Choosing Plastic Materials for Production: Cost, Shrink, and the Cheap Resin Trap

Resin sets 15% to 40% of a molded part’s piece price, and it sets nearly all of the failure modes. Polypropylene costs $0.75 to $1.15 a pound and shrinks 1.3% to 2.5%. Polycarbonate costs $2.00 to $3.10 and shrinks 0.5% to 0.7%. That shrink difference is why the resin has to be locked before the tool is designed: steel is cut oversize by the shrink factor, and switching resins after the cavity is machined can render a $26,000 mold unusable.

Pick the material for the load case, the environment, and the regulatory requirement. Then check the cost. Doing it in the other order is how the cheap resin ends up costing more.

The Cost and Shrink Table

Two entries deserve a second look. Glass-filled nylon shrinks differently along the flow direction than across it, which means the part warps in a predictable but hard-to-model way, and mold-flow analysis stops being optional. Acetal shrinks 1.8% to 2.5%, high enough that the cavity has to be cut noticeably oversize, and it releases formaldehyde at melt temperature, which pits standard P20 tool steel over time.

Selecting in the Right Order

Work through five filters in sequence. Cost is the fifth, not the first.

1. Load case. How much force, in what direction, how many times? A part that flexes 200,000 times is a fatigue problem and points to PP or an unfilled nylon. A part that takes a single high impact points to PC or PC/ABS. A part under sustained load points away from anything that creeps, which rules out PP and PE for structural members.

2. Environment. Temperature range, UV exposure, chemical contact, humidity. Outdoor products in unmodified ABS chalk and yellow within a season. ASA is the outdoor version of ABS and costs about 15% more. Carbon black pigment at 2% is the cheapest UV protection available and works on most resins.

3. Cosmetic requirement. Does it need a Class A surface, a specific color, a soft touch, transparency? PC/ABS takes paint and plating well. PP does not, without flame or plasma treatment that adds $0.08 to $0.25 per part. HDPE is close to unpaintable in production.

4. Regulatory. Food contact means an FDA-compliant grade with documentation. Electrical enclosures usually need a UL 94 flame rating, commonly V-0 or 5VA. Children’s products and anything sold in the EU carry additional testing. Flame-retardant packages add $0.40 to $1.50 per pound and often reduce impact strength by 20% to 40%.

5. Cost. Now, and only now, compare prices. If two materials pass the first four filters and one costs $0.90 less per pound, take the cheaper one.

When the Cheap Resin Costs More

Polypropylene is the classic case. At $0.90 a pound against ABS at $1.45, a 42-gram part saves about $0.05 in material. Then:

• PP shrinks 2.0% against ABS at 0.6%, so the part warps more and the tool has to be built with more shrink compensation and better cooling. Add $1,500 to $4,000 to the tool.
• PP needs a thicker nominal wall for the same stiffness, which adds material back and adds 15% to 30% to cycle time. That cycle penalty on a $75 per hour press costs more than the material saved.
• PP does not accept paint or adhesive without surface treatment. If the part gets a printed logo, add $0.08 to $0.25.
• PP has a lower service temperature and creeps under load, so a snap that holds fine in ABS relaxes over 18 months in PP.

Net effect on that part: PP saves $0.05 in resin and adds $0.09 to $0.22 in cycle, treatment, and tooling amortization. Meanwhile it wins outright on any part with a living hinge, where nothing else competes.

The lesson is not that PP is bad. It is that resin cost per pound is one line in a six-line equation, and the other five move more. The same logic applies to every substitution, and it is the same reasoning that governs what injection molding costs at the piece-price level.

Shrink and Why the Tool Cares

A mold cavity is machined larger than the finished part by the resin’s shrink factor. A 100.0 mm dimension in ABS at 0.6% shrink means a 100.6 mm cavity. The same dimension in POM at 2.2% means a 102.2 mm cavity.

Three consequences follow.

You cannot swap resins across shrink classes after the tool exists. Moving from ABS at 0.6% to PP at 2.0% on a cut tool makes every dimension undersize by 1.4%. On a 100 mm part that is 1.4 mm. There is no process adjustment that recovers it.

You can sometimes swap within a class. ABS at 0.6%, PC at 0.6%, PC/ABS at 0.6%, and HIPS at 0.5% are close enough that a tool built for one can often run another with tolerance verification. That flexibility is worth having, and it argues for specifying inside the ABS/PC family when the requirements allow it.

Filled resins shrink anisotropically. Glass fibers align with flow, so the part shrinks less along the flow path and more across it. The tool designer compensates for the expected orientation, which means gate location becomes a dimensional decision rather than a cosmetic one. Get that wrong and the part is out of spec in a way no process tuning fixes. This is a large part of why tooling cost rises with material choice.

Drying: The Step That Ruins Parts Silently

Hygroscopic resins absorb atmospheric moisture. Molding wet resin causes hydrolysis, which breaks the polymer chains and drops mechanical strength 20% to 50% while producing splay marks on the surface.

