Laminated vs Molded GPO-3 Sheets

Advantages, Disadvantages and Selection Guide

The same prepreg, two pressing processes: forming methods, cost structures and selection criteria

RDS Composite | November 2026 | Technical Information

Contents

1. Key Takeaways

2. What GPO-3 Sheet Is Made Of

3. How Laminated GPO-3 Sheets Are Made

4. How Molded GPO-3 Sheets Are Made

5. A Side-by-Side Comparison

6. Advantages and Disadvantages of Laminated GPO-3 Sheets

7. Advantages and Disadvantages of Molded GPO-3 Sheets

8. Which Process Should You Choose

9. Matching the Process to the Application

10. Five Details to Include in a Request for Quotation

11. Standards and Grade Designations

12. Frequently Asked Questions

13. Conclusion

GPO-3 sheet is a mainstay of switchgear insulation, combining UL 94 V-0 flame resistance with a comparative tracking index (CTI) of 600 V or higher. In the cost comparison considered here, its price is approximately one-third that of woven-glass epoxy sheet. Yet two quotations for “GPO-3 sheet” may describe products made by two different routes: lamination and compression molding.

Before comparing them, it is useful to address three common assumptions that can lead purchasing decisions in the wrong direction.

“Molded sheet is made from a different material.” For the two routes discussed here, it is not. Both use the same starting material: continuous glass mat impregnated with flame-retardant unsaturated polyester resin and partially cured to form prepreg.

“Molded sheets are smaller.” Not necessarily. Common molded GPO-3 sheet formats include 1010 × 2020 mm and 1250 × 2500 mm, putting them in the same size range as laminated sheets. The 1250 × 2500 mm format is even larger than a common laminated sheet format.

“Every thickness requires a separate mold.” It does not in the adjustable tooling arrangement discussed here. Sheet thickness is controlled by changing the internal cavity height, allowing one mold to cover a range of thicknesses without replacing the mold.

The practical differences lie in the cost structure created by the pressing cycle, the condition of the edges, the surface finish and dimensional consistency. These are the factors compared below.

1 Key Takeaways

The same prepreg and performance grade. Both routes use continuous glass mat and flame-retardant polyester resin. The grades compared here have UL 94 V-0 flame resistance and CTI ≥600 V.

Lamination is designed for volume production. A multi-opening press cures multiple sheets in one cycle, supporting a low cost per square meter for standard formats. Layup weight establishes the thickness, while sanding provides the final tolerance.

Molding provides near-net-shape panels. A closed mold produces one sheet per cycle in common formats such as 1010 × 2020 mm and 1250 × 2500 mm. The perimeter, both faces and edges are formed directly by the tool.

Both processes allow thickness changes. Lamination changes the layup; molding changes the cavity height. Neither requires replacement tooling for thickness changes within the supported range.

Both types are available from stock. Common thicknesses of laminated and molded sheet, including the 1010 × 2020 mm and 1250 × 2500 mm molded formats, are stocked. For stocked standard specifications, dispatch speed alone need not determine the choice.

A practical selection rule. Start with lamination for high-volume standard sheets; consider molding when edge quality, surface finish and dimensional consistency are the priorities.

2 What GPO-3 Sheet Is Made Of

GPO-3 is a glass-fiber-reinforced, flame-retardant unsaturated polyester material designated under NEMA LI 1. Its corresponding designation under IEC 60893-3-5 is UPGM 203. Within the GPO family, GPO-3 is specified when flame resistance, arc resistance and tracking resistance are required together. This combination explains its established role in switchgear, busbar systems and rail insulation applications.

Typical property ranges for the GPO-3 sheet grades discussed here are shown below. Use the supplier’s measured data sheet for the actual product, as values can differ substantially between grades.

Table 1 Typical Property Ranges for GPO-3 Sheet

Because the two pressing routes compared here use the same prepreg and layup structure, the same reference property data apply to both. The differences arise from how pressing affects the edges, surface, dimensional consistency and cost per sheet. Finished-sheet test reports remain the basis for purchase acceptance.

3 How Laminated GPO-3 Sheets Are Made

Figure 2 Laminated GPO-3 layup and pressing process

Impregnation. A continuous web of chopped-strand glass mat passes through a bath of flame-retardant unsaturated polyester resin. Woven roving may be added for mechanical grades. Metering rolls control the resin content.

B-staging. The impregnated mat is dried and partially cured to produce tack-free prepreg.

Layup. The required prepreg weight is calculated from the target thickness, and the layers are assembled into a stack or “book.” A 20 mm sheet typically requires more than 60 layers in the construction described here.

Pressing. Several books are loaded between the heated platens of a multi-opening hydraulic press. They cure together in one cycle at approximately 150–165°C and 5–10 MPa. Depending on thickness, the stated curing time is 30–90 minutes.

