Design of plastic products is similar to that of other materials such as steel, copper, aluminum, and wood; however, due to diversity of plastic material composition, structure, and shape, it has more ideal design characteristics than other materials; especially its shape design, material selection, and manufacturing method selection are incomparable to most other materials. Because most other materials, their designers are subject to considerable restrictions in appearance or manufacturing, some materials can only be formed by bending, welding, etc. Of course, diversity of plastic material selection also makes design work more difficult. As we know, there are more than 10,000 different plastics that have been used. Although only hundreds of them are widely used, formation of plastic materials is not composed of a single material, but a combination of a group of material families, each of which has its own characteristics, which makes selection and application of materials more difficult.

 

1. Determine shape, size, appearance, and material according to required function of finished product
2. Designed finished product must comply with molding principle, that is, mold is easy to make, forming and post-processing are easy, but still maintain function of finished product.

 

In order to ensure that designed product is reasonable and economical, close cooperation between appearance designer, mechanical engineer, draftsman, mold maker, molding plant and material supplier is necessary in the early stage of product design, because no designer can have such a wide range of knowledge and experience at the same time, and suggestions obtained from different business perspectives will be basic premise for rationalizing product; in addition, a reasonable design consideration process is also necessary; following will explain general design process:

In initial stage of product design, designer must list target use conditions and functional requirements for product; then, based on practical considerations, determine scope of design factors to avoid possible time and cost losses in later product development stage. Following table is a checklist for product design, which will help to confirm various design factors.

 

Product Design Checklist
General Data:
1. What is function of product?
2. How does product operate?
3. Can product combination be simplified by using plastics?
4. Is it possible to be more economical and efficient in manufacturing and assembly?
5. What tolerances are required?
6. What are space limitations?
7. What is product life?
8. What are product weight considerations?
9. Are there any approved specifications?
10. Are there similar applications already in existence?
Structural considerations:
1. What is state of load?
2. What is size of load?
3. What is life of load?
4. What is allowable amount of deformation?
Environment:
1. What is temperature environment in which it is used?
2. What is use or contact with chemicals or solvents?
3. What is temperature environment?
4. What is life of product in this environment?
Appearance:
1. Appearance
2. Color
3. Surface processing such as biting, painting, etc.
Economic factors:
1. Estimated product price?
2. Price of the currently designed product?
3. Possibility of reducing costs?

After functional requirements and appearance of product are determined, designer can start drawing preliminary product drawings based on properties of selected plastic material as a preliminary estimate, review and principle model production.

Prototype models give designers opportunity to see entity of designed product and actually check its engineering design. There are generally two ways to make prototype models. The first is to use plate or rod materials to process according to drawing and then connect them into a complete model. Model made in this way is economical and fast, but disadvantage is that quantity is small and it is more difficult to do structural testing; the other way is to use temporary molds for small-scale production, which requires higher mold costs and takes longer time, but products made are more similar to real mass-produced products (parts that require special mold mechanisms may be formed and then machined), general engineering tests can be done. Established molds and forming experience will help product to make correct corrections or evaluations for actual mold making and forming needs.

Every design must undergo some tests at prototype stage to verify calculations and differences between assumptions and entities during design.
Most of tests that need to be done when product is used can be effectively tested with prototype; at this time, all functional requirements of design are met and a complete design evaluation can be achieved.
Simulation use testing usually must be started at model product stage. Value of this type of test depends on degree to which use state is simulated.
Accelerated speed measurement of mechanical and chemical properties is usually regarded as an important item in evaluation of model products.

Reviewing design will help answer some fundamental questions: Does designed product achieve expected effect? Is price reasonable? Even at this time, many products must be discovered and improved for economic efficiency of production or for important changes in function and appearance. Of course, major changes in design may require a complete re-evaluation; if all designs are carefully reviewed, details and specifications of product can be established at this stage.

Purpose of specifications is to eliminate any deviations during production so that product meets requirements of appearance, function and economy. Specifications must clearly state requirements that product must meet, which should include: manufacturing method, dimensional tolerance, surface processing, parting surface location, burrs, deformation, color and test specifications, etc.

After specifications are carefully and practically determined, mold can be designed and made. Design of mold must be cautious and consult experts, because inappropriate mold design and manufacturing will increase production costs, reduce efficiency, and may cause quality problems.

It is a good inspection practice to set regular inspections for production products against a known standard, and inspection table should list all items that should be checked. In addition, relevant personnel, such as quality control personnel or designers, should also work with molding factory to establish a quality management procedure to facilitate production of products that meet requirements of specifications.

1. It should not be located in a position that obviously affects appearance
2. It should not form a dead angle (undercut) when opening mold
3. It should be located in a position where mold is easy to process
4. It should be located in a position where it is easy to process after finished product
5. It should be located in a position that does not affect dimensional accuracy (parts with important dimensional relationships should be placed on same side of mold as much as possible)

Demolding slope is set to facilitate product to be ejected from mold.
Demolding slope is 1~2 degrees, and minimum is not less than 0.5 degrees. Specific value will vary depending on shape of finished product, type of forming material, mold structure, surface accuracy, and processing method. The larger demolding slope, the better without affecting product quality.

