Determining the right material and process for part production represents one of the most crucial steps in product development. Quantities, lead times, budgets and design features must measure up against numerous variables, including market success and future design changes.
Liquid Injection Molding (LIM) is ideal for projects that require high-volume production, efficiency and tight tolerances. Dow Corning introduced the primary material used in the LIM process, Liquid Silicone Rubber (LSR), to the marketplace in the 1970s.
For increasing applications, LSR has become the preferred material over traditional silicone gum rubber. The excellent physiological properties of LSR — including its high thermal stability, the ability to produce intricate parts and components and shorter cycle times compared to traditional thermoplastic molding — has enabled it to gain a foothold in many markets. Read on to learn more about LSR and for key injection molding tips.
HOW LSR IS DIFFERENT
Liquid Injection Molding differs from thermoplastic molding in a few ways. The cycle can take as little as three to ten seconds, compared to molding and vulcanization, which takes from ten to ninety seconds or more. Some other attributes that distinguish LSR include:
- Complex geometries: The low viscosity of LSR allows the material to flow into thinner, more complex geometries.
- Automation: LSR allows for an automated process, requiring fewer workers.
- Improved safety: Human operators don’t need to enter the mold area to remove the part, which a conveyor, chute, or robot can handle.
- Easier material preparation: LSR does not require milling or mixing the base rubber with a catalyst.
Although LSR has been around for more than four decades, it has seen a resurgence across nearly every sector. LSR has become an essential material in automotive, aerospace, construction, electrical and electronics industries. It has also experienced growth in the cosmetics, textiles and paper, and medical industries. Its uses in healthcare devices include needleless injection sites, infusion pumps, membrane pumps, fluid dosage, ultrasonic blades, implantable products and skin contact items. The FDA has also approved silicone for use in the food and beverage industries.
The LSR process requires special machinery, intricate tool design, and product development and production expertise. The following injection molding guidelines will help you get a leg up on your LSR project.
DEVELOP A BUDGET FOR YOUR LSR PROJECT
The first step — and one of the most critical aspects of product development — is setting a budget. The key to creating an efficient budget, one that aligns with the strategic objectives and long-term planning of the company, is understanding the organization’s abilities and resources. If you do not have the R&D capabilities in-house, or if you are not using your resources efficiently, consider outsourcing this aspect of the development process to an experienced injection molding company.
The budget must include the incorporation of legal, technology and intellectual property. The complexities of collaboration and partnerships can also make it challenging to create a budget. However, these aspects must be included or the project risks possible failure.
Typically, the task of defining the budget involves cross-functional team members all estimating how much funding their particular function will require, per stage. Using this information, you can estimate an overall budget. Considerations should include:
1. Upfront Costs
- Injection molding design: Includes all preparations necessary before you can offer the product for sale on the marketplace, including new staff, training, designers and consultants.
- Product design: Includes all costs for producing your new product, including the silicone raw materials, tooling (tool’s useful life), computers, software, equipment, travel and expertise. Products you order require warehousing, shipping, delivery and packaging. You should also include special materials, processes, equipment and warehousing. This requires the product designer, tool designer and production people to work together to optimize the design, production levels and peak capacity to have the biggest impact.
2. Production Setup
This item tabulates the costs to prepare for a production run. You must account for some waste as you refine and test the process. With mass-production techniques, you will need to prepare for a production run of 10,000 or more units.
3. Productions Costs
- Component production cost: The incremental cost to generate one more item after it is in production, including raw materials, machine time cost, operator costs, other supplies and post-production finishing.
- Assembly setup and costs: Similar to the items under the component production category.
- Quality control: Pertains to the sample testing process required to assure quality and the components work as intended.
Develop a comprehensive budget that includes all costs, from development through delivery. Do not forget to include marketing expenses, such as advertising, press releases, signage and calls to potential customers. Decide on the mix of marketing approaches you will use and determine the costs of each. Even social networking can cost funding if you hire a specialist.
The budget provides an estimate of development and production costs based on prior experience and guesswork, which will likely result in an overestimate or underestimate of total costs. To protect yourself against underestimation, make sure you include a contingency fund of as much as 20 percent over the project budget. Once you have your budget, you will need to make refinements. Bring in staff who will be involved in the project to analyze the number further.
