ALWA MOULD D backfilling compounds

RTM pressure injections

Surfaces of:

Polyester gelcoat
Vinylester gelcoat

Foam moulds

Surfaces of:

Polyester gelcoat
Vinylester gelcoat
Epoxy gelcoat
Metal spray surface layer (zinc)

For example suitable for:

  • Polyurethane (PU) foam injection
  • Resin injection moulds
  • Reaction injection moulding (RIM) / Structural reaction injection moulding (SRIM)
  • PU reaction injection moulding
  • PU integral foam moulds
  • PU flexible foam moulds
  • Glass fibre mat reinforced thermoplastics (GMT) and fibre reinforced polymer (FRP) for prototypes and small series
  • Cold and hot pressure moulds up to 130 °C

The advantages:

  • Large volumes of more than 2500 kg can be cast in less than 45 min.
  • Good castability
  • Hardly measurable linear shrinkage
  • High temperature resistance
  • Demoulding process after approx. 90 min. (after adding the backfilling compound)
  • Chemical adhesion to polyester or vinylester gelcoat
  • Immediately after the first mould half has cooled down, the second mould half can be prepared
  • Copper pipes keep the mould surface on a nearly constant temperature
  • Post-curing is not necessary

Technical data at RT:

Density of filler mixture for RTM 2.0 kg/l
Viscosity > 10000 cps
Linear shrinkage < 1 mm/m
Temperature resistance approx. 135 °C
Reaction temperature approx. 50°C – 70°C
Pot life of backfilling compound at 21°C approx. 45 min.

Specific advantages for RTM pressure injection

Approx. 70 – 100 demouldings per release agent layer can be realised by using Frekote 700 NC.

Required materials and quantities:

ALWA MOULD COAT (gelcoat) art. no. 3600003 approx. 1000 g per m²
ALWA HARDENER, fluid, for ALWA MOULD COAT art. no. 3650000 approx. 15 g per m²
ALWA BARRIER COAT art. no. 3610006 approx. 2000 g per m²
ALWA HARDENER, fluid, for UP gelcoat + ALWA BARRIER COAT art. no. 3150001 approx. 20-40 g per m²
Glass mat approx. 40-80 g per m²
Spin split powder-coated glass fibre mat approx. 300 g per m²
ALWA MOULD D, component 1 art. no. 6000… approx. 100 g per l
ALWA MOULD D, component 2 art. no. 6001… approx. 200 g per l
ALWA HARDENER POWDER for ALWA MOULD D / P / PERM art. no. 5100001 approx. 6 g per l
ALWA FILLER MIXTURE for RTM and PU moulds art. no. 8530025 approx. 1700 g per l
Copper pipes (soft) – 10mm art. no. 9210000 approx. 10 m per l

Specific advantages for backfilling

Surfaces as required (different gelcoat and metal surface).

Density and compressive strength of the aluminium backfilling at RT:

