Design Guides-Urethane Casting

Wall Thickness

Wintech recommends a minimum wall thickness of 0.04”(1mm). However, there are cases where thinner walls down to 0.02”(0.5mm) are possible on small parts. The larger the part, the larger the wall should be to ensure proper support. Though the casting process makes it possible to vary wall thickness throughout a part, a uniform wall thickness will decrease the risk of deformation due to shrinkage and stress when curing.

Design walls to be at least 0.02” or 0.04” thick, depending on the size of the overall part, to ensure proper support.

Drafts

When urethane casting, incorporating drafts is not required, but it can extend the life of the mold. If possible, adding 3-5 degrees of draft will significantly reduce strain on the part when being pulled from the mold.

Meanwhile, be sure to take into consideration any other production methods that the prototype will be going through and design accordingly.

While drafts are not required, including them makes the cast part easier to remove from the mold.

Letters & Logos

Urethane casting produces an excellent quality of both raised and recessed letters and logos because of the accuracy that the silicone mold provides.

Wintech recommends leaving at least 0.05” space between letters. The width should be twice the measurement of the height.

The larger the radius of the letters, the more accurately they will read.

When casting parts with lettering, ensure at least 0.05” space between each letter.

Radii & Fillets

Wintech recommends a fillet with a radius of 0.125” on all inside corners, or surface transitions to increase the strength of the part.

Adding filets to inside corners significantly improves production quality, and gets you closer to an end-use part.

Add fillets to all inside corners to increase part strength and stability.

Inserts

Through holes: For simple through holes, dowel rods are placed into the silicone mold to define the feature. Maintaining a line of sight is requited when designing for urethane casting to ensure that a dowel rod can be inserted in the mold.

Thread: Threaded inserts can be added to a urethane casting mold in a variety of sizes.

Dowel rods inside a silicone mold.

Bosses

GUIDELINES

Bosses are an excellent feature to secure mating parts with screws or fasteners. The base radius of the boss should be 25% of the thickness of the part to prevent the screw or pin from burning when placed into the boss. To reduce the wall thickness and avoid sinking, inner corners of bosses can use a 0.060’’ radius for the fillet. Wintech recommends making the wall thickness for bosses less than 60% of the thickness of the part to ensure minimal shrinkage.

Notes:Creating a connecting rib between the boss and the wall, as well as implementing gussets around the boss can strengthen and further reinforce the boss. 

Radii & Fillets 

GUIDELINES

Wintech recommends a fillet with a radius of 0.125" on all inside corners, or surface transitions to increase the strength of the part.

Notes:Adding fillets to inside corners significantly improves production quality, and gets you closer to an end-use part.

Material

Urethane casting is conducive to many different types of polyurethane plastics ranging from rigid to flexible. Some of the most common materials used in the process include:

? Rigid, High-Impact Urethane

Hei-cast 8150 - ABS grade: well-balanced physical properties, excellent cure properties and superior dimensional stability.

Hei-cast 8550 - similar to PE or PP: low in Young's modulus in flexure and high in elongation.

Axson PX 527 - similar to ABS or PC: long term UV stability and weather resistance. Clear aspect. Good impact and flexural resistance.

? Rigid, Heat-Resistant Urethane

Axson PX 223: temperature resistance above 120°C. Good impact and flexural resistance.

Axson UP 6160: temperature resistance above 200°C. Good thermal resistance.

? Flexible, Cast-in color or Clear Urethane

Hei-cast 8400 - polyurethane elastomers: excellent rebound elasticity. Hardness in the range of Shore A 30~90.

Hei-cast T0387 - transparent polyurethane elastomer: hardness in the range of Shore A 30~90. 

Color

The coloring options for urethane casting include:

? Cast-in colors

? Painted after casting

Standard casting will result in a relatively smooth surface with excellent rigidity.There are certain advantages and disadvantages to each coloring option:

CAST-IN COLORS:

Advantages

? No chipping & peeling

? Specific texture

Disadvantages

? Surface finish is dependent on master pattern finish

? Color may not be exact to specifications 

PAINT

Advantages

? Variety of colors

? Gloss level

? Uniform surface appearance

? Eliminates visible lines left from parting object from mold

Disadvantages

? Prone to scratches in post processing

? Additional labor after casting. Typically resulting in a higher cost.

Helen Tan-Project Sales Manager   

WINTECH Tooling Co.,Ltd

E-mail:[email protected]

Skype:tan.helen9

Mobile&Wechat:0086 138 2436 2414

Website:www.wintech-t.com