LSR PROCESS: MATERIAL SELECTION
The Liquid Injection Molding process uses a two-part material delivery system, which consists of an “A” component (catalyst) and a “B” component (cross-linker). The cross-linker converts the raw rubber into a product with mechanical stability. The silicone material comes in ready-to-use compounds that allow the fabricator to add the cross-linker and color.
Both LSR components have a translucent to slightly opaque appearance when delivered from the manufacturer. The catalyst consists of a platinum curing agent that causes the cross-linking to occur at room temperature. The “A” component contains inhibitors that slow down the process, below a certain temperature. Above room temperature, cross-linking occurs freely and rapidly.
Similar to thermoplastics, product developers can choose from different grades and types of LSR. For example, the product may require a silicone material with a dry/slippery or rubbery/wet texture. In addition, buyers can select from a range of hardness, from 20 to 80 durometers. Durometer describes the indentation hardness of the material. Modification of the crosslink density, filler concentration, or both will change the hardness of the material.
General-purpose LSR does not contain a high amount of silica and works best for applications that require only the basic physical attributes. The addition of fillers and other additives make the LSR more resistant to heat, oil, and other fluids.
To stretch low-temperature capabilities, designers add phenyl units to the mixture. New grades of self-adhesive silicone or 2k molding do not require a separate primer step to bond to commonly used thermoplastics. Eliminating this step results in a cure cycle much like the cooling cycle associated with thermoplastics. It allows for in-mold bonding of Liquid Silicone Rubber to thermoplastics components.
The ability to eliminate the chemical primer, plasma or UV treatment step and assimilate LSR with different thermoplastic offers part designers a significant advantage — they can add LSR with precision without needing to cover an additional area to hold the LSR in place.
This advantage has a number of benefits, including:
- It does not add thickness to the final product.
- It makes the part more economical because it uses less material.
- It results in a smaller miniature part overall.
Self-adhesive silicone provides just enough initial bond strength for part removal from the mold. The bond’s strength increases over time, before the delivery of the part to the end user.
Often, designers add fillers — such as Pyrogenic silica, carbon black and precipitated silica — to reinforce the silicone network. Fillers have a significant effect on the properties of the raw and cured rubber. Designers also use non-reinforcing fillers to increase the cured rubber resistance to different media. To optimize heat and resistance to various media, the designer may add a stabilizer to the mix.
Making the product look great can also be important to designers. The additives used for coloring Liquid Silicone Rubber consist of compounds that include chemistry accessory ingredients: pigments, carrier resins, and dispersants. These ingredients must carry evenly in the resin.
Designers typically add color to the process at the point prior to static or dynamic mixing equipment, as you introduce the “A” and “B” parts. The pigments, which come in paste or liquid form, can be custom formulated via metered equipment to blend into the silicone at low or high percentages to obtain the desired color. Keep in mind that some additives already contain color pigments. Work with your injection molding design team to determine what LSR material has the properties that work best for your part/product.
PRODUCT DESIGN AND INJECTION MOLDING TIPS
In LSR molding, you cannot consider product design without also considering tooling design. Your tool designers must figure out the best way to meet the project’s requirements for dimensions and cavity geometries, as well as align those needs with the limited space within the mold.
After you select a material for final product performance, you will need to:
- Determine the molding process: How you mold the product will have a significant influence on the parts’ geometry. Some components work well with automation or the horizontal LSR molding process. Other products work better using vertical LSR molding or a manual process. The complexity of the geometry of a component has a bearing on the size of the mold as well as the size of the injection molding machine.
- Decide how you will demold the parts: Achieving the easiest demolding is an important aspect of the design process. Rubber has a natural tackiness that makes it difficult to remove, especially highly undercut components. The designer must give careful consideration to the undercuts and wall thicknesses during the design phase.
- Decide on the runner system: Traditional runner systems that feed material through the gate waste material, which cannot be recycled. In contrast, a cold runner system allows for direct gating, and it does not waste material. It works like the mechanics of the hot runner system for thermoplastics molding, but it consists of water-cooled nozzles within the mold that feed the material to each. It allows for direct gating, and the material doesn’t need to be cured within the cold nozzles.
- Select the gates: Once you decide on a type of runner system, choose the gates. A cold runner system may use pin, submarine or another automatic degating design. Typical runner systems may use the before mentioned gate style in addition to other gate types such as edge band or ring.
