Glue thickness (overall shell) of plastic products is usually around 0.80-3.00. Too thick is easy to shrink and produce bubbles, too thin is difficult to fully glue. Large products should be thicker, small products should be thinner, and general products should be 1.0-2.0. In addition, glue position should be as uniform as possible. In unavoidable circumstances, some places can be appropriately thicker or thinner, but it must be gradual and not sudden. Principle is to not shrink and be able to fully glue. Generally, it is difficult to glue when plastic glue thickness is less than 0.3, but soft glue and rubber can also be fully glued when glue thickness is 0.2-0.3.

Most plastic products have reinforcement ribs, because reinforcement ribs can greatly increase the overall strength of product without increasing the overall thickness of product. They are especially useful for large and stressed products, can also prevent product deformation. Thickness of reinforcement ribs is usually 0.5-0.7 times the overall thickness of glue. If it is greater than 0.7 times, it is easy to shrink. When height of reinforcement rib is large, a slope of 0.5-1 should be made (because of its large demoulding resistance), and no slope is required when height is short.

Plastic products must have a demoulding slope, except for those with a shallow height (such as a flat plate) and special requirements (but when side wall is large and there is no demoulding slope, a slide position is required). Demoulding slope is usually 1-5 degrees, and is often about 2 degrees. It depends on size, height, and shape of product, with principle of smooth demoulding and no impact on use function. Inclination of front mold of product is usually 0.5 degrees greater than that of rear mold, so that product can stay in rear mold when mold is opened. Usually, pillow position, insertion, and collision penetration are all required to be inclined, upper and lower break difference (i.e. difference between large end size and small end size) should be greater than 0.1 on one side.

Except for places where sharp edges are specially required for plastic products, fillets are usually made at edges to reduce stress concentration, facilitate flow of plastics and facilitate demolding. Minimum R is usually greater than 0.3, because it is difficult to achieve on a mold with too small R.

From perspective of mold processing, it is best to make hole into a simple round hole with a regular shape, and try not to make it into a complex special-shaped hole. Hole diameter should not be too small, and ratio of hole depth to hole diameter should not be too large, because thin and long mold core is easy to break and deform. Distance between hole and outer edge of product should preferably be greater than 1.5 times hole diameter, and distance between holes should preferably be greater than 2 times hole diameter, so that product has necessary strength.
Holes parallel to mold opening direction are usually formed by cores (can be inlaid, can be extended) or through-punching, and holes not parallel to mold opening direction are usually made into positions or lifters. Without affecting use and assembly of product, holes on side walls of product should be made into holes that can be pierced or inserted as much as possible.

Bosses are usually used for matching of shaft-hole form of two plastic products, or assembly of self-tapping screws. When BOSS is not very high and mold is ejected by a sleeve, it does not need to be inclined. When BOSS is very high, a cross rib (rib) is usually added to its outer side. Cross rib is usually inclined at 1-2 degrees, and BOSS should also be inclined depending on situation. When BOSS and column (or another BOSS) are matched, matching clearance is usually 0.05-0.10 on one side, so as to accommodate position error generated during processing of each BOSS.
When BOSS is used for assembly of self-tapping screws, its inner hole should be 0.1-0.2 smaller than thread diameter of self-tapping screw on one side, so that screw can be locked. For example, when assembling with M3.0 self-tapping screws, inner hole of BOSS is usually Ф2.60-2.80.

When an existing metal or plastic part is placed in a mold for molding again, existing part is called an insert. When a plastic product is designed with an insert, it is necessary to consider that insert must be able to be completely, accurately and reliably positioned in mold, and it is also necessary to consider that insert must be firmly connected to molded part. When rubber coating is too thin, it is not easy to be firm. It is also necessary to consider that there should be no glue leakage.

Plastic products can have smooth surface (mold surface light saving), spark pattern (mold cavity formed by copper electro-discharge machining), etched surface (sun-textured surface) and engraved surface. When depth of texture is deep and number is large, demolding resistance is large, and demolding slope should be increased accordingly.

Text on the surface of plastic products can be convex or concave. Concave characters are easy to make corresponding concave cavities on mold, while concave characters are more difficult to make convex cores on mold.

threads on plastic parts are usually not very accurate, and special thread removal mechanisms are required. For those with low accuracy requirements, structure can be simplified to a structure that can be forced to demold.

