Plastic manufacturing: which process fits your part, and what to specify

The main plastic manufacturing processes side by side, how part shape and volume decide between them, why the resin grade belongs on the drawing, and the problems that start after the parts ship.

Filed under Engineering and services

Short answer: Plastic manufacturing is the set of processes that turn plastic resin into finished parts: injection molding, extrusion, blow molding, thermoforming, rotational molding, and, at low volumes, urethane casting, CNC machining and 3D printing. The right process depends mostly on three things: the part's shape (solid, hollow, a continuous profile or a thin shell), how many you need, and how tight the tolerances are. Choosing the process sets the tooling bill, the lead time and the unit price before any supplier quotes, so make that choice first and then write a specification a factory cannot misread.

Plastic manufacturing is a big industry. The Plastics Industry Association reports that "The U.S. manufacturing industry by gross output in 2024 (the latest year for which data is available)." For a buyer, the size of the industry matters less than this: every process has a volume range where it is cheap and a range where it is the wrong tool. Most expensive mistakes we see start with a part quoted in the wrong process, or a right process quoted against a loose specification.

This note is a buyer's guide. It covers how the main plastic manufacturing processes differ, how volume decides between them, what to put on the order, and what goes wrong after the parts start shipping. It does not try to teach part design; for that, see design for manufacturability (DFM) before tooling.

The main plastic manufacturing processes

Injection molding. Melted plastic is forced into a closed steel or aluminum mold, cooled and ejected. It makes most of the solid plastic parts around you: housings, clips, caps, handles, gears. The catch is the tool. Formlabs, a 3D printer maker that publishes a neutral comparison of plastic processes, puts it plainly: "Creating new molds for injection molding can take months to complete, and their costs can run into five or six digits." The payoff comes after that: "Once the tooling is up and running, cycle times only take a few seconds, and millions of high-quality parts can be produced at a fraction of the cost of all other manufacturing processes." Our note on what an injection mold really costs breaks down where that tooling money goes.

Extrusion. Plastic is pushed continuously through a shaped die, like toothpaste from a tube. Dies are cheap compared with molds, but the shape has limits: "Forms and shapes that can be manufactured with extrusion are limited to products that have continuous profiles, such as T-sections, I-sections, L-sections, U-sections, and square or circular sections." Pipe, tubing, weatherstrip, trim and film are extruded. Blown film for poly bags is a variant: the extruded tube is inflated into a thin bubble and wound onto rolls.

Blow molding. A heated tube or preform of plastic is inflated inside a mold to make a hollow part. Formlabs again: "Blow molding is the most common process for creating hollow plastic products at scale." Bottles, jugs, tanks and hollow toy parts are blow molded. The molds run at lower pressure than injection molds and cost less, but the process controls the outside shape far better than the inside wall.

Thermoforming (vacuum and pressure forming). A heated plastic sheet is pulled or pushed over a single-sided mold. Tooling is cheap and fast, which suits trays, covers, large panels and packaging. Blister and clamshell packs are thermoformed; see blister vs. clamshell packaging. The limit, per Formlabs: "However, the process offers only limited form freedom and can only be used to manufacture parts with relatively thin walls and simple geometries."

Rotational molding (rotomolding). Plastic powder is tumbled inside a heated, rotating mold until it coats the walls. It makes large hollow parts such as kayaks, coolers and tanks with low-cost tooling, but slowly. Formlabs notes that "Rotomolding has some design constraints and finished products have looser tolerances." and that "The most common material for rotational molding is polyethylene (PE) which is used in 80% of applications, mainly because PE can be easily ground into a powder at room temperature."

Low-volume processes: urethane casting, CNC machining and 3D printing. These need little or no hard tooling, so they are how most products start. Urethane casting pours liquid resin into silicone molds, which wear out quickly: "Flexible molds made from latex rubber or room temperature vulcanized (RTV) silicone rubber are inexpensive when compared to hard tooling, but can produce only a limited number (around 25 to 100) of castings as the chemical reaction of urethanes, epoxies, polyester, and acrylic degrades the mold surfaces." Machining cuts parts from solid plastic stock: "Machining is ideal for low volume plastic part applications that require tight tolerances and geometries that are difficult to mold." And for 3D printing, "As 3D printers require no tooling and minimal setup time for a new design, the cost of producing a custom part is negligible in comparison with traditional manufacturing processes." The trade-off is the same for all three. The unit price barely falls as quantity rises, and the material and surface are not always the same as the molded production part.

How volume decides the process

The rule behind every process choice is tooling cost against unit cost. Formlabs states it simply: "Some manufacturing processes have high front costs for tooling and setup, but produce parts that are inexpensive on a per-part basis." Low-volume processes do the reverse: almost no setup, but every part costs about the same.

Thermoforming against injection molding is the clearest example. Productive Plastics, a US thermoformer, estimates that for a large part the total cost of thermoforming is lower below about 3,000 to 5,000 parts, and that injection molding usually wins above that (its comparison).

A rough way to think about it for a new product:

• 1 to about 100 parts: 3D printing, machining or urethane casting. Do not cut steel yet.
• Hundreds to a few thousand: a low-volume aluminum or simple steel mold, or thermoforming for large thin parts. Our note on low-volume injection molding covers where that range starts and ends.
• Thousands per year and up: a production injection mold, with the cavity count sized to the annual volume. Blow molding for hollow parts, extrusion for profiles.

