In the design process of plastic products, there are some basic considerations that cannot be ignored. Raw material manufacturers can be said to be important promoters of plastic product design. This paper introduces the basic skills and principles of plastic product design from the aspects of material selection, processing selection, strength consideration, mold design, etc.
1. Design checklist
The goal of new product development or product improvement is to make products have excellent performance while obtaining low production cost. Design tasks mainly include the selection of raw materials, processing methods, strength calculation and mold design.
Only by considering these factors and following up systematically can we produce high quality and commercial mould. It must be emphasized that the practicability and cost efficiency of plastics are not inevitable, and designers must pay great attention to the correct solutions of raw material selection and processing process.
2. Comparison of raw materials
Compared with other traditional raw materials, special attention should be paid to the design of plastic raw material parts.
The properties of plastics can vary in a wide range. By adding fillers, reinforcing materials and modifiers, their properties will be greatly transformed. Sometimes under the same operating conditions, plastic will show completely different performance from metal. Therefore, the economical and effective functional design of cast metal, if applied to plastics in a hurry, will easily fail.
When the temperature of the material is close to its melting point, the temperature and time will directly affect the deformation behavior of the raw material.
The properties of plastics are not just the properties of pure raw materials. If the raw materials are processed in an unsuitable range, the best design will fail. In the same way, products cannot be processed to solve design weaknesses. Therefore, the quality of plastic parts can be guaranteed only when all factors are considered.
3. Cost saving design
Designers are mostly responsible for the final cost of plastic parts. His decision predetermines the cost of production, mold making and assembly. Later revisions and optimizations are often expensive and infeasible.
To give full play to the advantages of plastic raw materials can save costs in many aspects. Multifunctional integrated design, low-cost assembly technology (such as buckle, welding device, fixed device, double material injection molding technology, etc.), self-lubricating characteristics, free surface treatment procedures, etc. Pay attention to wall thickness, mold, tolerance, raw materials, etc. to further save costs.
4. Basic assembly technology
Some simple assembly technologies recognized by all designers, such as buckle assembly, press assembly and thread assembly, can greatly save production costs by assembling components easily and quickly.
The biggest advantage of buckle assembly is that no additional assembly parts are needed. In the clasp design, the designer must ensure the geometric dimension of the fitting to avoid the loose of the assembly parts caused by the stress relaxation.
Press fitting can make plastic components assemble with high strength at the lowest cost.
Thread assembly is composed of the application of separate type, combined type screw or integral screw insert. In order to avoid the occurrence of unqualified components, it is a key step to ensure the correct shaft sleeve size. Many suggestions can be made by screw manufacturers in this regard.
5. Material selection
Generally speaking, there are no bad materials, only the wrong materials are used in specific fields.
Thermoplastic is the most commonly used in injection molding. It can be divided into amorphous plastic and semi crystalline plastic. Generally speaking, semi crystalline thermoplastic is mainly used for parts with high mechanical strength, while amorphous thermoplastic is often used for shell because it is not easy to bend.
Thermoplastics are divided into unreinforced, glass fiber reinforced, mineral and glass filled. Glass fiber is mainly used to increase strength, firmness and application temperature; mineral and glass fiber have lower reinforcement effect, mainly used to reduce warpage.
6. Reinforcing rib
In general, the rigidity of components can be enhanced by the following methods:
Increase the wall thickness;
Increase the elastic modulus (e.g. increase the content of reinforced fiber);
Stiffeners are considered in the design.
If the material used in the design cannot meet the required rigidity, the material with larger elastic modulus shall be selected. Large moment of inertia can be easily achieved by setting thick and high stiffeners. However, for thermoplastic engineering plastics, this method often produces surface dents, internal cavities and warps. Moreover, if the stiffener is too high, the structure may expand under load. For this reason, the size of the stiffener must be maintained in a reasonable proportion.
In order to ensure that the products with stiffeners can be easily ejected, it is necessary to design a proper taper of demoulding.
It is very important for components with very high surface requirements, such as the dimensions of wheel covers. The correct design of stiffeners can reduce the possibility of surface dents of components and improve the quality of components.
In plastic design, the cross structure is the best because it can cope with many different load arrangement changes. The correct design of the cross structure which can bear the expected stress can ensure the uniform distribution of stress on the whole product.
7. Tolerances
Injection molded products cannot have the same tolerance as machined products. Although most people are aware of this, they often specify inaccessible tolerances or make cost-effective production impossible.
