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Design Tips for Working More Effectively with Plastics

Today's high performance plastics make possible even the most complex designs with demanding performance requirements. This water control valve, previously machined from a brass casting, was redesigned into an injection molded component. A 75 percent weight reduction was achieved without sacrificing flow rates.
Photo courtesy of Minnesota Rubber and Plastics

By Michael Busker

Every engineer's aim when designing with plastic is to achieve a technically good design that functions well and that can be manufactured cost effectively. Following these basic tips will not only help in accomplishing these goals; it will speed up the designing process while making it easier.

When Your Material of Choice is Plastic
It is always wise at the outset of a particular design to consider why plastic was chosen in the first place. If the design project is a conversion from a previous metal part design to a plastic part, one must remember that plastic is not metal. Not only do strength and weight become considerations in the new design, but also durability, lubricity, and environmental effects will come into play. There are many more features that must be examined carefully, and there are always tradeoffs.

Today's high performance plastics bring many benefits to the design table that many previously thought existed only with metal. These start with durability. The high performance plastics of today have been proven in many applications to be much stronger than metal over time. Nearly every imaginable industry—ranging from plumbing, medical, and aerospace to many others—now has hundreds of applications where previously designed metal parts were converted to plastic.

Plastic Molding Experience is Crucial
Designing products with molded plastic components and assemblies is a more attractive option than ever before. New high performance plastics and innovative molding processes make this possible. Injection molded plastic parts offer an important combination of flexibility, toughness, and chemical resistance for cost effective, long-term performance in a wide range of applications.

However, not every plastic part design can be efficiently injection molded. So working with an experienced molder with many years of experience with high performance plastics and complex projects is important to a successful outcome. That way, you can be assured that your molded components and assemblies are both functional and within budget.

Knowing Plastic Materials and their Correct Selection
The selection of the right material is as important as any of the design tips discussed below. There are hundreds of plastic materials available, all with different performance characteristics. As a part designer, you need to understand what plastic will work best for your application. You need to understand the stresses that will affect the performance and the life of the part.

There are several key questions to answer. For example, will part stress occur repeatedly, or will it be a one-time occurrence? Will the stress in a moving plastic part experience a continual peak load and relaxation, or will it be under constant load? What chemicals will come into contact with the part? Some plastics are very sensitive to specific chemicals; whereas, others are not. Both the plastic material supplier and an experienced molder will prove helpful in selecting the correct material for the application.

Importance of Part Uniform Wall Thickness
Uniform wall thickness is critical in part design for an injection molded part. Non-uniform wall thickness causes dimensional control problems, warpage, and other part integrity issues. Wall thickness is dictated by multiple inputs to the part.

* Material selection. Some materials can flow through thin wall sections and give the proper strength for their application. However, some materials do not move freely or contain fillers to improve the strength of the material chosen. These types of materials will require a thicker minimum thickness.

* Mechanical characteristics needed for part integrity. What are the design requirements for the part? If this part is under constant load, it may require a thicker wall section than a part that is not under any load.

Seals and bushings in vehicle transmissions are a good example of the durability of today's high performance plastics. These components are molded of PEEK material and are subject to temperature ranges of -40F to +264F (-40C to 129C). They withstand dry and lubricated tribological contact with pressures up to 100 psi and velocities up to 8,000 ft/min. over many years.
Photo courtesy of Minnesota Rubber and Plastics

A good rule of thumb is to design all part cross-sections as thinly and as uniformly as possible. The use of ribs is an effective way of achieving rigidity and strength while avoiding cross-sectional thickness. In cases where it is impossible to avoid a thick cross-section, ribs may also help to minimize the distortion that can occur. Extremely complex shapes that must combine thick and thin cross-sections should be reviewed in advance so as to determine dimensional stability and tolerance changes that will occur during and after molding.

Material shrink also influences part integrity. Thermoplastic materials are heated at high temperatures in the barrel of the molding press and injected into the mold cavity. As the part cools in the mold, it shrinks. Thick cross-section areas cool at a substantially lower rate than thin cross-sections, and press cycle time is based on the cooling rate of the thickest cross-section.

Therefore, only one relatively thick cross-section area of the part will increase the press cycle time, thereby reducing the number of parts per hour and increasing the cost per part.

Also, the uneven rate of cooling of these thick and thin cross-sections is likely to result in distortion of the part after it has been removed from the mold. This distortion is often severe enough to prevent the part from meeting specifications.

A thick cross-section is also likely to result in a depression on the surface, called a sink mark, particularly if the cross-section is of varying widths.

Needed Wall Tapers for Plastic Molded Components
Amount of taper, or "draft," refers to the amount of taper of molded parts perpendicular to the parting line. The draft taper should be determined early in the plastic part design process.

