Although the process of plastic part design is best done through concurrent engineering procedures, several phases (some with parallel overlaps) are related to the design and development of plastic component. It is useful to consider the following basic stages of the component design for illustration purposes.

  • Step 1: Defining end-use requirements
  • Step 2: Create preliminary concept sketch
  • Step 3: Initial materials selection
  • Step 4: Design part in accordance with material properties
  • Step 5: Final materials selection
  • Step 6: Modify design for manufacturing
  • Step 7: Prototyping
  • Step 8: Tooling
  • Step 9: Production

Many design activities are performed in parallel in each area, but each step of the design process is detailed separately for discussion purposes.

A comprehensive and detailed description of product specifications and end use criteria starts the entire product development procedure.

This is perhaps the first stage of construction, as designers and engineers will create a product that is focused on these criteria.

If the specifications are incomplete or inaccurate, the product can not be used.

Designs are based upon requirements. It is essential, rather than qualitative, to define the end-use requirements of the product.

For misinterpretation, terms such as “strong” or “clear” make too much space to guess. it is better  to say that a 1-meter drop in the concrete at – 20 ° C must be able to withstand for the product test; or clarity must stay above 88% for a 5-year span, than simply to claim that the product should be solid or clear.

Unfortunately, all end-use criteria for a product , particularly when considering the possible misuse, can hardly be expected and quantified. There is a product background to be found in replacement applications ( e.g. metal conversions), but when completely new products are produced. It does not exist.

All the specifications for end-use goods in these emerging applications can be difficult to predict.

In this stage of design, prototypes (or models) are mostly used simply to help us understand the end-use specifications more thoroughly. Factors including structural loading, conditions for the environment, size specifications, standard requirements and industry specific problems are usually taken into account.

Structural and Loading Predictions: Loading type, load speeds, loading time, loading frequency, etc. should be taken into account and defined. The loads during mounting, transport , storage and end use have to be taken into account. The product is usually produced in conjunction with the plastic component design process itself in order to secure the packaging when shipping and storing. Part makers must take into account the normal loading case as well as the worst cases.
Probably the toughest choices here are those that pay the worst-case loading situation for this load vs the effects / liability of a catastrophic failure 

Products designed to meet unrealistic demand standards are more likely to be very costly, whereas those that do not consider misuse are more vulnerable to high failures. The manufacturer must be very cautious about the subject of reliability, especially if product failure can result in personal injury.

Anticipated Environmental Conditions: Because plastic material properties are very sensitive to environmental conditions, it is important to specify the temperatures, relative humidities, chemical environments, and radiation exposure that are associated with the end-use application. The environmental conditions that are associated with assembly and storage must also be considered (paint cure ovens, cleaning solvents, adhesives, etc.).High operating temperatures are indicative of creep and possible oxidative degradation problems,while low temperatures indicate a potential for impact problems. The types of chemicals (even household cleaners, etc.) must be clearly specified, along with the potential for ultraviolet radiation exposure (e.g., outdoor use). The problem once again is to anticipate misuse and specify the worst case scenario.

Dimensional needs: In the majority of applications, plastic part dimensions are important Practical tolerances must be defined as well as essential dimensions and surface finishes and the like. It is important to bear in mind that the tightness of dimensional tolerance criteria significantly affects tooling and development costs.

Regulations/Standard Compliance: Many plastic products are used in applications that are covered by one or more regulatory agencies. The agencies can be industry/trade groups or government organizations. The important step here is to determine which of these standards organizations have jurisdiction over the product being developed. Once this has been established, it is a matter of obtaining the published standards from these organizations and
complying with its requirements. Standards range from material sanctions (e.g., food grades, flammability, etc.) to dimensional standards (e.g., plumbing fittings, fastener dimensions, etc.) to end-use performance standard (e.g., EMI shielding capabilities, etc.). In many cases,pre-production or prototype products are required for evaluation by the standard agency.

Marketing Restrictions: There are also a variety of marketing or industrial design related requirements that must be specified during the initial stage of development. Items such as the anticipated production quantities, service life (replacement interval), and maximum product cost must be specified. Given all of this information, the product development team must develop the best possible design/product for the application at the lowest possible cost
(i. e., the most efficient design). Other market related restrictions that are related to esthetic considerations, such as color, size, or shape must also be clearly specified as quantitatively as possible. Models (non-functional prototypes) can be extremely useful as a medium for communication here because it can be very difficult to quantify esthetic variables.

