PC vs PMMA for Automotive Lighting: Comparing Clarity, Heat Resistance and Injection Molding

Q: Is "PC for the outer lens, PMMA for inner parts" a fixed rule?

No. Both PC and PMMA are used in automotive lighting. The choice depends on part location, optical requirements, heat resistance, weathering, surface treatment and the customer's material specifications.

Q: PMMA shows 92% transmittance and PC shows 89% on the TDS. Does that mean PMMA is clearer?

Not on that basis. The two datasheets use different transmittance test methods, so the values are only background for candidate grades. Actual clarity must be confirmed against the part's acceptance conditions.

Q: MVR is 34 for PC and 3 for PMMA. Does PC flow much better?

Not on that basis. The two MVR values were measured at different temperatures and loads, so they cannot be used to rank flowability.

Q: Once PC or PMMA is chosen, is the mold design settled?

Not yet. Material data must be reviewed together with part geometry, mold structure, molding conditions and trial planning. Multi-shot parts also need mold flow analysis and mold trial to verify interfaces and shrinkage differences.

Q: How is shrinkage compensated when the TDS gives no shrinkage value?

It should not be estimated. Obtain data for the specified grade from the material supplier, or list shrinkage compensation as a mold trial item and correct it based on actual measurement.

6 days ago

6 min read

The short answer: the PC grade Makrolon® LED2245 and the PMMA grade PLEXIGLAS® 8N both list optical or lighting-related applications in their official technical datasheets (TDS), but their typical values and recommended molding conditions differ. When tooling a lamp part, neither "PC" nor "PMMA" is a universal answer. The specified grade, test conditions, part drawing and acceptance basis should all be brought into the DFM discussion together.

This article compares only the typical values in the two official TDS for Covestro Makrolon® LED2245 and Röhm PLEXIGLAS® 8N. It does not represent all PC and PMMA materials, and it is not a part-level material selection conclusion.

Key Points

Both grades are candidates worth evaluating, but neither is automatically suitable for an outer lens, tail lamp lens, light guide or inner lens.
The two datasheets measure light transmittance differently, so 89% and 92% cannot tell you which is clearer.
The two datasheets measure MVR at different temperatures and loads, so 34 and 3 cannot tell you which flows better.
Vicat and HDT are useful as a first check on heat resistance, but they do not replace part validation.
Material data should be aligned before DFM, multi-shot station decisions, machining planning and mold trial.

Which Values Can Be Compared, and Which Cannot?

Property	PC: Makrolon® LED2245	PMMA: PLEXIGLAS® 8N	How to Read It
Listed applications	Injection molding, light guides, optical lenses, optics	Automotive, lighting, lenses and optical applications	Both are candidate grades, not a recommendation for a specific part
Light transmittance	>89% (3 mm; SCI, D65/10°; ISO/CIE 11664)	92% (D65; ISO 13468-2; specimen thickness per original TDS)	Different test methods; not a same-condition ranking
Haze	<0.5% (3 mm; C/2°; ASTM D1003)	<0.5% (ASTM D1003)	Both below 0.5%, valid only for each specimen and condition
Vicat B50	145°C (50 N, 50°C/h; ISO 306)	108°C (ISO 306/B50)	A first comparison of heat data, not finished-part or regulatory results
HDT (1.8 MPa)	125°C (ISO 75-1,-2)	98°C (ISO 75-2)	Revision and full conditions must still be checked against project documents
MVR	34 cm³/10 min (300°C / 1.2 kg; ISO 1133-1)	3 cm³/10 min (230°C / 3.8 kg; ISO 1133)	Different temperature and load; flowability cannot be ranked by these numbers
Molding shrinkage	0.65% parallel / 0.70% normal (60×60×2 mm; 500 bar; ISO 294-4)	Not listed in the public TDS	Not filled in or estimated; shrinkage compensation must be verified per grade and part

So this table does not support general conclusions such as "PMMA is clearer than PC" or "one grade is always easier to mold."

Clarity and Heat Data Are Only the First Round of Material Questions

If a project cares about transparent appearance or optically relevant zones, first confirm which transmittance, haze or appearance condition the drawing requires, then check the TDS specimen thickness, light source, geometry and test method. If those conditions don't match, TDS values serve only as background and cannot replace acceptance results on the actual part.

Vicat and HDT show how the two grades differ in heat resistance under their own test methods, but they cannot support claims about weathering, regulatory compliance or lamp service life. Whether a material suits an outer lens, tail lamp lens, light guide or inner lens is still decided by part function, drawings, applicable regulations and acceptance documents. Our PMMA, PC and ABS automotive lighting material FAQ is a starting point; the specified grade should then be checked against project documents.

材料數據須連同測試條件、零件圖面與多色成型界面，轉成可追溯的DFM與試模驗收資料。

Material Data Belongs in DFM, Not on a Separate Spec Sheet

Draft, Wall Thickness and Appearance Zones

The drawing should clearly mark appearance or optically relevant zones, parting-line constraints, acceptable geometry changes and the molding conditions tied to the material. This lets the design side evaluate draft, wall thickness and structural interference. Fixed angles and dimensions are decided per part; detailed review items are covered in our automotive lighting mold DFM review.

