The manufacturing of silicone rubber products involves a striking balance between efficiency and quality. To find this balance, different manufacturing processes must be considered and compared to determine which process best suits the unique needs of the product. For many plastic and rubber fabricators, the processes of choice include injection molding and transfer molding, each approaching the solution with a distinct set of advantages and disadvantages.
To help determine which process best suits your particular product, here is a comparison of the key features and benefits of injection molding and transfer molding.
WHAT IS INJECTION MOLDING?
Injection molding is one of the most common manufacturing processes for plastic and rubber parts. Patented in 1872, the original injection-molding machine worked like a large hypodermic needle, using a plunger to inject plastic through a heated cylinder and then into the mold. Over time, the industry expanded, increasing the demand for inexpensive, mass-produced products, which encouraged the injection-molding process to evolve.
In 1946, a screw machine replaced the plunger mechanism, helping to mix the material before injecting it into the mold and speeding up the process. Further innovations expanded the possibilities for injection molding machines, helping the process develop into what it is today.
APPLICATIONS
The injection molding process produces thin-walled plastic parts, most commonly in cylindrical, cubic or complex three-dimensional shapes ranging in size from 0.01 square inches to 80 square feet. Though the method commonly uses thermoplastics, it is also used with composites, elastomers and thermosets.
Manufacturers often use injection molding to create plastic housings, household appliances, consumer electronics, power tools and automotive interiors. This technique is also used to produce open containers like buckets, household products like toothbrushes, plastic toys, and medical devices like syringes and valves.
THE PROCESS
The injection molding process is as follows:
- The material, often in the form of pellets or granules, is fed into a heated barrel, where its temperature is raised to a molten state.
- The material, called a “melt” in its molten state, is forced through a channel called a sprue into the mold cavity, using a reciprocating screw or ram injector.
- The melt is left inside the mold and is either cooled down to solidify (thermoplastics) or heated up to cure (thermosets).
- When the material is solid, the mold is opened and the part is ejected.
COMMON POLYMERS
Injection molding is used for both thermoplastic and thermosetting materials. Some of the more commonly used materials include:
- Polystyrene (PS)
- Acrylonitrile Butadiene Styrene (ABS)
- Polyamide (PA)
- Polypropylene (PP)
- Polyethylene (PE)
- Polyvinylchloride (PVC)
- Other short fiber reinforced plastics
ADVANTAGES
Injection molding is one of the most popular and cost-effective molding methods for plastic and rubber products. This is primarily due to its set of unique advantages, including the following:
- Immediately Pre-Heats Material: The injection screw heats the material as it moves down the barrel of the injection machine, mixing the material as it moves. This decreases the viscosity of the material and allows it to flow more easily into the mold cavities. This results in faster cavity filling and rapid curing of thermosetting materials.
- Higher Capacity: Injection molding machines are able to accommodate more cavities, resulting in additional units per production cycle.
- Minimal Waste: Compared to transfer molding, injection molding produces much less waste due to a smaller sprue and lack of overflow channels.
- Fast: Injection molding cycles typically take anywhere from two seconds to two minutes, not including post-processes.
DISADVANTAGES
Though the injection molding process is ideal for large production runs, it does have certain limitations, including:
- Thin, Uniform Walls Only: Injection molds typically require uniform, thin walls with a thickness of 0.015 inches to 0.5 inches. Thick parts or non-uniform molds will typically result in defects.
- Rounded Corners: Injection molds are less capable of producing units with sharp corners, and require molds made with rounded corners.
- High Setup Costs: Injection mold machines are much more expensive than transfer molding machines, and the setup is much more involved.
POSSIBLE DEFECTS
All molding processes run the risk of defects. Possible injection molding defects include:
- Flash: This is the occurrence of the molten material seeping out of the mold cavity and solidifying. This can happen if the injection pressure is too high or the mold has too much clearance.
- Warping: This permanent bending of the part can occur if the part cools at a non-uniform rate.
- Bubbles: Air gaps can occur when the injection temperature is too high, there is too much moisture in the material, or if the material cools at a non-uniform rate.
- Unfilled Sections: This can occur if there is insufficient material or if the flow rate of the material is too low.
- Sink Marks: These voids in the mold can occur if certain sections solidify first.
- Ejector Marks: Marks made by the ejector can happen if the cooling time is not long enough to allow the part to fully set or if the ejection force is too high.
WHAT IS TRANSFER MOLDING?
Transfer molding is similar to injection molding and is widely considered to be a simplified variation of injection molding. The process has much in common with earlier iterations of injection molding before the introduction of the screw injector. The key differences between the two molding types lie in the processes.
Like injection, transfer molding pushes material into a mold through a sprue, but does so using a plunger instead of a screw injector.
APPLICATIONS
Transfer molding is a leading manufacturing process for encasing electronic components with rubber or plastic. Inserts, like metal prongs, semiconductor chips or ceramics can be placed within the mold before the material is injected, allowing it to “float” within the material as it cures. Products like pins, studs, connectors and molded terminals can be created using this method.
Because of this unique advantage, transfer molding is key to several different industries. The natural gas industry uses it to make metal-to-rubber face seals used to create interfaces for gas valves. The electrical industry employees use it to mold connector seals around electrical wires, like in the case of spark plug wires. The hydraulic industry also makes use of it because of the ability to achieve sharper cutoffs and edges, which is an advantage for making sharper lip seals.
THE PROCESS
The transfer molding process is similar to injection molding, with a few key differences. The process is as follows:
- The material (which may or may not be pre-heated) is placed into a holding chamber called the transfer pot.
