TECHNIQUES OF PACKING ACRYLIC RESINS

Acrylic resins are a group of related thermoplastic or thermosetting plastic substances derived from acrylic acid, methacrylic acid or other related compounds.[1] Polymethyl acrylate is an acrylic resin used in an emulsed form for lacquer, textile finishes, adhesives and, mixed with clay, to gloss paper. Another acrylic resin is Polymethyl methacrylate which is used to make hard plastics with various light transmitting properties.

Packing:Allow the flasks to cool to room temperature and paint all surfaces. EXCEPT THE TEETH, with tin foil substitute until shiny.Prepare a mix of acrylic resin sufficient for a denture (30 gr. polymer to 30 cc monomer is enough for the average case). Follow manufacturers' directions for proper mixing and packing consistency. See dental materials section at the end for characteristics of polymerization.

1-Standard Injection Molding

Successful molding of Acrylic resins begins with an appropriate choice of machine and screw.  Using an unsuitable molding machine or screw design

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can lead to poor material homogeneity, an inconsistent molding process and machine and/or screw wear.  All of these factors can adversely affect the quality of the molded parts.

Incorrect pre-drying, atypical machine settings, or machine and mold problems can lead to surface defects or part failures.

(a)Clamp Force Calculation:Viscous polymers require high pressure to force the melt through the nozzle, runner system, and gate into the mold cavity.  The pressure that develops in the mold cavity tends to push apart the mold plates.  The clamping unit must have adequate locking force to keep the mold fully closed during the several injection steps.  Insufficient locking force leads to flash on molded parts, dimensional variation, and can even damage the mold parting line or accelerate mold wear.  Depending on the polymer being processed, a reasonable estimate of required clamping force is three to five tons per square inch times the total projected area at the parting line of part(s) and runner(s). Higher viscosity grades, such as, highly filled compounds, may require higher pressure.

(b)Screw Speed Calculation:Screw rotation speed (rpm) has a major effect on the quality of the polymer melt.  Too low a speed increases the cycle time, while high screw speed may thermally degrade the polymer.  The required screw speed for a specific polymer depends on the polymer viscosity and the screw diameter.  Our Product Tool (link located on the upper right site of this page) contains recommend

2-Special Injection Molding

New Injection techniques have been developed,such as: multicomponent and hard/soft injection molding, MuCell’s microcellular foam process, and gas assisted injection molding.

(a)MuCell

Microcellular plastic foams can be made by mechanically or chemically dispersing a gas, usually carbon dioxide or nitrogen, into the polymer melt.

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A mechanical process employing supercritical fluids for this purpose, developed and licensed by Trexel Inc., is called MuCell (µCell).  Further physical foaming methods are Optiform (Sulzer Chemtech) and Ergozell (Demag Ergotech).

The critical point is the highest temperature and pressure at which a substance can exist as a vapor and liquid in equilibrium.  A gas above its critical temperature becomes a supercritical fluid. Carbon dioxide at about 1100 psi or nitrogen at about 750psi become supercritical and dissolve into the polymer melt.  As molding pressure decreases, the gas undissolves from the polymer to form a uniform cellular structure.  Such foams have cell sizes ranging from 5 - 100 microns and exhibit excellent mechanical properties and can provide thermal and acoustic insulation.  

The benefits of microcellular foaming include:

Reduced weight Improved flowability Lower apparent viscosity Less warpage, shrinkage and molded-in stress Improved cycle time Reduced clamping force

General observations regarding the MuCell process include:

Nitrogen yields smaller cells while carbon dioxide provides better flowability

Thin walled parts can be foamed Cycle time can be reduced by 40% Weight reduction up to 30% is possible, for technical demanding parts

the weight saving lies between 8 - 15% The advantages of physical foaming can be good combined with film-

or textile injection molding. To make best use of the physical foaming method, the tool needs to be

optimized (temperature, gating,first cut, ventilation) and potentially adapted to the process.

