HDPE & PP-like vacuum casting resins

These materials are thermoset polyurethane resins. They are particularly recommended for prototyping applications, with the technology of vacuum casting.

Unlike thermoplastic resins, these polyurethanes can withstand high temperatures and do not melt when exposed to heat. The chemical nature of the formula shows chemical bonding between the polyol and the isocyanate called cross-linking, which leads to a strong and durable polymer.

The purpose of this range is to simulate the mechanical properties of a future extrusion thermoplastic material like PP and HDPE, in order to create a functional and strong prototype part. Although the chemistry of the 2 polymers is different (thermoset and thermoplastic), the goal of the PR777, PR740 and HPR65 is to get closer to the injection-moulding series material, to get a realistic part that will support the development of the project. These thermoset urethane resins also make a particularly good choice for small series, when investing in an aluminium tooling is not a relevant option yet.

We recommend to use these PU resins in a polyaddition silicone mould. Moreover, the materials are formulated to have a very low aggressiveness to silicone, making them gentle materials to keep your moulds in good condition, and make multiple parts.

The PR740, PR777 and HPR65 have a specific chemical formulation base, that make them very gentle to silicone-rubber. Depending on your application and the complexity of your part shape, you can expect between 30 and 50 parts to be moulded in one single silicone mould.

For opaque colours, you can use Synthene pigments. Get in touch with our team to learn more about these additives. For translucent colours, we recommend to use solvent-based tints, that should not exceed 3% of the total weight of the moulded part. The pigments always have to be pre-mixed in the polyol component prior to mixing with the isocyanate.

Warning: these 3 materials can darken under the exposure to UV, without alteration of the other properties.

In order to reinforce the mechanical and thermal properties of the PR777, we can suggest the use of the Glass fiber filler SYNFILL G. Using this fiberglass filler will increase the flexural modulus and flexural strength of the material up to 64 MPa. The heat deflection temperature will be raised from 94°C to 131°C with the addition of 25% of Synfill G. The glass fiber rate can be adjusted depending on the requested properties of the project. This fiberglass filler should always be added to the isocyanate component before mixing with the polyol.

This range is composed of polyurethanes with a flexural modulus between 450 MPa and 930 MPa. They are therefore semi-rigid, or semi-flexible resins. Chemically speaking, the HPR65 is formulated as an elastomer, which is why it has no HDT. They are all good countertypes of PP or HDPE thermoplastics, that can be moulded in silicone moulds or aluminum moulds.

The PR777 is the most user-friendly material with a 1:1 mixing ratio and a very low viscosity. It benefits from a long pot-life and a short demoulding time, making it the winning combination.

The HPR65 also have a low viscosity. Its clear advantage is that it has a very limited exothermic reaction, and it can be used for massive parts with a curing at room temperature.

After a regular curing at 70°C, the material is not brittle and ready for using.

However, a post-curing greatly helps to speed up the chemical bonding and improve the performance of the material.

In case of big part dimensions, the HPR65 can get high mechanical properties even if it completely cures at room temperature.

The PR740 is designed for small dimensions. As the potlife is 7,5 min, sometimes it can be handy to use a differential pressure system.

The PR777 is suitable for larger parts, like car bumpers.

The HPR65 works for the fabrication of small, big or even very thick parts, with its ability to cure at room temperature without any brittleness phenomenon.

Our R&D laboratory has designed these PU resins to have a high resistance to shock. On the Charpy impact test, the PR740 and the HPR65 need to be notched, or else they do not break.

The impact resistance of these materials are respectively:

• PR740: 24 kJ · m -2 – notched
• PR777: 91 kJ · m -2 – unnotched
• HPR65: 30 kJ · m -2 – notched

After a proper post-curing and total polymerization of the resins, the PR777 can be used with high temperatures, with an HDT of 110°C. The PR740 also has a high thermal resistance with a Heat Deflection Temperature of 96°C.

Chemically speaking, the HPR65 is an elastomeric product. Therefore, we recommend a working temperature between -20°C and +85°C.

Because of their thermoset nature, these materials do not start softening or melting at elevated temperatures like thermoplastics might.

Pigments and solvent-based tints are compatible with these HDPE- and PP-like materials. For best results, we recommend pre-mixing the pigment or tint with the polyol component at a concentration of 1–3% of the total mixture, depending on the desired opacity. However, please note that this material range is not UV-stable and may darken over time. This discoloration does not affect the mechanical properties.

Our glass-fiber filler SYNFILL G can also be used to reinforce the properties of the PR777 and increase its toughness. This filler should be pre-mixed with the isocyanate component prior to doing the casting. Please refer to the dedicated Technical Data Sheet. There you will see how the filler rate influences the different parameters such as Flexural Modulus (from 900 to 2300 MPa), Flexural Strength (from 35 to 64 MPa), and HDT (from 110°C to 131°C)!

The PR777, PR740 and HPR65 are regular high performance semi-rigid resins. While the PR777 and PR740 are mostly adapted to prototyping and small series, the HPR65 has been created to make more massive parts, such as foundry models due to its exceptional impact resistance.

This versatility opens the door to a wide range of applications across various industries, including aerospace, automotive, medical, industrial design, and tooling, where durability and precision are critical.

These materials have properties that are close to polypropylene or High density PolyEthylene.

For seal and gasket-making, we recommend using our rubber-like HPE range.

This elastomeric system allows one to cast soft and durable parts between 40 Shore A and 55 Shore D. The casted parts benefit from a very high Abrasion resistance, chemical resistance and general wearproofness. They also have overall good adhesion properties and can easily bond to several types of metal, with the use of a primer.

No, all SYNTHENE materials comply with the REACH regulation and are mercury-free.

Although these thermoset resins have good self-degasification properties, we recommend to degas the products to avoid getting foam that would be detrimental to the casting of a perfect bubble-free part. The use of a vacuum chamber, or vacuum-casting machine, is highly recommended.

Although they are initially formulated for the vacuum casting application, the PR777 and PR740 have specific physical properties that make them suitable for a use in RIM machine.

All of these materials have been designed to strike an optimal balance between health considerations and product performance. They all feature a low content of free monomers. For further details, please consult the materials’ MSDS.

Tests have been conducted to evaluate the dielectric properties and CTI (Comparative Tracking Index) of our polyurethane resins PR777 and PR740. Regarding thermal conductivity, we also have a test report regarding the behaviour of PR777. If you would like more technical details or access to the full test data, feel free to contact us directly – we’ll be happy to assist you.