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Part 1 of 2 Designfax, Tech for OEM Design Engineers 6/7/11 By Dr. David C. Raia, Overmolding Marketing Manager, Star Thermoplastics, Broadview, IL Preface:
So much to pass along and so little time and space to offer you insight and
some of my secrets. Thus, I have chosen to present these overmolding tips in two
parts, with the first covering the benefits, key product design criteria for the product,
processing, achieving optimum end-product quality and marketing, a brief look at
various overmolding process techniques, and finally some solutions to reduce flashing.
Then, in Part 2, I will offer the many factors that influence bond strength – color and tactile
feel desired, among them. I will also delve into measurement of tactile peel strength, adhesive
failure, mold design considerations, a brief look at molding
conditions, mold shrinkage, resonance time, regrind, and purging.
What product development steps are
required to produce a successful
overmolded product?
The answer is not simple because it
involves numerous decisions involving
industrial design, engineering, marketing,
and sales. For a product to be successful
in the marketplace, the concept must
focus on actual functional customer
needs and balance specific product
benefits against cost. The benefits of overmolding
The overmolding process provides both
the product design team and the marketing
and manufacturing team very
unique product benefits not easily
achieved with normal assembly techniques. Overmolding simplifies the attachment of
one material component onto another without the use of fasteners and the application
of adhesives. The bonding of the two parts is accomplished in the injection mold. The
benefits are that overmolding enables the combination of two or more materials with
different hardnesses, potential for different colors, gloss, texture, and tactile feel to
enhance both product function and point-of-sale appeal. The process enables the bonding of soft-to-soft materials, soft materials to hard
materials, and hard-to-hard materials. When the design team focuses on the elements
that are important in overmolding, one can achieve robust reliability, point-of-sale
appeal, and very cost-effective benefits. The marketplace is filled with hundreds of
products that use overmolding to enhance their appeal. Typical examples can be seen
in products like power tools, hand tools, knobs, casters, office products, commercial
products, footwear, paintball masks, and pens with soft grips that improve function,
aesthetics, and point-of-sale appeal. Key product design criteria:
Product design
1. What substrate and TPE form an effective bond?
2. What desired elastomer hardness is required for the product?
3. What tensile peel strength do you need?
4. What is the minimum wall thickness desired?
5. Will the product be handled extensively or only a few times?
6. What are the maximum storage and use temperatures for the product?
7. What overmolding color is required?
8. What gloss level and texture is required?
9. What abrasion level is required for the product?
10. How will the product be cleaned?
11. Will this product be exposed to solvents?
12. Will this product be used indoors or outdoors and for what durations?
13. Is color shifting over the life of the product a problem? If so, how much can be tolerated?
Injection-molding processing
1. Where is the overmolding going to be gated for injection molding?
2. What is the maximum flow distance from the gate?
3. What size vents are being used in the mold?
4. What type of shut-off will be used with the overmolding?
5. What is the number of parts to be produced annually?
6. What size presses will be used in molding, and what are their shot capacities?
7. What is the overmolding resinʼs shot volume, and how does it match up with the injection-molding press to be used?
8. What is the scrap rate from runners and sprue?
9. Will the scrap be recycled?
10. Do you have access to desiccant drying equipment?
11. The volume of production determines what type of molding will be performed:
a. Low-volume production may be better suited for either two-stage injection molding, where the substrate is molded in one press, or in one side of a two-shot press mold, then inserted into another press or another cavity within the same mold to be overmolded with the TPE.
b. The other method involves rotating tooling, or rotary presses or presses with shuttle tables.
Quality
1. What is the minimum peel strength required, and how will you test for it?
2. What is the mean time to failure for the product and overmolding?
3. What is the resulting risk if delamination should occur during use?
4. How will the product be field tested, and how many units are required to provide the desired confidence level to meet your quality standards?
