Guide to Manufacturing Processes for Plastics - Formlabs

1. Print setup: Print preparation software is used for orienting and laying out models within a printer’s build volume, adding support structures (if needed), and slicing the supported model into layers.
2. Printing: The printing process depends on the type of 3D printing technology: fused deposition modeling (FDM) melts a plastic filament, stereolithography (SLA) cures liquid resin, and selective laser sintering (SLS) fuses powdered plastic.
3. Post-processing: When printing is complete, parts are removed from the printer, cleaned or washed, post-cured (depending on the technology), and the support structures removed (if applicable).

As 3D printers require no tooling and minimal setup time for a new design, the cost of producing a custom part is negligible in comparison with traditional manufacturing processes.

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3D printing processes are generally slower and more labor-intensive than manufacturing processes used for mass production.

As 3D printing technologies improve, cost per part continues to fall, opening up a wider range of low- to mid-volume applications.

While most plastic manufacturing processes require expensive industrial machinery, dedicated facilities, and skilled operators, 3D printing allows companies to easily create plastic parts and rapid prototypes in-house.

Compact desktop or benchtop 3D printing systems for creating plastic parts are affordable and require very little space and no special skills, enabling professional engineers, designers, and manufacturers to speed up iteration and production cycles from days or weeks to a few hours.

CNC machining includes mills, lathes, and other computer-controlled subtractive processes. These processes start with solid blocks, bars, or rods of metal, or plastic that are shaped by removing material through cutting, boring, drilling, and grinding.

Unlike most other plastic manufacturing processes, CNC machining is a subtractive process where material is removed by either a spinning tool and fixed part (milling) or a spinning part with a fixed tool (lathe).

1. Job setup: CNC machines require an intermediary step of generating and validating toolpaths (CAD to CAM). Toolpaths control where the cutting tools move, at what speeds, and any tool changeovers.
2. Machining: The toolpaths are sent to the machine where the given subtractive process begins. Depending on the desired shape of the final product, the workpiece may need to be set in a new position so that the tool head can reach new areas.
3. Post-processing: After manufacture, the part is cleaned and deburred, trimmed.

Machining is ideal for low volume plastic part applications that require tight tolerances and geometries that are difficult to mold. Typical applications include prototyping and end-use parts like pulleys, gears, and bushings.

CNC machining has low to moderate setup costs, and can produce high-quality plastic components with short lead times from a wide range of materials.

Machining processes have more part geometry restrictions than 3D printing. With machining, cost per part increases with part complexity. Undercuts, pass throughs, and features on multiple part faces all contribute to increased part cost.

Machining processes require allowances for tool access and certain geometries, like curved internal channels, are difficult or impossible to produce with conventional subtractive methods.

1. Mold preparation: The mold is coated with a release agent to facilitate demolding, and often preheated to a material specific temperature.
2. Casting: The synthetic resin is mixed with a curing agent and poured or injected into the mold, where it fills the mold cavity.
3. Cure: The casting cures in the mold until it solidifies (subjecting the mold to heat can accelerate the cure time for certain polymers).
4. De-mold: The mold is opened and the cured part is removed.
5. Trimming: Casting artifacts like flash, sprues, and seams are cut or sanded away.

Flexible molds made from latex rubber or room temperature vulcanized (RTV) silicone rubber are inexpensive when compared to hard tooling, but can produce only a limited number (around 25 to 100) of castings as the chemical reaction of urethanes, epoxies, polyester, and acrylic degrades the mold surfaces.

RTV silicone molds can reproduce even the smallest details, yielding high quality cast parts. Stereolithography 3D printing is a common way to create masters for molds directly from CAD designs, in part due to its high resolution and similar ability to produce fine features.

Polymer casting is relatively inexpensive, with little initial investment, but thermoset polymers for casting are usually more expensive than their thermoplastic counterparts and molding cast parts is labor-intensive. Each cast part requires some hands-on labor for post-processing, making the final cost per part high compared to automated production methods like injection molding.

Polymer casting is typically used for prototyping, short run production, as well as certain dental and jewelry applications.

1. Charging: Plastic powder is loaded into the mold cavity, and then the remaining parts of the mold are installed, closing the cavity for heating.
2. Heating: The mold is heated until the plastic powder melts and adheres to the mold’s walls, while the mold is rotated along two perpendicular axes to ensure a uniform plastic coating.
3. Cooling: The mold is slowly cooled while the mold remains in motion to ensure that the skin of the part does not sag or collapse before fully solidifying.
4. Part removal: The part is separated from the mold, any flashing is trimmed away.

Rotational molding requires less expensive tooling than other molding techniques as the process uses centrifugal force, not pressure, to fill the mold. The molds can be fabricated, CNC machined, cast, or formed from epoxy or aluminum at a lower cost and much faster than tooling for other molding processes, especially for large parts.

