Acrylic (PMMA) Injection Molding
Acrylic is the material for clear parts at volume, and moulding it is a different job from moulding an opaque plastic. Shrinkage is low, which is an advantage. Drying is critical, because moisture in an acrylic melt leaves silver streaks that are painfully visible through a transparent part. The tool has to be polished to optical standard. And the material is brittle, so the part geometry has to avoid the sharp corners and abrupt section changes that a tougher plastic would tolerate. Get those four things right and moulded acrylic delivers clarity, hardness and a low price per part.
Specification at a glance
- Material family
- Amorphous thermoplastic — polymethyl methacrylate
- Melt temperature
- Roughly 210–250 °C
- Mould temperature
- Roughly 60–90 °C
- Shrinkage
- Roughly 0.2–0.6% — the lowest of the common moulding plastics
- Drying
- Critical — around 80–100 °C for several hours. Undried material gives splay and silver streaks
- Wall thickness
- Typically 1.5–4.0 mm
- Draft angle
- 0.5–1° minimum; more generous than ABS because the material is brittle
- Mould finish
- Optical polish required for transparent parts — clarity is a tool property as much as a material one
- Impact behaviour
- Brittle and notch sensitive — sharp corners and abrupt section changes crack
- Regrind
- Usable in controlled proportions; must be dried like virgin material
- Tooling
- Ordinary mould steels are adequate; the tool needs a high polish and careful thermal design
- Service temperature
- ~80 °C
- Molding route
- Through our vetted partner network, managed under one quality process; metal inserts machined on our own floor
- Mould tooling
- Quoted per drawing — mould size, shot weight, gate count and polish standard reviewed before the tool is cut
- MOQ
- From 50 pieces, samples from a single piece
Moulding a transparent part is a different job
The process parameters for acrylic are unremarkable compared with ABS or polycarbonate. What makes it a different job is that every process defect is visible.
In an opaque material, a flow mark is a surface finish issue that may pass inspection. A weld line is hidden unless it is on a visible face. Splay is a cosmetic defect that a buyer may accept on a non-cosmetic part. In a transparent part, all three are immediately obvious from any angle, and none of them can be polished out — they run through the material.
That single fact rearranges the priorities:
| Priority | Why it matters more here |
|---|---|
| Drying | Moisture leaves silver streaks that are visible through the part |
| Mould polish | The cavity surface becomes the part surface |
| Gate position | The gate mark, flow pattern and weld line are all visible |
| Wall uniformity | Uneven cooling produces visible optical distortion, not just a sink mark |
| Material grade | Optical grades exist, and a general-purpose grade will not match them |
Low shrinkage is the compensation
Acrylic is amorphous, so there is no crystalline phase forming during cooling and pulling the part differentially. Shrinkage lands around 0.2–0.6% — the lowest of the common moulding plastics, and roughly a third of ABS’s.
That buys two things:
A first tool lands very close to the drawing. Correction cycles on the tool are fewer, which is worth real money on a part with a polished optical cavity, because reworking a polished cavity is expensive and risky.
Dimensional repeatability from shot to shot is good. For a clear part with a mounting flange, a snap feature or a sealing groove, that repeatability is the difference between an assembly that works on every part and one that works on most of them.
Brittleness sets the design rules
Acrylic is brittle and notch sensitive, and that changes the geometry rules compared with a tougher plastic.
Radius every corner generously. A sharp internal corner concentrates stress, and in a brittle material that concentration becomes a crack. There is no yielding to relieve it, which is exactly the difference between acrylic and ABS or polypropylene.
Keep the wall uniform and avoid abrupt section changes. A sudden change in thickness creates a stress concentration as the part cools, and it is a common origin of cracking during ejection or in service.
Use more draft than you would on ABS. A brittle part that grips the tool is more likely to crack on ejection than a tough one. 0.5–1° is a minimum; more where the surface is textured.
Do not design thin, unsupported features that will flex. Acrylic does not flex — it breaks. Snap features work in ABS and polypropylene precisely because those materials yield. In acrylic the same feature cracks, so a snap or a living hinge is the wrong design for this material and belongs in a different plastic.
Radius drilled and moulded holes at their edges. A moulded hole with a sharp edge is a stress raiser, and holes are where acrylic parts most often crack.