5. If the parts are going to be colored, how critical is the color match over the life of the product?
6. Does the part require color matching? To which light sources?
7. Are there other aesthetic parameters that must be characterized like gloss or gate blushing, and how will these be measured?
8. How will you track process parameters, regrind levels, and dryer performance for your application?
Marketing
1. What is the product life?
2. What is the length of product warranty or implied warranty?
3. What are customer requirements and expectations?
4. Does overmolding provide a cost-effective benefit?
5. Does overmolding provide cost-effective point-of-sales appeal?
Overview of overmolding process techniques
When there is no bond between two different materials, then the designer must use capturing geometry to keep the parts together. If application of capturing geometry is not properly designed, then the bond between the two different materials will not look aesthetic nor feel right because there can be movement between the two components that, in time, can lead to bonding failure through tear and peel. The use of physical interlocking designs is a useful technique to ensure that the two materials cannot separate under severe use. The diagram below is a cross-section through an overmolded component. This concept is one of many types of interlock designs for injection molding.
Example of capturing geometry.
The gray area in the illustration above depicts a cross-section of an overmolded TPE material physically bonded onto a substrate. The physical interlock provides a fastening technique when chemical adhesion of the overmold is minimal or nonexistent. An example of this type of bond is given in the following picture taken from an ice-shoe design, where the actual bond strength was reasonably strong, but the designer wanted to ensure that the two surfaces would not delaminate during rigorous use. A suspender-and-belt approach was used to ensure product reliability, preventing catastrophic material separation. The technique enabled the designer to use the second shot to hold carbide inserts and provide support for them in a harder material.
A series of experimental paper rollers for photocopy machines provides another example of interlocking geometry. The elastomer had minimal bonding, but the desire was to prevent the elastomer from slipping off the polycarbonate roller and rotation of the elastomer around the plastic substrate. The elastomer was injection molded onto the polycarbonate roller and was held by hoop stress from rotating around the substrate. The splines were good in theory, but resulted in voids due to melt turbulence during injection molding.
Xerox rollers.
A simpler solution to molding two materials together is to achieve chemical bonding between two or more materials to prevent delaminating under severe use. Overmolding has been used in many products like caster wheels, toothbrushes, razors, Power tool grips, automobile key entry fobs, and medical equipment to name a few where the overmolded resin adheres to the substrate.
Shut-off designs
The melt viscosities of many overmolding compounds for injection molding are low to facilitate filling of thin wall sections over long distances. These compounds are highly pseudo-plastic in nature, and their melt viscosity drops quickly with increases in both melt temperature and apparent shear rates. Optimum molding conditions for these resins usually require fast injection rates to fill thin wall sections, which can result in flashing between the substrate and the TPE overmolding resin. There are several methods to reduce flash at the parting lines between the overmolded TPE and the substrate.
Shut-off designs.
Solutions to reduce flashing of overmolded TPE
There are four basic methods for controlling flash in overmolded products. These methods often require interaction between the mold maker, molder, and compounder.
1. With sophisticated electric or hydraulic injection-molding machines, the converter can minimize flash by programming the injection speed, pressure, and injection time profiles carefully.
2. Balance off bond strength with flashing by reducing the melt temperature of the compound, thereby increasing the apparent melt viscosity.
3. The compounder can adjust the melt viscosity and shear sensitivity of the compound for a particular application.
4. The best solution is the use of good shut-off designs at the end of the TPEsubstrate interface. These designs can be very effective at controlling flash if properly implemented. The shut-off is designed to block and reduce flow of the melt front, thereby reducing flash.
Factors that influence bond strength
There are many factors that influence bond strength between two different materials and how they will be used:Material characteristics and selection
Material characteristics that are important to the designer using overmolding compounds are UV resistance, color, and tactile feel; wear resistance, bond strength, and environmental resistance.Color
The overmolding compounds developed by many TPE producers for polar substrates are opaque resins that range from white to off-white color. They generally can be colored with TPU or olefin-based colorants ranging from 2% to 3% depending upon the color saturation desired. Colors can range from standard colorants to fluorescent colors.
