Plastic Part Design for Economical Injection Molding – Part III

Editor’s Note:  This is the Third article in the series.   The first article can be found on the Product Design & Development Divisions website located here, and the second article here.

The first two articles in this series dwelt with the history of product design and the creative process that culminated in hypothetical new injection molded product sketches.  The next challenge is to reduce the concept sketches to detailed piece-part drawings or CAD data bases.  However, before starting that time consuming process it is highly desirable to determine whether or not the parts of the new product are moldable and what the molds and molded parts will cost.

The sketches, with their chosen material and approximate overall size, shape, and estimated wall thickness, can be used to secure tentative tooling and molded part cost estimates.  These preliminary estimates will indicate whether or not the product is moldable and is within an acceptable cost range.

If the concept passes this crucial test it is then safe to proceed with the detailed design of the individual parts that combine to produce the new product.  This is a critical part of the design process which defines the information the mold maker will use to build the cavities that will produce the required plastic parts.  The molded parts can only be as good as the cavity.  The cavity cannot be better than the part drawing.  This is why part design is just as important as product design.  However, it is in this area of part design where many perfectly good product designs begin to fail.  The product designer and the part designer may or may not be the same person.  If not, there is a possibility of incomplete transfer of important information and misunderstandings.

As the project progresses from the product design phase to the part design phase, the designer’s emphasis will change.  All through the product design phase, the designer was working in the creative realm of searching for a structure, process, and material combination that would satisfy the product’s functional requirements within an acceptable cost.  This free-thinking part of the project is basically undisciplined and there are few, if any, rules to guide or restrict the designer’s creative thinking.  The only important rule is that the resulting product concept must be commercially acceptable and is hopefully superior to competitive products.

Piece-part design, on the other hand, is highly disciplined.  By trial, error, and experience each of the industry’s different molding processes have evolved their own part design guidelines.  By following these guidelines, it is possible for both experienced and novice designers to produce an acceptable part design.

Designing for the Process

The injection molding process can be defined as a closed mold, melt flow, high temperature, high pressure, capital intense process that is capable of producing large quantities of complex, precision parts at a relatively low cost.  This is a wonderful process which has become the product design community’s favorite plastic molding process.  The process does however have its limitations.

For example, a designer could have designed a 7.600 in. long nylon part with a nominal wall thickness of 0.012 in.  This is twice the thickness required to provide the required electrical insulation.  Regrettably, the injection molding process cannot cause the nylon to flow that far at that thickness.  Other non-melt flow processes such as thermoforming and blow molding could produce even larger parts at that thin wall thickness.

Faced with this problem some molders would attempt to force the nylon to fill the cavity by increasing the injection pressure and speed.  Standard injection molding machines are capable of  developing cavity packing pressures at 20,000 PSI.  Some special machines intended specifically for thin wall molding are capable of developing pressures of 40,000 PSI and higher.

The molecules that make up plastic materials have a comfortable inter-molecular spacing.  In the melt phase these high pressures reduce these inter-molecular spaces.  Packing pressures will be the highest near the gate and the lowest at the periphery of the cavity.  The molded part will exhibit more shrinkage at the periphery of the part than near the gate.  Parts with non-uniform shrinkage will have increased levels of molded in stress.  Internal stress encourages warpage and make it difficult to maintain precision dimensions. Stress can also result in a decline of heat deflection temperature and resistance to aggressive chemicals.  It is good to remember that between two parts equal in every way except one was molded with higher injection pressure than the other the one molded with the higher pressure will always contain a higher level of molded-in stress.

A different molder faced with the same problem might attempt to fill that cavity by increasing the melt temperature.  Generally speaking the higher the melt temperature, the easier or further the material will flow through a cavity.

Overheating a thermoplastic material or holding it in an injection cylinder for a prolonged period of time will result in breaking the long molecular chains that form plastic material.  The longer the molecular chains, the stronger the plastic material will be.  We spend a lot of money for these high class engineering materials.  It is detrimental to overheat them and reduce their impressive physical properties.

Higher melt temperatures also result in longer molding cycles and higher part cost.  This is due to the added time required to cool the hotter plastic material until it has regained strength enough to retain its shape.

