液态硅胶射出包胶(P+R LSR overmolding) 脱胶问题
2015-10-14 11:45阅读：
液态硅胶射出包胶(P+R LSR overmolding) 脱胶问题
信越的KE2090-40 Select-Hesive LIMS 可以解决脱胶问题？
在以往的工作中，我曾导入欧洲某个知名智能腕带品牌的表带代工项目， 材料是液态硅胶包尼龙。图面要求材料是：Momentive LSR
2670 塑胶料是：Dupont PA 料。
但是在射出成型过程中出现脱胶问题。
曾经尝试了多种方法：如塑胶件包胶前清洁、加热、调配不同的胶水。似乎最终都没有彻底解决脱胶问题，不良率高达70%。
最后尝试了信越的材料。Shintsu CHIM-LIM，
以及同时使用了信越品牌的胶水，有所改善。但是最终还是改了产品结构，才使得脱胶问题彻底解决。
本人当时也参考了很多专业文献及报道。
尤其是下文提到的材料 信越 Shin-Etsu KE2090 Select-Hesive LIMS
silicon，号称可以彻底解决液态硅胶包塑胶成型过程中出现的脱胶问题，不需要塑胶件加热等包胶前的预处理。
当初我们也尝试了这款材料，据后来工程说有出油的现象，而且容易粘模。本人当初因为有事没有去参与试模过程，很遗憾，整个结论只是听说，当中的实情无法考证了。最后由于客户同意我们改结构。结构更改后这个脱胶问题就彻底解决了。我就没有纠结这个问题了。
但是有关液态硅胶包胶成型过程中的脱胶问题，似乎是个大问题，影响生产成本。一下这边文章提到的一些解决方案，如包胶前胶件清洁、加热处理、调配相匹配的胶水、不能使用脱模剂，脱模过程中，要轻柔，不能强拉扯等方案，的确可以改善包胶成型后液态硅胶跟塑胶间的粘合度，但是不良率似乎高的惊人，造成很大的浪费！使得制造加工成本剧增。
所以我个人认为如果产品涉及包胶的工艺的话，应该多从产品结构改良入手，从结构上彻底解决后续包胶工艺出现的脱胶问题。
以上论述仅仅代表本人的浅见，只是基于本人的以往经历体会。
欢迎同行专业人士的高见，让在下学习了，增长见识！
本人QQ: 744324088
参考文献出处：
http://www.ptonline.com/articles/do's-and-don'ts-for-overmolding-liquid-silicone-onto-ther
moplastics
Hard/soft overmolding has become a fundamental technique for
injection molders in recent years, and a growing number of
molders are extending that technology thermoset/thermoplastic
combinations.
Click Image to Enlarge
When one LSR material failed to achieve satisfactory adhesion to
this polycarbonate instrument housing, even after added
expense for UV pretreatment, a Select-Hesive LIMS material from
Shin-Etsu Silicones achieved a good bond with no
pretreatment and no rejects. (Details in sidebar below.)
Hard/soft overmolding has become a fundamental technique for
injection molders in recent years, and a growing number of
molders are extending that technology thermoset/thermoplastic
combinations. In some applications, liquid silicone rubber
(LSR) offers advantages over TPEs in its heat resistance, extreme
low-temperature flexibility, chemical resistance, and
inherent lubricity. While good long-term adhesion can be an issue
in overmolding TPEs onto rigid thermoplastic substrates,
the challenge is even greater with LSR on thermoplastics.
Preparing the substrate surface with chemical primers or plasma or
UV treatment has been one approach to the adhesion
challenge. To minimize processing steps, LSR suppliers have come
out with newer “primerless” or “self-adhesive” grades
that bond well on their own with thermoplastics (see accompanying
case history). Even so, there are some rules to observe
in order to obtain good results. Here are some key points of advice
offered by Eric Bishop, North American marketing
manager for Shin-Etsu Silicones of America, Inc. in Akron, Ohio
(shinetsusilicones.com). Shin-Etsu supplies Select-Hesive
LIMS for primerless adhesion to thermoplastics.
•Keep it clean. “Cleanliness is critical to adhesion, so
keep the substrate clean and dry,” says Bishop. That is less of
an issue if the substrates are molded in a two-shot mold with the
LSR, rather than molded separately and transferred from
one machine to another.