Designs where polypropylene homopolymers and copolymers are used can use many of the standard resins that companies produce. At least one firm has developed a number of ultra-clear overmolding compounds for propylene homo- and co-polymers that range in hardness from 15A to 70A.
Tactile feel
Overmolding a soft TPE compound onto a harder substrate like polycarbonate or polypropylene will produce a product that exhibits better grip and limited cushion depending upon the thickness of the overmolded TPE.Most overmoldings onto polar substrates use TPE compounds that range from 50A through 73A in hardness. When molded onto a hard substrate like ABS or polycarbonate, they can be molded from 0.040-in. thickness to 0.200-in. thick with no problem. Grades are available to extend filling and bonding into thinner areas.
Overmolding compounds for polypropylene can be designed to have extra tact for handles and power tools if the designer wishes to enhance that specific property.
Injection molding or extrusion process parameters
One or more of the more-advanced TPE suppliers has developed a line of highperformance overmolding TPE resins that can provide exceptional bond strength, color, and weather resistance at an affordable price. These overmolding TPEs can be quite successfully overmolded onto substrates including: ABS, ABS/Polycarbonate Alloy, Acrylic, ASA, Cellulose Propionate, Amorphous polyesters; Polycarbonate, Nylon 6, Polypropylene, Polysulfone, and Polyurethane.
The bonding results to different plastics vary according to TPE durometer, processing settings, and the material combinations.
The following chart illustrates some of the results from research and what customers have experienced.
Measurement of 90° tensile peels strength
When overmolding one material onto another, the bond strength will be both physical and chemical on most components. For example, if one molds an elastomer onto an ABS roller that is not functionalized to chemically bond to the ABS hub, the bond will be purely physical or hoop stress will occur from the higher-shrinkage elastomer. If you were to cut the TPE, you can easily strip the two materials apart.
Here at Star, we mold special overmolding grades onto a flat surface that essentially eliminates any possibility of physical attachment. The substrate is mounted into one jaw of a universal tensile testing machine along with a 90° tensile peel fixture that keeps the forces normal to the axis of pull. The overmolding TPE is mounted into the other jaw and the samples are pulled at 2 in./min.
Usually the bond strength is greatest at the gate and drops as the flow distance increases. The values generated are in lb/in. of peel. They are usually reported as high, average, or low values. Because every product design, flow distance, and molding condition is different, it is impossible to determine the exact bond strength on each part in every direction. It is the responsibility of the designer to fully test the product to ensure that the materials, design, and processing are working to expectations.
Tensile peel bond strength cannot be discussed properly without discussing modes of failure when the maximum bond strength is achieved.
Cohesive failure
All bonds between a substrate and a TPE overmolding compound are cohesive in nature. The delamination of the components always leaves TPE resin adhered to the substrate as the rubber is pulled off. The adhered layer can be only a fraction of a micron thick to a substantial thickness easily observed by eye. The thickness of the cohesive layer depends upon the melt temperature of the overmolding resin during filling and its apparent melt viscosity.
Mold design
Molds construction and design for performing overmoldings of two different materials depend strongly on the annual production volume expected for the product and the release quantities to be molded.
The production volume determines the type of press the part may be molded on and the actual construction design of the mold. The following table is a general guide for tool construction:
There are many options available to the molder and OEM with respect to tooling. Generally, if volumes are low, aluminum tooling may work satisfactorily for both the substrate resin and the TPE. As production volume climbs, it is important to design tooling for reliability and the required production volume that will be produced. Although aluminum provides exceptional heat transfer, it will not resist high clamping forces that occur in molding resins like polycarbonate or Nylon 6, as well as steel tooling. In addition, steel tooling will exhibit better wear resistance.
The following tooling mold design guidelines are suggested:
1. Use a minimum of 1.5° per side on deep-draw surfaces with higher draft requirements for softer-durometer overmolding TPE compounds.
