Short lengths of 7-cm ID polyethylene pipe are cast in a mold which has a core made of room-temperature-vulcanizable (RTV) silicone. Core expands during casting and shrinks on cooling to allow for contraction of the polyethylene

Elam, R. M., Jr.

English

NASA Technical Reports Server

February 1973	 B73-10032 
•	 NASA TECH BRIEF 
Ames Research Center
	 -4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Method for Casting Polyethylene Pipe 
The problem: 
To cast short lengths of 7-cm (2.75-in) ID poly-
ethylene pipe with 0.64-cm (0.25-in) walls; standard 
metal molds cannot be used because shrinkage around 
the metal core cracks the plastic.
Tube 
Bri
tinum 
The solution: 
Cast the polyethylene in a mold which has a sili-
cone core to allow for contraction of the polyethylene. 
How it's done: 
The mold for the outer diameter of the pipe is made 
of an aluminum cylinder which is 5 cm (2 in) longer 
than the required length of the finished polyethylene 
pipe. Since polyethylene shrinks when it cools, the 
outer diameter of the pipe mold is made 1.65 mm 
(0.065 in) oversize. The bottom of the mold is a re-
movable aluminum plate provided with a centered 
brass rod (secured by a bolt) which acts as a locator
for the core. 
The core is also 5 cm (2 in) longer than the finished 
product, and it is made by casting a room-temper-
ature-vulcanizable (RTV) silicone around a 2.2-cm 
(7-in) diameter piece of copper tubing which is about 
2.5 cm (1 in) longer than the RTV core. The bottom 
of the core has a 4-degree taper to keep air from 
being trapped under the core during casting of the 
pipe. Also, the core mold is made 0.127 mm (0.005 in) 
oversize; the silicone core expands during the casting 
of polyethylene and shrinks on cooling. After the 
RTV silicone cures, the copper tubing can be re-
moved; a collar is fastened flush around the upper 
part of the copper tubing and the lower part of the 
tubing is trimmed so that it protrudes about 0.25 cm. 
Cores can be used about 10 times before they soften 
enough and let the internal diameter of the pipe 
become oversized. 
The sections of polyethylene pipe are formed as 
follows: A thin film of silicone grease is wiped on all 
metal surfaces and then 135 g of polyethylene pellets 
are poured into the mold. The core is slid over the 
guide rod in the mold and rotated to distribute the 
pellets. .The assembly is placed in a vacuum oven and 
a 1-kg weight is set on top of the copper tubing; the 
weight forces the core down through the polyethylene 
as it melts. The oven is evacuated to about 47 kN/m2 
(14 in of Hg) and the temperature is brought to about 
138°C (280°F) where it is maintained for about 7 
hours. After the core reaches the bottom of the mold, 
the vacuum is released; 15 to 30 minutes later, the 
heat is turned off. The assembly is left in place to cool 
gradually (overnight). The castings come out of the 
mold easily, but the core must be forced out with the 
aid of a blast of air.
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19730000032 2020-03-17T06:37:01+00:00Z
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B73-10032
Patent status: 
NASA has decided not to apply for a patent.
	 I) Source: Ray M. Elam, Jr. 
Ames Research Center 
(ARC-10706)
I 
. 
B73-10032	 Category 08


Method for casting polyethylene pipe

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