•Serve it up hot. “It is vital for substrate parts to be hot,”
Bishop states. “The bonding of LSR to the thermoplastic
is a chemical reaction. It needs a combination of time,
temperature, and pressure. In general, he says, hotter is better.
Typical mold temperatures for LSR are 300 to 400 F and the minimum
is 250 F. The LSR has to achieve at least that internal
temperature to cure adequately. Hotter is also faster—meaning
shorter cure cycles.
You also don’t want to overmold LSR onto a cold substrate, which
will slow curing, Bishop cautions. In two-shot
overmolding that’s not a problem—residual heat in the thermoplastic
part will help cure the LSR. But if the substrate is
molded separately, it may need preheating in a conveyor oven or
with a hot plate.
•Watch out for additives, mold releases. Bishop advises against
usin thermoplastic substrates with internal mold releases
or “self-lubricating” additives, though they do not always create
an adhesion problem. External mold releases are a
definite no-no. And avoid any additives containing sulfur or
amines, which inhibit cure of LSR. That means amine-based
antistats can be a problem.
•Demold gently. Even though LSRs have good “green strength” right
out of the mold, when the mold opens, adhesion and
curing may not have achieved their final state. So avoid pulling or
stretching the LSR during demolding. Bishop notes that
a PTFE mold coating can help with release of the LSR.
•Consistency counts. If you follow all the advice above, you should
get good results with self-adhesive LSR regardless of
whether you use two-shot molding in one machine or separate molding
in two machines. But with two machines, it definitely
helps to use automated (robotic) transfer from one to the other,
Bishop says. That way ensures a consistent substrate
temperature for overmolding and also avoids any contamination from
a human operator handling parts.
•Mechanical interlock helps. Even with self-bonding LSR, it’s good
insurance to incorporate some form of mechanical
interlock between the materials into the part design. Allowing LSR
to penetrate through-holes onto the back side of the
part is a good example. A rough finish on the overmolding interface
area can help, but isn’t required with a good self-
bonding material.
•Do a preliminary test. To get a good idea of how well your
substrate material will bond to a particular LSR grade, it’s a
good idea to send a representative part or sample plaque of the
substrate material to the LSR supplier for testing.
Self-Bonding LSR Solves Adhesion Problem Without Need for
Pretreatment
Last year, Amerimold Tech Inc. was facing serious cost and time
overruns if it could not solve an overmolding adhesion
problem. The Marlboro, N.J., custom injection molder was producing
a thermal controller housing for a leading manufacturer.
The chassis, front cover, and keypad were molded in polycarbonate,
and then the keypad was overmolded with LSR in a second
tool and press. Amerimold’s client had an existing loyalty to a
particular material supplier, whose self-bonding LSR
material was already specified for the project. However, the
material’s adhesion to PC was insufficient without
pretreatment. The client had tried having the parts overmolded in
Asia. But testing of initial samples produced a 60% scrap
rate due to inadequate adhesion of the self-bonding LSR to the PC
keypad.
An alternative solution would have required Amerimold Tech to spend
$17,000 for variable-frequency light-pumping (VFLP)
equipment to UV pretreat the PC surface prior to overmolding. VFLP
was tested and provided some improvement but it still
failed.
Having already invested upward of $70,000 on tooling for the
two-cavity LSR overmolding tool, Amerimold Tech needed to find
a prompt and efficient solution to avoid incurring even more costs
for pretreatment, rejects, and waste. Company president
Michael Schon contacted Shin-Etsu Silicones to ship overnight a
5-gal sample pail of its KE2090-40 Select-Hesive LIMS
product for a test run in the LSR mold.
A handful of test shots were molded with Select-Hesive self-bonding
material without any pretreatment. According to Schon,
“Incredibly, the sample run was successful on the first shot,
creating a perfect bond with no UV light treatment required
—and more importantly, no rejects.” After passing additional tests,
the client gave the go-ahead for a 25,000-piece
initial run. Because of the time savings due to the absence of
rejects, that initial run was expanded beyond the 10,000
parts initially scheduled.
The KE2090 Select-Hesive LIMS silicone comes in 10 to 70 Shore A
hardness. It bonds to PC, PBT, PPO alloys, PEEK, and
polyphenylsulfone, according to Shin-Etsu’s Eric Bishop.