Polycarbonate is the one that punishes shortcuts. Under-dried PC parts look acceptable and test at half the impact strength of properly dried parts. Ask any molder quoting PC what their drying process is and whether they run a moisture analyzer. A shop that answers with a specific target number has done this before.

Drying also costs money: desiccant dryer time is press-adjacent overhead, and molders build it into the quote. That is part of why a PC part costs more than the resin price difference suggests.

Color and Regrind

Color. Most production runs use natural resin plus a color masterbatch metered in at 2% to 4%. Masterbatch costs $2.50 to $8.00 a pound, so at a 3% letdown it adds roughly $0.08 to $0.24 per pound of finished material. A custom color match costs $250 to $900 in lab work with a 10 to 25 pound minimum order.

Pre-colored compounded resin costs more per pound but delivers better consistency. For a cosmetic part where color variation between lots would be visible on the shelf, pay for the compound.

Regrind. Runners and rejects can be ground and blended back in, typically at 10% to 25%. Each heat history cycle degrades the polymer. ABS and PP tolerate regrind well. PC does not, and most PC specifications limit regrind to 10% or ban it outright on cosmetic parts. If your quote assumes 25% regrind and your spec bans it, the piece price will move when someone notices.

Matching Material to the Tool You Can Afford

Material choice constrains tooling, and tooling budget constrains material.

• Aluminum bridge tools run unfilled ABS, PP, PE, HIPS, and PC without complaint. They will not survive glass-filled resins for more than a few thousand shots.
• P20 pre-hardened steel handles everything unfilled and moderate glass loading, but pits with POM, PVC, and aggressive flame-retardant packages.
• Hardened H13 or 420 stainless is required for glass-filled resins in production volume, and for corrosive resins at any volume.

Choosing 30% glass-filled nylon on a project with a $9,000 tooling budget is choosing to replace the tool. That interaction, along with the low-volume paths that work with each resin family, is covered in the low-volume manufacturing options.

A Selection Worksheet

Write down the answers before you call anyone.

1. Peak load and direction, with a number in newtons or pounds
2. Number of load cycles over product life
3. Temperature range, storage and use
4. UV exposure: indoor, incidental, or continuous outdoor
5. Chemical contact: cleaners, solvents, oils, food, skin
6. Cosmetic class: A, B, or C surface
7. Color requirement and whether it must match a brand standard
8. Regulatory: UL rating, food contact, RoHS, medical
9. Annual volume, which sets the tooling class
10. Target piece price

That worksheet takes an hour and turns a material conversation from a preference argument into an engineering one. It also produces most of what a molder needs to return an accurate quote instead of a range.

Material choice interacts with wall thickness, rib geometry, and draft, so it is not a decision to make in isolation from the geometry. The relationships between resin, wall, and cycle time are worked through in design for manufacturability, and prototype-stage material selection follows different rules, covered in choosing materials for an invention prototype.

Enhance runs material selection as part of engineering and prototyping, before the tool is quoted, because a resin change after tool design is the most expensive kind of change there is. Where that spend sits relative to patent filing, design, and launch is mapped in the complete inventor cost breakdown. Before any of it, a $399 patent search confirms the concept is clear; the USPTO’s patent basics explains what that search covers, and the U.S. Small Business Administration offers free counseling for small manufacturers working through supplier qualification.

FAQ

What is the cheapest plastic for injection molding?

Polypropylene and HDPE, both between $0.70 and $1.15 a pound. Both come with tradeoffs: high shrink, poor UV resistance without additives, difficulty accepting paint or adhesive, and creep under sustained load. HIPS at $0.95 to $1.35 is the cheapest option that takes a good cosmetic finish, but it is brittle and has poor chemical resistance.

Can I change resins after the mold is built?

Only within the same shrink class. ABS, PC, PC/ABS, and HIPS all shrink between 0.4% and 0.8% and can sometimes run in the same tool with dimensional verification. Moving to PP, POM, or unfilled nylon changes shrink by 1% or more, which puts every dimension out of spec.

How much of my part cost is the plastic?

For a thin-wall commodity part, 15% to 25%. For a thick or glass-filled part, 35% to 50%. For a small part with a long cycle, as low as 10%. Calculate it directly: shot weight divided by cavity count, times price per pound, divided by 453.6 if you are working in grams.

What resin do most consumer products use?

ABS and PC/ABS for housings, PP for closures and anything with a living hinge, TPE for grips and seals, and nylon or acetal for moving mechanical parts. A typical handheld consumer product uses three to four materials across six to ten parts. Consolidating to two or three cuts supplier count and qualification time.

Does the resin affect how long my tool lasts?

Substantially. Unfilled ABS in a P20 tool commonly reaches 300,000 to 500,000 shots. The same tool running 30% glass-filled nylon may need cavity insert replacement at 100,000. Glass fiber abrades steel at the gate and along high-shear flow paths, which is why abrasive resins get quoted in hardened steel and why the tooling number climbs.