Marketing constraints: There are also a range of criteria relevant to the marketing or industrial design that have to be set at the initial level. Things such as the estimated quantity, service life and the overall cost of the commodity must be specified. In view of all this knowledge, the product development team must create the ideal design / model for use at the lowest possible costs (i.e. projects that are most efficient). Such market-related constraints pertaining to esthetic factors like colour , size must therefore be as transparent as possible.

Models can be incredibly helpful (non-function), since aesthetic variables can be very difficult to measure.

Once the end-use requirements for the product have been specified, the product development team will work with industrial designers to develop initial concept sketches of the product.

These sketches are typically 3D renderings rather than CAD drawings.

Fig. 3.5

Areas of the part that are of particular concern are highlighted and detailed.At this point, it is best to specify which functions and dimensions of the part are fixed and which are variable.

Fixed functions are those in which there is no flexibility from a design point of view (e.g., dimensions specified by standards, etc.), while variable functions are those which have not been specified in the initial stage of design.

As an example, consider the garden hose nozzles shown in Fig. 3.5.

The design task at hand is to design an all-plastic hose nozzle. If 10 designers are given equivalent product specifications and are asked independently to design the all-plastic hose nozzle, it is likely that 10 different designs will be created.

However, certain aspects of each design will be equivalent. For example, the inside dimensions in the areas of the inlet thread will be equivalent because these dimensions are governed by standards,and there is no room for variation or creativity.

However, other aspects, such as the product’s overall shape or the method used to valve the water flow, may vary from design to design.

One of the plastic nozzles shown in Fig. 3.5 is very similar in appearance to the die cast metal nozzle. It is very likely that the designer of the plastic part was greatly influenced by the existing metal design.

On the other hand, the other plastic hose nozzle in Fig.3.5 performs the same basic function,but does this using different methods. This product has a totally different look.

In fact, in situations such as this metal replacement application, it is best to work only with the product specifications rather than with the existing metal part alone.

Once a designer sees and evaluates the functionality of the metal part, it will be very difficult to avoid the tendency to simply copy the existing design.

If the designer is thinking of the existing design, creativity and innovation, which could lead to significant quality improvements and component/cost reductions are less likely.

In addition, the potential for infringing on patented designs is obviously more likely when existing competitive products are studied prior to the conceptual design process.

plastic material

Once the end-use requirements for a part have been specified, designers can begin searching for plastic materials that are suitable for the application.

The material selection or screening process is accomplished by comparing material properties with a“property profile” derived from the end-use product requirements.

Because there are literally thousands of plastic material grades available commercially, it is very likely that a designer will be able to find at least one material candidate that is suitable for the application.

It is generally best to select several potentially suitable material candidates (perhaps 3 to 6 specific material formulations/grades) during the initial material selection process.

Due to the shear number of materials grades that are available, the material screening process can be extremely difficult.

It is best to begin the material selection process by considering material properties that cannot be enhanced by design.

Properties such as coefficient of thermal expansion, transparency, chemical resistance, softening temperature, and agency approval are properties that cannot be enhanced by design.

For example, high density polyethylene cannot be used for glazing applications because it is translucent or opaque, while polycarbonate is not suitable for gasoline containers due to its limited resistance to hydrocarbons.

Altering the part design will not help in either case.

Using these types of properties, it is relatively easy to eliminate entire families of materials, thereby reducing the number of potential plastic material candidates.

Factors that complicate the materials selection process are coatings, additives, and co-injection technologies. In many cases, coatings are used to alter the chemical resistance, abrasion resistance, ultraviolet resistance, and general appearance of a part.

When coatings are used, it may be possible to use a material that would otherwise not be suitable for the application. Additives can also complicate the material selection process.

It is possible to selectively alter virtually any property of a plastic material by melt blending or compounding.

Unlike the properties listed above,most of themechanical properties of a polymer can in fact be enhanced by design, within the temperature limitations associated with the application.

For example, when designing a metal replacement application, designers generally consider the material modulus to be one of the most important properties when screening material candidates.

One problem here is that metals, such as steel, are both rigid and tough, while many rigid plastic materials are relatively brittle (e.g., many glass reinforced grades are rigid but brittle).

In many cases, superior performance is achieved when more lightly reinforced or unreinforced grades of engineering polymers are selected.

Even though these materials have lower modulus values (and may creep at a greater rate), they are tougher and the part geometry can be altered (by the addition of deeper ribs, etc.) to compensate for the reduction in modulus.

At this point in the design process, it is beneficial to have several candidate materials in mind for the application. Because there are differences in the properties of the individual material grades, there will also be differences in the product geometries associated with each of the different materials.