Rotomolding creates parts with nearly uniform wall thickness. Once the tooling and process are set up, the cost per part is very low relative to the size of the part. It is also possible to add prefinished pieces, like metal threads, internal pipes, and structures to the mold.

These factors make rotational molding ideal for short-run production or as an alternative to blow molding for lower volumes. Typical rotomolded products include tanks, buoys, large containers, toys, helmets, and canoe hulls.

Rotomolding has some design constraints and finished products have looser tolerances. As the entire mold has to be heated and cooled down, the process also has long cycle times and is quite labor intensive, limiting its efficiency for higher volume applications.

1. Clamping: A plastic sheet is clamped in a frame.
2. Heating: The sheet and the frame are moved close to heating elements, which makes the plastic soft and pliable.
3. Vacuum: The frame is lowered, and the plastic is stretched over the mold, while vacuum is activated to suck all the air out from between the plastic and the mold, thus forming the part.
4. Cooling and release: Once the part has been formed over the mold, it must be given time to cool before removal. Cooling system like fans and sprayed mist are sometimes used to reduce cycle times.
5. Trimming: Once the part is released, excess material is cut off either by hand or with a CNC machine.

Tooling costs for vacuum forming are low compared to other molding techniques, due to low forces and pressures involved. Molds are made from wood, plaster, or 3D printed resin for short production runs and custom parts. For high production volumes, manufacturers use more durable metal tooling.

Given the wide scale of thermoforming and vacuum forming machinery available and the possibilities for automation on the high end, thermoforming is ideal for any application from custom products or prototypes to mass production. However, the process offers only limited form freedom and can only be used to manufacture parts with relatively thin walls and simple geometries.

Commonly vacuum formed parts include product packaging, shower trays, car door liners, boat hulls, and custom products like dental aligners.

1. Mold setup: If the part has inserts, these are added either by hand or robotically. The mold is closed by a hydraulic press.
2. Plastic extrusion: Small plastic pellets are melted and extruded through a heated chamber by a screw.
3. Molding: The molten plastic is injected into the mold.
4. Cooling and release: The part cools in the mold until it is solid enough to be ejected, either mechanically or by compressed air.
5. Post-processing: Sprues, runners and any flash (if applicable) is removed from the part, often automatically as part of the mold opening.

Molds for injection molding are highly complex and need to be manufactured to tight tolerances to produce high-quality parts. Due to the high temperature and pressures involved, these molds are machined from metals like hardened steel. Softer aluminum molds are less expensive, but also wear faster, so are typically used for more moderate production runs.

Injection molding can be used to produce highly complex parts, but certain geometries will increase cost significantly. Following design for manufacturing (DFM) guidelines will help to keep tooling costs manageable. Creating new molds for injection molding can take months to complete, and their costs can run into five or six digits.

Despite the high initial costs and slow ramp up, injection molding has no match for high volume applications. Once the tooling is up and running, cycle times only take a few seconds, and millions of high-quality parts can be produced at a fraction of the cost of all other manufacturing processes.

1. Plastic extrusion: Plastic is heated and pushed through a heated chamber by a screw.
2. Molding: Plastic is forced through a die that creates the final shape of the part.
3. Cooling: The extruded plastic is cooled.
4. Cut or spool: The continuous shape is spooled or cut into lengths.

Extrusion machinery is relatively cheap compared to other industrial machines like CNC or injection molding as it is less complex and does not require such high levels of machine accuracy. Due to the simple shapes, dies are also less expensive, with tooling costs that are a fraction of molds for injection molding.

Much like injection molding, extrusion molding is an almost continuous process, which makes the price of extruded parts very low.

Forms and shapes that can be manufactured with extrusion are limited to products that have continuous profiles, such as T-sections, I-sections, L-sections, U-sections, and square or circular sections. Typical applications include pipes, hoses, straws, and window frame moldings.

Introduction

One of the primary challenges in plastic material selection for CNC machining is that not all plastic materials are suitable for a particular part Or application. It’s important to select the right material for any part, but selecting a plastic for machining is particularly tricky.

Some plastics also exhibit significant dimensional instability, impacting tolerances, and some also have limited color options. There are strength considerations, resistance to stress cracking, or heat expansion.  Application-specific considerations such as chemical resistance, food safety, or self-lubrication can also be an aspect.

The potential cost implications and performance issues associated with incorrect material choices cannot be overstated. Using the wrong material can lead to increased scrap rates, longer lead times, and higher overall production costs. It can also result in parts that do not meet the required specifications, leading to quality issues and customer dissatisfaction.

Fig. 1: CNC Machined Plastic Parts

How to Select a Plastic Material for CNC Machining

Selecting the right plastic material for CNC machining is indeed a critical decision, as it significantly influences the performance, cost, and quality of the final product. Several factors need to be considered when choosing a material:

Consider the part type

• Is it a bushing, an enclosure, a shaft, a gear?
• What are the characteristics of this type of part? Does it need to be smooth and frictionless, or is strength the priority

Consider Part Use

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Determine the specific requirements of the part, including mechanical properties (strength, hardness, etc.), dimensional tolerances, surface finish etc.

