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Plastic Component Design to Manufacturing Review

Plastics Design and Engineering

Prototyping the Design

In order to move a part from the design stage to commercial reality, it is usually necessary to build prototype parts for testing and modification. The preferred method for making prototypes is to simulate as closely as practical the same process by which the parts will be made in commercial production. Most engineering plastic parts are made in commercial production via the injection molding process, thus, the prototypes should be made using a single cavity prototype mold or a test cavity mounted in the production mold base. The reasons for this are sound, and it is important that they be clearly understood. The discussion that follows will describe the various methods used for making prototypes, together with their advantages and disadvantages.

Machining from Rod or Slab Stock

This method is commonly used where the design is very tentative and a small number of prototypes are required, and where relatively simple part geometry is involved. Machining of complex shapes, particularly where more than one prototype is required, can be very expensive. Machined parts can be used to assist in developing a more firm design, or even for limited testing, but should never be used for final evaluation prior to commercialization.

The reasons are as follows:

•  Properties such as strength, toughness and elongation may be lower than that of the molded part because of machine tool marks on the sample part.
• Strength and stiffness properties may be higher than the molded part due to the higher degree of crystallinity found in rod or slab stock.
• If fiber reinforced resin is required, the important effects of fiber orientation can be totally misleading.
• Surface characteristics such as knockout pin marks, gate marks and the amorphous surface structure found in molded parts will not be represented in the machined part.
• The effect of weld and knit lines in molded parts can-not be studied.
• Dimensional stability may be misleading due to gross differences in internal stresses.
• Voids commonly found in the center of rod and slab stock can reduce part strength. By the same token, the effect of voids sometimes present in heavy sections of a molded part cannot be evaluated.
• There is a limited selection of resins available in rod or slab stock.

Die Casting Tool

If a die casting tool exists, it can usually be modified for injection molding of prototypes. Use of such a tool may eliminate the need for a prototype tool and provide a number of parts for preliminary testing at low cost. However, this method may be of limited value since the tool was designed for die cast metal, not for plastics. Therefore, the walls and ribbing will not be optimized; gates are usually oversized and poorly located for plastics molding; and finally the mold is not equipped for cooling plastic parts. Commercialization should always be preceded by testing of injection molded parts designed around the material of choice.

Prototype Tool

Prototype molds made of easy-to-machine or cheap materials like aluminum, brass, Kristie, etc. can produce parts useful for non-functional prototypes. As the right molding conditions demanded by the material and the part geometry cannot be employed in most cases (mold temperature and pressure especially), such low-cost molds cannot produce parts that could be evaluated under operational conditions.

Preproduction Tool

The best approach for design developments of precision parts is the construction of a steel preproduction tool. This can be a single cavity mold, or a single cavity in a multi-cavity mold base. The cavity will have been machine finished but not hardened, and therefore some alterations can still be made. It will have the same cooling as the production tool so that any problems related to warpage and shrinkage can be studied. With the proper knock out pins, the mold can be cycled as though on a production line so that cycle times can be established. And most important, these parts can be tested for strength, impact, abrasion and other physical properties, as well as in the actual or simulated end-use environment.

Testing the Design

Every design should be thoroughly tested while still in the prototype stage. Early detection of design flaws or faulty assumptions will save time, labor, and material.

• Actual end-use testing is the best of the prototype part. All performance requirements are encountered here, and a completed evaluation of the design can be made.
• Simulated service tests can be carried out. The value of such tests depends on how closely end-use conditions are duplicated. For example, an automobile engine part might be given temperature, vibration and hydrocarbon resistance tests; a luggage fixture might be subjected to abrasion and impact tests; and an electronics component might undergo tests for electrical and thermal insulation.
• Field testing is indispensable. However, long term field or end-use testing to evaluate the important effects of time under load and at temperature is sometimes impractical or uneconomical.
• Accelerated test programs permit long-term performance predictions based upon short term “severe” tests; but discretion is necessary. The relationship between long vs. short term accelerated testing is not always known. Your DuPont representative should always be consulted when accelerated testing is contemplated.

Contributed by Johnathan Howard, PE