Finishing. All four edges are trimmed to size, both faces are sanded to thickness, and the sheets are inspected for voids, blisters and interlaminar defects.

Figure 3 Prepreg rolls stored on production racks
Figure 4 Laminated GPO-3 sheet surface and cut edge

Figure 5 Multi-opening presses in the production workshop

The defining feature of this route is production capacity: one press cycle produces many finished sheets. Layup weight sets the nominal thickness. This is why laminated sheet is often the economical starting point for standard material. The trade-off is that every edge must be cut afterward, and final thickness accuracy comes from sanding rather than pressing alone. Common laminated formats are 1020 × 2040 mm and 1220 × 2440 mm.

4 How Molded GPO-3 Sheets Are Made

Figure 6 Molded GPO-3 layup and pressing process

The same prepreg. As in lamination, continuous glass mat is impregnated with the same flame-retardant unsaturated polyester resin and partially cured to form prepreg.

Cutting to the mold footprint. Prepreg is cut to fit the mold. Common sheet formats are 1010 × 2020 mm and 1250 × 2500 mm.

Setting the cavity height. The internal cavity height is adjusted to the target sheet thickness. This is a setup adjustment rather than a mold replacement: one mold covers a defined thickness range.

Loading and closing. The layup is assembled to the specified charge weight and placed in the cavity. The mold closes and the sheet cures in one cycle at 140–165°C under molding pressure. The single-cavity arrangement produces one sheet at a time.

Demolding. When the panel leaves the mold, its perimeter, edges and both faces have already been formed by the tool. Only minor finishing is required.

Figure 7 Panel inside a flat compression mold
Figure 8 Stacked molded GPO-3 sheets

Figure 9 Hydraulic compression molding press

The defining feature is the mold itself. It controls the outer dimensions, forms compact edges under pressure, removes the need for subsequent perimeter cutting and transfers its surface finish to both faces. The trade-off is output per cycle: a single-cavity mold produces one sheet, compared with multiple sheets in a multi-opening laminating press.

5 A Side-by-Side Comparison

Table 2 Laminated and Molded GPO-3 Sheets Compared

6 Advantages and Disadvantages of Laminated GPO-3 Sheets

The strengths of lamination are production economies and thickness flexibility. Its limitations concern downstream finishing and control of consolidation. Each is considered below.

Advantages

Economies of scale. A multi-opening press cures several sheets in a single cycle, giving standard formats a somewhat lower cost per square meter.

Thickness controlled through layup. The production range described here is 0.5–100 mm, with thickness changed by adding or removing layers rather than changing tooling. The actual qualified range should be confirmed for the supplier and grade.

Accurate thickness after sanding. Double-sided sanding provides a consistent tolerance across the sheet, which matters for automated punching or CNC nesting. After sanding, tolerance depends on the sanding equipment rather than whether the sheet was laminated or molded.

Large standard formats. Sheets measuring 1020 × 2040 mm and 1220 × 2440 mm allow wide barriers to be made without joints.

Stocked standard sizes. Common thicknesses in 1020 × 2040 mm and 1220 × 2440 mm formats are routinely stocked by many suppliers and distributors.

An established reference route for UPGM 203. The IEC designation and its published performance framework relate to rigid laminated sheet, making lamination an established reference process for this material.

Disadvantages

Edge trimming is required. Every sheet must be cut to final dimensions after pressing. Cut edges expose glass fibers, which may provide moisture-entry paths in humid conditions. Edge sealing is often specified where the application requires it.

Custom outlines create cutting waste. Cutting nonstandard shapes from a large sheet can produce the 10–30% nesting loss described in this process comparison. The exact loss depends on the layout. It may not appear as a separate line in the quotation, but it affects the margin on the finished part.

The surface depends on the platens and sanding. Appearance surfaces may need sanding. Precision thickness comes from the finishing operation rather than pressing alone.

Layup interfaces remain in the material. The Z direction is the weakest direction in the sheet construction described here. Poor press control can result in blisters, voids or delamination. The layered construction is a material consideration, while the occurrence of these defects also reflects supplier quality.

Thick sheets need additional attention. Above 50 mm, cure exotherm and resin shrinkage can increase the risk of a porous core and weak interlaminar regions. This applies to either process. Full cross-section inspection is recommended.

Consolidation can vary with the cure and pressure schedule. The staged heating and gradual pressure application described for lamination suit step-growth epoxy curing systems. By contrast, the unsaturated polyester matrix in UPGM 203 cures by a free-radical chain reaction. In the process comparison presented here, the shorter gel window leaves less time to apply pressure, increasing the risk of variation in material compactness. The result must be assessed against the supplier’s actual resin system and cure schedule.