The need for part surface tapering facilitates ejection of the part from the mold, especially in high speed, high volume production applications. Surfaces to be tapered include holes, cavities, and internal grooves, as well as the outside diameter.

Cost-Saving Hole Design
A hole or inside diameter is created in a part by inserting a core pin in the cavity. Holes at a right angle to the mold parting line are relatively easy to produce, since the core pin is parallel to the injection path. The normal shrinkage process, however, can cause the part to cling to the core as it cools in the mold. In order to facilitate ejection of the part from the mold, a draft should be incorporated along the length of the hole.

Holes that are parallel to the mold parting line call for the use of a sliding core that automatically retracts from the part as the mold opens. The use of sliding cores adds to the cost and complexity of tool design and construction. If a hole does pass completely through a part, or if the part contains holes on more than one side, the mold must be designed to hold the part on a specific side of the open mold to facilitate automatic parts unloading.

Long, fragile cores tend to warp or break under continuous use due to the heat and pressure of their operating environment. The size of the core pin, and thus the diameter of the hole, should therefore be maximized whenever possible, particularly at the base, to ensure the stability of the pin. A useful rule of thumb to remember when designing part holes is the "2:1 rule." The height of the hole should not be more than twice its diameter.

External Part Features and Parting Lines
As a rule of thumb, parts with external features need to be designed so that the part can span across a parting line. A witness line is required where two pieces of the mold come together; therefore, if the surface finish is cosmetic, this should be taken into account. If the part cannot span across a parting line, slides are needed to generate the feature. Also, direction of pull must be considered in the plane that will be pulled with the sliding steel. Parts requiring features that need to be removed from the tool with sliding steel require a parting line.

An assembly of many injection molded plastic parts. It is an electronic testing device requiring high tolerance components that facilitate the device function. Included are a snap-together design that has a vacuum sealing chamber that isolates and protects circuitry, bosses that position and hold electrodes, and an actuating system.
Photo courtesy of Minnesota Rubber and Plastics

Expect Part Warpage
Some warpage should be expected with any molded plastic part. The amount of warpage will vary with the type of thermoplastic material used, the gate location, and the wall section of the part. A consistent part wall section usually will have the least warpage and provide the best overall outcome.

Eliminating Knit Marks
Knit marks are weak points where material flows together when a core pin blocks the normal path of the molten material as it enters the mold. This weak point occurs on the "back side" of the pin where the material flows together. These weak points can be eliminated by proper placement of the gate, or material entry point. It is very important to specify locations on the part where knit marks cannot be tolerated. This will eliminate potential problems in the mold design stage.

Achieving the Best Molded Part Corners
It is important to remember the following when designing part corners. Since the mold is machined from steel, it is easier and less costly to machine a radius than a square corner. Whenever possible, parts should have round corners when viewed from the top of the mold. When viewed from the side, part edges should be square.

Note that due to the flow characteristics of the molten plastic during the molding process, square corners tend to be weaker than rounded corners. To ensure dimensional stability, a minimum radius of 0.010 (.254mm) is recommended.

Selecting a Part Surface Finish
Molded plastic parts may be designed with a variety of surface finishes, from a high gloss to a rough texture. The choice of surface finish is usually based on cosmetic considerations. A glossy finish can enhance the appearance of a part, while a textured surface may help to mask sink marks or parting lines.

Surface finish should be specified so it does not interfere with ejection of the part from the mold. The smoother the finish, the more easily the part is ejected from the mold. An extremely rough surface may hinder ejection from the mold. Rough finishes tend to function much like an outside dimension undercut, thus preventing the part from slipping easily out of the mold.

When not otherwise specified, the surface finish standards of the Society of Plastic Engineers (SPE) and The Society of the Plastic Industry (SPI) should be followed. 
   
Molding in Part Inserts
Inserts made of steel, brass, and aluminum components are commonly inserted into plastic parts during or after the molding process. Having a knurled, ribbed, or abraded surface on the metal part helps to ensure a strong, permanent bond between metal and plastic so that the insert remains locked into the molded part. Design engineers who work with plastics can provide you with more information and assistance in designing your inserts.

Summary
Designing with plastics holds many exciting possibilities as more new high performance materials, automated molding equipment, and creative know-how enters the scene almost daily. Original equipment manufacturers should follow a regimented, data-based approach. Selecting the material and testing and validating the designs are all essential in a successful high performance plastic parts program. Such a program requires the early assistance of a company with deep knowledge and experience with plastics. This requires a team approach to reach a common goal.

Michael Busker is a materials engineer at Minnesota Rubber and Plastics (www.mnrubber.com), Minneapolis, Minnesota.