- Choose the best gate location: During the process, it is best to achieve cavity filling from the thickest area of the component. When it constitutes part of the assembly, the gate’s location should not interfere with the assembly or action of the final product.
The high pressure of the Liquid Injection Molding process requires tooling with double-sided design. It must be made of aluminum, steel or a beryllium-copper alloy. Although the tooling cost can be expensive, the lowper-unit molded part cost offsets the initial tooling expense.
While the LSR process has a number of similarities to the thermoplastics process, it has some very important differences. Only an experienced operator will understand its nuances, such as how to heat the LSR mold, properly vent the mold, obtain a desired surface finish and maintain the mold properly.
ESTABLISH LSR TIMELINES AND BENCHMARKS
Depending on your LSR project, you can define many major or minor benchmarks to track and monitor the progression of key activities and dates for your project, including:
- Design and engineering: Typically CAD and CAM software convert your concepts into virtual prototypes. They provide the opportunity to evaluate the design from multiple angles and make changes.
- Prototype and model making: Choose a prototype that emulates the size, shape, and cosmetics of the component. You can use this prototype to introduce the product to potential buyers.
- Building the mold: After you finalize the design, build aluminum or steel molds.
WHAT YOU SHOULD KNOW ABOUT PROTOTYPES
Prototyping may be one of the most important stages of development. Do not rush through this step or underfund this phase. The ability to prototype injection molded components and assemblies quickly can give you a competitive edge.
It helps to have a clear and comprehensive understanding of all aspects of the rapid prototyping process, including terminologies, methodologies, and the pros and cons. This will cover all considerations necessary for successful new product prototyping under tight time constraints.
In the Liquid Injection Molding process, the fabrication of prototypes saves you both time and money. You can evaluate the product form, fit and functionality. Prior to paying out for tooling, the prototype also enables you to see the part, which product marketing, product engineering, tooling and product manufacturing teams can use. The key is to get the prototype right so you can continue to production seamlessly and without issue.
Today, the marketplace offers a number of technologies for prototype models. For example, Fused Deposition Modeling (FDM) can provide a functional model with full testing capabilities. If you need to evaluate part size, Stereolithography (SLA) makes a good choice. Some other prototype options include conventional CNC Machining and 3D printing. Your injection molding design team can help you decide which prototype model works best for your project.
DETERMINE THE OPTIMAL PRODUCTION QUANTITY
Product manufacturers must determine the optimal production quantity or economic order quantity for production batches. To do so, you need to figure out material costs and the expenses associated with manufacturing a particular batch size.
The goal is to minimize the total costs required to deliver equal shipments of the product, at fixed intervals, to the end users. These costs should also include any waste associated with the machine setup and any tooling discarded after a production run. Some items to consider for determining your optimal production level include:
- Raw material inventory carrying cost.
- Finished goods inventory carrying cost.
- Raw material ordering cost and manufacturing set-up cost.
Carrying costs can include interest, insurance, taxes and storage. You are already paying for the operator and machine overhead, so you should take advantage of the savings by increasing or decreasing the production quantity.
Vendor Selection: Your Partner Can Make the Difference
The growth and maturity of companies and their markets can change the complexity of parts or products, materials or annual volumes. In many cases, thermoforming technologies have reached their limits.
LSR fabrication can help take a company’s operation and production levels to new heights. Many companies considering the LSR process find it challenging to find an experienced partner for their LSR molding program. While many thermoplastic companies have converted their equipment to Liquid Silicone Rubber molding, or actually own a silicone machine, it does not make them qualified and experienced LSR molders. An experienced molder will understand the dynamics of a wide variety of methods for molding silicone, to help you choose the one that works best for your part.
When choosing an injection molding company, look for an LSR vendor with proven core competence in the fabrication of Liquid Silicone Rubber, over-molding, and two-component molding for both parts and components. Collaborating with a company experienced in silicone product development and production will prove invaluable along every phase of the project, including part/product design, mold design, and the molding process. Check out vendor production capabilities, because this will help to determine the vendor’s ability to turn around large orders.
If you’re interested in initiating an injection molding project or have questions concerning your LSR needs, please contact SIMTEC for more information.