How to Select the Optimal Plastic Manufacturing Process

Example Plastic Part

Example Plastic Part

Selecting the optimal manufacturing process for a new plastic part is one of the quintessential mechanical engineering exercises. It requires a clear identification of all the part requirements and balances those needs within a thorough understanding of all the available processes. This is a topic which occupies volumes of engineering books, but I will try to condense it down to one blog post by performing the exercise on the example part pictured to the right.

The example part is a basic plastic cap measuring 2.7” in diameter and 1.0” deep. It is used in a specialty product, so I only need to make 200. Material is specified as Nylon with UL94 V-0, but I can substitute an equivalent material so long as it is comparable to nylon’s strength/durability and UL94 V-0 flame retardant. I also am required to source the part domestically.

Plastic manufacturing processes available:

CNC machining
Cast Urethane
DDM rapid prototyping
Injection Molding
Rapid Injection Molding
Reaction Injection Molding
The First Step :: Describe the Part

The first step in process selection is clearly identifying some basic physical requirements of the part. Definition of these characteristics will quickly narrow down your process selections. Below is a list of common requirements for a part, and I’ve input information for the example part:

Material Nylon (or durable UL94-V0 equivalent)
Color Teal (color match required)
Painting Allowed? No paint allowed. Color must be in the material.
Surface Texture Good Quality. Smooth or light stone.
Tolerances Light
Feature Complexity Low, but 2-sided features.
Undercuts Yes, one side-hole.
Example Plastic Part in Section

Example Plastic Part in Section

So let’s eliminate some processes by talking down the list.

  • Nylon: No problem… all of those processes can produce in a UL94 V-0 material close to Nylon’s durability.
  • Teal and no painting allowed: Problem… CNC machining, thermoforming, and DDM require stock material to fabricate from. I can’t find plastic block (for machining), sheet (for thermoforming), or SLA/SLS/FDM material (for DDM) in the teal I need. They would need to be custom blended and manufactured at potentially large minimums. The rest of the processes can handle color matching without much difficulty.
  • Smooth surface texture: Not a problem for any process. DDM and CNC can be rough, but the parts could be bead blasted.
  • Light tolerances: No problems.
  • Low but 2-sided complexity: Thermoforming could have a problem with this requirement. For a part this size, thermoforming can mold detail on only one side. However, the interior details on this part could be added/machined after forming the exterior, but that will add to part cost.
  • One side-hole undercut: Not a problem for any of the processes, but the feature will add some cost to injection molding tools (for slide actions) and thermoformed parts (post-machining).
Second Step :: What is important?

Next you should define what deliverable is most important to you when producing the part in question. Achieving the lowest total cost? High quality surface finish? Fastest production timeframe? Tightest tolerances? I’ve listed the best processes for each deliverable below.

Best Processes for given deliverables:

  • Fastest timeframe: DDM, CNC machining, Rapid Injection Molding, Cast Urethane
  • High quality surface finish: Injection Molding, RIM molding
  • Tightest tolerances: CNC machining, Injection Molding
  • Lowest cost: Depends on quantity manufactured, see below

For the example part, cost is the most important deliverable. I need to make these parts for the absolute least amount of money. I can be lenient on surface finish and production timeframe, and the part does not have very restrictive tolerances. Total costs of the processes vary by production quantity needed, and that brings us to…

Third Step :: How many will be made?

This can be a simple or hard question to answer. In the example part, I need a defined quantity of 200 parts. But other situations might call for an undefined number of parts per month for an unknown number of months. Clear or indefinite, you should do your best to define or realistically project this number because it will have direct influence on process selection.

Best processes for given part quantities:

  • Low quantity (1-50): CNC machining, DDM, Cast Urethane
  • Medium quantity (50-500): Rapid Injection Molding, Reaction Injection Molding, Cast Urethane, DDM, Thermoforming
  • Intermediate Quantity (500-5,000): Rapid Injection Molding, Injection Molding
  • High Quantity (5,000+): Injection Molding

The 200 unis I need of the example part is at a sweet spot where there are many processes to choose from. But from step one I know that DDM will not be able to produce the color I need, and Thermoforming will have large material minimums and multiple post-form machining operations. I will quote my part at multiple vendors for the processes of Rapid Injection Molding, Reaction Injection Molding, and Cast Urethane. Although thermoforming does not seem to be a good fit for this part, I’ll have it quoted as well to verify my predictions.

So which process is the best?

Below I tabulated average quotes for the sample part per the various processes. Tooling and part costs are itemized and total included part cost is at the bottom (all tooling amortized into the 200 parts). Notice thermoforming was not quoted. My vendor informed me that the custom color would require a 1,000 pound minimum order of sheet plastic (enough for almost 20,000 parts), and it would not be cost effective for only 200 parts.

Rapid Injection Reaction Injection Cast Urethane Thermoform
Tool $6,000 $7,500 $2,700 n/q
Parts $5 $25 $50 n/q
Unit $35 $63 $64 n/q

For the physical requirements, deliverable requirements, and production quantity specific to the example part, rapid injection molding would seem to be the best process.

And actually, this part is a simplified version of a real part we recently designed for a client, and this is the same process I used to determine the appropriate manufacturing process. However, I used a preliminary CAD model and performed the analysis before designing the actual part; that way I was able to tailor the part’s geometry to the process I would eventually use to manufacture it. I’ll go over that in more detail in a future blog… how to design a part with a particular manufacturing process in mind.

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