Choose the cut from the material and the feature

Separating removes material. A saw makes a blank, a drill makes a round hole, and a router or mill can make an outline, pocket or slot. Identify the resin and grade before choosing the process: two clear sheets can be acrylic and polycarbonate, with very different cutting behavior.

See Milling Operations for tool paths and feature access. Machine use belongs with Equipment Operating Procedure and Machine Hazards and Housekeeping.

Make chips instead of rubbing the plastic hot

Heat builds up near the cut because plastics generally conduct heat poorly. Softened chips can stick to the tool or weld back onto the edge. The symptoms are smeared surfaces, strings of softened material and chips packed into the cutter.

Slowing the feed alone can make melting worse. Feed is how quickly the tool advances through the work; spindle speed is how quickly it turns. Too little advance per tooth lets the edge rub instead of taking a useful chip. Start with cutting data for the resin and tool, keep the tool sharp, and maintain chip clearance. Reducing excessive spindle speed may help, but feed, speed and cut depth must be considered together.

Deep holes need a planned way to clear chips. Coolants and cleaners must be compatible with the plastic; an unsuitable liquid can cause fine cracks called crazing. Use Feeds and Speeds for the relationships, not its metal cutting values as plastic settings.

Supporting detail: Addendum 1 — Why a slower cut can produce more heat.

Acrylic, PVC and polycarbonate behave differently under a laser

Acrylic (PMMA) absorbs the infrared energy of a CO₂ laser. Concentrated heating removes material along the cut and can leave a smooth, glossy edge. That result depends on the sheet grade, thickness and process settings; it is not a promise for every clear plastic or every laser.

PVC is not a laser-cutting substitute for acrylic. Heating PVC can release hydrogen chloride; with moisture it forms hydrochloric acid, creating an exposure and corrosion concern. This is a chemical incompatibility, not simply a poor-looking edge.

Polycarbonate is often a poor choice for a clean CO₂-laser-cut sheet edge. It can yellow, char and leave a degraded edge instead of acrylic's clear finish. Specialized processes can cut some polycarbonate, so “polycarbonate cannot be laser processed” is too broad. Routing or sawing is often the better starting choice for a clear polycarbonate profile. Do not assign PVC's chlorine chemistry to polycarbonate.

Material approval and operating controls belong with Equipment Operating Procedure and Chemicals and Safety Data Sheets. Appearance or a trade name is not enough to identify an unknown sheet.

Supporting detail: Addendum 2 — Manufacturer references and the limits of a material list.

Trim the molding excess without cutting into the part

A gate is the entry from a molding flow channel into the part cavity. Degating separates the part from that feed connection. Flash is a thin unwanted fin, often at a mold joint; trimming removes it. A burr is a raised edge left by cutting. These are different defects, even when the same trimming tool can remove them.

Support the part near the cut and leave the required contour intact. Cutting into the wall creates a notch that may start a crack. For batches, tumbling with suitable media can remove small burrs, but it can also round wanted edges or damage thin features. Inspect the dimensions and surface after the trial.

Practice diagnosing a melted cut

Solvent cement softens the surfaces; adhesive adds a bond layer

Solvent cementing uses a compatible liquid to soften or dissolve the surfaces so polymer chains can mix across the joint. As solvent leaves, the joint develops strength. It is chemical softening, not heat melting. Acrylic-to-acrylic is a common application; PVC and ABS also have specific cement systems. The cement must match the resin and fit of the joint.

Adhesives bond through a separate layer. Epoxies, cyanoacrylates and other formulations have different gap, flexibility and temperature limits. “Plastic glue” does not mean it bonds every plastic. Allow the specified cure time before loading; a joint that holds itself together may not have reached service strength.

Polyethylene (PE) and polypropylene (PP) resist many ordinary adhesives. Their low surface energy makes it difficult for an adhesive to spread into intimate contact, called wetting. Specialized adhesives or a controlled surface treatment can help; sanding alone is not a universal fix. See Chemicals and Safety Data Sheets for chemical handling.

Supporting detail: Addendum 3 — Why a clean-looking bond can peel off.

Welding needs compatible thermoplastics and a designed joint

Plastic welding heats the joint region so compatible thermoplastics can fuse. Cooling while the joint is held in position completes the connection. Cured thermosets do not remelt into a comparable fusion weld. Even two thermoplastics are not automatically weld-compatible.

