Dip Molding Medical Device Products
To mold products with emulsions of liquid rubber, it is necessary to complete a series of process steps to ensure proper formation, vulcanization, and finish treatment to meet the customer’s needs in the final application. We ensure dip molding rubber recipes meet FDA guidelines and also meet the requirements set by the FDA for Medical Devices.
The term “dipping” is associated with the manipulation of the dip form. In fact, the forms are dipped into the materials as the sequence is performed.
Coagulant Dip >>> Rubber Dip >>> Leach Dip >>> Cure or vulcanization >>> Finish
The dipping 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 dipping:
- 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:
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
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.
What does the customer want or need for their product to be successful?
- Cure Time
- Cure Temperature
Examples of dip molded products:
Anesthesia bellows, ultrasound probe covers, neck seals for escape hoods, heart balloons, breather bags, wound drains, attenuation gloves, gastric pressure balloons, hearing aid covers, syringe covers, finger cots, prostate balloons, colon balloons, tourniquets, neck seals for barometric chambers, exam and surgeon’s gloves, vein stripping.
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’s in a word – (Neoprene is Resistant). Neoprene stands up against a multitude of factors which makes it a 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 nonallergenic.
Synthetic Polyisoprene has superb resistance and is a nonallergenic. Polyisoprene is 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.
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