Injection Compression Molding — How Do You Use the Coining Process to Reduce Packing Pressure, Minimize Residual Stress and Improve Dimensional Accuracy on Your Injection Molded Parts?
Last updated: 2026-09-03 · Reading time: about 6 min
Injection compression molding — also called coining — injects into a slightly open cavity, then closes it to final thickness. The compression replaces high-pressure packing, so residual stress drops 40–70%, warpage drops 30–50%, and clamp tonnage drops 20–40%. It is the standard fix for optical, thin-wall and tight-flatness parts.
See also related article.
Key facts
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Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.
How does the injection compression process work — and what are the steps?
Four steps turn conventional injection into a hybrid process. Each one changes what the melt sees.
- Inject into an open cavity. The mold starts open by 2–5 mm (10–30% extra volume). The melt flows freely without fighting a thin gap, so injection pressure drops 30–50% versus conventional molding.
- Compress (coin) to final thickness. Once the cavity is filled or partially filled, the moving half — or a coining plate — closes to final dimensions. Compression force is applied hydraulically or by the clamp unit. Pressure is uniform across the part instead of graded from gate to far end.
- Cool and eject. Cooling runs as in conventional molding. Because the melt was compressed uniformly rather than force-packed, flow-induced orientation is minimal and shrinkage is even across the part.
- Measure the result. Residual stress drops 40–70% (photoelasticity or solvent cracking), warpage drops 30–50% (CMM), clamp tonnage drops 20–40%. Cycle time is comparable — compression adds 1–3 s.
What are the mold design requirements — and how does the mold differ from a conventional one?
The mold carries the compression mechanism. That single change ripples through cavity depth, parting line behavior and clamp tonnage.
| Feature | Conventional mold | Injection compression mold |
|---|---|---|
| Cavity depth | Final part thickness | Open position 2–5 mm thicker than final |
| Parting line | Fixed (closed position) | Moving (open → closed during compression) |
| Compression mechanism | None | Hydraulic cylinders, coining plate or toggle |
| Clamp force | Full clamp for packing | 20–40% lower (no high-pressure packing) |
| Mold cost | Baseline | 15–30% higher (moving parts + mechanism) |
Break-even sits at 100,000–300,000 parts. The 15–30% mold cost adder pays back through a smaller press ($0.01–0.03/part), less scrap ($0.02–0.05/part) and no annealing ($0.05–0.10/part). Below that volume, conventional molding with tuned packing is usually the more economical route.
“We quote injection compression when the drawing has a flatness callout conventional packing cannot hold, or when birefringence rules out an optical part. We tell you the added mold cost, the press tonnage you save and the scrap you stop making — with the numbers on the quote, not in a brochure.”
— Project Engineer, aCatMould
Frequently asked questions
What resins are suitable — and where are the limits?
Most thermoplastics work: PC and PMMA for optics, PP and PS for thin-wall containers, ABS for low-stress housings.
Amorphous grades (PC, PMMA, PS, ABS) are the easiest because they do not crystallize and shrinkage is predictable.
Semi-crystalline resins (PP, PA, POM) need the compression ratio tuned for crystallization shrinkage. Very low melt-strength grades can sag during compression — move to a higher melt-strength grade if that happens.
How do you control compression force and distance — and what if compression is uneven?
Hydraulic cylinders with pressure and position sensors, or a coining plate on precision ground stops. Force typically runs 30–100 tons; distance is the 2–5 mm gap between open and closed.
Uneven compression means uneven density and shrinkage — the part warps, sinks or shows flow marks. Hold parallelism within 0.01 mm across the parting line and apply force symmetrically.
Can it run on a standard injection molding machine?
Yes, with two conditions. The press needs a coining function — a second hydraulic circuit that controls clamp movement during compression. Most modern hydraulic and all-electric machines offer it as an option.
The mold must carry the compression mechanism. Because packing pressure is eliminated, a 500-ton conventional job can run on a 300–400 ton press, and cycle time stays comparable.
Key takeaways
- Inject into an open cavity, then compress to final thickness — compression replaces high-pressure packing.
- Residual stress drops 40–70%, warpage 30–50%, clamp tonnage 20–40%; cycle time stays comparable.
- Mold cost is 15–30% higher, but break-even is 100,000–300,000 parts on scrap, press-size and annealing savings.
Injection compression is the next step when conventional process tuning cannot hold flatness, birefringence or warpage. It costs more in steel and saves more per part — the calculation is worth running on any optical, thin-wall or tight-flatness program above a few hundred thousand shots.
Sources
Society of Plastics Engineers — compression and hybrid molding fundamentals
Plastics Technology — injection compression process practice
Kistler — cavity pressure sensing for compression control
RJG Inc — scientific molding and process control training
Autodesk Moldflow — packing and compression simulation
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet