By Steve Gerbig, DSM Engineering Plastics
Frequently Asked Questions
- I’ve been making this plastic part for a long time and never had a problem. Now the parts are failing. What’s wrong?
- Whose hot runner system should I use?
- How much regrind can I use?
- How do I dry material for processing?
- When building a mold what type of steel should I choose?
- I know that I should use fillet radii when designing a plastic part in order to improve toughness. What size radius should I use?
- I make connectors out of LCP material and I need to reduce costs. What plastic material do you recommend?
I’ve been making this plastic part for a long time and never had a problem. Now the parts are failing. What’s wrong with this material?
Many things can change from one run to the next to cause part failure. First, check all aspects of the process to ensure nothing has changed. Were there any changes to the tool or molding machine? Are the parameters set properly? Beyond these, the most likely causes have to do with failure of some part of the process. Check the molding machine, the mold temperature controller, and the dryer to ensure proper functioning. If no root cause is found in these areas, contact your technical support representative. You will probably be asked to submit samples of the parts as well as the pellets from which these parts were made. The materials analysts will examine the samples using the latest in laboratory analytical equipment to determine the root cause of the problem.
Whose hot runner system should I use?
There are several hot runner manufacturers and in general, the systems are designed and built with quality materials and manufacturing equipment. Select a supplier with whom you are comfortable working then design the system to meet the following criteria:
- Allow for the melt characteristics of the material. (i.e. amorphous or crystalline, and melting point).
- Balance the flow. Multi cavity molds must fill equally. Flow paths must be the same length and have the same pressure drop from sprue bushing to the gate.
- Round all corners to eliminate dead spots.
- Size melt channels according to type of polymer being molded. In general, larger melt channels are used for shear sensitive materials. Smaller channels are used to reduce residence time.
- Maintain uniform temperature. This is very critical. Surface areas should be evenly heated. If drops are utilized thermocouples should be placed close to the tip to ensure accurate temperature control. Ensure there are no areas where the mold base is drawing excessive amounts of heat from the manifold.
How much regrind can I use?
The amount of regrind you can use depends on the material used, the care exercised in handling the regrind, and the requirements of the application. As a general rule, the more reinforcement used in a material, the more susceptible it is to damage due to shear during processing. This is not always the case however as we have run 40% carbon fiber reinforced materials through several generations of 100% regrind with no deterioration of mechanical properties. On the other hand, the most critical factor in using regrind is the condition of the material. Regrind must be kept clean, dry and free of fines. It is important to remember that the performance of the finished product is really what determines how much regrind may be permitted. Always test the finished product to see whether regrind-containing molded parts will meet the required standards. Furthermore, if the OEM customer specifies no regrind, do not be tempted to use regrind no matter how attractive the cost incentives may be.
How do I dry material for processing?
Many materials, including nylons, polyesters, and polycarbonates, are hydroscopic and, therefore, require drying prior to processing. Some drying of filled, non-hydroscopic materials, like polypropylene, is also suggested. If they are not dried a significant loss in physical properties, as well as appearance defects such as splay can result. For nylons, 2 to 4 hours of drying at 180°F is typically required. However, there is more to drying than simply time and temperature. Efficiency of the desiccant, operable rotation of desiccant, adequate and clean airflow, operable regeneration elements, and immediate and consistent use of regrind are also key factors for properly drying material. Ultimately, a consistently low moisture content in the pellets is the goal. For nylon 6 and 66, a moisture content between 0.08 and 0.12 % is preferable. Polyesters and polycarbonates require less with <0.01%. Nylon 46 requires less than <0.05%.
When building a mold what type of steel should I choose?
The type of mold steel to choose is dependent on resin selection, part geometry, and product life. Before selection is made it is important to have knowledge of what additives or reinforcements are in the plastic and an understanding of the steel composition. The following are steel selections that have been used with success in molding a variety of materials:
– P-20 is a prehardened steel that comes in a Rc of 30-36. Easily machined and polished, it is used for mold base plates, large cores and cavities, and manifold blocks. This steel is not recommended if you’re looking for abrasion and/or chemical resistance.
– H-13 steel can be hardened after machining to a 46-50Rc. It is used for medium cores and cavities, ejector lifts, and pins. It is excellent for abrasive wear from glass filled materials and it polishes and machines very well.
– S-7 and D-2 are very hard steel with a 54-58Rc. They offer excellent abrasion resistance, and, due to their hardness, some corrosion resistance. Recommended DSM materials include all glass fiber reinforced material. Higher hardness also means they’re more brittle. They can be used for core, cavities, runner and gate inserts, and stripper rings.
– 420-SS is a stainless steel that can be hardened to 46-50Rc. It can be used for mold base plates, large cores, and cavities. In the hardened condition you will get good corrosion and abrasion resistance from glass filled materials. Machining and polishing are very favorable with this steel.
– CSM 21-SS is a stainless steel furnished in the prehard condition around 32Rc and can be heat treated to a 44Rc. CSM 21, which can be used for mold base plates, main core, and cavity blocks or inserts, is easily machined and polished.
I know that I should use fillet radii when designing a plastic part in order to improve toughness. What size radius should I use?
The reason to use internal radii is to reduce stress concentration. There are ways to calculate the optimum radius vs. wall thickness, however, a radius of 1/2 the wall thickness generally gives good distribution of the stress, and hence, good toughness. While larger radii may reduce stress further, this improvement is incremental.
I make connectors out of LCP material and I need to reduce costs. What material do you recommend for this application?
LCPs are typically selected for connectors because of their high thermal performance, ease of mold filling in thin cross sections and their dimensional stability. Polyamide 46 has both been successfully used in existing tools built for LCP. These glass reinforced, flame retardant 46 nylon products offer thermal performance near that of LCP and can generally be used in the same molds without extensive modification. The cost savings resulting from making this switch typically amounts to about two dollars per pound.
About the Author
|Steve Gerbig, DSM Engineering Plastics
P.O. Box 3333
2267 West Mill Road
Evansville, IN 47732-3333
|With 40 years of experience in the plastics industry, Steve Gerbig has collected a vast amount of expertise in the area of engineering plastics. Since 1981, Steve has worked within DSM Engineering Plastics as a technical service engineer, market development specialist, product manager, and currently the technical support manager.Prior to DSM, Steve worked as both product and tool design engineer, sales engineer, manufacturing manager and general manager of several molding companies. Steve graduated from the University of Evansville, IN. Over the years he has presented papers at SPE Annual Technical Conference (ANTEC) and SPI Plastic Parts Innovation Conference, and other national technical conferences.Steve has written the chapter on reinforced thermoplastics for the Encyclopedia of Polymer Science and Engineering, and had papers published in numerous technical journals including Plastics Engineering and Design News.|
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