Supporting detail: Addendum 4 — Why heat alone does not guarantee a strong or sealed weld.

Hot-melt adhesive is not a fusion weld

A hot-melt adhesive is applied molten and usually gains handling strength as it cools. The adhesive melts; the two parts do not have to melt together. Choose the formulation for both surfaces, the available assembly time and the service temperature. An ordinary hot-melt joint may soften or creep under sustained load in warmth. Fast setting is not the same as suitability for structural service.

Use mechanical joints when access and disassembly matter

A snap fit bends a feature during assembly so it catches behind another feature. It needs room to flex, rounded transitions and a resin that tolerates the required strain. Some snap fits release repeatedly; others are intentionally difficult to undo.

Thread-forming screws for plastics create their mating thread in a designed hole. Repeated removal can wear plastic threads. A threaded insert supplies a metal thread; a heat-set insert is installed by locally softening a thermoplastic around it. Hole size, surrounding wall thickness and installation conditions come from the insert supplier. A metal insert cannot rescue an undersized plastic wall.

Through-bolts and suitable washers can spread the clamping load. Excessive tightening can crack brittle plastic; sustained load can cause creep, a gradual change in shape, and reduce clamping force. Allow for movement and thermal expansion rather than trapping a sheet tightly at every hole.

Compare two cases: a clear permanent acrylic corner is a candidate for compatible solvent cement; a PP cover that must be removed regularly is a candidate for screws and a suitable thread arrangement. An adhesive that works on the first pair tells you little about the second.

Supporting detail: Addendum 5 — Insert dimensions come from the chosen insert system.

Practice choosing a removable joint

A slicer turns a solid model into deposited layers

Material extrusion is the standard family in which material is selectively dispensed through a nozzle. This page covers thermoplastic filament printing, often called fused filament fabrication (FFF). Fused deposition modeling (FDM) is a registered trademark of Stratasys for an implementation of this mechanism. The machine feeds filament into a heated nozzle and lays down narrow roads of softened material that bond as the part builds. Relative motion between the nozzle and build plate advances the layers; the nozzle itself does not have to move upward on every printer.

A slicer converts the model into tool paths, including walls, internal fill and any temporary supports. Preview those paths before printing. An enclosed shape on a CAD screen does not tell you how its underside will be supported.

Compare material extrusion with the other standard routes in Additive Manufacturing Process Families.

Supporting detail: Addendum 6 — Where additive manufacturing fits the five course categories.

Layer height, walls and infill do different jobs

Layer height is the vertical increment between layers. A smaller height makes steps on sloping surfaces finer and usually adds print time. It does not make all small horizontal features printable: nozzle size, road width and motion accuracy also matter.

Walls or perimeters trace the boundary. Top and bottom layers close the part. Infill is the pattern inside those boundaries. More infill generally adds material and time, but strength also depends on walls, orientation, bonding and load direction. A 20% infill setting does not mean the entire part uses 20% of its solid volume, because the walls and skins remain.

Simple layer-count example: for a 24 mm height with every layer 0.20 mm thick, 24 ÷ 0.20 = 120 layers. Real slicers may use a different first-layer height or variable layers. Halving layer height roughly doubles the layer count, not necessarily the total print time.

Choose filament for service conditions and printer capability

PVA is used as a water-soluble support material in compatible multi-material systems; it is not a universal companion for every filament. Use the exact filament manufacturer's settings and the printer's limits. Consult Equipment Operating Procedure and Chemicals and Safety Data Sheets for process controls.

Supporting detail: Addendum 7 — Material guidance is grade-specific.

Estimate mass from deposited volume, not infill alone

Mass = deposited volume × density. If a simplified model has 8 cm³ of solid walls and skins plus a 32 cm³ interior at 25% infill, its estimated deposited volume is 8 + (32 × 0.25) = 16 cm³. At an assumed density of 1.24 g/cm³, that is 19.84 g. Supports, brims and purge material add more.

This estimate treats infill percentage as a volume fraction of the interior. A slicer estimates the actual paths more closely. Time also includes travel, acceleration, heating and cooling constraints; volume alone does not give an exact printing duration.

