Dip Molding Medical Device Products

What is Dip Molding?

Dip molding is a manufacturing technique used to create durable, precise products with hollow interiors.

Dip molding dates back thousands of years, when ancient Egyptians and ancient Romans used it to make candles. Both civilizations would dip wicks into wax, allow the wax to cool, and repeat the process until they achieved the size they wanted.

Humans evolved the dip molding process over time, coating clay molds with beeswax to create the desired shape. The wax mold was packed with sand to keep its shape and dipped into metal or plaster. During the baking process, the wax would melt away, leaving behind the final metal or plaster product. This is called “lost wax molding” and is the earliest known method for creating hollow products using dip molding.

In the past 100 years, dip molding techniques have become significantly cheaper and faster to execute. Advancements in plastics and polymers have given manufacturers more options for creating heat-safe molds and dip-molded products, opening up a vast world of possibilities.

These days, plastisol and other liquid polymers have replaced wax and ceramic to allow for a more precise dip molding process. Heat resistant plastic molds are dipped into the desired solution, often repeatedly, to produce the desired shape. Once the solution has cooled, the mold can be removed, leaving behind a hollow component or product with precise internal dimensions.

Dip molded products can be made from natural rubber latex, synthetic polyisoprene, and neoprene polychloroprene, among others. Manufacturers have the luxury of choosing materials based on budget and desired properties, such as overall resilience, resistance to certain stressors, and more.

Successful dip molding relies on a series of process steps to ensure property formation, vulcanization, and finish treatment to ensure an exact match with customer specifications. A variety of additives, such as cure systems, antidegradants, and fillers, may be included in the material formulation to achieve the desired outcome. Additionally, some products may require adherence with FDA guidelines for medical devices and other products.

The Process

Coagulant Dip >>> Rubber Dip >>> Leach Dip >>> Cure or vulcanization >>> Finish

The dip molding process can be characterized as a conversion sequence. The rubber is converted from a liquid to a solid, and then chemically converted into a vulcanized network of molecules. But then, more importantly, the rubber is chemically converted or linked from a very fragile film into a networked group of molecules that can stretch, can be deformed to some extent and then return to their original shape.

Coagulation: Changing a liquid to a solid

This coagulation process is not absolutely necessary for all “dip” processing, but critical to our processing sequence. The rubber can be allowed to change from a liquid to a solid through air drying, but it will take much time. Some thin walled parts are produced in this manner. The coagulation process uses chemicals to force this physical state change.

The coagulant is a mixture or solution of salt(s), surfactant(s), thickener(s) and release agent(s) in a solvent, typically water. Alcohol can be used as the solvent in some process. Alcohol evaporates quickly and leaves very little residue. Some water-based coagulants will require help from an oven or other means to dry the coagulant.

The main component of the coagulant is the salt (Calcium Nitrate). It is an inexpensive material and provides the best uniformity of coagulation over the dip form.

Surfactants are used to wet out the dip form and assure a smooth, uniform coating of coagulant onto the form.

Release agents such as calcium carbonate are used in the coagulant formula to aid in the removal of the cured rubber part from the dip form.

Keys to coagulant performance:

• Uniform coating
• Fast evaporation
• Material temperatures
• Entrance and retrieval speeds
• Easy change or maintenance of the calcium concentration

The Rubber Dipping Step

This is the stage where the rubber is converted from a liquid to a solid. The chemical agent which facilitates the solidification, the coagulant, is now applied to the dip form and is dry.

The form is “dwelled” or held immersed in the tank of liquid rubber. As the rubber makes physical contact with the coagulant, the calcium from the coagulant causes the rubber to destabilize and turn from a liquid state to a solid state. The longer the form is immersed, the thicker the wall will develop. This chemical reaction will continue until all the calcium is consumed from the coagulant.

Keys to latex dip molding:

• Entrance and exit speeds
• Temperature of latex
• Uniformity of coagulant coating
• Controlling PH, viscosity and total solids of the rubber

The Leach Dip Process

The leaching process is the most effective stage to remove unwanted, water-based chemicals which are not wanted in the final product. The most opportune time to remove the unwanted materials from the dipped film is the leach before cure.

What is removed? Most residual salts, surfactants, and proteins.

Main material components:

• Coagulant: Calcium nitrate.
• Rubbers: Natural (NR); Neoprene (CR); Polyisoprene (IR); Nitrile (NBR)
• Rubber solids
• Dwell time

What happens if you have inadequate leaching?

• “Sweating,” a sticky film on finished product
• Adhesion failure
• Increased risk of allergic reactions

Leaching keys to performance:

• Water quality
• Water temperature
• Dwell time
• Water flow rate

The Cure Stage

This step is a two-step activity.

The water in the rubber film is being removed and the temperature of the oven along with time is activating the accelerators starting the cure or vulcanization process. Times and temperatures are optimized to give the best physical properties for the different types of rubbers.

Curing keys to performance:

• Cure time
• Cure Temperature

Finish

Several options are available to treat the surface of a dipped part so that the part does not stick to itself, such as a powder part, urethane coating, silicone rinse, chlorination, and soap wash.

Finishing keys:

What does the customer want or need for their product to be successful?

• Cure Time
• Cure Temperature

Why synthetic rubber over natural rubber latex?

Natural rubber has outstanding resilience and high tensile strengths making it an ideal fit for applications that require high resistance to abrasion, superior flexibility and excellent tear strength. The one issue with natural rubber latex is that it carries proteins that can cause an allergic reaction in humans.

Synthetic Neoprene is resistant and stands up against a multitude of factors which makes it great for use in a wide range of industries. Neoprene rubber features resistance to flames, oil (moderate), weather, ozone cracking, and abrasion and flex cracking, alkalis and acids. Neoprene is also a safe alternative for applications where latex allergies are a concern.

Synthetic Polyisoprene also has superb resistance and doesn’t contain latex-allergens, making it a safe replacement for natural rubber. Polyisoprene is a close replacement for natural rubber as far as feel and flexibility. It has a better resistance to weather than natural rubber latex, although it does sacrifice some tensile strength, tear resistance and compression set.

Why dip molding?

• A wide selection of shapes, sizes and wall thicknesses can be dipped. Specialty line of latex and non-latex compounds to choose from.
• Small to medium run batches, helping you meet your tough deadlines.
• Low cost to prototype and develop.
• Tooling can be made from several materials, stainless steel, aluminum or polypropylene. Prototype tooling can be made with a 3D printer.

The Importance of Dip Molding for Medical Grade Equipment

Dip molding is an excellent option for medical components and parts. The dip molding process is highly compatible with many materials that are easy to sterilize, such as natural rubber latex. This is vital in a medical setting, where patients must be protected from contaminants and drug solutions must be free from foreign bodies. Dip molding is also ideal for manufacturing products that need consistent dimensions. The use of a master mold prevents variance and discrepancy between units that should be identical. This can be vital for product quality and performance, especially in a medical setting, where precision is so important.

Dip Molded Products from KEP

Kent Elastomer Products (KEP) is proud to manufacture top-quality dip-molded products for the medical and surgical industries. We use durable compounds and materials to ensure consistent performance of your dip-molded products, even in a rigorous service environment. Our domestic manufacturing facilities in northeast Ohio are ISO 13485:2016 certified for medical parts, and we can help you meet the demands of FDA approval. Our goal is to make the production process as smooth as possible for you, which is why we follow lean manufacturing practices and offer on-site labeling and packaging services.

To discuss a dip molding project and learn more about our capabilities, get in touch with a member of the KEP team >