3ERP’s polyurethane casting service lets you make small batches of plastic parts at a much lower cost than injection molding. These high-quality prototypes have a great surface finish and can provide a pathway to mass produced molded parts.

Many product designers are, however, unfamiliar with polyurethane casting, which requires a unique set of design considerations — both for the making of the master pattern and for the plastic castings themselves. Designing polyurethane cast parts is not the same as designing injection molded parts, so designers must familiarize themselves with the process before getting stuck into their CAD designs.

This article provides information on how to design polyurethane castings. It provides advice for feature design, looks at typical polyurethane casting tolerances, and notes the differences between polyurethane casting design and injection molding design.

What is polyurethane casting?

Otherwise known as vacuum casting, polyurethane casting is a prototyping and low-volume manufacturing technique suitable for creating batches of up to 25 units. It is the same as urethane casting, an alternate term for the process.

The polyurethane casting process comprises three main steps: making the master, making the silicone mold, and making the castings.

1. Master: Using a process like CNC machining or SLA 3D printing, the designer creates a positive master pattern of the parts. Alternatively, a pre-existing part, made of virtually any solid material that can withstand temperatures of 40 °C (for the silicone curing stage), can be used as the pattern.
2. Mold: A casting box is half-filled with liquid silicone with the pattern in the center. The silicone is cured at low heat in an oven for 16 hours until it becomes solid, making the first half of the mold. Then the rest of the casting box is filled with liquid silicone, which is cured to make the second half of the mold. The mold halves are removed and the pattern set aside for future use.
3. Casting: The flexible silicone molds are filled with a polyurethane casting resin, which solidifies within the mold and assumes a solid shape identical to the original master. The silicone mold can be used around 25 times before it needs replacing.

Polyurethane casting is a valuable manufacturing technique because the silicone molds are incredibly inexpensive to make compared to metal tooling. They only last for around 25 castings, but this is usually sufficient for prototyping of small plastic parts.

Although casting polyurethane resins does not lead to strong functional parts, it does have unique strengths, such as producing an excellent surface finish, allowing for a range of colors and transparencies, and being highly economical in low volumes.

Making the master pattern

The first thing a product designer must consider during polyurethane casting design is the master pattern. Common techniques used to make casting patterns include 3D printing (SLA, PolyJet, etc.) and CNC machining. Traditional patternmaking techniques like manual sculpting can also be used, though this comes at a greater cost.

When using a digital manufacturing process like 3D printing or CNC machining, designers must effectively consider the design constraints of two separate processes at once: the patternmaking process and the casting, though this article will focus primarily on design tips that lead to effective casting (rather than effective 3D printing or CNC machining).

The advantages of 3D printing polyurethane master patterns are:

• Higher level of geometrical freedom
• Faster to make than CNC machining

The advantages of CNC machining master patterns are:

• Tighter tolerances than 3D printing
• Less surface finishing required
• Longer lifespan than 3D printed patterns

Tolerances & dimensions

When designing polyurethane cast parts, designers should be aware of standard tolerances and minimum/maximum dimensions, which are not the same as those for injection molding.

However, when using polyurethane casting as a prototyping technique for future injection molded parts, it might make sense to create a design that can also be injection molded (by incorporating draft angles etc.).

• Tolerance: 3ERP can make polyurethane parts to an accuracy of ± 0.3%, with a lower limit of ± 0.3 mm on dimensions smaller than 100 mm.
• Shrinkage: Thick parts are more susceptible to shrinkage, which occurs due to temperature shifts during the molding process. Designers should plan for a shrinkage rate of around +0.15%.
• Part size: Polyurethane cast parts can have dimensions of up to 1900 x 900 x 750 mm and a maximum volume of 10 liters.

Wall thickness

The polyurethane casting minimum wall thickness is 0.75 mm — adequate for proper mold filling, although 1.5+ mm will lead to the best results. If walls thinner than 0.75 mm are required, contact us to discuss your design.

