Although the process of designing plastic components is a most efficient way to conceive and design plastic parts, there are several phases (some of which overlap with each other) that are needed to conceive, design and develop a plastic component.
For illustration purposes, let’s take a look at the basic steps of the component design.
- Step 1: Define what the end-user requirements are.
- Step 2: Make a concept sketch
- Step 3: Material selection
- Step 4: Design part based on materials
- Step 5: The final selection of materials
- Step 6: Make it better for manufacturing
- Step 7: Make a prototype
- Step 8: Tooling
- Step 9: Production
Many design activities are carried out simultaneously in various areas of the design, but each step of the design process is independently outlined for discussion.
- Step 1 –Defining End-Use Requirements ->>
- Step 2 –Preliminary Concept Sketch
- Step 4 – Design Part In Accordance with Materials Selected
- Step 5 – Final Material Selection
- Step 6 – Modify Part Design for Manufacturing
- Step 7 – Prototyping
- Step 8 – Tooling and Step 9 – Manufacturing
The entire process of product development starts with a comprehensive and detailed description of the products specifications and end use criteria.
It is evident from the fact that this is the first stage in the construction process, because designers and engineers will create a product based on these criteria.
If a product does not meet the specifications, it cannot be used.
The designs of the product should be based upon its end-use requirements, rather than its quality.
It’s better to define terms such as “strong” or “clear” than simply claiming that the product is solid. This provides more room for misinterpretation because it is not as straightforward to guess what the product should withstand or how it should look.
Nevertheless, when considering the possible misuse of a product, it is difficult to quantify it with all its end-use criteria. When replacing existing products (e.g. metal conversion), there is product background, but not when entirely new products are produced.
A lot of these specifications are hard to predict.
Typically, prototypes (or models) are created during this stage of the design process mainly in order to help us understand the end-use specifications more thoroughly.
As a general rule, structural loading, environmental conditions, size specifications, standard requirements, and industry specific problems are often considered.
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 manufacturers must consider both the normal and worst loading scenarios.The toughest decision here is which side gets hit the hardest if the load fails.
Poorly designed products are more likely to cause major failures, while products whose design does not consider misuse will be more likely to fail. Product designers must be very careful to ensure their designs are reliable, especially if product failure is likely to cause serious injury.
Anticipated Environmental Conditions: Because the plastic material’s properties are very sensitive to the environmental conditions, it is extremely important to specify the environmental conditions in which the product will be used. These conditions include the temperature, relative humidity, the chemical environment, and the exposure to radiation.
A careful examination of the environmental conditions required for assembly and storage (paint cure ovens, cleaning solvents, adhesives, etc.) is also necessary. High equipment operating temperatures indicate a potential for creep and oxidative degradation, while low temperatures suggest a potential for impact problems.
The problem again is to anticipate misuse, specify worst case scenarios, and specify specifics. The list of chemicals must be clear, along with the chances of UV exposure if used outdoors.
Dimensions: In many cases, plastics part dimensions are important Practical tolerances and dimensions must be defined along with surface finishes and such. It is important to remember that the tightness of dimensional tolerance criteria affects tooling and development costs.
Plastics are used in applications that are covered by a number of regulatory agencies. It’s important to know which regulatory agencies have jurisdiction over the product.
Once you have this part down, it will be a simple matter to get the standards from these organizations and comply with them. You can get everything from material sanctions (food grades, flammability, etc.) to dimensional standards, to performance standards (EMI shielding, etc.).
The standard agency often requires pre-production or prototype products for evaluation.
Also specified during the first stage of development are marketing and industrial design parameters such as the expected production volume, the replacement interval, and the maximum product cost.
The product development team needs to come up with the best design/product at an affordable price.
(i. e., the most efficient design).Besides color, size, and shape, other market related restrictions must also be well quantified. Models (nonfunctional prototypes) are an excellent form of communication as it is difficult to quantify esthetic variables.
Marketing constraints: There’s also a bunch of things that have to be decided on the basic level, like what kind of stuff you’re going to make, how long it will last, and what the total cost of it is gonna be.
To create the ideal design, the product development team must keep in mind the lowest possible costs (i.e. projects that are most efficient). This means that market related constraints such as colour or size must be transparent to the consumer as much as possible.
Models are incredibly helpful since aesthetic variables are difficult to measure if you decide to use them (non-functional).
The product development team and industrial designers will work together to develop early concept sketches of the product once the end-use requirements have been specified.
These sketches are usually 3D renderings as opposed to CAD drawings.
Areas of the part that require special attention are highlighted and detailed. As a result, it is best to identify whether a specific dimension or function is fixed or variable.
A fixed function is one in which the designer is not able to express his own creativity regarding the design of the product (e.g., dimensions defined by a standard, etc. ); while a variable function is one that is being developed in the stage that it must be designed in.
Fig. 3.5 shows a typical nozzle of a garden hose.
The task is designing an all-plastic hose nozzle. If 10 designers are given the same set of specs and independently design the hose nozzle, it’s likely to come up with ten completely different designs.