The mistake to avoid is buying a production mold for a product that has not sold yet, or the opposite: paying prototype prices on every part for two years because nobody revisited the process once volumes were known.

Thermoplastic or thermoset, and why the grade matters

Most plastic manufacturing uses thermoplastics: ABS, polypropylene, polycarbonate, nylon, acetal, polyethylene. Romeo RIM, a US molder, gives the short version of the difference: "Put simply, a thermoset cannot be re-melted and a thermoplastic can." Thermosets, such as epoxies, phenolics and liquid silicone rubber, cure by chemical reaction. Of liquid silicone rubber, Protolabs writes: "The material cures in the presence of heat forming a bond that can’t be undone." That is why thermoset sprues and scrap cannot be reground, and why thermoset parts generally cannot be reshaped or welded with heat.

Material choice matters more than most buyers expect. As Protolabs puts it, "After all, the material is your part." "ABS" on a drawing is not a specification. There are many ABS grades with different flow, impact strength, heat resistance and flame ratings, and they shrink differently in the same mold. Name the grade (supplier and product code), or name the properties it must meet and an approved equal, and say whether a flame rating, a food-contact rating or UV resistance is required.

What to put on the order

Whichever process you choose, the factory builds what the documents say. A complete request usually includes:

• A 3D model and a 2D drawing. The model defines the shape. The drawing defines the tolerances, the critical dimensions and which surfaces are cosmetic. Without a drawing, every dimension is open to interpretation.
• The resin grade and color. A named grade, as above, and a color reference (a color number or a physical color plaque), not "black". Black from one resin lot can look different from the next under store lighting.
• Surface finish or texture. Polished, matte or a named texture standard, and on which faces. Texture is cut into the mold steel, so changing it later is a tooling change.
• Cosmetic rules. Where gate marks, ejector pin marks and parting lines may and may not appear, and how sink marks or flow lines on visible faces will be judged.
• Regrind policy. Whether reground runners and rejects may be mixed back in, and at what percentage. If you say nothing, assume some regrind is used.
• Quantity, annual volume and packaging. Annual volume sizes the mold. Packing spec (bags, dividers, carton counts and marks) prevents scuffed parts on arrival.
• Inspection terms. First article approval before production, and the sampling plan for each lot. Our first article inspection note covers what that report should hold.

Also settle who owns the mold, in writing, before paying for it. Tooling custody explains why that clause matters the day you want to move production.

What goes wrong, and how to prevent it

The resin quietly changes. A cheaper grade, or a higher share of regrind, can pass a visual check and fail in the field: brittle clips, warped lids, faded color outdoors. Prevent it with a named grade on the drawing, a material certificate for each lot, and a rule that any change in resin needs your written approval.

Molding defects. Short shots, sink marks, flash, warpage and weld lines come from either the tool design or the process settings, and the fix depends on which. Injection molding defects: tool or process? walks through how to tell the difference.

Color and gloss drift between lots. Keep a signed reference sample from the approved first article and compare every lot to it under the same light.

The process outgrows the product. A part that started in urethane casting or a soft mold at 500 a year may need a production mold at 20,000. Review the process whenever annual volume changes sharply, and ask for a quote in the next process up; at the higher volume, the per-part savings can pay for the new tool.

Two parts that should have been one, or one that should have been two. A soft grip on a rigid handle can be glued on, overmolded or two-shot molded, at very different costs. See two-shot molding vs. overmolding before you commit the design.

Where JCH fits

JCH runs injection molding, insert molding and overmolding programs through vetted partner factories, with mold engineering, first-article measurement and pre-shipment inspection managed for US buyers. If a part belongs in a different process, we will tell you that before you pay for a mold. The capability page lists what we make, and the injection molding page shows how a molding program runs from drawing to first shipment.

Straight answers

What is plastic manufacturing?

Plastic manufacturing is the group of processes that turn plastic resin into finished parts. The main ones are injection molding, extrusion, blow molding, thermoforming and rotational molding, plus low-volume methods such as urethane casting, CNC machining and 3D printing.

What is the most common plastic manufacturing process?

Injection molding makes most solid plastic parts, such as housings, caps, clips and handles. Blow molding is the usual process for hollow parts like bottles, and extrusion is used for continuous profiles such as pipe, tubing and film.

How do I choose between injection molding and thermoforming?

Mainly by part shape and volume. Thermoforming suits large, thin-walled parts and has much cheaper tooling, so it often wins at lower quantities. Injection molding suits complex solid parts and becomes cheaper per part at higher volumes; one US thermoformer puts the crossover for a large part at roughly 3,000 to 5,000 parts.

What is the difference between a thermoplastic and a thermoset?

A thermoplastic softens when heated and can be remelted and reshaped, which is why its scrap can be reground. A thermoset cures by a chemical reaction and cannot be remelted afterwards. Most molded consumer and industrial parts are thermoplastics; liquid silicone rubber and epoxies are common thermosets.

What should I specify when ordering plastic parts?

A 3D model plus a 2D drawing with tolerances and cosmetic surfaces, the resin grade by name, a color reference, the surface finish or texture, where gate and ejector marks may appear, the regrind policy, quantity and annual volume, packaging, and first-article and lot inspection terms. Settle mold ownership in writing before paying for tooling.

How this note was made: researched and drafted with AI assistance, checked against the sources listed below and edited at JCH Design & Manufacturing.