As with injection molding, consistent wall thickness leads to better results, as it decreases the changes of temperature-induced deformation. However, varied wall thickness is less damaging than it is with injection molding.

Draft

One of the advantages of polyurethane casting is the flexibility of its silicone molds. Because the molds are flexible, the finished polyurethane parts can easily be removed from the mold without ejector pins. This also means that draft — tapered sides that allow for easy ejection — is not necessary like it is with injection molding.

That being said, a small draft angle of up to 5° can slightly prolong the lifespan of the silicone mold, so draft may be advantageous when carrying out medium-volume production runs on parts that do not require perfectly straight sides. This also makes the polyurethane casting design more adaptable to injection molding (for scaling up production).

Radii

When designing polyurethane parts, sharp inside corners should be avoided to maximize the strength and stability of the part. We recommend adding fillets to inside corners with a radius of 3 mm.

Note that using CNC machining to make the master pattern is ideal for adding fillets to polyurethane cast parts, because CNC cutting tools have a cylindrical shape and create natural rounded internal corners when cutting a pocket.

Ribs

To add stiffness to polyurethane casting parts without increasing wall thickness, designers can incorporate ribs, which increase the moment of inertia. Generally, it is better to use multiple short ribs than a single tall rib, and it is important to orient the ribs properly relative to where the part could potentially bend.

Ribs should have large corner radii and a thickness of less than 60% of the wall thickness to avoid shrinkage and sink marks. The height of each rib should be less than 3x its thickness to avoid breakage.

Bosses

Cast polyurethane parts can have bosses — small protrusions that accommodate fasteners or threaded inserts. These bosses can be supported by gussets or connecting ribs.

To prevent sink marks, the wall thickness of a boss should be less than 60% of the wall thickness of the part (as with ribs). The base radius of the boss should be at least 25% of the wall thickness of the part.

Threads & holes

Polyurethane casting can accommodate through holes and threads, and these are best achieved with the use of inserts rather than being added to the master pattern. However, these complex features can increase the cost of the project and may not be necessary for prototypes.

For through holes, dowel rods can be placed in the silicone mold. For threads, it is best to incorporate metal threaded inserts via insert molding.

Joints

Polyurethane casting can be used to make multi-components parts with tongue-and-groove joints. A small reveal gap should be incorporated into the joint between components.

Surface features

Surface features like text and logos can be incorporated into polyurethane cast parts. In general, these features — especially in recessed (engraved) form — are much easier to add using CNC machined master patterns than with 3D printed ones. 

For best results, letters and numbers should be spaced at least 1.3 mm apart.

3ERP is a prototyping and low-volume manufacturing specialist which also functions as a polyurethane casting company. Contact us for a free quote.

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Chemshun Ceramics’s Return For Work on Feb 10, 2020.

Affected by the novel coronavirus, our government announced all enterprise close still February 9.

On February 10th, Pingxiang Chemshun ceramics co.,ltd is in normal business operation after our government approve us. All the people of Chemshun ceramics and the families are health. The government made a series strict check to our factory for the work of prevent novel coronavierus.

In the following days, Chemshun Ceramics adopts the suggestion from the expert for prevention and control of the coronavirus and executes strictly the rules for the clean and clear of the office and production environment, guaranteeing the safety and segregation of the workers and avoiding close contact among people.

Thus,our production and operation activity go on well, we can normal provide the wear resistant ceramics for customers , Welcome the customers’ enquiries.

We believe that China can overcome this coronavirus.

Chemical filling beads is an inert ceramic ball. called "ceramic balls" because of their inner shape like ceramics sphere.

Inert ceramic ball does not participate in chemical reactions. It is calcined from alumina powder into industrial powder, and then calcined into dense structure at high temperature through rotary kiln and tunnel kiln. This dense and stable chemical structure looks like a ceramic, so it is called alumina ceramic ball. Different from general ceramics, alumina ceramics have a Mohs hardness of 9, second only to diamond, and its compression and impact resistance are far from ceramics. At the same time, it has the same advantages as ceramics: acid and alkali resistance, corrosion resistance. These good properties make inert alumina filler balls more and more widely used in industry.

