Our PMI (polymethacrylimide) foam is a closed-cell rigid foam plastic with the following key properties:
Excellent specific strength and stiffness, ideal for lightweight sandwich structures.
Outstanding heat resistance, withstanding processing temperatures up to 180–190°C and compatible with a wide range of resin systems.
Good solvent resistance, compatible with most commercial adhesives.
Reversible moisture absorption: Environmental moisture diffuses into the foam (physical plasticization), but drying fully restores original properties.
Storage and Handling
Storage Conditions: PMI foam should always be stored in a dry environment, avoiding direct exposure to high-humidity air.
Moisture-Proof Packaging: For pre-dried or heat-treated materials (e.g., -HT series), factory packaging uses diffusion-proof aluminum bags. If not fully used after opening, reseal the bag to protect remaining material.
Consequences of Moisture Uptake: Moisture absorption causes slight dimensional expansion, reduced mechanical properties (especially creep resistance), and when processed above 100°C, internal steam formation can severely damage the bond between core and skins.
Pretreatment: Drying and Heat Treatment
Depending on the subsequent processing temperature, appropriate pretreatment must be selected. All pretreatments are best carried out in a circulating-air oven to ensure temperature uniformity.
1. Drying (for processing temperatures > 100°C)
Purpose: Remove physically absorbed moisture to prevent steam defects during high-temperature processing.
Process Parameters:
Temperature: 130°C
Time: Minimum 3 hours (extend for sheet thickness exceeding 25.4 mm)
Heating rate: 2–3°C/min
Spacing: Sheets should be separated by at least 35 mm to ensure air circulation.
Reversibility: After drying, the material regains its original mechanical properties and dimensions. If not used immediately, store in moisture-proof bags.
2. Heat Treatment (for processing temperatures > 150°C or requiring maximum creep resistance)
Purpose: Further enhance the foam's compression creep resistance at high temperatures, adapting it for processing up to 190°C.
Two-Step Process:
Step 1: Drying (135°C, minimum 4 hours) – remove moisture.
Step 2: Heat Treatment (immediately after Step 1, hold at 150–180°C for 48 hours).
Important Notes:
Heat treatment causes minor changes in volume and surface area; therefore, all final machining (e.g., milling to final dimensions) must be performed after heat treatment.
Heat-treated sheets must also be sealed in moisture-proof aluminum bags.
Out-Time Management: The allowable accumulated time out of the bag for processing is limited. If exceeded, the material can be re-dried (but with caution for machined parts due to potential dimensional changes). Heat-treated formed parts cannot be re-heat-treated.
Shaping Processes
PMI foam can be shaped into desired geometries through both machining and thermoforming.
1 Machining
Applicable Methods: Drilling, planing, milling, sawing, sanding, etc.
Equipment Requirements: Standard wood or plastic processing machines with high-speed operation can be used.
Cooling/Lubrication: No lubricants are required; machine dry.
Dust Management: All generated dust must be removed by vacuum suction to prevent contamination and ensure subsequent bonding quality.
Custom Services: We can supply fully machined, ready-to-use cores according to your drawings, with quality-assured support from prototype to mass production (ISO 9001:2008 certified).
2 Thermoforming
Application: Transforming flat sheets into single or double-curved parts, such as radomes, leading edges, etc.
Heating Parameters:
Heating temperature range: 165–205°C
Heating time estimation: Approximately 1 minute per mm of thickness (adjust based on oven type).
Heating methods: Oven (air circulation) or infrared heaters.
Cooling and Transfer:
Due to low heat capacity and increased surface area from cut cells, the sheet cools rapidly. During transfer from oven to forming tool, use a protective cover (e.g., cotton cloth, breather, glass fabric, or silicone rubber) to minimize heat loss and maintain forming temperature.
Formed parts should be cooled to below 100°C before demolding.
Tooling Requirements: Because forming temperature is lower than the sheet's heating temperature, molds do not need to withstand high heat. Acceptable materials include wood, polyester resin, epoxy resin, or fiberglass.
Thermoforming Methods: Choose among three primary methods based on part complexity and precision requirements:
2.1 Compression Molding
Characteristics: Uses a press to apply pressure, allowing precise thickness control. Suitable for both developable and non-developable surfaces.
Tooling Configurations:
Double-sided hard mold: Provides excellent thickness control but requires both male and female tools.
Single-sided hard mold: Suitable for large thickness or deformation, but thickness control is more difficult.
Single-sided hard mold + soft mold: Typically used for unidirectional deformation (developable surfaces).
Note: Usually requires post-thermoforming machining to achieve final design dimensions, which can be complex for precise positioning.
2.2 Vacuum Forming
Characteristics: Uses vacuum to draw the heated, softened foam onto a single-sided mold. Capable of forming non-developable curves.
Tooling: Single-sided hard mold + soft mold (vacuum bag).
Limitations: Deformation is more constrained compared to compression molding due to vacuum force limits. Post-forming machining is still needed.
2.3 Cold-Deforming-Heat-Shaping
Process:
Machine the foam sheet to precise developable dimensions.
Fix it onto a mold and apply vacuum.
Place in an oven, heat to forming temperature, and hold for about 1 hour.
Cool slowly in the oven to room temperature before removal.
Advantages: Excellent process control; if positioning is accurate, no post-machining is required.