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(b)Multi-Component Molding

Two-shot molding, co-injection, and hard-soft combinations of thermoplastics are all variations of Multi-Component molding. This is a multi-step molding process that produces an assembly, comprised of two or more integrated components.  Ticona’s expertise in this area means that we can provide part design recommendations, possible material combinations, tool and machine recommendations, and other useful processing parameters. We look forward to working together with you.

Multi-component processes enable one to:

Produce parts not possible with single shot molding Produce multi-colored or multi-resin parts and assemblies for

functionality Produce parts with movable segments or components using “in-mold

assembly” Eliminate some post-molding assemblies such as snap-fits, ultrasonic

welding, adhesives, screws, or bolts Eliminate other operations

Typically co-injection or “sandwich” molding is the injection of a skin to partially fill a cavity, followed by the core component to pack out the part. This process can use two injection units and rotary molds designed for sequential injection, or a robot transferred mold.

Two-shot injection molding allows the first material to cool before the second one is injected.  Critical variables such as draft and mold temperature must be considered when using either of these processes.

Applications possible with two-component molding include: gaskets, parts with molded-in seals, parts with shock-absorbing or soft grip features, acoustic dampening, flexible hinges, multi-color parts, toys with movable parts, and automotive AC or heater louvers (frame, vanes and connecting pieces).

Combining hard polymers with soft materials in a 2-shot molding process can enable new functions:

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Haptic elements on hard surfaces   Inject damping elements New designs (such as complicated gaskets) are made possible Balance tolerances

Hard/Soft combinations of materials currently are used in applications such as: control elements for venting, levers, radio, or lights; conveyor chain links; clips with a gasket or damping element; a sensor housing head lamp regulator; a clamping device for an automotive CD player; linear drive gaskets; a thermos mechanism; a housing with rubber gasket, and a lamp socket for a washing machine.  Our work with soft component suppliers has resulted in new patented hard-soft combinations(with outstanding adhesion) for hard materials such as: Hostaform® /Celcon® POM, Fortron® PPS, Celanex® PBT, and Celstran® LFRT. We can provide testing data and “know-how” for resin combinations to obtain a synergistic and complementary system.

Normal Steps for Multicomponent Molding

1. Resin A is used in Shot #1.2. Resin B is used in Shot #2.  The molding machine can be constructed to

achieve this or the mold can be actuated in some fashion to accommodate Shot#2.  The Shot #2 resin may form an adhesive (or mechanical) bond with the Shot #1 resin. It could also be constructed to move in some fashion in, around or about the part molded from Shot #1.

3. A third (or fourth) shot can be employed to make an even more complex assembly.

4. The assembled part is ejected from the mold.

Processing Considerations

Machine/Mold:

The injection molding machine will need to have 2, 3, or 4 plastication/injection units (one unit for each different resin).

The molds are more expensive than standard molds since the cavity blocks may rotate between Shots #1 and #2, or other mold components

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may need to actuate/articulate in some fashion between shots.  The required control systems may also increase the mold costs.

The machine’s multiple screws can be arranged in a Vertical position (90 deg.), an L-position, or a “Piggyback” arrangement as shown below.

Tool Designs:There are also several types of tool designs such as:

Core Back  - A sliding core is first closed and Melt 1 is injected, then the sliding core is opened and Melt 2 is injected.

Rotating Plate - This two-station tool rotates in a vertical or horizontal direction for the injection of Melts 1 and 2.

Index - The mold is physically transferred from one point to another.

In addition to the tool design, one should also consider the wall thickness, the surface structure of the part from Shot #1 (for venting problems), the tool surface and temperature (for demolding), the gating location (for adhesion in dependence of flow path), the kind of contact (flat or overflow), and how the part will be demolded (force in the adhesion area).

Materials:

Resins A, B, C, etc. should be generally compatible, with no antagonistic effects between resins.

Where movement of assembly components is desired, resin shrinkage becomes an important factor.

Code and agency requirements (such as FDA, UL, etc.) should be factored into decisions regarding materials. It is possible that the complete assembly would need to be tested.