Additionally, certain aspects of every design will be exactly the same because certain dimensions are governed by standards and no room for variation or creativity. For instance, the inside dimensions of the inlet thread will be exactly the same because these dimensions are governed by standards.
However, other features vary from design to design, such as the shape of the product or the way it shuts off the water flow.
The die cast metal nozzle shown in Fig. 3.5 looks a lot like the plastic nozzle. It’s likely that the plastic part designers were greatly influenced by existing metal designs.
On the other hand, the other plastic hose nozzle is a completely different design from the other hose nozzle in Fig.3.5. This product has a totally different image.
The fact is that in a replacement metal part application such as this it is best to stick with the specifications rather than the existing metal part alone.
As soon as a designer sees the functioning of a metal component, it will be extremely difficult to avoid copying the current design.
If the designer does not think outside the box, then innovation and creativity are less likely to result in significant quality improvements and component or cost reductions.
In addition, the possibility of infringing upon patented designs is evidently greater if the design process begins without a detailed analysis of existing competitive products.
After a specific end-use requirement for a part has been defined, designers can begin searching for plastic materials to meet those requirement in the material selection or screening process. These decisions are based on whether a particular plastic material’s physical properties meet the end-use requirements.
More plastic materials are available in the marketplace than ever before, so it is likely that a designer can find a material that is suitable for the application.
During the initial material selection process it is generally preferable to identify several potentially suitable material candidates (e.g., 3-6 specific formulations/grades).
The sheer number of material grades available can sometimes make the screening process extremely challenging. In order to determine the most appropriate material for the application, it is best to consider the material properties that cannot be enhanced by design.
The coefficient of thermal expansion, transparency, chemical resistance, softening temperature, and agency approval are characteristics that can’t be altered.
For example, polycarbonate injection molding is not suitable for gasoline containers because it does not provide adequate resistance to hydrocarbons. High density polyethylene is not suitable for window applications because it is translucent or opaque.
In both cases, altering the design of the part will fail to solve the problem.
By utilizing these kinds of features the process can be fairly easily extended by eliminating entire families of materials that share the same characteristics, thereby elimating the number of potential plastic material candidates.
Coatings, additives and co-injection technologies complicate the materials selection process. Coatings can change chemical resistance, hardness, and abrasion resistance, and make parts look nice.
For coatings, a material that would not otherwise be suitable for the application can be utilized. In addition, adding additives to the material choice process can complicate material selection.
Melt blending or compounding is a technique that can be used to selectively modify the properties of plastic materials.
Unlike the mechanical properties listed above, most of the mechanical properties of a polymer can be enhanced by design, provided the specific application temperature is met.
Generally, designers evaluate a material’s modulus when selecting a candidate for metal replacement applications.
One of the problems with metals in this regard is that they are both rigid and tough, whereas most rigid plastics are relatively brittle (e.g., many glass reinforced grades are both rigid but fragile).
In many instances, engineering polymers with a lower degree of reinforcement or without reinforcement are superior to others.
Although the modulus values may be low (and they may creep more rapidly), the part geometry can be altered (by making the ribs deeper, for example) in order to compensate for this reduced modulus.
At this point in the design process, the application would benefit from knowing about and comparing several candidate materials. There are various properties that make each individual material grade unique, as well as different geometries that pertain to each material.
For example, a designer is considering applications involving static loads and organic solvent exposure in high density polyethylene, polypropylene, and nylon 6/6.
The designer feels that each material has its own individual advantages. A final decision on the use of a particular material will have to be made once each part has been designed, because the material consumption and manufacturing cycle times will differ in each case.
Despite the fact that nylon 6/6 is a more expensive material per unit weight or volume, the results of lessening the material’s wall thickness and decreasing the cycle time may partially offset the higher price per unit of material.
Figure 3.6 shows two part geometries with equivalent stiffness values,namely, they have sections with the same section modulus or moment of inertia value (where section is any dimension), and these values have been corrected for the material differences.
While the example given here has a relatively simple geometry, in practice many other geometrical features associated with the performance or assembly of a device would vary based on the materials specifications.
At this stage in product design development, the designer is not required to commit to a particular primary material at this stage as long as they retain the option of remaining flexible materials in case an unanticipated problem occurs later in the development process (e.g., during prototyping or production).
It’s unlikely any of these candidates’ stuff fits the job perfectly.
The materials that are proposed for consideration have their own advantages and limitations. The designer may have a favorite material based on past experience. While it may be useful to work with a familiar material, the other candidate materials may be better suited for the application.
Nevertheless, decisions formulated solely based on cost of materials and manufacturing do not take into account performance or processing advantages.
It is recommended that candidates be evaluated in terms of their processing costs, end-use performance, and overall characteristics of manufacturing.
By weighing properties or characteristics, designers can select the best candidate materials based on an essentially unbiased grading system.
Figure 3.7 presents the three materials for the chemical and static-load application polypropylene, HDPE, and nylon 6/6.
Despite the fact that the individual numerical ratings associated with a given house have a tendency to be somewhat arbitrary, I believe that, most of the time, these ratings are based on actual numerical data about the house.
All of the important factors will be considered, and then a semi-quantitative method will be used to choose the best material candidates on balance.
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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