7 Advantages and Disadvantages of Molded GPO-3 Sheets

Molding offers advantages in forming quality and consistency between batches. Its limitations are flexibility in panel footprint and output from a single press. The individual considerations are set out below.

Advantages

Sheet sizes are comparable. The 1010 × 2020 mm and 1250 × 2500 mm formats are in the same size range as laminated sheets. The latter is larger than the common 1220 × 2440 mm laminated format. Thick molded panels still need assessment for edge cracking under load or at elevated temperature.

Near-net dimensions. The mold defines the outline, so a matching panel can leave the tool at its final external dimensions without perimeter cutting or the associated cutting loss. Sheet-to-sheet dimensions are highly consistent. Inserts or additional tooling within the mold can also be used to produce specific panel sizes.

Compact molded edges. The perimeter is formed under pressure rather than sawn through. This avoids exposed cut-fiber bundles at the molded edge and reduces potential moisture-entry paths.

A mold finish on both faces. The tool finish transfers directly to the panel surfaces. Appearance faces and surfaces defining electrical clearances may need no further machining when the as-molded finish meets the drawing.

Thickness flexibility without replacing the mold. Adjusting the cavity height covers a defined range instead of restricting the tool to a single thickness.

Very little cutting waste. Charge weight is matched to panel weight. When the molded perimeter is the required final perimeter, there is no saw kerf or nesting waste from cutting the panel out of a larger sheet.

Standard formats are also stocked. Common thicknesses in 1010 × 2020 mm and 1250 × 2500 mm are held in stock in the supply arrangement described. For stocked standard formats, dispatch speed need not differ from laminated sheet.

The same specified electrical and flame performance. With the same prepreg system, the grades compared here target the same electrical and flammability performance, including UL 94 V-0 and CTI ≥600 V. Certification and test evidence must apply to the actual finished grade ordered.

Controlled material consolidation. The molding route described uses a constant tool temperature and applies full pressure in one step. This is well suited to the unsaturated polyester matrix of UPGM 203 in the comparison presented. The closed tool contains the charge and reduces the problems of material escape or layup movement, allowing full pressure to be applied immediately and supporting a more compact sheet. Actual density and void content still depend on the qualified process.

Disadvantages

One sheet per cycle. A single-cavity mold produces fewer sheets per cycle than a multi-opening press, so standard sheet cost is usually higher in this comparison. However, the lack of repeated tool heating and cooling and subsequent sheet cutting means overall output is not necessarily lower.

The footprint is tied to the mold. Stock covers standard formats, and the panel outline follows the available tooling. A new outline requires a new or modified mold and the associated lead time.

Thickness tolerance depends on tooling accuracy. An adjustable cavity offers flexibility, but final accuracy depends on mold condition and charge control. Critical panels may require light machining.

Supplier process discipline matters. Charge consistency and uniform mold temperature influence void content and flatness. Qualify the molding operation itself rather than relying only on a data sheet.

Figure 10 Palletized sheet shipment
Figure 11 Sheet inventory in the warehouse

8 Which Process Should You Choose

Table 3 Process Selection Guide

The economic question is whether you are buying sheet area or a finished outline. For high-volume standard sheet purchased by the square meter, lamination benefits from producing many sheets in one cycle. For a specific outline with compact edges, finished faces and consistency between batches, molding can take over work otherwise done by the saw and sanding machine. Savings in material and finishing time can then pay for the tooling.

9 Matching the Process to the Application

Laminated GPO-3 sheets suit arc barriers and phase barriers, switchgear covers and panels, terminal boards, thin slot and coil insulation where suitable for the design, transformer insulation components, large busbar barriers and standard blanks for general machining.

Molded GPO-3 sheets suit the same applications: arc and phase barriers, switchgear covers and panels, terminal boards, thin slot and coil insulation where suitable, transformer insulation components, large busbar barriers and general machining blanks. Additional reasons to select molded sheet include exposed insulation with specified edge and appearance requirements, panels requiring consistent assembly dimensions across batches, near-net blanks that avoid excessive sawing waste, and 1250 × 2500 mm panels with both faces supplied without further machining.

Both types often appear in the same switchgear design. Laminated sheet supplies standard barriers and machining stock; molded sheet supplies repeated panel outlines. This assigns each process to the work it performs most effectively.

Figure 12 Switchgear insulation application
Figure 13 Insulation components around a busbar assembly

10 Five Details to Include in a Request for Quotation

Whether you buy laminated or molded sheet, a vague inquiry produces a vague quotation. Include these five details.

1. Process requirements. State whether both routes are acceptable and whether the perimeter must be near-net molded. The latter is a key reason to select molding.

