Components of a Successful Injection-Molding Product

A successful plastic product depends on the optimization of each of the following four principal components: part design, material selection and handling, tool design and construction, and processing/machine capabilities.

These interact with each other and they are equally important. One or more is often neglected by most in the molding industry, which causes delays in production and significant profit loss throughout the entire life cycle of the product.

To employ all four from the beginning of a project is “concurrent” engineering

and their implementation is scientific injection molding (SIM). SIM is a detailed

strategy of optimizing each one of these components and not allowing errors to be

compensated through processing. The synergy of working with all four from the

beginning of a project has shown profit increases of 100% and faster development

times to market and significantly fewer manufacturing problems.

Part Design. A successful plastic product must begin with a good part design.

Because injection molding is a comparatively new and a rapidly expanding

industry, it is often the case that an engineer with little plastics training is forced

into plastic part design. This combination of no plastics experience and a tendency

to apply rules for metals to complex shapes is ill-advised and often leads to project

delays, failures, cost overruns, and production problems. Issues such as draft angles,

weldlines, and polish in the direction of draw are singular to plastics. Plastics

are unique, different than the materials most designers are familiar with and do

not have a uniform set of precise design rules. A common mistake for the uninitiated

designer is to increase nominal wall thickness to gain strength.With plastics,

generally thicker means weaker due to more internal stresses. Thicker walls add

weight and higher production cost because of longer cycle times. To gain stiffness

or strength in a plastic wall, properly designed reinforcing ribs can be added.

Different plastics have different amount of shrink upon cooling, higher for

semicrystalline resins (eg, polypropylene can shrink 0.030 in./in.) and lower for

amorphous plastics (eg, polystyrene can shrink 0.006 in./in.). Further, shrinkage,

especially for semicrystalline materials, varies due to thickness, cooling rate,

and often color. Whatever stays hotter for a longer time will shrink more. Therefore,

changes in nominal wall thickness cause differential cooling, which causes

differential shrinkage that causes warp.Warp distorts the part out of dimensional

tolerance. This distortion may be immediate after ejection from the mold, or after

assembly in the application environment. For example, the fit and shape of an

interior trim piece of a car’s dashboard are fine in production and assembly, but

once on the road and after a few thermal cycles (cool nights and hot days) the

trim piece warps out of shape. With plastics, nominal wall-thickness changes and

sharp corners should be avoided. If nominal wall-thickness changes are necessary

the variation must be minimized. How fast the wall changes thickness will also

influence properties and performance. Blending different thickness wall sections

will minimize internal stresses. The first rule in plastic part design is uniform

nominal wall thickness, yet few parts are designed on this basic premise. Figure 9

shows an application of uniform nominal wall thickness.

During part design one must take into consideration material, tool, and processing

issues. When designing a part the mold is also being designed. For example,

to have a dimensionally stable, high performance part it must be cooled as

uniformly as possible. Is it possible to get water channels to all sections of the

part so that it will cool evenly? Can the part be removed from the mold? Part design

forces these and other tool construction details. Often the gate or entry point

for the resin is determined by the cosmetic or performance requirements; however,

it may not be possible because of mold building requirements. A good plastic

part designer knows and comprehends the other three components and makes the

difficult compromises to meet material, tooling, and processing requirements.

Schools and training programs have not kept up with demand for plastic

part designers because of industry growth. Lack of proper training and drive for

short product development times are two of the many causes for “engineering

changes” as the mold is being built. That is, the design is changed after work

has started on making the mold. Often a design change is made and there is no

steel to accommodate the change. The mold builder cuts it to the previous design.

Figures 10 and 11 depict the cost of engineering changes at different stages of a

project and the impact design has on the profit margin of a project. Engineering

changes must be completed in the design phase before production tooling is made.

Unfortunately, the industry norm is to start cutting the tool (mold) before the

design is finalized.

Industry trends will continue to challenge design. Trends to consolidate

multiple functions into few parts for easier and faster assembly results in more

complex plastic part geometries. Thinner walls to save plastic and longer flow

paths in larger parts are other trends that test design limits. It is imperative that

part design heed fundamental design principles as outlined in References (11)

and (12). Better performance is often tied to tighter tolerances. This forces more

attention to the details in each of the four components for making a successful

plastic part.

“Injection Molding” in EPSE 2nd ed., Vol. 8, pp. 102–138, by I. I. Rubin, Robinson Plastic

Corp

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