At present, inert alumina ceramic balls are widely used in high-purity products, such as 90/92/95/99, which are widely used in chemical, petroleum and other industries. The materials required in these industries need to be resistant to corrosion by organic solvents such as strong acids and bases, resistant to high temperature and high pressure, and cannot participate in the reaction. These requirements are precisely that alumina filler balls can meet the needs, so they have attracted much attention.

In addition to high inert alumina balls, Chemshun Ceramics also provides other medium-aluminum, medium-high-aluminum, and low-aluminum chemical ceramic balls. Filler balls with these contents should have an advantage in price, but there will be a noticeable difference in content and ratio compared to high aluminum. Generally, the lower the content, the lower the ratio, and the worse the compressive strength and impact resistance. Therefore, customers should choose the appropriate content of alumina balls according to the actual industrial environment needs

 On 2015 May, Pingxaing chemshun Ceramic Team had a pleasant trip to Taiwan. 

Chemshun Ceramics afford the industrial ceramics especially the inert ceramic balls, chemical packings and ceramic honeycombs to Taiwan area for many years. 

Ceramic balls are round spheres, made of ceramics material.It can be classified into two types according to their purpose: filler ceramic balls and grinding ceramic beads.

Packing ceramic balls, also known as inert alumina ceramic balls, are widely used in petroleum, chemical, fertilizer, natural gas and environmental protection industries. They are used as the covering support material and tower packing for the catalyst in the reactor. The main function of the packing ceramic balls is to increase gas or liquid. Distribution points, support and protection of active catalysts with low strength. Chemshun ceramic packing ball products include: normal ceramic balls, medium-alumina ceramic balls, high-alumina ceramic balls, ceramic balls with hole, etc.

Grinding ceramic balls are grinding bodies used in fine grinding equipment such as ball mills, pot mills, and vibration mills. Grinding ceramic balls have the advantages of high hardness, high bulk density, corrosion resistance, etc., and their crushing efficiency and wear resistance are much better than ordinary balls or natural pebbles. They are widely used in ceramics, glass, enamel, pigments, chemicals, mining and other industries. According to the composition of the ceramic ball, it can be divided into: alumina ball, ZTA ceramic bead, zirconium silicate sphere, Yttria-stabilized zirconia grinding beads, etc.

(I) Safety of lifting device during filling random tower packing:

1. All construction personnel must wear safety helmets when entering the tower, personnel outside the tower and on the ground must wear safety helmets, and personnel on the tower must wear safety belts;

2. Safety devices such as the switch and overload alarm display of the elevator must be kept intact, and an empty load inspection must be carried out before daily use. It can only be used after it is normal to prevent any accidents caused by electrical failures;

3. During the lifting process, it is strictly forbidden to stand within the radius of the hanging basket to prevent accidents.

(II) Safety and fire prevention:

1. Fireworks are strictly prohibited when entering the construction site;

2. Electric welding shall not be used during the installation of packing;

(II) Packing installation method:

1. Install the random packing according to the tower equipment diagram to ensure that the packing installation height and quantity are correct;

2. Put 10 meters of tap water into the tower to buffer the force of the packing falling into the tower bottom.

3. The filling material is naturally piled into the tower;

4. When filling, it should always be poured from a certain height away from the filling layer. People should not stand directly on the filling material to prevent the filling material from being deformed by pressure and uneven density. Wooden boards should be laid on the filling material to disperse the force;

5. After each section of filling material is installed, the upper end of the chemical filling material should be checked to see if it is flat. If there is any unevenness, it should be naturally flattened with wooden strips;

6. During the filling process, other debris, packaging bags, etc. are not allowed to be left in the tower together with the filling material.

High alumina ceramic balls, also known as inert ceramic balls, are ceramic balls made of 99% alumina. These balls are widely used in various industrial applications due to their excellent properties and advantages.