Plans for a multi-shot molded part should be reviewed with all of the material suppliers involved. Information such as the melting point, energy transfer, surface tension, molecular wt, rate of crystallization,

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mold release, internal lubricants, reinforcement ingredients, pigments, and stabilizers should all be considered.

Process Variables:

Use the recommended molding conditions for each material at the outset and then refine them if necessary (molding conditions can vary widely between materials).

The resin having the highest melting point or glass transition temperature should normally be “shot” first  

It is often desirable to preheat parts to be overmolded to achieve better adhesion

Coating parts to be overmolded occasionally can be useful to achieve better adhesion.

NEVER try to use multi-shot regrind unless it has been thoroughly studied (as severe degradation could occur).

Other processing variables that should be studied are the melt temperatures of the two materials, the tool temperature, the injection speed, the hold pressure, screw retraction, and air traps

Testing:Multicomponent parts should be tested at temperatures above those expected in end-use and exposed to thermocycling to expose any problems with expansion/contraction.  The tests should also rule out any antagonistic relationships between the two resins.

3-Fluid Assisted Injection Molding

Injection molding processes can often be improved by pressurizing the melt in the mold cavity with either gas or water at an appropriate point in the molding process.  Both of these processes can be used with Ticona’s engineering resins.  Ticona can provide processing assistance at molding trials.

4-Gas-Assisted Injection MoldingLarge stress-free components can be produced in this process.  In operation,

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the mold is partially filled and then gas is introduced into the melt stream (near the point of injection) at a very high pressure.  The gas pressure pushes the plastic into the tool to pack it out, and a hollow cross-section is created in the part.  Gas assisted injection molding offers several advantages including weight savings, fast cycles, improved flow length and dimensional stability, as well as reduced warpage and sink marks.  Gas injected parts also have a higher strength/weight ratio.

In general, the minimum amount of resin, gas injection time and gas pressure necessary to produce a good part should be used.  Gating should be designed to facilitate melt flow and gas entry points should allow the gas to flow in the same direction as the resin.  The gas injection point leaves a hole in the part and so may need to be located in an inconspicuous location.  Gas channels should be cut steel safe until the tool has been sampled and the process is optimized.  

5-Water-Assisted Injection MoldingThis process is somewhat similar to the gas assist process, however water is used as the medium.  Since water is incompressible, it can produce better-shaped hollow parts with good control and smoother surfaces.  It can also reduce the cycle time due to the additional cooling from the water.

6-Blow Molding

Many of Acrylic resins are capable of being processed via blow molding, and are used in such applications as: small fuel tanks for lawn care tools, fuel floats, chemically resistant containers, HVAC or coolant ducts, multilayer structures, and pharmaceutical containers.  

There are 4 basic types of blow molding:

(a)Continuous Parison Extrusion:In this process an extruder produces a parison continuously. Two alternating molds or a number of molds in a rotary table (carousel) capture a length of parison where it is then blown into a part.

(b)Intermittent Parison ExtrusionAn extruder continuously produces molten plastic, which fills an

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accumulator.  A piston is used to push the plastic through a die into a parison shape.  This parison is then captured by a mold where it is blown into a part.

( c)Injection Blow Molding:A two-step process in which a preform is molded and kept in a near-molten condition.  It is then indexed into another mold and blown into a finished part.

(d)Stretch and Blow Processing: An amorphous preform is molded and then heated to a temperature above its glass transition temperature.  The preform is stretched (elongated) by a push rod and then blown into a container (bottles for soft drinks are produced from PET using this process).

Parison programming can also be used to vary the wall thickness over the length of the part or to maintain a uniform wall thickness for a complex part with areas of high blow up ratio.

Material Requirements

Good melt strength (high zero shear rate viscosity & low melt index) Good melt stability (retention of molecular weight at processing

temperatures) Reasonable “blow ratio” capabilities Good weldline strength

Most resins can be accommodated using injection blow molding, whereas the stretch blow molding process demands a high viscosity at the processing temperatures.

