Author Archives: Veasna

About Veasna

After interning for more than a year at Albright Technologies, Veasna gained experience in many aspects of silicone manufacturing including understanding of materials, mold making, and silicone molding. Veasna received a Bachelors of Science in Plastics Engineering from University of Massachusetts in Lowell in 2010, shortly after he accepted a full time position at Albright Technologies. In the spring of 2011, he received a Graduate Certificate in Plastics Design from UML. Veasna is mainly responsible for managing customer’s projects which begin from signed purchase orders to accepted and on-time part deliveries, he is also actively involved in research work on silicone bonding.

What is the difference between silicone compression molding and silicone injection molding?

Question: What is the difference between silicone compression molding and silicone injection molding?

Answer: In the compression molding process, the mixed silicone is filled into an open mold. For a core and cavity mold, the cavity side is usually filled with excess material. Once filled, the two halves are then uniformly compressed under hot platens. After a certain amount of curing time, the two halves are either manually or mechanically opened. The final part is removed after excess material is detached.

In the injection molding process, the A and B component materials are mixed from the mixing unit, and immediately travel through the cool barrel. The mixed material is then injected in a hot closed mold. After the curing and mold-open phase, the final part is removed from the mold.

What is HCR?

Question: What is HCR?

Answer: This heat cure rubber or high consistency rubber is an old technology. Its physical form appears to be a solid block of rubber as pre-cure material. As a result of thixotropic untreated silica and a very high viscosity polymer, HCR is a very high consistency material. Since untreated silica is very thixotropic, it requires high shear to break the hydrogen bond. This leads to a reduction in viscosity and flows into molds. High pressure presses are typically used to mold HCR. The properties of HCR in comparison with other silicone materials are illustrated in the table below.

Type of Silicone Rubber Tensile Strength Tear Strength Elongation

Units: Mpa Units: kN/m Range

HCR – High Consistency Rubber 4 to 13 9 to 55 90 to 1120%

FSR – Fluorosilicone Rubber 9 to 12 18 to 46 160 to 700%

LSR – Liquid Silicone Rubber 4 to 12 11 to 52 220 to 900%

RTV – Room Temperature Vulcanize 6 9 370%

(160-60)

What is the typical price for producing an over-mold of silicone on an injection molded part?

Question: What is the typical price for producing an over-mold of medical silicone on an injection molded part? (not including the mold development)

Answer: It is difficult to estimate a typical price for a medical siliocone over-molded part because the complexity of a part needs to be accounted for. A molded part is carefully placed inside a medical silicone over-mold tool. Depending on the design of the tool, the molded part should be in a fixed position inside the tool, and shut-off consideration is also evaluated.

In addition to the complexity of the part, cycle time also comes into play for the pricing. If a molded medical silicone part is quite simple in geometry and does not require significant liquid silicone material injection, curing time will be reduced. Various processing parameters for liquid silicone molding can be either reduced or increased, depending on mold design and certain silicone materials. Another unforeseen factor is what can be the potential problem of molding. By going towards liquid silicone production molding, one may run into the risk of molding unacceptable parts. With the aid of the prototyping an over-mold part from the concept, it will assure the capability of the design.

If you have any other questions, please email Veasna Nhem directly at veasna@albright1.com.

How long does it take to perform over-molding?

Question: How long does it take to perform the over-molding? What are the typical curing durations inside the mold?

Answer: Curing time of over-molding process depends on two major factors:

  1. Although an increase in temperature will reduce the cycle time, it can deflect the secondary over-molded piece such as plastics. Temperature should be adjusted according to both materials’ processing specifications.
  2. Larger part dimensions require longer cycle time. An increase in wall thickness of the part geometry will consume more curing time.

The majority of molding time in each cycle is spent during a curing process. Once the part is cured, it can be immediately ejected. Similarly to silicone curing time, thermoplastics cooling time is also the longest phase in the molding cycle.

If you have any questions please feel free to post a comment or email me  directly at veasna@albright1.com.

Are you capable of over-molding medical liquid silicone to thermoplastic materials?

Question: In your prototyping experience, are you capable of over-molding medical liquid silicone onto thermoplastic materials? If so, what is the lowest melt temperature material that you have been successful without deforming the substrate (ABS, Nylon, Polycarbonate, etc.)? Additionally, do you find that a silicone primer is always necessary when over-molding?

