2-support column
Avoid triangular threads

 

Reasonable design of threaded parts

 

Beginning and end of thread should be smooth

 

Avoid surface collapse caused by screw connections

 

Plastic parts connected by bolts need metal sleeves

 

Markings should be easy to process and replace

 

Avoid excessive tensile stress on bonding surface

 

Reduce movements required when assembling plastic parts

 

Special mechanism designed by utilizing elasticity of plastic parts

 

Avoid simply imitating structure of steel or wooden parts

 

Avoid making threaded holes directly on plastic parts