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Design of plastic parts for medical device shell

Release time:2021-10-29

Selection of materials

There are many kinds of plastic materials, which have different requirements for different types of products. For the product shell of medical devices, the design should be discussed with the corresponding supervisors to meet the fire prevention or biocompatibility requirements proposed by the regulations.

For monitoring portable products and portable B-ultrasound: it is recommended to use PC materials with fire rating above V2 or ABS PC (c2950, c6600) materials.

For desktop or trolley products: ABS materials with fire rating above V2 are recommended. (anc120 desktop, pa-765a Chimei).

Selection of wall thickness

The wall thickness of plastic parts has a great impact on the quality of parts. If the wall thickness is too small, the forming flow resistance is large, and it is difficult for large and complex parts to fill the cavity, the plastic wall thickness of smaller size should meet the following requirements:

1. Sufficient strength and rigidity;

2. It can withstand the impact and vibration of the demoulding mechanism during demoulding;

3. It can bear enough tightening force during assembly.

Plastic parts are required to have a small wall thickness value, which varies according to the plastic variety and brand and part size. For machine shell parts of medical equipment, the following wall thickness is recommended:

1. ABS, PC ABS, PC, transparent PC, transparent ABS, wall thickness: 2.0-3.5mm

2. Some small appearance parts (such as key cap, lampshade, knob) can be 1.2-2.0 mm

The wall thickness of the same plastic part should be as consistent as possible, otherwise the wall thickness will shrink due to uneven wall thickness.

Draft angle design

On the inner and outer surfaces of plastic parts, design enough draft angle along the demoulding direction, otherwise it is difficult to demould or damage the plastic parts during ejection. In addition, the draft angle is small, and the etched lines will be relatively shallow, which is easy to cause dirty appearance.

For the company's products, it is recommended to adopt the following draft angle: (the design angle is 0.5 degrees higher than the angle required by the grain).

1. For the rear shell of the monitoring instrument, if the travel position is required, it is recommended that the front mold on the side of the travel position is 0-0.5 degrees and the rear mold is 0.5-1 degrees.

The recommended draft angle of the front shell is: according to the thickness of the grain, the front and rear molds are 2-3 degrees respectively.

2. For large ultrasonic products, such as host shell, keyboard shell, front and rear shell of display, and surface shell of blood cell products, it is recommended that the front and rear molds be 3 degrees. For small ultrasonic products, such as knobs, it is recommended that the front mold be drawn 1-2 degrees.

3. When the profile of the side is composed of surfaces, it is necessary to ensure that the angle between the tangent direction of the surface at the parting and the draft direction is not less than the draft angle, otherwise there will be strains near the P.L. surface of the product.

Design of column position

Role of column position:

Usually, the column position of plastic parts is used to support PCBA, fix PCBA or plastic parts themselves, fix electronic components, or connect the front and rear shells of products. One of its biggest advantages is that the height is easy to adjust. Therefore, generally speaking, we should try to use the plane formed by the end face of the column as the supporting surface. The plane formed by several ribs is used as the support surface.

Draft and height of column:

When the height of a column exceeds 10 mm, it is usually pushed out with a cylinder, so the draft angle can be very small, or 0 degrees. The inner hole can be taken as 0 degrees, and the outer surface can be taken as 0.25 degrees. When the height of the column is less than 10mm, inserts can be used on the mold, and the draft angle of the inner hole is 0.5 degrees, and the outer surface is 1 degree. If you want to adjust the height, please mark it on the drawing. When you need mold processing, please consider adjusting the height. Under normal circumstances, the height of M3 self tapping screw column should not be greater than 30 mm. If it is too high, the cylinder needle of the column is easy to bend and deform due to the flow of glue.

Size and size of column: since the column position with a height greater than 10mm is usually pushed out with a cylinder, the outer diameter and inner diameter of the column are limited.

Reinforcement position design

The function of reinforcing rib is to increase the strength of plastic parts and avoid deformation of parts. It is often unreasonable to increase the strength of plastic parts only by increasing the wall thickness. First, it is easy to shrink; Second, the injection molding cost is increased. The stiffener should not be designed too thick, otherwise the root is easy to shrink and should not be too thin. If it is too thin, it is easy to get uneven glue. The recommended thickness is: when the mold is processed into a large nozzle, the thickness of the stiffener root of the part is less than 1/2 of the wall thickness (such as the rear shell of the monitor and the rear shell of the display). The top thickness of the stiffener shall not be less than 1mm. When the mold is processed into a small nozzle, the thickness of the stiffener root of the part should be less than 2/5 of the wall thickness, and the thickness of the stiffener top should not be less than 0.8mm.

About the draft angle of stiffeners: for ordinary stiffeners, the draft angle on both sides can be taken according to the above thickness requirements. For stiffeners with special height requirements, it is best to have 0.1-0.25 degrees on both sides, and then use the stiffeners as inserts. If possible, adding column positions to the stiffeners can help demould.

Stiffener on column position: if the structure allows, the stiffener must be designed for the rubber column. The stiffener on the column position can be much higher than the ordinary stiffener, because it is pushed out together with the column position, and its height direction is 1-3 mm lower than the end face of the column position. At the same time, the stiffener of the column position should be processed symmetrically as far as possible to ensure that the deformation of the column position is as small as possible.

Fillet design

Except for sharp corners, all corners of plastic 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 in the process of demoulding, cracks will appear. Generally speaking, we recommend the following principles for machining fillets:

Both ends of the top of the reinforcement, r>1mm.

The square hole in the middle of the plastic part is surrounded by rounded corners, which are greater than 1/4 of the wall thickness of the part.

Make rounded corners on the four corners of the square parts to match the holes above the plastic parts.

The root of the specially stressed column is rounded, about r0.5.

Design of common transparent parts

Transparent materials: there are many kinds of transparent materials in common use, among which transparent ABS, PC and PMMA are commonly used in our design.

The transparent parts commonly used by the company (except biochemical and blood analysis) include lampshade, key cap and decorative sheet (pm7000).

Lens (PM600), etc. due to the poor fluidity of transparent materials, it is easy to produce appearance problems in the injection molding process.

Transparent key caps and other products are easy to produce water marks on the top. Decorative films, lenses and other products are prone to nail lines and serpentine lines near the nozzle.

For the above problems, please pay attention to:

Transparent key cap and other parts: the thickness of the top should be designed to be thicker than the surrounding. If the cavity is too deep, glue should be reduced locally near the nozzle to make the glue flow to the top of the part first, and then to the periphery.

Decorative parts, lenses and other parts: during the design, please reserve a wide (more than 6-8 mm) and thick nozzle position (more than 1-1.5 mm). When processing the mold, try to use a wide fan-shaped nozzle, and the flow channel is short and thick.

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