Gate position is a cosmetic decision
Every gate leaves a mark — a gate blush, a flow pattern, or a weld line where two flow fronts meet. In a transparent part all of them are visible.
So the gate is placed where the marks will not be seen: behind a flange, under a mounting surface, at an edge that an assembly covers, or at a location the design naturally conceals.
This is decided at tool design stage, which means the drawing has to say which surfaces are visible and which are not. That is a small addition to the drawing and a large saving on the tool, because relocating a gate after the cavity is polished is a rework operation on an expensive part of the tool.
The same reasoning applies to the wall and the cooling layout: uneven cooling produces visible optical distortion through a transparent part, not merely a sink mark on a hidden face. Cooling design on an optical tool is a quality parameter, not just a cycle-time one.
Molding route
Plastic molding runs through our vetted partner network, managed under one quality process, while metal inserts and machined features are done on our own floor in Qidong City, Jiangsu Province, China. Mould tooling is quoted per drawing, and on an acrylic part the review includes mould size, shot weight, gate count and the polish standard of the cavity — because on a clear part the polish is part of the specification rather than a finishing detail.
Moulding holds part-scale tolerances agreed per drawing in DFM; it does not match machined metal. Where a clear part needs a precision bore or a thread, that feature is normally carried by a machined insert rather than by the plastic.
Where molding acrylic is the right choice
- Light covers, lenses and light guides produced in volume
- Display and signage components — see enclosures and housings
- Transparent covers and housings with mounting features
- Cosmetic panels and bezels where clarity and hardness both matter
- Instrument windows and inspection covers
- Fluidic and microfluidic components where the flow has to be visible
- Complex transparent geometry that would need machining and bonding if made from sheet
- Any clear part whose volume justifies a polished tool
Where molding acrylic is the wrong choice
The part has to absorb impact. Acrylic is brittle and shatters. Polycarbonate is the impact plastic, at the cost of clarity and scratch resistance.
The service temperature is above roughly 80 °C. Acrylic softens and distorts; polycarbonate holds up to about 120 °C.
The part needs a snap fit, a living hinge or any flexing feature. Those designs rely on the material yielding, and acrylic does not. Polypropylene is the material for living hinges; ABS for snap features.
The part meets alcohol-based cleaners or solvents. Alcohols, ketones, esters and chlorinated solvents attack acrylic and cause crazing, and alcohol wipes are the default cleaner in many workplaces.
The volume does not justify a tool. Below the break-even point, machining from sheet is cheaper, and simple flat parts are usually cut rather than moulded regardless of volume.
The part is outdoors long-term and unprotected. Acrylic has better weather resistance than most plastics but it can chalk and lose clarity over years; a UV-stabilised grade is the specification.
What to put on the drawing
| Instead of | Write |
|---|---|
| Plastic | Acrylic (PMMA), for a clear part at volume |
| Acrylic, transparent part | Say which surfaces are visible — it decides gate position and polish standard |
| Acrylic, optical requirement | State it — the cavity polish and the material grade both follow from it |
| Acrylic, sharp internal corners | Radius them — acrylic is brittle and does not yield |
| Acrylic, snap fit or living hinge | Wrong design for this material — use ABS or polypropylene |
| Acrylic, cleaning regime | Name it — alcohol-based cleaners cause crazing |
| Acrylic, service temperature | State it — the limit is around 80 °C |
| Acrylic, low volume or still changing | Machine it instead — tooling is a fixed cost |
The three lines that decide most acrylic moldings are which surfaces are visible, how the material is dried and run, and whether the geometry relies on the part flexing. The first is a drawing note that shapes the tool. The second is the process discipline that produces clarity. The third decides whether acrylic is the right material at all — and on a part with a snap feature, it is not.
Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the polish standard, the gate position, the wall and draft, and a comparison against machining the same part where the volume is borderline.
Frequently asked
- What is acrylic injection molding used for?
- Clear and coloured transparent parts at volume: light covers, lenses and light guides, display and signage components, transparent housings and covers, cosmetic panels, instrument windows, and fluidic or microfluidic components where the flow has to be visible. It is also used for opaque parts where hardness and scratch resistance matter more than impact strength.
- Why must acrylic be dried before molding?
