The process of dip molding technology encompasses a wide variety of materials and products. It remains a viable production process in that tooling costs are low compared to injection molding, compression molding, and rotational molding. Additionally, because of the dip molders ability to manufacture multiple numbers of pieces per machine cycle, production and rotational molding. Additionally, because of the dip molders ability to manufacture multiple numbers of pieces per machine cycle, production costs are typically lower than others molding technologies.
There are many types and styles of dip molding plants, dependent upon many factors. At first, the evaluation criteria for selection of the right dip molding machine style may appear confusing. But, one can generally follow a step-by-step process for every application to make the right choice.
Today’s market offers many more choices for dip molded products (See Figure – 1) and polymers. When first evaluating an application for dip molding there are many factors one should consider as part of the proper machine selection process, including the following;
- Type of polymer(s) to be used.
- Type of product or products (wall thickness, shape, design).
- Desired production output for the system.
- Dedicated product versus multiple product system consideration.
- Maturity life cycle of the polymer/ product for dipping.
Types of machines
Properly designed dip molding plants are generally customized for the application, with a few exceptions. This does not necessarily mean that the dip molding manufacturer should customize all aspects of the machinery. For example, continuous chain glove and condom machines are offered by several machinery suppliers, and can generally be purchased as “standard” units. However, extreme caution should be exercised if buying a “standard machine”. For example, available oven space associated with the equipment may be insufficient for use with latex cold compounds as compared with prevulcanized latices. Some dip tank designs and product leaching systems offered on standard lines may prove insufficient for some products. The two dipping formats for consideration are as follows:
1. Batch machines – Formers (dipping molds) are mounted on pallets, often as 1.2 meters by 2.4 meters. The pallets are moved intermittently, normally on a consistent time cycle, to each machine station. For actual dipping stations, the pallets move over the tank, and the pallet is dipped into the tank, or alternatively, the tank is raised to the pallet of formers. After completing the dip cycle, the pallet indexes to the next station, with a fresh pallet indexed to the same dipping station. Generally, the formers are affixed to the pallet in rigid fashion. However, on rare occasions, formers can spin axially about the pallet axis.
Vertical dipping using the batch indexing technique can occur through use of electrical motors, hydraulics, or compressed air (See Picture -1) .
2. Continuous chain machines -Formers are Mounted on Multiple former racks, or individually and attached to a continuous moving conveyer chain (roller chain or transmission chain). The line moves continuously at a constant speed, carrying formers through dip tanks, ovens, and associated process stations. Some continuous chain machines feature re-rotating forms, whereas for others, the forms are fixed (See Picture- 2)
With the understanding of these two basic machine formats, the evaluator must then work through the following selection factors so that proper machine selection can occur.
Flow chart analysis assists decision making: As for most engineering and technical disciplines use of a basic flow chart with “yes-no” responses can generally lead the analyst to the proper selection of dip molding technology. Using surprisingly short list of six questions, the writer estimates that approximately 98% of all accurately assessed using this simple flow chart (See Figure- 3)
Selection factor #1 – Product family: Most dipping plants worldwide generally are designed for production of a single product. The application of continuous chain technology for latex gloves and latex condoms is commonplace. The prime incentive for employing continuous chain technology is that of economics and productivity. Glove dip lines today can produce from 1 million to 1.5 million gloves per day. Product markets such as latex exam gloves, latex condoms and latex novelty balloons are extremely competitive and must be manufactured in high volumes to be profitable.
Contrary to this scenario is the firm who desires to produce multiple dipped products on a single system. A typical plant may produce specialty dipped goods using a “private label” strategy, offering their services for lower volume, but higher profit margin items. Examples of products typically produced with this type of plant would be probe covers, cast covers, specialty gloves, boots and bellows. One dipping plant in Europe today not only produces over 50 types Of products on one dipping machine, but also processes them simultaneously. At any given time during the production day, between 3 to 8 different dipped products are being produced on the dip cell. The advantages reduced dipping mandrel inventory, reduced work in process, and reduced finished goods inventory. Additionally, shelf life is seldom in issue with this plant.
In general, batch dipping technology is employed for multiple product families within the same plant. Contrary to this, is, high volume dedicated products are most economically produced on Continuous chain lines, providing they meet the other selection criteria to follow.
Selection factor # 2 -pilot dipping plants lab scale units: It is possible that no other technology in the rubber and plastic industry, varies more widely than that of dip molding Machines come in all shapes and sizes and can vary with price significantly, dependent upon the business objective of the project.
As mentioned in previous text, continuous chain technology has significant benefits in reducing production costs. Many firms today prepare themselves for the ever changing market for alternative polymers through the use of a computerized controlled Laboratory Dipping Unit (See Picture 3). These machines are invaluable for verifying and improving dipping and temperature profiles on a practical scale, prior to introducing modifications to the production equipment. Even for the plant which manufactures products only by Continuous Chain processing, laboratory scale dipping units are normally designed as batch dipping units to optimize flexibility. The ability to alter dipping profiles, Cure oven specifications, and environmental issues, greatly favors the use of batch dipping. Research scale machines should be designed in this format.