Drying, Melt and Mold Temperature Start From Different Points

The recommended starting points in each TDS are:

LED2245: dry at 120°C for 2–3 hours; melt 250–300°C; mold 80–120°C.
8N: dry at a maximum of 98°C for 2–3 hours; melt 220–260°C; mold 60–90°C.

These are the material supplier's starting recommendations and must still be validated against the part, mold, machine, colorant and project conditions. Validation conditions and methods should be written into the project record so they can be compared at mold trial.

How Howmin Turns Material Data Into Mold Design

Howmin does not decide which grade a customer should use. What we do is turn the molding conditions of the customer's specified grade into design decisions the mold side can execute:

Shrinkage compensation: cavity dimensions are compensated based on the specified grade's shrinkage data and part geometry. Where a TDS gives no value, we do not estimate one; we list it as an item to verify at mold trial.
Flow and cooling: before cutting steel, we run Moldex3D mold flow analysis to review filling, weld-line locations and cooling layout, moving risk forward into the design stage.
Multi-shot interfaces: Howmin has made red / clear / amber multi-shot tail lamp lens molds. In our experience, the shrinkage difference and interface compatibility between two materials cannot be judged from a single grade's TDS. They must be verified with mold flow analysis plus mold trial.

Multi-Shot: Confirm Interfaces and Station Count Before Material Pairing

In a multi-shot lamp project, beyond each material's data, confirm whether each material's interface, molding sequence and station count meet the drawing and acceptance requirements. Two single-grade datasheets cannot prove the bonding result or multi-shot production quality of a PC/PMMA combination. 2K, 3K and 4K are mold architecture decisions, not something the words "PC" or "PMMA" decide. For how to choose a station count, see our guide on choosing 2K/3K/4K automotive lighting molds.

At this stage, list what is still unconfirmed: whether the material grade is fixed, whether approved material-pairing data exists, which interfaces or appearance zones need verification, and who updates the drawing and conditions after a change.

Material Data Does Not Decide the Machining Method

Material data affects design review and trial planning, but it does not directly decide how the mold is machined. 5-axis machining, deep-hole drilling, EDM and die spotting each have their role; whether to use them, where, and how to verify them depends on mold structure and drawings. For the overall flow, see our automotive lighting mold development process; for machining differences, see our comparison of 5-axis and 3-axis machining.

What Should the Material Supplier, Molder and Mold Maker Align on Before Mold Trial?

Material grade, lot and supply status, and the original TDS available.
The source, revision and recording method for drying and molding conditions.
The appearance, dimensional, assembly and optically relevant items to verify on the drawing, and the corresponding acceptance documents.
Responsibility for multi-shot interfaces, change records and issue feedback.
The samples, condition records and judgment method to compare after trial.

This is a list of questions for communication; actual values, test methods and responsibilities follow the project documents.

FAQ

Is "PC for the outer lens, PMMA for inner parts" a fixed rule? No. Both PC and PMMA are used in automotive lighting. The choice depends on part location, optical requirements, heat resistance, weathering, surface treatment and the customer's material specifications.

PMMA shows 92% transmittance and PC shows 89% on the TDS. Does that mean PMMA is clearer? Not on that basis. The two datasheets use different transmittance test methods, so the values are only background for candidate grades. Actual clarity must be confirmed against the part's acceptance conditions.

MVR is 34 for PC and 3 for PMMA. Does PC flow much better? Not on that basis. The two MVR values were measured at different temperatures and loads, so they cannot be used to rank flowability.

Once PC or PMMA is chosen, is the mold design settled? Not yet. Material data must be reviewed together with part geometry, mold structure, molding conditions and trial planning. Multi-shot parts also need mold flow analysis and mold trial to verify interfaces and shrinkage differences.

How is shrinkage compensated when the TDS gives no shrinkage value? It should not be estimated. Obtain data for the specified grade from the material supplier, or list shrinkage compensation as a mold trial item and correct it based on actual measurement.

Conclusion: Turn Material Questions Into Verifiable Mold Inputs

The PC vs PMMA discussion should first narrow down to a traceable, specified grade. LED2245 and 8N show identifiable differences in transmittance, heat resistance and molding starting points, but these cannot be extended into a general material ranking or a part-suitability conclusion. For multi-shot lamp molds, the practical path is to organize material and drawing data first, then close each open item through DFM, architecture decisions, machining planning and mold trial records.

If you are preparing lamp part drawings, material grades or multi-shot mold requirements, talk to Howmin. We will work from your actual data to confirm how material conditions translate into mold design and trial verification.

References

Covestro, Makrolon® LED2245 Technical Datasheet
Röhm GmbH, PLEXIGLAS® 8N Product Information
Howmin Co., Ltd., Mold Material FAQ: PMMA, PC, ABS and Aftermarket Lamps
Howmin Co., Ltd., What Is a DFM Review? The Cost Key Before Tooling an Automotive Lighting Mold
Howmin Co., Ltd., How to Choose a Multi-Shot Lamp Mold: A 2K/3K/4K Station Guide
Howmin Co., Ltd., Automotive Lighting Mold Development Process
Howmin Co., Ltd., The Advantages of 5-Axis Machining Compared With 3-Axis CNC