Choosing a Plastic Forming Process
Use geometry, material, quantity and tooling cost to narrow the process choice, then estimate cost per part and production rate.
Check geometry and material before comparing prices; Checklist
Check geometry and material before comparing prices
A low quoted price is irrelevant if the process cannot make the required shape or use the required material. Start with Plastics as Materials and the capability cards in Plastic Forming Processes.
- Describe the geometry: hollow container, open sheet-like shell, constant cross-section or discrete solid part.
- Choose a material type and candidate grade that meet the load, temperature and environment.
- State the quantity, expected repeat orders and required delivery rate.
- Check how the part leaves the tool: draft, undercuts and access to internal spaces.
- Compare tooling, per-part cost, achievable detail and likely defects for the remaining processes.
Use part shape to build a short list; Practice
Use part shape to build a short list
| Need | Start with | Then ask |
|---|---|---|
| Many thin-walled hollow containers | Blow molding | Can the hot tube or preform stretch into this shape with an adequate wall? |
| A large hollow shell in a modest quantity | Rotational molding | Are the longer cycle and achievable detail acceptable? |
| An open tray or sheet-like cover | Thermoforming | Will the sheet thin too much, and can it release from the tool? |
| A long constant cross-section | Extrusion | Which end features or side holes would need later operations? |
| Many detailed discrete parts | Injection molding | Can walls stay reasonably uniform, and does quantity justify the mold? |
| Repeated thermoset parts or composite panels | Compression molding | Can the charge fill the matched tools before curing? |
| A few detailed resin parts | Casting | Will mold life, trapped air and cure time meet the need? |
| A custom fiber-reinforced shell | Hand layup | Is the labor acceptable, and can the reinforcement reach every region? |
“Solid” does not mean “make every region thick.” An injection-molded housing is a discrete part, yet hollowing out bulky sections often improves it. Several processes can make a tank; size, quantity, resin and detail determine the better candidate.
Spread tooling cost over the number of acceptable parts; Practice
Spread tooling cost over the number of acceptable parts
A simple comparison is average cost per part = tooling cost / quantity + variable cost per part. Variable cost includes the costs that grow with production, such as material and processing time. Tooling cost is paid even if only a few parts are made.
Illustrative estimates, not supplier prices: suppose two processes can both meet a cover's specification. Route A costs $900 for tooling and $14 per part. Route B costs $18,000 for tooling and $2 per part.
| Quantity | Route A | Route B |
|---|---|---|
| 100 | $900 / 100 + $14 = $23.00 | $18,000 / 100 + $2 = $182.00 |
| 5,000 | $900 / 5,000 + $14 = $14.18 | $18,000 / 5,000 + $2 = $5.60 |
Find the quantity where the two estimates are equal
Set total costs equal: 900 + 14N = 18000 + 2N. Then 12N = 17100, so N = 1,425 parts. Below that quantity A costs less; above it B costs less under these assumptions.
This model leaves out financing, mold replacement, setup changes, inspection, shipping and rejects. A flexible mold that must be replaced during a run adds another tooling cost. Recalculate when the assumptions change.
Cycle time and cavity count set the ideal production rate; Practice
Cycle time and cavity count set the ideal production rate
A cavity makes one part per cycle. With cycle time in seconds, ideal parts per hour = 3600 / cycle time × cavities.
For a 30-second cycle and two identical cavities: 3600 / 30 × 2 = 240 parts/h. That assumes uninterrupted operation and every part acceptable. Use the complete repeating cycle time, including opening and ejection; do not add overlapping cooling time twice.
Actual useful output is lower when setup, stoppages or rejects occur. More cavities may increase tooling cost and require a larger machine, so a higher ideal rate does not by itself settle the process choice.
Explain the choice with both a benefit and a limitation
Explain the choice with both a benefit and a limitation
A useful selection sounds like this: “For 300 large hollow PE garden reservoirs, investigate rotational molding because the shape fits and tooling can be economical at modest quantities. Confirm wall control and cycle time before committing.”
The quantity alone does not prove that choice. A narrow neck, precision sealing face, delivery deadline or different resin could change it. State the assumptions, identify a credible alternative, and compare the features that matter.
Further process-selection reading
Formlabs' manufacturing-process overview compares form, volume and setup cost. Treat published volume ranges as examples, not universal cutoffs. The linked process references in Plastic Forming Processes explain individual capabilities.