Plywood vs MDF vs HDPE vs Acrylic for CNC-Cut Parts

Most routed parts start as one of four sheets: plywood, MDF, HDPE or acrylic. All four cut on the same CNC router from the same file, but they behave nothing alike once the part leaves the machine — one holds a screw near its edge, one takes a flawless paint finish, one shrugs off water, one carries light. This comparison goes property by property and ends with a pick for each kind of part. Building a booth or a set? The exhibit and scenic material guide covers that work in more depth.

The four sheets at a glance

• Plywood: thin wood veneers glued with their grain alternating. Strong for its weight, good with screws and stiff across the sheet. Grades run from Baltic birch, with many thin plies and few voids, through birch and maple hardwood faces to ACX construction plywood and MDO sign board.
• MDF: wood fiber and resin pressed into a dense, even sheet. No grain and no plies — dead flat and smooth, the best paint surface of the four, and the weakest with screws and water.
• HDPE: a tough, waxy plastic, the same color all the way through. Waterproof and impact-resistant; glue and most paints won’t stick to it.
• Cast acrylic: a hard, glossy plastic that’s clear or carries light. Stiff, but brittle next to HDPE — it cracks from sharp notches and stressed holes.

Strength and screw-holding

Plywood is the structural sheet of the four. Its cross-laminated plies resist splitting, so screws, cam locks and connectors hold close to an edge, and a slot-together frame stays rigid. MDF is heavy but weak where hardware counts: screws driven into its edge strip easily, so it wants a plywood frame, glue blocks or through-bolts wherever it carries load. HDPE is tough rather than stiff — it bends and springs back instead of cracking, and holds coarse-thread screws well — but it sags over a long unsupported span. Acrylic is stiff and holds its shape in a frame, but a stressed screw hole or a sharp inside corner is where it fails.

Routed edges and detail

• Baltic birch shows a tidy stripe of thin plies, often left exposed under a clear coat. Other plywoods show their core — sometimes with a void — and usually get banding, edging or filler.
• MDF cuts a smooth, uniform edge and holds a sharp profile or a V-carved detail better than any plywood, but the edge is porous and drinks paint until it’s sealed.
• HDPE comes off the router clean and finished in its own color; a light deburr softens it for hands.
• Acrylic leaves a smooth, frosted edge with light tool marks. A glass-clear edge is a separate polishing step — ask the shop before ordering if you need one.

Whichever sheet you choose, the round bit leaves its radius in every inside corner — 1/16″ at the least, from the 1/8″ bit — so a square part that has to seat in a routed opening needs dog-bone relief. The tab-and-slot design guide shows how.

Moisture, heat and movement

Wood products move with humidity and plastics move with temperature, and Las Vegas shows off both. Wood cut in dry desert air can swell slightly when it travels to a humid venue, and plastic mounted in full sun sees a big swing between a July afternoon and the night after.

• MDF soaks up water at any exposed edge and swells for good. Keep it indoors, dry and sealed on every face.
• Plywood tolerates the odd splash. Exterior-glue grades can live outside if every face and edge is painted or sealed — exterior glue keeps the plies together, it doesn’t waterproof the wood.
• HDPE absorbs no water at all, but it grows and shrinks with heat far more than wood does: slot the mounting holes on long parts and let them float.
• Acrylic absorbs almost no water either, and it also moves with heat; leave clearance in frames and around light boxes.

Paint and finish

MDF is the paint-grade sheet: seal the edges, prime, and it finishes smoother than anything else here. Birch, maple and Baltic birch take stain and clear coats, ACX takes paint once its edges are filled, and MDO’s paper face exists to be painted. HDPE is the opposite — paint and glue barely bond to it — so it’s used in its own color; ask the shop which colors are stocked. Acrylic usually stays clear, or is painted on its back face so the front keeps its gloss, and lines engraved on the back read through the front. Sanding, painting and edge finishing aren’t part of a routing order, so plan them with your finisher.

Outdoors

• HDPE is the most weatherproof of the four, but years of sun call for a UV-stabilized grade. Ask for one and confirm it with the shop before ordering — it isn’t assumed for every sheet.
• Plywood: MDO is made for painted outdoor signs; other plywoods last outside only when every face and edge is painted or sealed.
• Acrylic: cast acrylic stands up to sun well, but mount it with slotted holes and room to expand, and expect scratches wherever people can reach it.
• MDF: not outdoors. Standard MDF swells and breaks down once it gets wet.

For a printed sign face that lives outside, Dibond aluminum composite is often a better fit than any of the four. The graphic itself is vinyl printing, quoted on request.

Weight

The plastics are the heavy ones. For the same size and thickness, acrylic is the heaviest of the four, then HDPE, then MDF, with plywood the lightest — construction plywood especially. That surprises people who expect plastic to be light, and it matters for anything hung, carried or shipped. When a big painted panel needs to lose weight, ultra-light MDF or EPS foam does more than switching between these four. For a sculpted shape rather than a panel, carved foam block is lighter still — CNC foam carving is quoted on request.

What drives the cost

The sheet’s price is only one part of what a routed part costs:

• Material: you pay for the whole sheets the parts nest onto, so the sheet’s price and how fully your parts use it both matter. Prices vary a lot between these four, and between plywood grades.
• Cutting: priced by the length the bit travels, at a rate set for each material — dense or thick sheets cut slower.
• Thickness: thicker sheets cost more to buy and take longer to cut. Use the thinnest one that’s strong enough.
• Grain matching: locking plywood parts to one grain direction can add a sheet, so it’s off by default. Turn it on only where the look or the strength needs it.

Because one file prices in any material, the quickest way to settle it is to upload once and switch the material on the part — the routing quote re-prices each time. The material library lists every sheet’s stocked thicknesses and sizes, and for a sense of scale, the pricing page prices typical Baltic birch jobs live.

Which one for your part

• Carries screws, hinges or load, or gets assembled and taken apart → Baltic birch or another plywood
• Painted, indoors and seen up close → MDF, on a plywood frame where it carries load
• Wet, washed down or knocked around → HDPE
• Clear, glowing or diffusing light → cast acrylic
• A painted sign that lives outside → MDO plywood
• A cutting board, food-contact or chemical-contact surface → HDPE in a grade made for it, confirmed with the shop
• A small part with real three-dimensional shape → FDM 3D printing for tough parts or resin for fine detail, both with an instant online quote