Plastic products usually do not use the entire surface as support surface, but make bosses, convex points, and ribs separately for support. Because it is difficult for plastic products to have a relatively large absolute plane, they are prone to deformation and warping.

1. Ultrasonic wire joint assembly method, which is easy to do on mold, but a special ultrasonic machine is required in assembly process, cost is increased, and it cannot be disassembled.
Cross section of ultrasonic wire is usually made into a triangle with a width of 0.30 and a height of 0.3, and a length of 5-10MM in length direction. 2MM;
2. Self-tapping screw assembly method, which is easy to do on mold, but it increases assembly process, cost is increased, and disassembly is troublesome;
3. Hook-button assembly method, which is characterized by complex mold processing, but easy assembly, can be repeatedly disassembled and used multiple times. There are many forms of hooks. It is necessary to avoid local glue position of hook being too thick, and mold making convenience of hook should also be considered. Hook should be properly matched, easy to assemble and disassemble. Matching surface is fitted, and the other surfaces have appropriate gaps.
4. BOSS shaft-hole assembly method, which is characterized by easy mold processing, easy assembly, and easy disassembly, but its disadvantage is that assembly is not very firm.

Nozzle is usually made at mating contact surface of two plastic products. Depth of nozzle is usually about 0.8-2.5, and a gap of about 0.1 is left on the side. When depth is deep, slope is 1-5 degrees, usually 2 degrees, and no slope is required when depth is shallow. Upper and lower mating surfaces of nozzle are usually fitted (i.e. 0 gap).

Aesthetic line is usually made at mating surface of two plastic products. Width of aesthetic line is usually 0.2-1.0, depending on the overall size of product.

Common methods include spraying, silk screen printing, hot stamping, printing, electroplating, engraving, etching, polishing, adding color, etc.

Stainless steel, copper alloy (brass, bronze, phosphor copper, red copper), spring steel, spring, aluminum alloy, zinc alloy.

a. ABS:
High fluidity, cheap, suitable for parts with low strength requirements (parts that are not directly impacted and do not withstand structural durability in reliability tests), such as internal support frames (keyboard brackets, LCD brackets), etc. It is also commonly used in electroplated parts (such as buttons, side keys, navigation keys, electroplated decorative parts, etc.). Currently, Chimei PA-757, PA-777D, etc. are commonly used.
b. PC+ABS:
Good fluidity, good strength, and moderate price. Suitable for high-rigidity, high-impact toughness parts, such as frames, shells, etc. Commonly used material codes: Bayer T85, T65.
c. PC:
High strength, expensive, and poor fluidity. Suitable for shells, buttons, transmission racks, lenses, etc. that require high strength. Common material codes include: Teijin L1250Y, PC2405, PC2605.
d. POM has high stiffness and hardness, excellent fatigue resistance and wear resistance, small creep and water absorption, good dimensional stability and chemical stability, good insulation, etc. It is commonly used in pulleys, transmission gears, worm gears, worms, transmission components, etc. Common material codes include: M90-44.
e. PA is tough and absorbs water, but becomes fragile when water evaporates completely. It is commonly used in gears, pulleys, etc. Critical gears that are subject to greater impact force need to be filled.
f. PMMA has excellent light transmittance. After 240 hours of accelerated aging by light, it can still transmit 92% of sunlight, 89% after ten years outdoors, and 78.5% of ultraviolet rays. It has high mechanical strength, certain cold resistance, corrosion resistance, good insulation performance, stable size, easy to form, and brittle quality. It is often used for transparent structural parts with certain strength requirements, such as lenses, remote control windows, light guides, etc.

a. Wall thickness should be uniform, and thickness difference should be controlled within 25% of basic wall thickness. Minimum wall thickness of the entire component shall not be less than 0.4mm, back of this part is not a Class A appearance surface, and area shall not be greater than 100mm².
b. Thickness of shell in thickness direction should be as close as possible to 1.2~1.4mm, and side thickness should be 1.5~1.7mm; thickness of outer lens support surface is 0.8mm, and minimum thickness of inner lens support surface is 0.6mm.
c. Battery cover wall thickness is 0.8~1.0mm.
d. Minimum wall thickness and common wall thickness recommended values of plastic products are shown in table below.