Selection factor # 3 -product film thickness: One key quality for nearly all dipped products is that of consistent wall thickness and uniformity. Simply Put, the larger the wall thickness, the more one would tend to employ batch dipping technology. Generally, any product With wall thickness in excess of 15-mil should fall into this category. Examples of products generally dipped on batch dipping machines because of thicker wall dimensions include electricians’ gloves, baby teats and pacifiers, baby feeding nipples, Foley catheters, and rebreather bags.
Products such as thin film nitrile gloves, medical latex gloves, condoms and novelty balloons perform well on continuous chain machines, in part because of the ability to maintain consistent wall thickness with lower viscosity dipping materials. Some dipping companies may wish to explore the ability to make a product With 15 mil or greater wall thickness, on a continuous Chain machine. This can be practical providing that the multiple dips to achieve this total not more than 3. If employing more than 3 dip tanks to achieve this wall thickness, the conveyer begins to extend greatly in length, requiring a larger capital investment. Furthermore, dip tank management becomes more difficult to maintain consistency across 4 or more dip tanks. Some latex gloves, latex condoms and unsupported nitrile gloves are Manufactured employing 3 dips on a continuous chain system.
Selection factor #4 – dipping mandrel shape: The dipping process can utilize many different mandrel shapes and materials. Material selection alone can be a thought provoking question, the subject of which warrants publishing of future detailed information under separate cover.
In general, symmetrical mandrel shapes dip well using continuous chain machines. Dipping mandrel examples of this type include glass condom formers glove formers, and standard bulb shaped balloon mandrels. However, the more intricate and non-symmetrical the former, the more difficult it becomes to dip with on continuous chain machines. Many novelty balloons are designed to replicate old shapes for marketing purposes. For example, “bird shaped balloons” are typically dipped on batch machines, because of its odd shape. The very recognizable “Mickey Mouse” balloon by Disney, is best dipped using batch machine technology, to ensure optimum cosmetic appearance.
Selection factor # 5 – process maturity: One disadvantage of continuous chain machines, is its inherent inflexibility. Product performance becomes dependent entirely upon line speed. If the manufacturer discovers methods by which the line speed can be increased because of advancement in the dipping tank, the manufacturer must also ensure that all other operations on the dipping line, call also withstand the speed increase. A 10% speed increase in the chain speed may not be tolerated because of reduced oven exposure time. The Manufacturer Must adjust temperatures and/or air flow (if the machine has a convection oven design) to enable the finished product to maintain acceptable physical characteristics. Alternatively, additional off line curing via tumbler/dryer may prove feasible for this condition.
New product design often requires between 2 to 5 years of maturity in process definition, chemistry, etc. before stabilizing the process. Installing a continuous chain machine, too early in the product life cycle, can make manufacturing scale up difficult and expensive. The manufacturer may become mired in making changes to the chain machine, which takes time and labor. Dipping profile changes for some continuous chain machines are nearly impossible to alter. Chain machines that have dipping cams which can be altered are only done so by the “heat and beat” method. More simply put, cams must be torch cut and-welded into place which can idle a machine for 24 hours.
Alternatively, batch machine dipping profiles can be modified through the use of proximity switches and speed dials, which can be adjusted in 15 minutes to 30 minutes time. PLC control technology, made commonplace 10 years ago by ACC Automation Co., can enable a manufacturer to alter dipping and oven profiles in a matter of minutes. Newer computer motion controls now offered by DipTech Systems, Inc. provide even higher precision at lower cost.
In projects where process definition is vague ‘and undefined, the use of batch dipping machines is advised.
Selection factor # 6 – type of polymer: Today’s market offers far more polymer alternatives than 30 years ago, when latex was nearly the only material available for dip Molding. Protein issues associated with natural rubber latex have opened up opportunities for new polymers, especially for medical dipped goods that are invasive to the body. Many catheter products and medical balloons today are constructed of either silicone or polyurethane for this reason.
Based upon the writer’s experience, the industry breakdown for material usage is also shown in parenthesis next to the polymer identification, in Figure – 2. As can be seen, natural rubber latex still is the dominant material used in the industry today, for reasons of comfortable fit at a competitive price.
Water based materials (Such as natural rubber latex, nitrile, and neoprene) are normally adaptable to both types of machines, dependent upon film this thickness and component shape. Solvent based polymers (such as polyurethane, silicone, styrene butadiene, and polycarbonate), are seldom dipped on continuous chain machines.
There are exceptions to this statement, in that a few solvent based gloves and condoms are manufactured on Continuous chain machines. However, manufacturing start up for these solvent based start up for these solvent based continuous chain plants can be extremely difficult and lengthy. Moving formers continuously in two axes (horizontal and vertical) simultaneously, as occurring in continuous chain dip lines, can disturb the polymer in the dip tank, causing solvent gassing (bubbles in solution), leading the pinholes and weak spots in the film.
Most dipping projects require significant investment capital for equipment. Making the proper choice in machinery style will enable the manufacturer to more quickly realize full Production levels, reduce start up frustrations, and more importantly, reduce investment and working capital costs.