7-Compression Molding

Originally developed to manufacture composite parts for metal replacement applications, compression molding is mostly used to make larger flat or moderately curved parts such as hoods, fenders, scoops, spoilers, lift gates and the like for automotive end-uses.  Although this technology is the main processing method used with thermoset resins, it can also be employed to process themoplastic resins such as Ticona’s Compel® and Celstran® long

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fiber reinforced thermoplastics (LFRT), and GUR® ultrahigh molecular weight polyethylene (UHMWPE).  

The schematic below shows the process equipment.

Thermoplastic resins may be loaded into the mold either in the form of pellets or sheet, or the mold may be loaded from a plasticating extruder. Thermoplastic materials are heated above their melting points, formed and cooled.  For both thermosets and thermoplastics, the better the feed material is distributed over the mold surface, the less flow orientation occurs during the compression step.

8-Rotational Molding

Rotational molding (or Rotomolding) is a method that involves the slow tumbling, heating, and melting of a thermoplastic powder in a biaxially rotating mold to produce seamless, hollow plastic parts.  This process is typically used to mold hollow parts, especially those with complex and varied shapes not easily obtainable by other hollow-art processes.  It is a virtually shear-free and pressure-free process.  The wall thickness uniformity and part weight can be easily maintained.  There is very little waste of material due to scrap.  Rotomolding molds are often less expensive than other types of molds.

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Lower viscosity resins are typically used in this process as they are more readily sintered to ensure a good surface finish.  The material must be capable of being ground into a 35-mesh powder that flows like a liquid. It must also be able to adhere to the hot surface of the cavity and fuse together without pressure.  The material should be stabilized to resist oxidation, or a nitrogen purge must be used during processing.

Process Steps

1. The rotomolding process is begun by placing a pre-measured amount of plastic material (in either liquid or powder form) in a cavity.  The mold is then closed and indexed into an oven where it and its contents are brought up to the molding temperature.  As the mold is heated, it is rotated continuously about its vertical and horizontal axes.  This biaxial rotation brings all surfaces of the mold in contact with the plastic material.

2. The mold is rotated within the oven until all the plastic material has been picked up by the hot inside surfaces of the cavity and densifies into a uniform layer.

3. While continuing the rotation, the machine moves the mold out of the oven and into the cooling chamber.  Air, or a mixture of air and water, cools the mold and the layers of molten plastic material. This cooling process continues until the part has cooled sufficiently to retain its shape.

4. The machine then indexes the mold to the loading and unloading station.  The mold is opened and the part removed.  A new batch of material is then placed in the cavity, the mold is closed and the process is repeated.

9-Thermoforming

Thermoforming involves heating a flat sheet of thermoplastic material until it softens and is stretchable. The hot sheet is then forced against the contours of a mold by vacuum, pressure, mechanical means, or a combination of all three. After cooling, the plastic sheet retains the mold shape and detail.

Thermoforming is most commonly used to produce mass parts for the packaging industry.  Thermoforming is also used to produce large parts

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where the volume is too small to economically injection mold.  Tooling costs for thermoforming parts when compared to injection-molded tools are significantly lower.

Optimizing process window is critical to achieving high quality parts. Temperature distribution, wall thickness distribution, minimum parts thickness, surface finish, and possible material degradation must be considered. The size of the parts is limited by the size of the plastic sheet stock available and the geometry of the thermoforming machine. Draw depth and stretch ratio, draft angles, stiffening details, trim lines, and undercuts are also important considerations for this process.Plastic materials used for thermoforming must have enough hot melt strength to support themselves during the forming process

REFRENCES

[1] http://www.ticona.com/homepage

[2] http://www.plasticmoulding.ca/techniques.htm

[3] http://www.epa.gov/ttn/uatw/hlthef/methylme.html

[4] Compression Molding, ASM Handbook 2001, volume 21 Composites, Peterson, Charles W, Ehnert G, Liebold R and Kühfusz R pp516–535, ISBN 0-817170-703-9

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