2. Material standard. Specify NEMA LI 1 GPO-3. Add UPGM 203 where the drawing follows the IEC system.

3. Thickness and tolerance. Nominal thickness alone is insufficient. A tolerance of ±0.1 mm and one of ±0.3 mm describe different requirements at different prices. For molded sheet, state whether light machining is permitted.

4. Format and edge condition. Give the outer dimensions, edge-quality requirements and whether the faces require precision finishing. These factors influence the process route as much as thickness does.

5. Electrical requirements and demand. Specify dielectric strength, CTI, arc resistance, required certificates and annual volume. Demand determines whether the unit economics of molding are attractive.

11 Standards and Grade Designations

Table 4 Standards and Corresponding Designations

Finished sheets from either route must meet the same specified performance requirements. Qualified suppliers can provide reports for laminated and molded sheets against the relevant NEMA GPO-3 and UPGM 203 requirements. Because the two routes described use the same raw materials and layup construction, the same IEC performance framework applies. Whichever pressing method is used, request the test report for the sheet you actually buy.

Do not confuse GPO-3 with G-10 or FR-4. These are woven-glass epoxy materials. In the comparison presented, they offer higher mechanical strength and better dimensional stability, but cost approximately two to three times as much, while their tracking resistance is often lower. For the medium-voltage insulation conditions considered here, GPO-3 offers a favorable balance of performance and cost. Compare the actual grades and current quotations before substitution.

12 Frequently Asked Questions

The following ten questions cover common purchasing and design issues.

1 What is the practical difference between laminated and molded GPO-3 sheets

The two routes described use the same prepreg: continuous glass mat impregnated with flame-retardant unsaturated polyester resin. Laminated sheet is made by stacking layups in a multi-opening press, curing several sheets per cycle, and then cutting and sanding them. Molded sheet is cured one panel at a time in a closed flat mold that directly forms the perimeter, edges and both faces.

2 What sizes are molded GPO-3 sheets available in

Common formats include 1010 × 2020 mm and 1250 × 2500 mm. These are in the same size range as common laminated formats of 1020 × 2040 mm and 1220 × 2440 mm. The 1250 × 2500 mm molded format is larger than the latter laminated format.

3 Does every thickness require a separate mold

No, not within the adjustment range of the mold described. Thickness is controlled by changing the internal cavity height, allowing one mold to cover a range of thicknesses without replacement.

4 Are both types available from stock

Yes, in the standard supply arrangement described. Common thicknesses of 1020 × 2040 mm and 1220 × 2440 mm laminated sheet and 1010 × 2020 mm and 1250 × 2500 mm molded sheet are stocked. Inventory depth varies by format and thickness. Confirm the actual range available before ordering rather than assuming that only one process produces stock items.

5 Which type costs less

For high-volume standard formats, lamination benefits from producing several sheets per cycle. Molded sheet has a higher sheet cost in the comparison presented, but removes perimeter trimming, nesting loss and many downstream operations. For repeatedly produced formats, the difference can narrow substantially or reverse.

6 Is molded GPO-3 stronger or weaker

Neither follows from the process name alone. With the same prepreg and layup, the grades compared are specified for equivalent flexural strength, dielectric strength, CTI, arc resistance and UL 94 V-0 classification. The main comparison concerns edges and surfaces rather than a change of material grade. Verify the finished-sheet results for each supplied product.

7 Do both types have a layered structure

Yes. Both routes described stack prepreg layers, so they retain the same layered construction and through-thickness considerations. For thick sheets made by either route, request cross-section inspection records.

8 Can laminated GPO-3 sheets delaminate

A properly controlled process should produce sheets free of manufacturing delamination. Where delamination occurs, investigate inconsistent resin content, moisture in the prepreg or incomplete curing. It reflects manufacturing quality rather than an unavoidable feature of the material. For thick sheet in particular, request cross-section photographs and void-inspection records.

9 Can both types be used in the same project

Yes. Many switchgear manufacturers use laminated sheet for standard barriers and machining stock, with molded sheet for repeated panel outlines. Specify the process by part number rather than leaving it unresolved.

10 What IEC grade corresponds to GPO-3

UPGM 203 under IEC 60893-3-5. Because the two routes described use the same prepreg and layup, the relevant performance requirements apply to both. Use the supplier’s test report for the actual sheet as the basis for acceptance.

13 Conclusion

Choosing between laminated and molded GPO-3 sheet is principally a process decision. Both routes cover comparable sheet sizes, support thickness changes without replacing tooling within the available range, and offer stocked standard specifications in the supply arrangement discussed.

The most important differences in this process comparison fall into three areas.

Table 5 Main Differences Between the Two Pressing Routes

Specify the process clearly on the drawing. With the appropriate grade and manufacturing controls, GPO-3 can continue to provide reliable insulation as it has for decades at a cost that woven-glass epoxy sheet may not match in the applications compared.