One of the main advantages of high-alumina inert ceramic balls is their high thermal and chemical resistance. They can withstand high temperatures and harsh chemical environments and are suitable for processes such as petrochemical refining, gas processing and chemical manufacturing. Their inert nature also makes them resistant to corrosion, ensuring a longer service life and minimal maintenance requirements.

Additionally, these ceramic balls offer excellent mechanical strength and wear resistance, making them ideal for grinding and milling applications. Their high density and hardness enable them to effectively break and grind materials, which makes them valuable in industries such as mining, cement production and pharmaceuticals.

The high alumina content of these ceramic balls also provides excellent insulating properties, making them suitable for high temperature insulation applications. They are commonly used as fillers and support media in high-temperature furnaces and reactors, where their thermal stability and inert properties are highly beneficial.

In addition, high-aluminum inert ceramic balls are widely used in the field of catalyst carriers. Their high surface area and porosity allow them to effectively support catalysts for a variety of chemical reactions, including hydrogenation, oxidation and reforming processes in the petrochemical and refining industries.

In summary, high-aluminum inert ceramic balls have a wide range of advantages and applications in various industries. Their high alumina content, inertness and excellent physical properties make them valuable materials for use in high temperature, corrosive and abrasive environments. Whether as catalyst supports, grinding media or thermal insulation materials, these ceramic balls play a vital role in improving process efficiency and performance in industrial operations.

The specific application of tower packing balls in environmental protection equipment is mainly used as filler in various chemical and environmental protection equipment, such as carbon dichloride degassing towers, contact reaction towers, desulfurization, sewage treatment and other devices. They can increase the surface area of the equipment and provide a variety of channels, so that the fluid can be well mixed and distributed within the packing layer, thereby improving the gas-liquid mass transfer efficiency, reducing fluid resistance, reducing fluid pressure drop, and improving flow and mass transfer efficiency. 

In sewage treatment equipment, the specific application of chemical packing balls is to use multi-faceted hollow ball fillers. This filler has a large specific surface area and can fully solve the problem of gas-liquid exchange, thereby improving the efficiency of sewage treatment. Multi-faceted hollow ball packing is generally made of polypropylene plastic, which has the characteristics of light weight, high strength, high temperature resistance, corrosion resistance, etc., and is suitable for various sewage treatment equipment.

In addition, chemical packing balls can also be used in other environmentally friendly equipment, such as air purifiers, ozone generators, etc., to improve the mass transfer efficiency and operating efficiency of the equipment, reduce production costs, and reduce energy loss.

In short, Inter packing balls are widely used in environmental protection equipment, which can improve the efficiency and performance of the equipment and contribute to environmental protection.

(I) Safety of lifting device during filling random tower packing:

1. All construction personnel must wear safety helmets when entering the tower, personnel outside the tower and on the ground must wear safety helmets, and personnel on the tower must wear safety belts;

2. Safety devices such as the switch and overload alarm display of the elevator must be kept intact, and an empty load inspection must be carried out before daily use. It can only be used after it is normal to prevent any accidents caused by electrical failures;

3. During the lifting process, it is strictly forbidden to stand within the radius of the hanging basket to prevent accidents.

(II) Safety and fire prevention:

1. Fireworks are strictly prohibited when entering the construction site;

2. Electric welding shall not be used during the installation of packing;

(II) Packing installation method:

1. Install the random packing according to the tower equipment diagram to ensure that the packing installation height and quantity are correct;

2. Put 10 meters of tap water into the tower to buffer the force of the packing falling into the tower bottom.

3. The filling material is naturally piled into the tower;

4. When filling, it should always be poured from a certain height away from the filling layer. People should not stand directly on the filling material to prevent the filling material from being deformed by pressure and uneven density. Wooden boards should be laid on the filling material to disperse the force;

5. After each section of filling material is installed, the upper end of the chemical filling material should be checked to see if it is flat. If there is any unevenness, it should be naturally flattened with wooden strips;

6. During the filling process, other debris, packaging bags, etc. are not allowed to be left in the tower together with the filling material.