- A hydraulically powered plunger pushes the material through a channel, called the sprue, into the mold cavity.
- The material remains inside the mold and is either cooled to solidify (thermoplastics) or heated up to cure (thermosets). Any material still within the sprue remains attached to the part as it solidifies.
- When the material is solid, the mold is opened and the part is ejected.
- The extra material from the sprue is trimmed off.
COMMON POLYMERS
The transfer molding process commonly uses thermosetting materials, though it is possible to use thermoplastics as well. Some of the most common materials used in this molding process include:
- Epoxy
- Polyester (Unsaturated)
- Phenol-formaldehyde plastic (PF, Phenolic)
- Silicone rubber (SI)
ADVANTAGES
The simplicity of the machine’s design and process allows for a few unique advantages over injection molding:
- Allows for Inserts: This process is ideal for products with metal inserts.
- Fast and Cheap to Set Up: Inexpensive machinery that is easily assembled means transfer molding is ideal for a quick start-up.
- Lower Maintenance Costs: The machinery involved in transfer molding is much less costly than that of injection molding, meaning maintenance is cheaper.
- Sharper Edges: The transfer molding process, due to the higher pressures, can achieve sharper edges and cutoffs than injection molding.
DISADVANTAGES
Although transfer molding is cheaper and faster to set up, the simplicity of the machine comes with a few disadvantages:
- Waste Material: Transfer molding typically produces more waste, due to the larger sprue and overflow channels. If using thermosetting materials, the scraps produced from this are not reusable.
- Slow: The production speed is much slower than injection molding because of the time needed to preheat the materials before transfer, as well as the lower capacity.
- Smaller Quantity: Transfer molding machines aren’t able to accommodate as many cavities as injection machines, which means fewer units are produced per cycle.
POSSIBLE DEFECTS
A properly made transfer mold is as likely to result in a defective product as an injection mold, and the two processes can result in similar defects. Some possible defects include:
- Flash: This is the occurrence of the molten material seeping out of the mold cavity and solidifying. This can happen if the injection pressure is too high or the mold .has too much clearance.
- Warping: This permanent bending of the part can occur if the part cools at a non-uniform rate.
- Voids: This is often the result of non-uniform pressure distribution, which results in the material folding in on itself and creating voids.
- Ejector Marks: Marks made by the ejector can happen if the cooling time is not long enough to allow the part to fully set, or if the ejection force is too high.
INJECTION MOLDING VS. TRANSFER MOLDING: WHICH IS BETTER FOR YOUR PRODUCT?
Although these two processes share numerous similarities, their differences are most important in determining which method is best to produce a specific product. Here are the most important differences to consider when deciding which method to use:
INITIAL INVESTMENT
Both of these processes require a toolmaker or machinist to build the molds, which is an expensive process by itself. However, the cost of the machinery involved is the biggest factor in determining startup costs. The injection molding machine is significantly more expensive than the press needed for a transfer mold, primarily because of the complexity and specialization of the components within the machine.
It also takes much more time to set up compared to a transfer mold machine, which means it will take longer for a project to get underway. The cost and complexity also means maintenance costs are substantially higher for injection molding machines.
SPEED OF PRODUCTION
Injection molding has a very short process cycle, running anywhere from two seconds to two minutes, depending on the size of the product. The overall production time increases with the removal of any excess material, such as flash or the sprue, but is still much shorter than the production time for transfer molding. The primary disadvantage in transfer molding is that the material is prepared before placement in the machine, increasing the time of the cycle significantly.
COST OF PRODUCTION
Cost of production does not typically favor one method over the other in all cases, but instead depends on the geometry of the product. Materials requiring a high injection pressure would require a more powerful injection machine, meaning it would be more expensive to use an injection machine instead of a transfer machine.
The same can be said for a larger part. However, injection machines are able to accommodate more cavities, increasing the production per cycle. Injection molding also involves more automation than transfer molding machines, meaning long-term labor costs are reduced significantly for high-quantity projects.
PRODUCT SHAPE AND ACCURACY
Both processes allow for impressive accuracy, and both provide very consistent results. However, injection molding doesn’t handle sharp edges very well, and can end up rounding off edges that were meant to be sharp. Additionally, even though both processes can produce units with very complex forms, the cost of doing so with transfer molding and is significantly lower than with injection molding, primarily because complex injection molds require more intricate and expensive injection systems to produce.
AMOUNT OF WASTE
Although flash and sprue waste does happen with injection molds, transfer molding produces much more waste on average. This is primarily due to the presence of a wider sprue, air holes and overflow grooves that are not present in injection molds. If the material involved is thermosetting, this can result in substantial material waste.
PRODUCT SIZE
Both of these methods operate well for small- to medium-sized products, but injection molding has the capacity to create much larger products, up to 80 square feet. Transfer molding is best suited to small and medium part sizes, primarily due to limitations in press sizes.
PRODUCT VOLUME
In terms of the quantity of products desired, injection molding is vastly superior to transfer molding. The relatively automated systems, combined with faster cycle times, make this method much more cost-effective in the long run for high volume projects.
Both injection and transfer molding can produce high-quality products and do so in a very similar fashion. However, injection molding is much better suited to higher quantities of larger, thin-walled parts, while transfer molding is better suited to encasements and small quantities of simpler molds.
Is your company looking to produce custom-made silicone rubber parts? Contact SIMTEC today to speak with one of our experienced representatives about our superior manufacturing capabilities.