In the world of plastic part fabrication, every process, be it injection molding, machining or extrusion, has an important place. Confusion regarding these processes persists in many industries. That confusion can have profound effects on the final product, both – in cost and- performance. Fortunately, selecting the right material and process for a particular component production run is not difficult with some tips from the plastics experts. With a handful of questions and a little knowledge about the processes, an engineer should have no trouble getting a part that offers strong performance at a targeted cost.In the world of plastic part fabrication, every process, be it injection molding, machining or extrusion, has an important place. Confusion regarding these processes persists in many industries. That confusion can have profound effects on the final product, both – in cost and- performance. Fortunately, selecting the right material and process for a particular component production run is not difficult with some tips from the plastics experts. With a handful of questions and a little knowledge about the processes, an engineer should have no trouble getting a part that offers strong performance at a targeted cost.

Design Considerations

Before settling on a process or a resin to work with, a few things need to be understood. These include:

  1. Performance requirements – What will the part have to do once it is installed, and what kind of forces and stresses will it have to endure?
  2. Dimensional attributes – What is the required size and shape– of the fabricated component -, and what surface qualities will it need?
  3. Target cost – What kind of budget is there to work with? How can that budget maximize part performance while remaining at or under the target?

 

For design engineers, the first two are elementary, and require little more than applied knowledge to answer. However, cost is something that is rarely understood well. That’s because there is a pervading assumption in many industries that plastic components are always less expensive than components made from other materials, specifically metal. That is a poor assumption.  Many high- performance polymers are on a volume basis much more expensive than the most expensive metal alloys.  Although process options like injection molding offer molding to final form options helping to reduce unit manufacturing costs, engineers must account for the cost of the mold itself. Injection molding is often assumed to be a cost effective option because the expected volume of a production run pushes down the cost per part.  However, molds need to be machined out of extremely durable metals to withstand thousands of parts which can drive total project costs up unexpectedly.

While injection molding can still be the right process even when cost is considered, it bears mentioning that machining components from stock shapes reduces the need for expensive tooling while speeding up delivery of initial parts. Also, machining maintains a precision advantage, which often leads to better performance and dimensional versatility.

Selecting a Material

There are hundreds of resins available, most of which come in many grades and viscosities. Narrowing down this wide field into a select few resins, grades and viscosities is the primary mission of material selection, but getting there means answering several important questions.

A material is selected for its ability to provide several performance characteristics in the presence of several environmental stressors. Some of those performance characteristics include strength, wear resistance and transparency, while environmental stressors may include extreme temperatures, abrasive or impact forces, corrosive chemicals and sunlight. Putting it all together, these are the kind of questions commonly asked by polymer providers:

  1. Is the part going to experience a lot of wear, or is it a structural component?
  2. What temperature range will the part operate at?
  3. What environmental stressors need to be considered?
  4. What are the minimum strength requirements the part must meet?
  5. Are there any appearance requirements, like color or transparency?

The answers to these questions will be instrumental in narrowing the field of candidate polymers.

However, that’s only the start, as every resin comes in a variety of grades that will affect performance. Each grade is made with a different composition, and resin composition must be closely considered. Here’s a quick review of the different grades, and what they are best suited for:

  1. Glass or carbon reinforced resin grades – These grades are designed for superior strength and stiffness. They also offer strong dimensional stability.
  2. Wear grades that contain graphite and PTFE – These grades are designed for maximum wear resistance and produce little friction.
  3. Unfilled or glass-reinforced grades – These grades offer superior thermal or electrical insulation, which makes them an ideal material choice for isolators.
  4. Non-reinforced grades – These grades provide the best chemical resistance, and if the max stress level is low enough, they can also offer excellent fatigue resistance.

Selecting a Process

At this point, grade selection is mostly settled on, but the final resin selected will depend on the process selected as well. Design engineers should note that process selection influences the resin choice and can greatly impact the cost of the project, as well as the parts’ performance. Again, another quick recap is warranted, this time between injection molding and machining:

  1. When to choose machining – If the parts need to be fabricated to at a large size (wall thickness greater than .5” or 12mm), machining is the right choice. Also, if impact strength and toughness are priorities, or if the parts need to be fabricated within tight tolerances with no drafting, machining makes the most sense. Finally, if production is going to be kept to smaller runs (less than 5,000 parts or less during a single year), then machining will be a cost-effective option.
  2. When to choose injection molding – If the parts are designed with features that are difficult to machine, or if larger production runs are required (more than 5,000 parts annually), then injection molding is the right choice.

Even after the resin and process are both selected, there may be some fine-tuning left. This will likely require input from the polymer supplier or manufacturer.

A common issue arises is when a resin grade that is not designed for machining is nonetheless selected for machining. Extruded shapes are nearly always produced using high viscosity resins, as they offer superior stiffness and strength both during and post extrusion processing. Make sure that if machining is the preferred part production process, that a high viscosity resin is selected, as this will ensure that there are stock shapes can be produced to machine from. It can be tricky to decipher industry nomenclature without some professional guidance. For example, some polymers, like Torlon 4203, denote low viscosity grades designated for injection molding thin walled parts with an L. Most polymer grades are number identified as well with a specific number series are reserved for low viscosity resins.  An example is Victrex’s 150 grade PEEK which is a low viscosity grade for thin walled injection molded parts while the 450 grade is higher viscosity targeting parts with thicker cross sections and all extruded shapes.

In short, injection molding is the fastest and most efficient conversion route, but there can be challenges namely start-up costs and lead time.  Machining parts from extruded shapes may be the fastest route to prototyping parts but also deserves consideration even once parts become standard production. The material selected may sound the same and may even carry the same resin tradename but process differences and seemingly subtle grade differences can result in unexpected outcomes when these considerations are ignored. The safest route is to involve your processor as early in the material selection process as possible. Their experience with and understanding of the many polymer grades is invaluable and must be considered before an engineering drawing is released for production.

A Final Note on Production Phases

Many a design engineers have been frustrated while transitioning parts between injection molding and machining. Using the same resin does not guarantee equal performance. Examples include machined prototypes work great but injection molded production parts do not to injection molded parts costing tens of thousands of dollars more than better functioning machined parts. It is important that a design engineer has a strong grasp on what both processes entail and the underlining differences between the processes and resin grades being considered. In addition to those already mentioned, these differences can include fiber orientation, fiber length, coupling agents, coloring additives, wear rates, toughness and a host of other performance attributes.

Material and process selection when designing with plastics can be tricky but with an integrated consideration of both material and process your opportunity for design success is maximized.