Molding process and use requirements of plastics have important restrictions on wall thickness of plastic parts. If wall thickness of plastic part is too large, it will not only increase cost due to excessive material consumption, but also bring certain difficulties to process, such as extending molding time (hardening time or cooling time).
It is not conducive to improving production efficiency, and it is easy to produce bubbles, shrinkage holes, and depressions; if wall thickness of plastic part is too small, flow resistance of molten plastic in mold cavity is large, especially for complex shapes or large plastic parts, which are difficult to mold. At the same time, because wall thickness is too thin, strength of plastic part is also poor.
While ensuring wall thickness of plastic part, wall thickness must be uniform, otherwise it will cause uneven shrinkage during molding and cooling process, which will not only cause bubbles, depressions and warping, but also cause large internal stress inside plastic part. When designing plastic parts, it is required to avoid sharp angles at junction of thick and thin walls, transition should be gentle, and thickness should gradually decrease along direction of plastic flow.

 

There is no specific criterion for size of demolding angle, most of them are determined by experience and according to depth of product. In addition, molding method, wall thickness and plastic selection are also considered.
Generally speaking, any side wall of a molded product needs to have a certain amount of demoulding slope so that product can be removed from mold. Size of demoulding slope can vary from 0.2° to several degrees, depending on surrounding conditions, and is generally ideal between 0.5° and 1°.
When selecting specific demoulding slope, following points should be noted:
a. Direction of slope is generally based on small end of inner hole, in accordance with drawing, and slope is obtained from expansion direction. Outer shape is based on large end, in accordance with drawing, and slope is obtained from reduction direction.
As shown in Figure 1-1 below.

 

b. For plastic parts with high precision requirements, a smaller demoulding slope should be used.
c. For higher and larger sizes, a smaller demoulding slope should be used.
d. For plastic parts with large shrinkage, a larger slope value should be used.
e. When wall thickness of plastic part is thicker, molding shrinkage will increase, and demoulding slope should use a larger value.
f. Generally, demoulding angle is not included in tolerance range of plastic parts.
g. Demoulding angle of transparent parts should be increased to avoid scratches. Generally, demoulding angle of PS materials should be greater than 3°, demoulding angle of ABS and PC materials should be greater than 2°.
h. Side walls of plastic parts with leather grain, sandblasting and other appearance treatments should have a demoulding angle of 3°~5°, depending on specific depth of texture. Required demoulding angle for reference is clearly listed on general texture plate. The deeper texture depth, the larger demoulding angle should be. Recommended value is 1°+H/0.0254° (H is texture depth). For example, demoulding angle of texture of 121 is generally 3°, and demoulding angle of texture of 122 is generally 5°.
i. Slope of insertion surface is generally 1°~3°.
j. Demoulding angle of outer shell surface is greater than or equal to 3°.
k. Except for outer shell surface, demoulding angle of other features of shell is 1° as standard demoulding angle. In particular, it can also be taken according to following principles: demoulding angle of ribs below 3mm high is 0.5°, 3~5mm is 1°, and the rest is 1.5°; demoulding angle of cavity below 3mm high is 0.5°, 3~5mm is 1°, and the rest is 1.5°

In order to ensure strength and rigidity of plastic product without thickening wall of plastic part, reinforcement ribs are set at appropriate parts of plastic part, which can not only avoid deformation of plastic part, but also improve plastic flow during molding of plastic part in some cases.
In order to increase strength and rigidity of plastic part, it is better to increase number of reinforcement ribs rather than increase its wall thickness.

 

 

a. When designing columns, whether glue position will shrink should be considered.
b. In order to increase strength of column, additional reinforcement ribs can be added around column. Width of reinforcement ribs is shown in Figure 3-1.
Improvement methods for shrinkage of column are shown in Figures 4-1 and 4-2: before improvement, glue of column is too thick and easy to shrink; after improvement, it will not shrink.

 

a. Distance between holes should generally be more than 2 times hole diameter.
b. Distance between hole and edge of plastic part should generally be more than 3 times hole diameter. If it is limited by design of plastic part or used as a fixing hole, edge of hole can be reinforced with a boss.
c. Design of side hole should avoid thin-walled sections, otherwise it will produce sharp corners, hurt hands and easy to lack materials.