Answer: In our experience, we have had success bonding medical silicone with certain thermoplastic materials. Since different types of materials require different temperatures to work with, it would solely depend on that specific material. Over-molding medical silicone to plastics is limited to either using low temperature or faster cycle time. Plastic substrates would not deform under low temperature, but it will increase the cycle time during the over-molding process. In contrast, using high temperatures can lower the cycle time, but can potentially damage the plastic substrate. Using silicone primer helps the over-molding process, there are some other options available such as adhesive grade medical silicone and surface treatments.

If you have any questions please feel free to post a comment or email me  directly at veasna@albright1.com.

What types of injection molding polymers can medical silicone be over-molded on?

Question: What types of injection molding polymers can medical silicone be over-molded on?

Answer: Medical silicone-plastics bonding has increasingly become a hot topic in the bonding industry and very limited research has been conducted to prove compatibility of the two materials. A ShiEtsu adhesion strength graph below illustrates a limited amount of polymers used in this experiment.

If you have any questions please feel free to post a comment or email me  directly at veasna@albright1.com.

What are the catalysts in medical silicones?

Question: What are the catalysts in medical silicones and what is the definition of medical grade silicones?

Answer: In medical silicones, only a platinum cure system is used due to the fact that this cure system does not produce any extractable. Peroxide silicone cure systems produce acid residue and out gassing of the peroxide as by-products and therefore, are not suitable for medical applications because they do not pass biocompatibility testing.

The term “medical grade silicone” simply implies to a silicone which is rigorously tested to certify into a Class VI material. There are three types of medical silicone: non-implantable grade, restricted grade, and unrestricted grade.

  • Non-implantable grade medical silicone: Although it is classified as Class VI material, its applications should not be implantable. It can still be in the direct contact with skin and blood, as long as it is outside the human body.
  • Restricted grade: This liquid medical silicone has exhibited superior compatibility with human tissues and body fluids, and an extremely low tissue response when implanted. The length of a medical silicone implant is generally less than 29 days, although it may vary accordingly to the liquid medical silicone manufacturers.
  • Unrestricted grade: This medical silicone material is vigorously tested to ensure its capability and human use. It also meets or exceeds all USP Class VI and many ISO-10993 test requirements. In contrast with the restricted grade, the length of implant will be considered more than 30 days.

If you have any questions please feel free to post a comment or email me  directly at veasna@albright1.com.

Is over-molding onto thermoplastic materials possible?

Question: In your prototyping experience, are you capable of over-molding onto thermoplastic materials? If so, what is the lowest melt temperature material that you have been successful without deforming the substrate (ABS, Nylon, Polycarbonate, etc.)? Additionally, do you find that a silicone primer is always necessary when over-molding?

Answer: In our experience, we have had success bonding silicone with certain thermoplastic materials. Since different types of materials require different temperature to work with, it would solely depend on that specific material. Over-molding silicone to plastics is limited to either using low temperature or faster cycle time. Plastic substrates would not deform under low temperature, but it will increase the cycle time during the over-molding process. In contrast, using high temperature can lower the cycle time, but can potentially damage the plastic substrate. Using silicone primer helps the over-molding process, while there are some other options available such as adhesive grade silicone and surface treatments.

If you have any questions please feel free to post a comment or email me directly at veasna@albright1.com.

Do I need different molds to have the same geometry parts made in different durometers of silicone?

Question: Do I need different molds to have the same geometry parts made in different durometers of silicone?
Answer: For the same part geometry, different durometers of liquid silicone can be made using the same mold. Part dimensions should be scaled to compensate the shrinkage after silicone molding. Different durometers have different shrinkage values. If the overall part size is small (less than 0.500 inch), the impact of the shrinkage value would not make a significant difference. In addition, if the part has a larger tolerance, the same mold would still meet the dimension specifications. In some cases, different durometers of the same grade material even have the same shrinkage values, the same mold can be then used for all. During prototyping stage, getting a feel of the part with different durometers can become more critical than meeting a tight dimension tolerance. Therefore, the same mold can be used to produce the same part geometry with different durometers.

If you have any other questions you can comment or ask me directly at veasna@albright1.com