For example, a designer is considering high density polyethylene,polypropylene, and nylon 6/6 for an application involving static loads and organic solvent exposure.

The designer feels that each of the three materials has it own merits. It is impossible to make a final choice (based on economic considerations) until each part has been designed, because the material consumption and manufacturing cycle time will be different in each case.

The nylon 6/6 is a more costly material per unit weight or volume, but the wall thickness and cycle time reductions may offset the higher raw material cost.

The part geometries shown in Fig. 3.6 have equivalent stiffness values because the section modulus or moment of inertia values have been adjusted to compensate for the different modulus values of the individual materials.

Fig. 3.6

While this is a simple example, in practice, many other geometric features associated with either end-use performance or assembly would vary according to the specific material properties.

At this stage of the design process, the designer must commit to one primary material for the product, while keeping the remaining candidate materials in reserve in the event that an unanticipated problem is detected at a later stage of development (e.g., during prototyping or production).

It is unlikely that any of the candidate materials are perfectly suited for the application.

Each particular material candidate will have its own advantages and limitations. The designer may have a preference for one of the candidate materials based on past experience.

While it is an advantage to work with familiar materials, the other candidate materials may be better suited for the application.

On the other hand, decisions based solely on material and manufacturing costs do not take performance or processing advantages into account.

At this point, it is best to consider the overall characteristics of each candidate material in terms of manufacturing cost, end-use performance properties, and processability (both primary and secondary processing characteristics).

By rating each of these characteristics, designers can make an essentially unbiased selection of the best candidate material on balance. Properties or characteristics that are particularly important can be given a weighted rating.

Consider the three materials, polypropylene, high density polyethylene, and nylon 6/6 that are slated for the chemical/static load application described in the previous section, see Figure 3.7.

Figure 3.7

While the individual numerical ratings associated with a given property can be somewhat arbitrary, they are hopefully based on actual numerical property data.

The technique provides a semi-quantitative method for selecting the best material candidates on balance if all of the important characteristics have been considered.

Once the material and initial design have been established, the design must be altered for manufacturing. Input from tooling and process engineers is extremely valuable.

Fig. 3.8

The part geometry that has been developed must be moldable. Designers must consider the impact that the various phases of the injection molding process can have on the part design.

Each stage of the injection molding process, namely mold filling, packing, holding, cooling, and ejection, has its own special requirements.

Consider the part shown in Fig. 3.8. The part has been designed with ribs to support the service loads.

In practice, the part must be modified with radii to assist in flow (and reduce stress concentrations), draft angles to assist with part ejection, and surface texturing to improve the visual appearance of the sink marks (due to material shrinkage) on the wall opposite the ribs.

These are but a few of the possible design modifications that are required from a manufacturing point of view.

The effect of these modifications on the end-use performance of the part should be evaluated after they are made, because design changes, such as the addition of draft angles to ribs, can have a significant influence on the maximum deflections and stresses that occur due to service loading.

Part design checklists, such as the one shown in Fig. 3.9, can be useful during planning stages or as final checks to ensure that all aspects of manufacturing and assembly have been considered.

The final part design, modified for manufacturing is generally prototyped at this point in an effort to evaluate both the manufacturability and performance capabilities of the part.

Prototyping is required because all process (e.g., molding simulations) and performance design work (e.g., structural analysis) done up to this point in time have been “theoretical” and must be verified.

This is particularly important for molded plastic parts because there are a number of manufacturing related problems that are difficult to predict in advance (weld line appearance and strength, warpage, sink marks, etc.).

In order to obtain realistic results, it is necessary to mold prototype parts using the intended production material. This typically involves building a single (unit) cavity tool for smaller parts or soft (often simplified) tools for larger parts.

The prototyping process can take a great deal of time and money; however,it is best to detect manufacturing or end-use performance problems with a unit cavity or soft tool, rather than a multi-cavity, hard production tool.

Steel safe practices should be followed to minimize the cost of tool rework.

While molded prototypes provide the type of information that is required to verify engineering functions and manufacturing, other more easily fabricated prototypes (rapid prototypes, etc.) can be produced very quickly (within hours or days) and provide invaluable hands-on models for communication and limited functionality long before the prototype tool is built.

Once the prototype tools and parts have been evaluated and modified as necessary, pre-production or production tools are built.

In order to save time, it is common to begin the more basic work on the production tools long before this point in time. Once the tools are built and debugged, the part goes into the initial stage of manufacturing.

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