1. Plastic parts

When designing plastic parts, try to succeed once. For some places that are difficult to guarantee, given that it is difficult to add materials to mold and easy to remove materials when repairing mold, you can reserve a certain clearance for plastic parts in advance.

Introduction to plastics

The commonly used plastics are mainly ABS, AS, PC, PMMA, PS, HIPS, PP, POM, etc. The most commonly used transparent plastics are PC, PMMA, PS and AS. The case of high-tech electronic products is usually made of ABS+PC; The display screen is made of PC, and if PMMA is used, surface treatment is required to harden it. Most midsole electronic products used in daily life use HIPS and ABS as shells. Due to its better anti-aging properties, HIPS tends to gradually replace ABS.

Introduction to common surface treatment methods

Surface treatment includes electroplating, sputtering, screen printing, pad printing. ABS, HIPS and PC materials have good surface treatment effect. However, surface treatment efficiency of polypropylene material is low, and a pre-treatment process is usually required. In recent years, in-mould transfer technology (IMD), injection molding surface decoration technology (IML), and magic mirror technology (HALF MIRROR) have been developed.

Difference and advantages of IMD and IML:

1. Most IMD diaphragm substrates are made of high peel PET, while most IML diaphragms are made of polycarbonate.

2. With IMD injection, only ink on diaphragm is bonded to resin, while with IML, entire diaphragm is on resin.

3. The IMD is automatically transported and positioned by film feeder, while IML is hung manually.

1.1 Shape Design

For plastic parts, if mold design is wrong, mold may be defective, so be careful. The shape design requires product to have a beautiful and smooth appearance, and surface transition is smooth and natural, which is in line with ergonomics. For most electronic products used in real life, case is mainly composed of upper and lower cases. Theoretically, shapes of upper and lower cases can overlap, but in reality, due to factors such as mold manufacturing precision and injection molding parameters , upper and lower shapes. Size mismatch, i.e. surface scraping (surface shell larger than bottom shell) or bottom scraping (bottom shell larger than surface shell). Allowable surface scraping <0.15 mm, allowable bottom scraping <0.1 mm. So when it's not possible to guarantee a zero level difference, try doing product: surface shell > bottom shell.

Generally speaking, since upper shell has more buttonholes, mold shrinkage is greater, so choose a higher shrinkage ratio, typically 0.5%.

The shrinkage of lower shell is small, so shrinkage factor should be small, typically 0.4%. That is, shrinkage rate of surface shell is typically 0.1% greater than that of lower shell.

1.2 Build Design

Refers to an assembly dimension design between parts with an assembly relationship. Focus mainly on clearance fit and tolerance control.

1.2.1 Stop

Refers to fit between top and bottom of case. The nominal size of structure should leave a gap of 0.05~0.1mm, and seating surface should have a slope of 1.5~2°. The ends are beveled or rounded for easier loading. The upper shell is aligned with fillet seam of lower shell. The angle R of mating inner corner should be increased to increase gap between rounded corners and prevent interference in rounded corners.

1.2.2 Buckle

It mainly refers to clasp between top and bottom of case. When considering number and location of buckles, overall dimensions of product should be taken into account, requiring an average number and a balanced position. The buckles at corners should be as close to corners as possible to ensure a better fit at corners and prevent corners from designing. It is easy to appear due to a seam problem.

The design of buckle must take into account the reserved clearance.

When designing position of buckle, consider whether there is enough clearance for pumping from filter side.

1.2.3 Screws

As a rule, self-tapping screws with a diameter of 2–3 mm are used.

The table above lists common screw hole sizes for HIPS and ABS materials. For different materials, size of screw holes varies. Generally speaking, softer and stiffer materials have smaller d values ​​and are brittle. The chosen d value of material must be larger.

1.3 Structural Design

When designing main thickness, it should not be too thin, otherwise external strength will not be enough, which will easily lead to problems such as deformation and breakage. If it is too thick, material will be wasted and injection molding will be affected. As a rule, wall thickness of housing is 1-2 mm. The overall thickness of casing should be medium and excessive, and there should not be designs with large differences in thickness, otherwise it will easily lead to shrinkage of appearance, especially at bottom of ribs and screw posts. To prevent shrinkage, thickness of ribs is controlled at 0.6–1.2 mm.

1.3.1 Shell

Keyhole design. There are three options for entering keyhole.

Method A is convenient for making molds, but sharp edge at puncture site can cause key to get stuck; method B avoids stuck key problem, but on impact

With eccentric threading, keyhole becomes smaller, resulting in sharp edges. Method C increases angle of button and at same time maintains a margin of eccentricity

This amount can effectively prevent problem from occurring, and B or C is now commonly used.

1.3.2 Button Design

Gap. Pay attention to gap between button and keyhole on cover when designing button. Generally speaking, if button is made of silicone, gap between button and keyhole of cap is 0.2~0.3mm. If button uses a cantilever beam, you need to take into account deflection clearance when pressed. If surface of button needs to be machined, effect of different surface treatments on clearance should be considered. The thickness of water (plating) coating is usually 0.1mm, and thickness of sputtering and vacuum coating is usually 0.05mm.

Key Top Arch: If key surface is to be screen printed, key surface arc should not be too large and arc height should be smaller. than 0.5 mm.

Rounded corners. Round off edges around top of button so you don't pinch button.

Different cantilever beam designs affect button effect differently

The button shown in picture above flexes a lot when pressed, and a large gap must be left between button and keyhole in panel

The button shown in picture above has a slight flex when pressed, button moves mostly vertically, and there is a slight gap between button and keyhole on panel.

On other hand, length and thickness of cantilever beam also directly affect action of button. If it is a connected button, it is necessary to avoid button clutching (that is, when one button is pressed, other buttons will also move. It happens that other buttons are triggered, resulting in incorrect operation)

Key feel: Light touch buttons should generally have a force of 100-200g, which is flexible and pleasant to touch.

Key lifetime. The key lifetime should normally be 100,000 times.

Deformation control: For cantilever beam keys, deformation of key must be controlled during production, transport and storage, since a small deformation can lead to a significant decrease in effectiveness of use of key. .