Density of aluminium backfilling 2,2 kg/l
Compressive strength of aluminium backfilling 70 – 80 N/mm² = 700 – 800 kp/cm
  1. Laminate the surface of the pattern with a polyester spray filler and treat it with sandpaper (fine sanding 200 grain).
  2. Pre-treat the surface of the pattern with ALWA RELEASE LACQUER. Apply three layers of honey wax and vent each layer for approx. 30 min. Polish the surface after each applied layer. For instance, Oskar‘s M500 can be applied with an atomizer in one or two layers. After a layer of Oskar’s M500 has been applied, the surface does not need to be vented. The surface can be directly polished. Apply a thin release layer with a spray gun after approx. 1 hour. When the pattern is dry, the gelcoat can be applied.
  3. First, stir up the sprayable version of ALWA MOULD COAT (art. no. 3600006) in its original  container with a wooden spatula. The usage is approx. 1.1kg/m². Mix very well the calculated amount with 1.5 – 2 % hardener for ALWA MOULD COAT.
    Attention: The gelcoat and the pattern should have a temperature between 19 °C – 22 °C. The gelcoat is sprayed in two layers of 0.5 mm/m² (0.6 kg/m²) each time. A good cup spray gun with a 2.5 mm nozzle is required. The injection pressure is set on 3.5 – 4 bar with an injection distance of 400 – 500 mm. As before, the first layer of gelcoat must be gelled before the second layer can be applied. The period of gelling is approx. 90 min.
  4. Use a star mixer to mix ALWA BARRIER COAT (approx. 0.5 kg/m²) together with 3 % glass fibre short cut and 0.5 % hardener for ALWA BARRIER COAT (art. no. 3150001) mechanically until the substances turn into a thick white paste. Please use only 0.5 kg of ALWA BARRIER COAT per batch and stir it with 0.5 % fluid hardener. Coat the mixture of the final gelcoat layer with a broad surface brush and vent it with a Teflon venting roll (Ø 15 mm). Small areas should be rolled out evenly. The wall thickness of the ALWA BARRIER laminate should be approx. 0.5 mm. The gelling
    occurs after approx. 60 min.
  5. Two layers of spin split glass-fibre mat 225 g/m² (art. no. 8610000) are laminated behind the gelcoat with ALWA BARRIER COAT. The laminate should be vented very well. After approx. 60 min. the resin gels.
The pattern should not have any undercuts and has to be clean, smooth and dry. Choose stable material for the pattern and casting box, which are suitable to carry the weight of the backfilling. Fastenings, polyurethane displacer, guide pins and loose parts of the mould have to be fixed at the pattern or the casting box before the backfilling is cast. Porous surfaces have to be sealed with ALWA ACRYLIC SEALER (art. no. 9535000). In order to achieve a high-gloss surface, the surfaces should be coated with lacquer (EP or PU lacquer) and treated with a very fine grade of wet sandpaper. ALWA SURFACE TENSION REMOVER (art. no. 9500000) is applied on the surfaces of the pattern and the casting box with a cloth in order to remove the tension of the surfaces (prevents the formation of drops when applying ALWA RELEASE LACQUER). Afterwards, ALWA RELEASE LACQUER (art. no. 9510000) is applied with a sponge, cloth or a spray gun (at least three layers). ALWA RELEASE LACQUER dries after approx. 30 – 60 min. depending on the room temperature. Hot air accelerates the drying process (e.g., by using a dryer). Errors can be avoided by using materials with similar thermal conductivity for the pattern and the casting box. Heating and cooling systems (e.g., copper pipes) should have a distance of 5 cm between each other and a distance of 2 cm to the surface. Only aluminium perforated sheets should be used as reinforcement material (2 – 3 mm). The sheets must be fixed to the pattern with a distance of 2 cm to the pattern’s surface. Please consider the different thermal conductivities when something is later attached to the mould. Once the pattern is dry, gelcoat can be applied.
Coat or spray polyester or vinylester gelcoat for PU foam moulds and press moulds (layer thickness of approx. 1 – 3 mm) bubble-free on the pattern. After the gelcoat has cured, the backfilling is cast carefully and directly on the gelcoat layer. ALWA EPF 10 (art. no. 3300005) (approx. 2.7 kg/m²) is preheated to 30 °C and stirred in the original container. Afterwards, the needed quantity is taken out of the container and mixed very well with 5 % hardener H 10 (art. no. 3310001). The pot life is approx. 40 – 50 min. at 30 °C. ALWA EPF 10 for moulds with abrasion-resistant surface is coated in two layers. When the first layer is cured but still sticks at the backside, the second layer is coated and immediately sprinkled with aluminium granulates 2 – 4 mm (art. no. 8024025) as mechanical bond of the backfilling. After the gelcoat layers are fully cured, one can start to cast. Loose aluminium particles must be removed before the backfilling is cast. Metal sprayed zinc surfaces (layer thickness 1 – 3 mm) are used for RIM and other PU moulds. The backsides of the metal spray surfaces are first coated with a thin layer of polyester gelcoat in order to get a better bond between the metal surface and the backfilling. Afterwards the backfilling is immediately cast on the gelcoat, which has not yet been cured. Galvano moulds are additionally equipped with a mechanical bond for the backfilling compound.

Preparation of the backfilling

  1. Calculate the complete casting volume in litres (e.g., calibration with water or sand) and multiply the amount with the density = complete quantity of backfilling.
  2. Bend the copper pipe over the pattern and fix it to the prefabricated casting box with a distance of 20 –
    40 mm from the pattern’s surface. The pipes should have a distance of approx. 60 – 80 mm to each other.
  3. Screw the casting box with the fixed copper pipes to the model and then begin to seal.