Limitations: Cold deformation is limited; not suitable for non-developable surfaces.
3 Correcting Sheet Warpage
Minor warpage may occur due to moisture absorption or internal stress release (not a product defect). The following correction process can be used:
Flatten at room temperature: Place the foam horizontally and evenly distribute weights (e.g., steel plates) to press it flat elastically. Ensure uniform weight distribution to avoid surface indentations.
High-temperature setting: Place the weighted sheet in a circulating-air oven at 200°C and hold for 8–10 hours (adjust based on thickness).
Cooling: Allow to cool in the oven to room temperature before removing weights.
Bonding and Lamination
Reliable bonding between PMI foam and skins is critical for sandwich structure performance. The following guidelines are based on the specific characteristics of our PMI foam.
1 Surface Preparation
Cleaning: Before bonding, thoroughly remove dust from foam surfaces using a vacuum cleaner or oil-free compressed air.
Enhancement for High Strength: For applications demanding extremely high bond strength, the foam surface can be treated with a needle roller before adhesive application to increase mechanical anchoring.
2 Adhesive Classification and Process Guidelines
Our PMI foam is solvent-resistant and heat-resistant, making it compatible with the vast majority of commercial adhesives. Process recommendations based on adhesive type are summarized below:
| Adhesive Type | Examples | Process key points | Key Parameters |
|---|---|---|---|
| Solvent-based & Emulsion | Rubber-based adhesives | Apply adhesive to both sides, allow sufficient drying/venting before mating under pressure; bond line remains slightly elastic with good peel strength. | Mating temperature: 80–120°C; Pressure: ≤ 40% of foam's compressive strength at 20°C; Hold time: 1–5 min; Cool to <80°C before releasing pressure. |
| Solvent-less Systems | Epoxy, polyester, methacrylic resins, hot-melts | Cure under sufficient pressure to ensure adhesive fills opened cells, or ensure low viscosity during application; bond line becomes hard and rigid. | Curing pressure: 0.05–0.3 N/mm²; Heat acceleration possible up to 160°C. |
| Adhesive Films & Hot-Melt Films | Epoxy films, phenolic films, hot-melt films | Require heat for bonding. For films releasing volatiles (e.g., phenolic), briefly "bump" the press during initial heating to allow volatiles to escape. Pre-perforating hot-melt films can help avoid air bubbles. | Film weight: 100–200 g/m²; For difficult de-airing, grooves 1–1.5 mm deep x 2 mm wide on foam surface can be beneficial. |
3 Adhesive Selection for Different Skin Materials
Metals, Laminates: Epoxy, rubber-based, or polyurethane (PU) adhesives.
Acrylic Glass (Plexiglas): Methacrylic resins.
Wood, Paper: Epoxy, PU, polyester.
Glass Fiber Reinforced Polyester (GRP): Polyester, epoxy, or PU adhesives.
4 Principles for Warp-Free Sandwich Panel Manufacturing
Symmetrical Bonding: Both sides of the foam core must be bonded to skins simultaneously. Skins should be of the same material and thickness.
Uniform Heating and Cooling: Both sides must experience identical heating and cooling conditions to prevent thermal stress-induced warping.
Dimensional Compensation: It is recommended to use foam sheets with a slight oversize (0.5–1 mm) and press to a firm stop in the mold. This compensates for thermal expansion or creep, ensuring final dimensions meet tolerances.
Compatibility with Mainstream Composite Processes
Thanks to its excellent thermomechanical properties, our PMI foam is compatible with various composite manufacturing processes, allowing co-curing of core and skins.
Process Combinations: We can combine different methods of pressure application (autoclave, vacuum, press), heating (oven convection, tool conduction, infrared), and resin introduction (prepreg, liquid infusion, resin film) to suit specific needs.
Typical Processes:
Prepreg Autoclave: Uses autoclave pressure, vacuum, convection heating; resin is pre-impregnated on fibers.
Compression Molding / RFI (Resin Film Infusion): Uses mold pressure, resin film, and tool heating.
RTM (Resin Transfer Molding): Can be combined with prepreg; uses mold heating and pressure.
Key Advantage: Our PMI foam withstands the high temperatures (up to 180°C) and pressures involved in these processes and is compatible with epoxy, polyester, phenolic, and other resin systems, enabling true co-curing.
Quality Control and Common Issues
1 Post-Pretreatment Time Window
Material that has been dried or heat-treated and sealed in moisture-proof bags has a limited cumulative out-time once the bag is opened. If the accumulated out-time exceeds the recommended limit, the material should be re-dried (heat-treated parts can be re-dried but NOT re-heat-treated).
Re-drying of already machined parts must be approached with caution due to potential dimensional shifts.
2 Dimensional Stability
Any heat treatment causes minor dimensional changes. Therefore, final finishing machining must be scheduled after heat treatment.
If high precision is required after thermoforming, it is advisable to leave a small machining allowance and perform final trimming after forming.
3 Warpage Correction
If finished sheets exhibit minor warpage, they can be corrected using the process described in section 4.3 without affecting final performance.
Custom Services from Us
We offer the following tailored services to meet your specific needs:
Supply of pre-dried or heat-treated (-HT) sheets, packaged in moisture-proof aluminum bags.
Precision-machined, ready-to-use cores manufactured to your drawings.
Quality-assured support from prototype development to mass production (ISO 9001:2008).