The ideal part design is one which has been proportioned so that it can be molded at a modest injection pressure and melt temperature.  This combination of molding conditions is different for each type of plastic material and part design.

A new product development project could call for the creation of a no frills, low cost in hotel room coffee maker.  Eleven of the coffee maker’s components are to be injection molded polypropylene.

Another project might result in a one piece, low cost polypropylene lawn chair.  At this stage of the project the lawn chair and the coffee maker exist only as a design check list and/or a specification sheet and some sketches.  In order to proceed it is now necessary to finalize the design and prepare detailed engineering drawings or CAD databases.

The coffee maker components and the lawn chair are both injection molded polypropylene.  Their design requirements are however, very different.  The coffee maker is an assembly of plastic and metal parts.  These parts have to fit together to provide a desired function.  Some of the parts will require close tolerances for water tight fitments.  The design must allow for efficient assembly with metal fasteners, snap-fits, and welding.  Easy disassembly for repair or recycling may be required.  Coffee makers are not normally subjected to heavy loads, except perhaps by being dropped or by the stresses created during assembly.  It is not likely that the coffee maker will be used out-of-doors.  The assembly will however be subjected to elevated temperatures.  Some of the parts will be in contact with hot water, coffee, cleaning fluids, and maybe sugar, cream and their substitutes.

The lawn chair has very different design requirements.  As a one piece product there is no consideration of ease of assembly or co-operation between mating parts.  Precision dimensions are not required.  The only dimensional considerations are that the chair be of a size to comfortably accommodate a human being.  Another desirable feature is that the chairs be stackable to minimize shipping cost and reduce storage space for both the manufacturer and the end user. 

These chairs can be used indoors and out-of-doors where they will be subjected to ultraviolet light.  They are at risk of coming into contact with all sorts of lawn and garden chemicals and insect repellent sprays that are stress cracking agents for some plastics.

The primary challenge in designing lawn chairs is to create a structure that meets the market’s demand for relatively low cost while providing the necessary strength to safely support a full grown man with a wiggling child in his lap.  If these chairs are used on grass, or a stone or brick patio, the bottom free end of the chair legs tend to be held in there as molded positions.  If the same chair is used on a waxed tile recreation room floor, the free ends of the chair legs can easily spread apart when heavily loaded.  The back legs of a chair are subjected to the highest loads.  If they fail, the person in the chair falls backwards.  This has resulted in severe injuries when the back of that person’s head strikes the floor.

Design engineers have been successful in designing the shapes of these chair legs to maximize the stiffness of a non-engineering material like mineral filled polypropylene.  The Business and Institutional Furniture Manufacturers Association has established recommended strength values and test procedures for chairs.  From a liability perspective, chairs should be designed to meet or exceed these recommendations.  Unfortunately some manufacturers have been unable to resist the temptation to ignore these recommendations and are marketing lower cost, dangerously weak chairs.

The coffee maker also presents opportunities for personal injury.  Burns from contact with hot surfaces or hot water.  Every effort must be made to protect the user from electrical shock throughout the life of the product.

Both products require careful attention to appearance design and human engineering.

Both the coffee maker components and lawn chair are to be made of injection molded polypropylene.  Is this a suitable combination of process and material?  An observant plastic product designer should already know that injection molded polypropylene is the first choice for one piece lawn chairs and coffee makers and hot water kettles.  It is a safe bet that injection molded polypropylene is a good combination for these two new products.

Using the same material and process combination as a competitor is not very creative, but it is very efficient.  Several months or years of success in the market place is a better indicator of the suitability of a material and process than any pre-production laboratory testing. 

The lawn chair and the coffee maker share many common part design characteristics.  However, from a product design point of view they are very different from each other.  The coffee maker is made up of a multiplicity of relatively small, thin walled, precision parts.  the lawn chair is a one piece, relatively large, thick walled part with no precision dimensions.  In spite of these differences in product design, the part design guidelines will be exactly the same for both products.

The next article in this series will introduce the core subject of this series which is how to design parts for the injection molding process.