As the name suggests, the hand-made moulding process for polyurethane elastomers does not require the use of complex and expensive machines. However, to create any polyurethane moulds, the following equipment is necessary: a laboratory scale, a mechanical mixer, a vacuum pump, an oven and/or a heating table with a temperature control up to 120°C, preferably with forced air circulation to speed up the material melting process (for systems processed hot).
Additionally, containers for weighing are needed, such as polypropylene buckets, mixing spatulas (shown in the picture below), disposable latex or vinyl gloves, and optionally 250ml cardboard cups, which are useful for pouring excess raw material from the buckets.
Once the basic equipment and polyurethane system ingredients (polyol, isocyanate, and hardener for three-component systems with variable hardness) are gathered, the preparation of raw materials can begin. The temperature and the mixing process of the ingredients must be carefully considered.
Polyol is typically required to be heated and mixed before use. The recommended melting temperature of polyol is usually 50–55°C and should not be exceeded, as this could degrade the catalyst contained within the polyol. It is also not advisable to store polyol at this temperature for several days.
Isocyanate should be melted if the material has been supercooled. In such cases, it becomes cloudy, and lumps precipitate until the entire content solidifies. If this happens, the raw material should be melted in an oven at 70°C, then cooled to the processing temperature (specific processing temperatures are provided in the technical datasheets of the system). Isocyanate that has not been supercooled is ready for use once it reaches the recommended processing temperature and does not require mixing due to its homogeneity.
Once melted, polyol needs to be thoroughly mixed. (Be sure that the material has been completely melted.) Polyol is a heterogeneous mixture that tends to sediment. For a 25kg bucket, it is recommended to use a roller mixer. In the case of containers with a top lid, a mechanical mixer set to low speed should be used to avoid aeration (mix for approximately 15 minutes). The roller mixer and the mechanical mixer with an anti-aeration tip are shown in the pictures below.
All components (polyol, isocyanate, hardener) are sensitive to moisture. If the contents of the package are not used entirely at once, it is recommended to protect the remaining material from moisture by filling the package with dry technical nitrogen (purity 5.0). If nitrogen is not used, the packaging should be tightly sealed after pouring out a portion of the raw material (to avoid the closure sticking in the isocyanate package, wipe the threads dry and lubricate them with technical vaseline or silicone oil) and avoid situations where there is little raw material in the package and a lot of air (for example, you can pour the contents into smaller containers).
Once the raw materials are prepared, the next step is to weigh the individual components. Typically, the first material to prepare is the polyol portion (polyol, possibly hardener, pigment paste, and sometimes other additives). It is important to recalculate the proportions if pigments, catalysts, or other additives are added. The recommended accuracy of proportions for the components is usually 1% by weight, as indicated in the technical datasheet. The weighed polyol portion is then mixed—special attention must be paid to thoroughly mix the material along the walls and at the bottom, for example using a long spatula. The prepared polyol portion can be stored (protected from moisture) as its components do not react with each other.
The weighed raw materials: the polyol portion and isocyanate (prepolymer)—separately—should be degassed in a vacuum vessel connected to a vacuum pump. The degassed material can increase in volume by up to three times, which is why the bucket should be filled to no more than one-third of its volume to prevent overflow. Whether or not degassing is necessary depends on the quality and aesthetic requirements of the final product—it is not absolutely necessary, but it is recommended.
If the polyurethane is to bond with a metal element, the element should be sandblasted, degreased, and coated with a primer (adhesive). The adhesive must be fully dried—solvents must evaporate, otherwise delamination will occur. The metal element should be heated to the same temperature as the mould. Care must be taken to ensure that the surface coated with primer does not become contaminated with the anti-adhesive agent. If the anti-adhesive agent is applied by spraying, it should be done in a separate room to prevent the aerosol from settling on the adhesive-coated parts.
A suitable amount of isocyanide (pre-polymer) has to be poured into the bucket with deaerated polyol part, put on the floor. To avoid any error exceeding 1%, the required accuracy has to be calculated beforehand. It is important this and consequent actions are carried out without any delays, as the reaction is started once polyol and isocyanide are combined. A stopwatch should be used to control the duration; the time counting has to be started as soon as the polyurethane system components are combined. The gelling time (the time upon which the mixture begins to densify) is provided in the datasheet. The gelling time is the shorter, the higher is the temperature and the weight of the processed raw material. It is also shorter for higher hardness values.
Depending on whether the components are to be deaerated or not, mix them briefly and intensively (30-45 seconds) or more gently and then pour into another empty bucket. Mix briefly again (for a few seconds). This procedure aims to eliminate the line of poorly mixed raw material on the surface of the product.
The degassing time (usually ~2 minutes) should be determined experimentally to ensure that there is enough time to pour the raw material into the moulds. The vacuum chamber connected to the vacuum pump is shown below.
The mold should be filled within no more than 75% of the gelation time. When designing the mold, it is important to include elements that facilitate quick filling.
The mold should be leveled, coated (either lubricated or sprayed) with a release agent, and heated to the temperature specified in the technical datasheet. The mold can be heated and filled with raw material either in an oven or on a heating table. The heating table should be adjustable to allow leveling. It should also be equipped with a cover to prevent temperature differences between the upper and lower surfaces of the mold (a sample heating table is shown in the picture below).
Air bubbles that may appear in the mold on the surface of the still-liquid raw material can be removed by gently heating them with a gas or electric heat gun (shown in the picture below). However, do not overdo it with the heating, as the surface can remain sticky for a long time even after the rest of the product has dried.
After the time specified in the technical datasheet has passed (it is necessary to confirm organoleptically that the product has reached an adequate degree of reaction), the casting can be removed from the mold.
The product should be left to cure for the time specified in the technical datasheet. Curing (if allowed by the technical datasheet) can be done for several (5-7) days at room temperature or for 12-24 hours in an oven (usually at a temperature of 80°C). The casting may also remain in the mold during this time. To achieve the best possible mechanical properties (especially in parts of the casting thinner than 5mm), it is recommended to cure in the oven.
The necessary equipment such as a heating oven, vacuum pump, laboratory scale, and similar items are not available for sale as part of our offer. However, based on our experience, we can recommend manufacturers of suitable equipment.