Consider the application

• Does your target application have any special requirements for the parts in terms of electrical conductivity, self-lubrication, etc.?
• What kind of environment is the application – is it highly corrosive? Is there a high degree of exposure to UV or weathering? High heat?
• Are there regulatory or certification requirements?

Finalize Requirements & Selection Criteria

Based on the product and application requirements, establish criteria for selecting the plastic type, considering factors such as machinability, cost, availability, and suitability for the application.

• Shortlist Material Options - Shortlist your potential materials and then evaluate trade-offs between material properties, cost factors, and application requirements.
• Consult References: Refer to material selection guides, handbooks, and databases to compare the properties of different plastics and their suitability for CNC machining.
• Cost Analysis: Compare the cost of each material, including raw material cost, machining costs, and any additional costs (e.g., surface treatment, post-machining operations).
• Final Selection: Based on the above considerations, select the plastic material that best meets the requirements for the CNC machining process.

Fig. 2: Simple CNC Machined Parts Made of Plastic

Factors for Selecting a Plastic Material for CNC Machining

Material Properties

Tensile strength, yield strength, hardness, and ductility are important mechanical properties. Higher tensile strength and hardness are desirable for parts subjected to high loads, while ductility is important for parts that require deformation without breaking.

Thermal Properties

Thermal conductivity and coefficient of thermal expansion are important for parts subjected to temperature variations. High thermal conductivity helps in dissipating heat generated during machining, while a low coefficient of thermal expansion ensures dimensional stability at different temperatures.

Chemical Properties

Corrosion resistance and chemical compatibility are crucial for parts exposed to corrosive environments or chemicals.

Machinability

Factors such as material machinability, complexity of the part, and machining time. Materials that are easy to machine, may result in shorter machining times compared to harder materials.

Costs

Cost-effectiveness in material selection involves evaluating the cost per kilogram of material and the machining time required to produce the part. Some materials may have a higher initial cost but offer cost savings in terms of reduced machining time.

The Komacut Advantage

Many companies struggle to find reliable and cost-effective suppliers for low and medium-volume production – prototyping companies charge high set-up costs and per-piece pricing; larger manufacturers are not interested in orders of several hundred or even thousand pieces. Not to mention the time and expertise needed to manage and coordinate with these suppliers.  Komacut is a solution to bridge the gap between prototyping and mass production, with instant quotes, easy management, and flawless execution from quote to order to delivery.

For more information on designing and manufacturing CNC machining and machined parts, please visit the Komacut website. Instant quotations and design feedback, a range of materials, engineering design guidelines, and more - visit Komacut today for superior CNC machining solutions.

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FAQs

What are the most commonly used plastics in CNC machining?

Commonly used plastics in CNC machining include ABS, polycarbonate, nylon, PEEK, Delrin (acetal), and PTFE (Teflon). Each of these materials offers unique properties suitable for various applications.

How do I determine which plastic is best for my project?

To determine the best plastic for your project, consider factors such as mechanical properties, thermal resistance, chemical compatibility, and cost. Additionally, the specific requirements of your application, such as load-bearing capacity, wear resistance, and environmental conditions, should guide your selection.

What are the advantages of using plastic over metal in CNC machining?

Plastics offer several advantages over metals, including lighter weight, corrosion resistance, lower cost, and ease of machining.

How does the machinability of plastics compare to metals?

Plastics generally have better machinability compared to metals due to their lower hardness and density. They require less cutting force and can be machined at higher speeds, reducing tool wear and production time. However, some plastics may have specific challenges, such as melting or deforming under high temperatures, which need to be managed during machining.

What surface finishes can be achieved with CNC-machined plastics?

CNC-machined plastics can achieve a variety of surface finishes depending on the machining process and tools used. Common finishes include a smooth, polished finish, matte or textured finishes for improved grip or reduced glare, and precision finishes for tight tolerance parts.

Are there any special considerations for machining reinforced plastics?

Reinforced plastics, such as those with fiberglass or carbon fiber, require special considerations due to their abrasive nature. These materials can cause increased tool wear and may require the use of carbide or diamond-coated tools.

How does production volume impact the choice of plastic material for CNC machining?

For low production volumes, CNC machining is a cost-effective method for producing plastic parts and is often used for prototyping or pilot productions due to the lack of any requirement for mold or tooling investment. However, for medium to high-volume production, other manufacturing methods such as vacuum casting or injection molding will have a definite cost advantage.

Can recycled materials be used in CNC machining?

Yes, certain recycled materials, such as recycled PET (rPET), can be used in CNC machining. They can offer cost savings and environmental benefits but should be evaluated for consistency and performance compared to virgin materials. In general, plastics with a high (25%+) percentage of recycled content tend to be more brittle and have degraded physical properties.

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