- Because PMMA is hygroscopic and picks up moisture from the air, and moisture in the melt turns to steam and leaves splay — silver streaks and surface defects. In an opaque material splay is a cosmetic problem; in a transparent part it is a rejection, because a streak is visible from any angle and cannot be polished out. Drying at around 80–100 °C for several hours, with the dried material held in a dryer hopper, is the standard control, and on an optical part it is a process parameter rather than a precaution.
- How does mould polish affect an acrylic part?
- Directly and permanently. In a transparent part the mould surface becomes the part surface, so every scratch, polish mark or imperfection in the cavity is reproduced on every part the tool makes. That is why an optical acrylic tool is polished to a much higher standard than a general-purpose tool, and why tool handling and maintenance matter more on this material than on almost any other. The investment in the polish is what the clarity is bought with.
- Why is acrylic's shrinkage so low, and does that help?
- Because it is amorphous — there is no crystalline phase forming during cooling and pulling the part differentially. Shrinkage lands around 0.2–0.6%, the lowest of the common moulding plastics. The benefit is twofold: a first tool lands very close to the drawing, and dimensional repeatability from shot to shot is good. For a clear part with a mounting flange or a snap feature, that repeatability is worth real money, because those features have to work on every part from the first to the millionth.
- What design rules matter most for moulded acrylic?
- The rules that come from brittleness. Radius every corner generously, because a sharp internal corner in a brittle material is a crack waiting to start. Keep the wall uniform and avoid abrupt section changes, which create stress concentrations as the part cools. Use more draft than you would on ABS, because a brittle part that grips the tool is more likely to crack on ejection than a tough one. And avoid designing thin, unsupported features that will flex, because acrylic does not flex — it breaks.
- Can moulded acrylic be as clear as cast acrylic sheet?
- It can be very close, with the right tool and process — a highly polished cavity, correctly dried material, and a gate position that puts flow marks where they cannot be seen. What moulding adds is the ability to produce complex geometry and features that would have to be machined or bonded into a sheet part, and to do it at a low piece cost. Where absolute optical perfection is required and the geometry is simple, cast sheet remains the benchmark. Where the part is complex and the volume is high, moulding is usually the better route.
- How should the gate be positioned on a transparent acrylic part?
- Where the marks it leaves will not be seen. Every gate leaves something — a gate mark, a flow pattern, a weld line where two flow fronts meet — and in a transparent part all of them are visible. So the gate is placed behind a flange, under a mounting surface, at an edge that is hidden in assembly, or wherever the design provides cover. This is decided at the tool design stage, which means the drawing needs to say which surfaces are visible and which are not.
- What is the service temperature of moulded acrylic?
- Around 80 °C — the same as the machined material, and lower than polycarbonate's 115–120 °C. It is adequate for indoor, display and light-fitting service and inadequate near a heat source, a lamp with poor ventilation, or a dark surface in direct sunlight. Where a transparent part has to run hotter, polycarbonate is the material, at the cost of lower clarity and higher scratch sensitivity.
- Does moulded acrylic scratch?
- Yes, and more easily than glass. Acrylic is harder than most plastics — which is why it is chosen over polycarbonate for visible parts — but it is still much softer than glass and it marks in handling, packaging and cleaning. That matters most on a clear part, because a scratch is permanent. Protective film until the last possible moment, soft packaging, and cleaning with a soft cloth and mild soap rather than anything abrasive or solvent-based are the standard controls. Hard-coated grades are available where scratch resistance is a specification.
- When should an acrylic part be moulded rather than machined?
- Moulding wins on complex geometry at volume — a lens with a mounting flange, a light guide with an integrated boss, a cover with snap features. Machining wins for prototypes, low volumes, simple flat parts cut from sheet, and design iterations. There is also a quality difference worth knowing: a machined acrylic part needs annealing after cutting to relieve stress, whereas a well-run moulded part has less locked-in stress to begin with. Our [machined acrylic page](/materials/acrylic-pmma-cnc-machining/) covers that route.
Related pages
Acrylic (PMMA) in other processes
Other Engineering Plastics materials
Have a part in Acrylic (PMMA)?
Send the drawing, the grade and the quantity. You get DFM feedback and a price within 24 hours, and orders start at 50 pieces per style and material.