Calculation of the batch quantity

  1. Calculate the casting volume in litre.
  2. Multiply the volume with the density of the compound = batch quantity in kg.
Example of an aluminium backfilling 10 litres volume 10 l x 2.2 kg = 22 kg batch
Example of a mineral backfilling 10 litres volume 10 l x 2.0 kg = 20 kg batch
The mixture of resin and filler should have a temperature of 19 – 22 °C before casting.
(low viscosity mixture; in case of bigger volumes we recommend using a concrete mixer)
Mix resin and filler, add the hardener and mix for approx. 3 – 5 min. Cast the first backfilling into the deepest
part of the pattern (possibly place the pattern with the casting box diagonally). Gelcoat layer or metal mask
should be well moistened. The backfilling has a pot life of approx. 60 min. at 19 – 22 °C. The backfillings can be mixed and cast one by one.

5 % ALWA MOULD D comp. 1 art. no. 6000…
10 % ALWA MOULD D comp. 2 art. no. 6001…
55 % Filler mixture for RTM art. no. 8530025
30 % 30 % of the filler mixture for RTM is deferred art. no. 8525025
2 % Hardener (2 % on the resin content) art. no. 5100001
5 % ALWA MOULD D comp. 1 art. no. 6000…
10 % ALWA MOULD D comp. 2 art. no. 6001…
85 % Filler mixture for RTM art. no. 8530025
2 % Hardener (2 % on the resin content) art. no. 5100001
5 % ALWA MOULD D comp. 1 art. no. 6000…
10 % ALWA MOULD D comp. 2 art. no. 6001…
20 % Aluminium spray grit art. no. 4065025
15 % Aluminium granulates, round – 0.5 – 2.5 mm art. no. 8525025
25 % Aluminium granulates, round – 2.0 – 4.0 mm art. no. 8525025
25 % Aluminium granulates 2 – 4 mm are placed back for spreading after the last backfilling
2 % Hardener (on the resin content) art. no. 5100001
5 % ALWA MOULD D comp. 1 art. no. 6000…
10 % ALWA MOULD D comp. 2 art. no. 6001…
20 % Aluminium spray grit art. no. 4065025
15 % Aluminium spray granulates, round – 0.5 – 2.5 mm art. no. 8525025
50 % Aluminium spray granulates, round – 2.0 – 4.0 mm art. no. 8525025
2 % Hardener (referring to the resin content) art. no. 5100001

Processing instructions

Cast all the following mixtures with the complete filler content one by one into the first mixture until the pattern is almost full. After the last backfilling has been cast, the pure resin will settle at the surface. The placed back part of the filler of the first backfilling is mixed into this pure resin.

Next working steps

After the first mould half is cured, the following working steps can be taken:

  • Building of the wall thickness (distance)
  • Ventilation systems (riser)
  • Building of the second mould half


After cold curing (the reaction temperature of the backfilling can reach approx. 60 °C) the two warm mould halves will be spanned with screw clamps against each other and demoulded after cooling. Now, a water heater is adjusted to approx. 80 °C and fixed to the copper pipes. The mould surface is covered and the mould is tempered for 10 hrs. at 80 °C. In order to increase the temperature resistance of the mould it can be tempered up to 130 °C.

Finish of the mould surfaces

We recommend to wet grind the mould surfaces with 600 – 2000 grain and, afterwards, to polish them.

Quantity ALWA MOULD D ALUMINIUM Hardener powder
comp. 1 comp. 2 spray grit granulates, round
art. no. 6000… art. no. 6001… art. no. 4065025 art. no. 8525025 art. no. 8024025 art. no. 5100001
1 0.05 0.10 0.20 0.15 0.50 0.003
2 0.10 0.20 0.40 0.30 1.00 0.006
5 0.25 0.50 1.00 0.75 2.50 0.015
10 0.50 1.00 2.00 1.50 5.00 0.030
20 1.00 2.00 4.00 3.00 10.00 0.060
30 1.50 3.00 6.00 4.50 15.00 0.090
40 2.00 4.00 8.00 6.00 20.00 0.120
50 2.50 5.00 10.00 7.50 25.00 0.150
60 3.00 6.00 12.00 9.00 30.00 0.180
70 3.50 7.00 14.00 10.50 35.00 0.210
80 4.00 8.00 16.00 12.00 40.00 0.240
90 4.50 9.00 18.00 13.50 45.00 0.270
100 5.00 10.00 20.00 15.00 50.00 0.300