Product shape should be streamlined as much as possible to avoid sudden changes, so as to avoid defects such as bubbles caused by poor flow of plastic here during molding; and mold is prone to wear here.
Main factors that determine thickness of wall:
1. Whether structural strength is sufficient
2. Whether it can withstand demoulding force
3. Whether it can evenly disperse impact force
4. Whether it can prevent rupture when there are embedded parts, such as whether fusion line will affect strength
5. Whether fusion line at forming hole will affect strength
6. Make wall thickness as uniform as possible to prevent shrinkage
7. Whether edges and thinner parts will hinder flow of material, thereby causing insufficient filling
Impact of uneven wall thickness on formability:
1. Cooling time of molded product depends on thicker part, which prolongs forming cycle and reduces production performance
2. Uneven wall thickness will cause uneven shrinkage of finished product after cooling, causing shrinkage, internal stress, deformation, rupture, etc.
Materials we often use are: PC, ABS, PMMA, etc., and their standard wall thickness is as follows:
PC: 1.5-5.0 ABS: 1.2-3.5 PMMA: 1.5-5.0

Methods:
1. Adding R to corners
Sharp corners of plastic products are often the biggest factor causing product damage. Eliminating sharp corners of products can not only reduce stress concentration at that location and improve structural strength of product, but also make plastic material have a streamlined flow path when forming, and make finished product easier to eject. In addition, from perspective of mold, rounded corners are also beneficial to mold processing and mold strength.
All inner and outer peripheral corner arcs of product must be as large as possible to eliminate stress concentration; however, too large an arc may cause shrinkage, especially at corner arc at the root of rib or protrusion. In principle, minimum corner arc is 0.020 to 0.030 inches.
In summary, rounded corners have following advantages for design of molded products:
(1) Rounded corners increase strength of molded products and reduce stress.
(2) Elimination of sharp corners automatically reduces possibility of cracking, that is, it improves resistance to sudden vibration or impact.
(3) Flow state of plastic will be greatly improved. Rounded corners allow plastic to flow into all sections of mold cavity evenly without retention and less stress, and improve uniformity of density of molded product section.
(4) Mold strength is improved to avoid sharp corners in mold, which cause stress concentration and cracking. Rounded corners are more important for parts that require heat treatment or are subject to higher stress.
The larger rounded corners, the less stress concentration.
Inner fillet R<0.3T ---- Stress increases
Inner fillet R>0.8T ----Almost no stress concentration

 

2. Add reinforcement ribs
Thickness of rib root is about (0.5~0.7)T

 

Inter-rib spacing>4T
Rib height L<3T
3. Use variable thickness and shape
1) Side wall reinforcement
It can prevent deformation and improve fluidity
2) Edge reinforcement
Use variable edge shape to strengthen and prevent deformation
3) Peripheral reinforcement
Larger planes are prone to warping and deformation, use peripheral concave and convex or wavy shapes to prevent deformation
4) Bottom reinforcement
Bottom of box-shaped part is often strengthened and prevented from deformation by following methods:

1. Length of BOSS is generally not more than twice its own diameter, otherwise reinforcement ribs must be added. (If length is too long, it will cause air holes, burning, insufficient filling, etc.)
2. Position of BOSS cannot be too close to corner or side wall

 

3. Shape of BOSS is mainly round, and 3 are used when designed at the bottom (other shapes are difficult to process and have poor fluidity)
4. Thickness around BOSS can be removed to prevent shrinkage and sinking

 

Shape and position of holes must be chosen to avoid causing product fragility and production complexity.
In the general method of forming holes, plastic is ejected from mold cavity and then flows along periphery of core tip to form a hole. Therefore, when plastic meets at one end of core tip, a joint line will be formed. Position of these joint lines becomes potential fragility of finished product itself.
1. Distance between holes is more than 2 times hole diameter
2. Distance between hole and edge of finished product is more than 3 times hole diameter
3. Distance between hole and side wall is more than 3/4 times hole diameter

 

4. Thickness of hole periphery should be strengthened (especially for holes with assembly and stress), and periphery of cut hole should also be strengthened

 

5. For blind holes perpendicular to material flow direction, when hole diameter is less than 1.5mm, hole depth shall not exceed 2 times hole diameter (mold tip supported at only one end will have a 48 times higher deformation than mold tip supported at both ends)

 

6. Through holes with constant diameter should not be designed to be formed by aligning two sides, which will cause eccentricity. Diameter of either side can be increased, or it can be designed as a hole that does not need to be aligned to form

 

Comparison of hole shape design:

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Precautions for forming thread design:
1. Avoid using threads below 32 teeth/inch (pitch 0.75mm), and maximum pitch can be 5mm
2. Long threads will cause pitch distortion due to shrinkage, so they should be avoided. If structure requires it, self-tapping screws can be used for locking
3. Avoid using when thread tolerance is less than shrinkage of forming material
4. Thread must not be extended to the end of finished product, because sharp part produced in this way will cause end face of mold and thread to crack and reduce its life, so at least 0.8mm of flat part should be left

 

5. Thread needs to have a draft angle of 2~4 degrees
Roller patterns are usually grooves parallel to demolding direction, and roller pattern spacing is usually 3.0mm, and minimum is 1.5mm. To prevent mold from cracking and to facilitate post-processing, leave a flat portion of at least 0.8 mm between roller pattern and parting surface.