2. Where possible, use vented pins to avoid gas entrapment. Blind corners can result in adiabatic burning of resins, resulting in not only inconsistent filling but also scorching of the overmolding resins.
3. Tool cavities should be draw-polished to avoid hang-ups of material in the cavities.
4. Tool cores and cavities should be coated with a lubricated coating. There are many available, from Teflon to Zirconium Nitride.
5. Vents should be 0.0005-in. deep to avoid gas entrapment, as the overmolding resin requires high-speed injection burning due to entrapped gasses.
6. Round runners are best because they result in optimum flow with minimum shear and frictional heating.
7. Balanced runners are essential for multi-cavity molds to ensure even filling pressures, times, and heat to all cavities.
8. Tab and submarine gates should not have excessive land lengths because they will result in poor pressurization of the overmolding melt.
9. Gate diameters should not be under 0.40 in. in diameter.
10. Tab gates should not be less than 0.030 x 0.30 in. cross-section and have land lengths to exceed 0.060 in.
General molding conditions for typical Star-branded TPE resins
Here we provide some general guidelines and insight:
1. A mold release should never be used in the molding of selected TPE resins (at least from Star). If release is required, it is advisable to contact the producer for assistance.
2. Most TPE overmolding resins do not usually need to be dried in order to achieve good bonding.
3. Some resins, however, do require 4 hr of drying in a desiccant dryer (with a capability of achieving a dew point of -40°F) at 170°F to 180°F until moisture levels are in the 0.02% range or lower.
4. When using certain types of resins, it is only necessary to surface dry substrates that are hydroscopic like CAP, polycarbonate, or Nylon 6 to remove surface moisture. This does not apply to molding where the substrate is molded in one molding cycle and is immediately overmolded in the next cycle.
5. For select TPE resins, one should employ high injection rates to minimize the melt front from cooling during injection molding.
6. Some overmolding TPEs should employ high first-stage injection pressures slightly less than those that will flash the mold, then reduce the pressure only 20% for the balance of the cooling cycle until the melt has frozen. Cooling should continue until it is safe to eject the part without distortion or premature peeling.
7. Overmolded parts will not achieve a stable peel strength for at least 6 to 8 hr after molding, so they should be handled with care. Within 24 hr they will achieve maximum strength and physical properties.
Mold shrinkage
It is suggested that molders contact the overmolding TPE producer for specific shrinkage rates of individual overmolding resins. In general, shrinkage of TPEs from a company like ours ranges from 0.025 to 0.035 in./in. Shrinkage is a function of melt temperature, mold temperature, injection speed, injection pressure, and composition of the TPE.
Resonance time
It must be noted that TPE materials, like nearly all thermoplastics, can degrade at process temperatures required to melt them. As the process temperature increases, and the length of time at that temperature is extended more, degradation can occur. Our firm, like other TPE producers, heat stabilizes our resins very well to minimize this effect with antioxidant systems. However, the processor can minimize these effects by choosing the proper press. The shot size should be matched up so it takes between 30% to 60% of the barrel capacity. Resonance times should be kept to 5- to 8-min. maximum times. If the equipment is out of cycle for more than 5 min., purge the machine. Upon restarting the cycle, throw away the first five to six shots to minimize degraded resin from contaminating the product and reducing physicals.
Regrind
The use of regrind overmolding compounds from sprue and runner systems can safely be added up to 20%. Many TPEs can be added uniformly back to virgin resin up to the 25% to 30% range without problems, but many TPE resins should not exceed 20% to ensure good physicals, rheology, and reduced color shifting.
Purging
Upon completion of the manufacturing run, purge the resin from the barrel with polypropylene at the process temperatures used, and then lower the temperature to 380°F and continue to purge until the melt is clean. In conclusion, there are many variables to a good overmolded product. Any TPE supplier stands ready to help you come up with a quick, easy, and hopefully cost saving solution.
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