Post-processing choices are in Conditioning and Finishing Plastics. Practice estimating filament mass

Conditioning changes properties; finishing changes the surface

Conditioning changes the material's state or properties: relieving internal stress, developing a thermoset cure, or controlling absorbed moisture. Finishing changes the surface for appearance, protection or function. A part can need both. Removing a support is Separating; sanding the resulting surface is Finishing.

Start with the resin, dimensions and service requirement. A glossy surface does not prove that a part is strong, properly cured or dimensionally correct.

Annealing relieves stress, but can change dimensions

Annealing uses a controlled heating, holding and cooling schedule. In acrylic it can reduce stress left by machining, bending or polishing. It is not a general instruction to heat near molding temperature. Use the resin supplier's schedule for the grade and thickness, including support of the part during treatment.

Printed PLA can also be annealed. Depending on its formulation and thermal history, heating can allow more crystalline order and improve heat resistance. Shrinkage, expansion in another direction or warping can spoil a fit. Measure before and after; do not assume every strength property improves.

Supporting detail: Addendum 8 — Stress relief and crystallinity are different changes.

Curing creates a network; drying removes moisture

A thermoset cure is a chemical reaction that builds links between polymer chains. Resin and hardener, heat or light may initiate the reaction, depending on the system. Correct proportions, time and temperature matter. A prescribed post-cure can develop properties further; an already cured thermoset will not simply remelt like a thermoplastic.

Drying moisture-sensitive filament addresses a different problem. Moisture can affect extrusion and part quality, but drying does not replace annealing or cure. Use a drying schedule intended for the material; excessive heat can deform filament or its spool.

Supporting detail: Addendum 9 — Other ways processors change plastic properties.

Remove scratches progressively before adding gloss or paint

Sanding replaces a coarse scratch pattern with progressively finer scratches. Start with the finest abrasive that will remove the defect efficiently. Within the same grit system, a larger number generally means a finer abrasive. A sequence such as P240 → P320 → P400 → P600 illustrates progression; it is not a required recipe for every plastic or coating.

Remove the previous scratch pattern before moving on and clean away abrasive residue between stages. Wet sanding with suitable waterproof abrasive can reduce loading and heat where the material and equipment permit it. Keep corners and fit surfaces from rounding over. Paint may need a controlled scuffed surface rather than a polished one; follow its preparation specification.

Tumbling can finish many parts together, but media and time affect edge rounding and small features. Buffing can improve gloss, but excessive heat or pressure can smear the surface. Check a representative sample rather than equating more polishing with a better part.

Flame and solvent smoothing are material-specific processes

Flame polishing can smooth suitable acrylic edges by briefly softening the surface. It can also round edges and introduce stress. It is not a general treatment for clear plastics: polycarbonate is not interchangeable with acrylic, and a process must be validated for its exact grade. A clear edge is not necessarily an optically accurate or low-stress edge.

Solvent or vapor smoothing softens a compatible polymer's surface chemically. Acetone smoothing is associated with ABS and ASA; do not transfer that recipe to PLA or PETG. Smoothing can erase fine lettering, change dimensions and alter mechanical performance. It does not guarantee that weak layer bonds inside a print have been repaired.

These descriptions explain process selection, not operating procedures. Refer to Chemicals and Safety Data Sheets, Fire Safety and Equipment Operating Procedure for the relevant controls.

Supporting detail: Addendum 10 — Why a clear surface can crack later.

Coatings need adhesion as well as the right appearance

For paint, remove contamination with a compatible cleaner, prepare the surface as specified and use a primer or adhesion promoter when the coating system calls for one. PE and PP often need special preparation or products because of low surface energy. A primer is not automatically compatible with every plastic. Trial the complete system, including cure, on a representative sample.

Dip coating deposits a liquid film as the part is withdrawn, followed by drying or curing as the formulation requires. Withdrawal rate and liquid properties affect thickness. Powder coating plastics requires a suitable way to attract or deposit the powder and a cure that the part can withstand; an insulating plastic does not become conductive merely because a grounding wire touches it.

Electroplating needs a conductive surface. Plastics therefore require preparation and a conductive layer before electroplating. Decorative options also include printing, decals, in-mold labels and dyes for receptive polymers. Choose them for the required abrasion, outdoor exposure and chemical resistance; decoration is not automatically protection.

Supporting detail: Addendum 11 — Powder coating needs a complete substrate and cure system.

Practice choosing a finish