Structural Foam versus Injection Molding: Advantages and ...

Design engineers responsible for creating equipment enclosures often face the challenge of choosing the best manufacturing process for custom injection molded parts. Structural foam and straight injection molding are two popular options. Ferriot practices both methods of fabrication and can offer its expertise to help determine which option will best suit any given application, to meet your project’s objectives and specifications.

With competitive price and timely delivery, WIT MOLD sincerely hope to be your supplier and partner.

Factors that design engineers must consider regardless of the part requiring manufacturing include tooling costs, production volume, material selection and part complexity. The chosen injection molding method and plastic injection molder must also be capable of producing parts that meet the required quality standards and tolerances, as well as production timelines and deadlines. Careful evaluation of the advantages and disadvantages of each method should be weighed carefully against the specific project requirements.

What defines structural foam molding and injection molding?

Structural foam injection molding involves injecting thermoplastic material into a mold. The packing stage is augmented with a chemical blowing agent mixed with the resin. Heat triggers the chemical blowing agent. It expands the material, creating a microcellular structure at the core of the object or product, with an integral outer shell or skin.

Structural foam can be used for parts ranging in size from small to large but is particularly well-suited for larger parts with thicker walls. The cost-effectiveness of the process increases with larger production runs, to spread out the impact of the tooling cost and production set up time.

This results in a part that has:

• High strength to weight ratio: the foam core reduces the weight of the part while maintaining its strength and rigidity. While actual weight savings can vary, parts built using structural foam molding can be 10-30% lighter than other parts. The integral skin contributes to added strength and the entire process creates parts with low stress.
• Cost effectiveness: the process can manufacture large parts for a relatively low cost.
• Good dimensional stability: the level of stability of the finished part shows good resistance to warping and/or distortion.
• Good thermal and acoustic insulation: the foam core lends the finished piece excellent thermal and acoustic insulation properties making them useful in underground and outdoor applications.
• A few of the limitations of structural foam manufacturing for design engineers to consider include:  Longer cycle times: compared to other manufacturing methods, this process can take longer than other types of injection molding methods, as the foam core takes additional time to expand and fill the mold.
• Produces a swirling pattern on the surface which might not be desired where cosmetic finishes are required but can be sanded and painted when needed.

Traditional injection molding is typically done in two stages. There’s an injection stage where melted thermoplastic is injected into a mold, and a packing stage where pressure is built, and as the molten material solidifies, it takes on the shape of the mold cavity.

Straight Injection molding is ideal for small to medium-sized parts that require tight tolerances and a smooth surface finish. It is also often used for parts that require high production volumes, as it can be automated for efficient and consistent production. This process offers design engineers:

• High production efficiency: The process is highly automated and can produce large quantities of parts with consistency and accuracy.
• High accuracy and precision: Parts can be created within tight tolerances and with intricate geometries.
• Versatility: Injection molding can produce a wide range of parts from small components to large pieces with complex structures.
• Cost efficiencies over time: Once the mold is set up, the process runs automatically, requiring minimal labor.

Some of the limitations of plastic injection mold manufacturing can include:

• High upfront tooling costs: The design and particularly tooling can be expensive, with costs rising alongside the size of the piece, or for smaller production runs.
• Long lead times: The tooling and mold design can take several weeks or even months.

Factors that tip the balance to one method over another

As a custom injection molder with broad expertise in multiple methods of fabrication, we can point our customers to structural foam molding when traditional injection molding may not meet part design requirements.

The structural foam process creates parts with a high strength-to-weight ratio and is often used for large parts and in metal to plastic replacement.

Structural foam molding also scales well, allowing large or bulky parts to be manufactured while still retaining the superior production speed offered by injection molding. Regardless of size, structural foam parts can be worked post-production in a manner similar to parts constructed of wood or metal. The surface of a finished part is smooth, offering the potential of easy cleaning and can be painted over.

An additional benefit to parts manufactured using structural foam molding is part durability. As a result of the difference in process, parts built employing structural foam molding are sturdy while remaining rigid, and possess greater internal flexibility than parts built using traditional injection molding.

There are many ways to achieve cost savings when considering structural foam:

• Design flexibility allows for part consolidation – stronger, greater wall thickness, lighter weight.
• There is great flexibility in resin selection as even commodity resins can be used.
• The low-pressure process allows large parts to be produced with low tonnage machines which are less costly to run
• Converting a metal part to a plastic part can save weight and manufacturing cost.

Ferriot employs structural foam molding to build a variety of parts, including gas pump front facia. Alongside durability, material options available for structural foam molding also means a finished product can be chemical resistant and/or offer electrical or thermal insulation, suitable for outdoor applications.

Working with an experienced injection molding company

Ferriot has focused on thermoplastic injection molding since the late s, earning a leadership position among plastic injection molding companies due to our focus on quality and service. Our clients turn to us for professional assistance that spans the entire project lifecycle from product design assistance to custom mold design and manufacturing, resin selection, custom injection molding, painting and assembly.

At our manufacturing facilities we offer painting and decorating as part of our value-added services. Within this category, we provide EMI/RFI Shielding of parts, to protect medical equipment and other devices from electromagnetic and radio frequency interference. Methods employed for medical injection molding can successfully transfer to supplying similar protection for sophisticated business and technical equipment as well.

In conclusion, design engineers should work with their custom molder to evaluate the specific requirements of their equipment enclosures to determine whether structural foam or injection molding is the best process for their custom injection molded parts. Structural foam is ideal for larger parts that require strength and durability with a lower weight, while injection molding is ideal for smaller parts that require tight tolerances and a smooth surface finish.

Considering structural foam molding as an alternative solution? We encourage you to contact us to discover how structural foam molding can benefit your business.

Learn more:

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 , 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 , 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:

For more information, please visit custom plastic molding.

1. 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.
2. 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.
3. The melt is left inside the mold and is either cooled down to solidify (thermoplastics) or heated up to cure (thermosets).
4. 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:

1. The material (which may or may not be pre-heated) is placed into a holding chamber called the transfer pot.
2. A hydraulically powered plunger pushes the material through a channel, called the sprue, into the mold cavity.
3. 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.
4. When the material is solid, the mold is opened and the part is ejected.
5. 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.