Production flow improvements: a simulation study of the Frame factory at Volvo Trucks

Abstract: In Tuve outside of Gothenburg one of Volvo Trucks’ factories for final
assembly is located. In the same plant the Frame factory is located,
manufacturing frame sides for the Tuve assembly lines, as well as other
foreign plants. Companies in the heavy trucks market have been operating in
high capacity utilization for several years and the Frame factory at the
Tuve plant is no exception. Customer demand is exceeding current production
rate at the Tuve Frame factory and volume increases have been incremental
since last major reconstruction in 2005. Shifting bottlenecks are a major
issue at the Frame factory and an investigation of the production line and
the planning process is requested to detect possibilities for capacity
increases without major investments in new machines.

The purpose of the thesis is to investigate Volvo Trucks’ frame side
production line and give recommendations for controlling order releases and
increasing line capacity. The goals are to define the current state of the
production line and its’ constraints, develop an order release method, and
to build a simulation model for testing of order release methods and
examining constraint behavior.

The authors examined the production line with first hand observations of
the process flow and work methods. Logging statistics, process documents,
time studies and interviews with personnel at the Frame factory were used
as input for the empirical description, constraint analysis and simulation
testing. A tool for analysis and experiments was discrete event process
simulation in AutoMod™.

The current state of the Frame factory is a high volume general flow shop
with small buffer stocks and closely arranged resources similar to lines
described under the Just-in-time ideology. Highest utilization is found in
two early stations for punching and plasma cutting. The Overall Equipment
Effectiveness (OEE) is close to 60 percent in both stations which show high
levels of down time and speed losses. Product types with varying operating
times in Capacity Constraint Resources (CCR) create shifting bottlenecks
unless scheduling of order releases is leveled. Since buffers are small in
the Frame factory, unleveled scheduling will risk starving or blocking CCRs
which reduces throughput.

Recommendations are to focus on the punching machines and the plasma cutter
as bottlenecks. According to Theory of Constraints, concentrating on other
resources will not increase throughput but could reduce lead time and WIP.
A short setup time in the Rollformer is critical to avoid starving the
punching machines. Keeping the Punching buffer stocks filled before the
setup begins allows setup time variations without decreasing throughput.
The largest contributor to differences in lead time, WIP and throughput is
high variations in availability in CCRs. The current manual logging of
machine losses cannot account for all losses at current rate and
recommendations are to install automatic logging in CCRs. The product mix
should be monitored to maintain a leveled workload in resources to avoid
the shifting bottleneck phenomenon. Releasing orders with consideration of
short term loading in CCRs gives small increases in throughput of the
production line.